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RT/CE/C/025 Issue: 1 Date: February 2001

RAILTRACK LINE CODE OF PRACTICE The Structural Assessment of Underbridges

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and information relating thereto, is Kim Teager, Professional Head Of Structures Engineering

Accepted for Issue by:

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Authorised by

Published & Issued by Railtrack plc Railtrack House Euston Square LONDON NW1 2EE © 20 01 RA ILT RA CK PLC

RAILTRACK LINE CODE OF PRACTICE

The Structural Assessment of Underbridges

RT/CE/C/025 Issue: 1 Date: February 2001 Page 2 of 11

SUMMARY This Code of Practice provides recommendations for the parameters and methods to be used for the assessment of underbridges owned by Railtrack. ISSUE RECORD This Code of Practice will be updated when necessary by distribution of a complete

replacement revised black sections. Amended or additional parts of revised pagesofwill be marked byora vertical line in the margin. Due to the extensive number revisions compared with Issue 1 such changes have not be marked in this Issue. ISSUE 1

DATE

February 2001 COMMENTS: New Code of Practice to provide a limit state code for assessment of underbridges in respect of steel, wrought iron, and concrete and composite bridges, and to codify permissible assessment parameters and methods for under bridges formed from other materials of construction.

RESPONSIBILITIES AND DISTRIBUTION This Code of Practice should be used by persons undertaking the assessment of underbridges and by those responsible for managing the process of bridge assessment carried out by others. IMPLEMENTATION This Code of Practice should be complied with from April 2001. DISCLAIMER Railtrack PLC has used its best endeavours to ensure that the content, layout and text of this document are accurate, complete and suitable for its stated purpose. It makes no warranties, express or implied, that compliance with the contents of this document shall be sufficient to ensure safe systems of work or operation. Railtrack PLC will not be liable to pay compensation in respect of the content or subsequent use of this document for any purpose other than its stated purpose or for any purpose other than that for which it was prepared except where it can be shown to

have acted in bad faith or there has been wilful default. SUPPLY Paper copies of this document will be available to Railtrack staff on request to the Document Controller. Copies of this document will be available to other organisations from Technical Indexes (01334 404409).


The Structural Assessment of Underbridges


CONTENTS SECTION1

INTRODUCTION

SECTION 2

ASSESSMENT PHILOSOPHY

SECTION 3

INSPECTION FOR ASSESSMENT

SECTION 4

LOADING FOR ASSESSMENT

SECTION 5

STEEL AND WROUGHT IRON STRUCTURES

SECTION6

MASONRYARCHES

SECTION 7

CONCRETE STRUCTURES

SECTION 8

COMPOSITE STRUCTURES

SECTION 9

CAST IRON STRUCTURES

SECTION 10

TIMBER STRUCTURES

SECTION11

SUBSTRUCTURES

SECTION12

BEARINGS

APPENDIX A

ASSESSMENT OF STEEL AND WROUGHT IRON

APPENDIX B

ASSESSMENT OF CONCRETE STRUCTURES

APPENDIX C

ASSESSMENT OF COMPOSITE STRUCTURES

APPENDIX D

FATIGUE ASSESSMENT OF STEEL AND WROUGHT IRON

APPENDIX E

MODEL BRIDGE ASSESSMENT REPORT

APPENDIX F

INFORMATIVE ANNEX


The Structural Assessment of Underbridges Section 1 - Introduction


CONTENTS

1. INTRODUCTION...................................................................................................................4 1.1 Purpose................................................................................................................................4 1.2 Scope....................................................................................................................................4 1.3 ..................................................................................................................................... 5 1.4 Units Definitions and Abbreviations.........................................................................................6 1.5 Competency ....................................................................................................................... 6 1.6 Procedures for Quantitative Assessment.....................................................................7 1.7 Qualitative Assessment Procedures............................................................................10 1.8 Railtrack s Technical Approval Procedures ...............................................................10 1.9 Reporting...........................................................................................................................10 1.10 Informative Annex.........................................................................................................11 1. INTRODUCTION 1.1 Purpose

The purpose of this Code of Practice is to recommend applicable standards and analytical methods which may be used to determine the load carrying capacity of existing Railtrack underbridges, in terms of British Standard Units of Type RA1 loading. The load carrying capacity is determined in the context of the performance requirements of an underbridge. The requirements are that the bridge meets safety and serviceability criteria whilst regularly carrying rail traffic up to a level of traffic load and speed in accordance with operational system requirements. 1.2 Scope

This Code of Practice may be used for the assessment of all Railtrack owned underbridges and is applicable for permissible speeds up to a maximum 125 mph. This Code of Practice provides recommendations for the assessment of underbridges constructed from steel, wrought iron, cast iron, concrete, timber, or composite steel/concrete construction. Recommendations for masonry arches, substructures and bearings are also included. Limit state principles are used for underbridges of steel, wrought iron, concrete and steel/concrete composite construction. Permissible stresses or allowable loads are used for other materials and forms of construction. Where appropriate, guidance on the use of simple and more rigorous methods of analysis is given. Unusual forms of construction such as cable stayed, moveable or combined road/rail bridges are not specifically covered, but the principles outlined may be applied in checking the elements of such structures.




Page 5 of 11

Requirements for the assessment of superstructures and supports under accidental loading conditions are excluded from this document. 1.3 Units

The S.I. system of units is adopted throughout this Code of Practice unless otherwise stated. In the course of assessment frequent reference may have to be made to existing records which may be presented in Imperial Units. Great care should be exercised in the conversion between the two systems of units. The following table gives conversion factors for some of the most commonly occurring units.

PROPERTY

IMPERIAL UNIT

METRIC equivalent of IMPERIAL UNIT

METRIC UNIT

Length

inch foot yard mile chain

2.5400 0.3048 0.9144 1.6093 20.1168

cm m m km m

Area

inch² inch² foot² yard²

645.1600 6.4516 0.0929 0.8361

mm² cm² m² m²

Volume

inch³ foot³ yard³

16.387 0.0283 0.7646

cm³ m³ m³

Mass

lb ton ton

0.4536 1016 1.0160

kg kg tonnes

Modulus

inch³ inch³

16387 16.387

mm³ cm³

Inertia

inch4 inch4

416200 41.62

mm4 cm4

Speed

mph

1.6093

kph




Page 6 of 11

Table 1.1 Conversion Factors

1.4 Definitions and Abbreviations

For the purpose of this Code of Practice the following definitions apply: Bridge means a structure of one or more spans whose prime purpose is to carry traffic or services over an obstruction or gap. PSR means Permanent Speed Restriction. Provisionally Sub-standard Bridge means a Bridge that has been assessed at the Level 1 assessment stage of the Bridge Assessment process to have a safe load capacity less than the RA Capacity of the route. The Bridge remains Provisionally Sub-standard until it is confirmed on completion of the Bridge Assessment that the safe load capacity is not less than the RA Capacity of the route or the Bridge is classified as a Sub-standard Bridge.

Serviceability Limit State (SLS) means the condition at which the behaviour of a Bridge becomes unsatisfactory to the extent that it can no longer satisfactorily perform its function under service loads. Sub-standard Bridge means a Bridge where, following completion of a Bridge Assessment, action(s) is (are) required to protect the safety of the Bridge. A Bridge remains classified as Sub-standard until actions are taken to remove the applied controls, or the RA Capacity of the route is amended to not more than the safe load capacity of the Bridge. TSR means Temporary Speed Restriction. Ultimate Limit State (ULS) means the condition at which the Bridge, or one of its constituent parts, would fail due to loss of equilibrium, fatigue induced deterioration, or exceedance of its collapse strength. Railtrack Director s Nominee means the Structures Engineer with formally delegated responsibility for the assessment of underbridges within the Railtrack Zone. 1.5 Competency

The skills, expertise and training of those persons responsible for, and carrying out, the assessment should be appropriate to the nature and complexity of the structure under consideration.




Page 7 of 11

1.6 Procedures for Quantitative Assessment

The assessment should commence with a definition of the assessment objective. A clear statement of the required load carrying capacity should be made. In particular, it should state specifically the existing RA capacity of the structure, the existing RA capacity of the route and whether an increased structure capacity greater than that of the route is required. The initial assessment (Level 1) should generally comprise three distinct phases as follows: 1.

Desk Study All available information relevant to the structure, including record drawings, inspection and maintenance records, details of past performance and previous assessments, and any available ground investigation data should be collated and examined. The documents should be verified for correctness and in particular, whether they were updated after previous works on the structure.

2.

Inspection for Assessment A detailed examination of the structure is required to verify the form of construction, its dimensions and the nature and condition of the structural parts.

3.

Analysis Based on the information obtained from the first two phases of the assessment process, structural analysis to determine the distribution of forces within the structure and the load capacity of the structural parts is required in most cases.

In order to determine the adequacy of a particular structure with the minimum degree of effort, the assessment should be carried out in levels of increasing refinement and complexity, with the initial level (Level 1) being based on the most conservative distributions of loads and analytical assumptions. If the structure is shown to be inadequate in relation to the required load carrying capacity at this level, assessment work should continue, with subsequent levels seeking to remove conservatism in the assessment where this can be justified. Subsequent more detailed levels may use:

• • • •

more refined structural analysis; more precise estimates of loading based on real vehicles; material properties based on testing of materials samples; supplementary load testing.

As illustrated in Figure 1.1 the process is cyclical in nature, each cycle being at an increasingly refined level until a decision on the adequacy of the bridge is reached. Conceptually it is useful to envisage levels of assessment as follows:




Page 8 of 11

Level 1

Simplest level using assumptions known to be conservative.

Level 2

Use of more refined analysis and better structural idealisation. This level may also include use of data on materials strengths based on mill test certificates or recent material tests on another similar structure.

Level 3

Use of a bridge specific live loading based on the known traffic and/or the use of tests on materials samples or the use of worst credible strengths or the use of load tests.

Where, by inspection, it is considered that greater benefit may be gained by the adoption if live loading based on real trains than from a more refined analysis, the assessment may progress from Level 1 directly to level 3. The conclusion from the assessment should be subjected to a plausibility check. In particular, discrepancies between the results of structural analysis, indicating inadequacy say, and the real structural condition, for example no sign of distress or failure, should be explained.




Page 9 of 11

Review past performance and inspection data. Decide assessment objective.

No

Analytical assessment? Yes

Qualitative assessment •Review structure

Pass

Fail

Level 1 assessment •Analysis

Pass

Fail Provisionally sub-standard structure

Yes

Urgent safety measures?

Urgent safety measures

Yes

Implement measures

No

No Further investigation and review

Pass

Level 2-3 assessment

Pass

Fail Fail Review safety measures Review assessment objective

Implement measures

Review assessment objective Assessment report

Sufficient capacity/ adequate condition?

No Operational restrictions /repair/upgrade?

Figure 1.1 Assessment Process Flow Diagram

Yes

Bridge management programme •Periodic inspection •Maintenance •Performance review




Page 10 of 11

1.7 Qualitative Assessment Procedures

For some types of structure where no established method of quantitative theoretical assessment exists and where increased capacity is not required, assessment may be made qualitatively on the basis of satisfactory past performance. Structures for which this procedure may be considered are spandrel and dry stone walls, retaining walls, jack arches, substructures and foundations. The requirements for assessment on this basis are:

•

the structure has demonstrated satisfactory performance over a long period of time (over 5 years) since any significant repairs or alteration;

•

careful inspection does not reveal significant damage, distress or deterioration;

• • •

review of the structure confirms its force transfer system; predicted future deterioration will not jeopardise safety; no significant changes in the loads and actions on the Bridge are anticipated.

Where the assessing engineer proposed a qualitative method of assessment, this shall be justified and recorded in accordance with Railtrack s Technical Approval Procedures. 1.8 Railtrack s Technical Approval Procedures

All assessments shall be subject to Railtrack s Technical Approval Procedures for assessment. Irrespective of whether the assessment is to be carried out on a quantitative or qualitative basis, the chosen method should be recorded and justified within the Form AA. Where a qualitative method is proposed for the assessment of one of the structure types identified in Clause 1.7, reference to this document may be deemed to be sufficient justification for adoption of the method. For the assessment of Bridges or structural elements which are outwith the scope of this document, the method of assessment should be agreed within the Technical Approval Procedure by Railtrack s Professional Head of Structures Engineering. 1.9 Reporting

When the assessment has been completed, a report should be prepared detailing the various stages of the process, together with the results. A suitable format for the assessment report is given in Appendix E. Summary tables for reporting the assessment results have been included in Appendix E for metallic structures, masonry arches and concrete structures. These summary tables should be completed and incorporated in the final report where applicable.




1.10 Appendices

Additional notes and further information relating to the assessment of underbridges are contained in Appendices A to F. Clauses within each appendix are numbered sequentially from 1.0 and are followed by a letter denoting the appendix to which they belong. For example Clause 4.1.1B indicates Clause 4.1.1 of Appendix B. 1.11 Informative Annex

Background information on the derivation of certain clauses of this code of practice and guidance on its usage is contained in Appendix F. It should be noted that this Appendix is not intended to give comprehensive guidance, and should not be assumed to indicate all aspects of a structure that should be checked in the course of an assessment.


The Structural Assessment of Underbridges Section 2 - Assessment Philosophy


CONTENTS

2. QUANTITATIVE ASSESSMENT PHILOSOPHY...............................................................1 2.1 Applicability.........................................................................................................................1 2.2 Basis for Quantitative Assessment.................................................................................1 2.3 Assessment Situations ...................................................................................................... 1 2.4 Limit States..........................................................................................................................3 2.5 Assessment Load Values .................................................................................................. 4 2.6 Load Factors ....................................................................................................................... 4 2.7 Assessment Load Effects .................................................................................................. 6 2.8 Assessment Resistance.....................................................................................................7 2.9 Verification of Structural Adequacy...............................................................................8 2. QUANTITATIVE ASSESSMENT PHILOSOPHY 2.1 Applicability

The analytical procedures for quantitative assessment given in this Section are applicable to most structural forms. They are not applicable to structures where analysis is impractical and where the srcinal design was based on good construction practice of the time and no codes existed. In these cases assessment can be based on qualitative judgement of satisfactory past performance and the information obtained from assessment inspections. In all cases the purpose of assessment is to determine whether the bridge meets relevant safety and serviceability criteria, see Clause 1.1. 2.2 Basis for Quantitative Assessment

Assessment of steel, wrought iron, concrete and steel/concrete composite Bridges should be undertaken by the application of limit state principles. Bridges and structural elements constructed from cast iron, timber or masonry should be assessed on permissible stresses or loads. Irrespective of the basis on which a Bridge is to be assessed, the bridge is required to satisfy the Operational Safety Limit State requirements given in Clause 2.4(d). 2.3 Assessment Situations

The circumstances in which the Bridge is required to fulfil its function should be taken into account by selecting relevant situations for assessment. The situations should encompass all conditions that can reasonably be foreseen during use of the Bridge by rail traffic. The situations should be determined by making a critical selection of conditions arising due to dead and imposed load, live traffic loads and where relevant temperature and wind effects. The situations chosen, characterised by a dominant live load and one or more coexistent loads, should include the most adverse live loads as follows:




Page 2 of 8

Situation (1)

Maximum vertical live load with coexistent transverse and longitudinal live loads;

Situation (2)

Maximum longitudinal live loads with coexistent minimum vertical and transverse live loads;

Situation (3)

Maximum transverse loads with coexistent minimum vertical and longitudinal live loads.

In the above situations other live loads where required by Section 4 such as those due to wind and temperature should also be included where a more onerous loading may result. The values of maximum and minimum live loads for each situation are determined by multiplying the nominal live loads given in Section 4 by the applicable factors given in Table 2.1. The coexistent loads should be taken as zero if this results in a more onerous loading of the Bridge.

Railway Live Loading Component

SITUATION (1) Maximum Vertical + coexistent

SITUATION (2) Maximum Longitudinal +

SITUATION (3) Maximum Transverse +

Longitudinal Transverseand

coexistent minimum Vertical and Transverse

coexistent minimum Vertical and Longitudinal

Vertical: Type RA Loading

1.0

0.5

0.5

1.0(0)

1.0

0.5(0)

Longitudinal: Traction&Braking Transverse: Nosing

1.0 (0)

0.5 (0)

1.0

Centrifugal

1.0 (0)

0.5 (0)

1.0

Table 2.1 Factors for Combinations of Components of Railway Live Loading

Partial factors for use in commonly occurring situations are given in Table 2.2. In special cases, other situations may arise and govern the assessment.




2.4 Limit States

Where Bridges are to be assessed under the selected situations using limit state principles, the following should be considered: (a)

Ultimate Limit State (ULS) The ULS is generally underbridge capacity.the governing condition for the assessment of This condition relates to the collapse strength of individual elements of the Bridge, and to the stability of a part or the whole of the Bridge when considered as a rigid body. To verify that an ultimate limit state is not reached, it is necessary to demonstrate that the criteria in the relevant Section of this Code of Practice are not exceeded under the application of ULS assessment loads.

(b)

Serviceability Limit State (SLS) Serviceability limit states are those situations where excessive deformations or a deterioration in structural condition may lead to a loss in utility of the Bridge such that remedial action may be required. Circumstances in which it may be necessary to carry out checks against SLS criteria are defined in

(c)

Clauses 4.2.2A, 4.1.1B and 4.3.2C. Fatigue Limit State The limit state for fatigue may be either an ULS or SLS. Where an assessment situation exists requiring fatigue evaluation (see Clause 4.3.2A) it should be checked taking the load factors γ fL and γ f 3 equal to 1.0. For cast iron Bridges, see Section 9.

(d)

Operational Safety Limit State (OSLS) These conditions are attained when specified limits which govern the safe operation of the railway are reached. These limits will generally be related to the changes in structural deformation that occur under the passage of a train and which, if exceeded may lead directly to derailment, or to degradation of the track which may, in time, have the same effect. They are limits of serviceability beyond which Bridge is operationally information regarding OSLS arequirements is given in unserviceable. Appendix F. Further

For the bridge structure as a whole, an Operational Safety check should be made relating to track twist in accordance with Section 4. For some structures, Serviceability Limit States, such as bridge deflections and rotations, may also need to be checked. Appropriate criteria should be agreed in accordance with Railtrack s Technical Approval Procedures.




2.5 Assessment Load Values

The assessment loads, Q *A , are determined from the nominal loads, QK according to the equation:

Q *A =

γ fL ⋅Q K

Equation 2.1

where:

γ fL

is a partial factor for each type of loading as given in Table 2.2.

Nominal dead and superimposed dead loads may be determined using the information given in Section 4. Details of the nominal live loading and its application are also given in Section 4. 2.6 Load Factors

Dead and superimposed dead loads should be taken together with live loads using the factors given in Table 2.2 and in accordance with Section 4. Where it is necessary to consider loads, such as those due to wind or temperature, which are not defined in Section 4 of this Code of Practice, reference should be made to BD37/88: Loads for Highway Bridges in accordance with Clause 4.4.



The Structural Assessment of Underbridges Section 2 - Assessment Philosophy LO AD

Limit State

Page 5 of 8

γfL to be considered in 1

Dead: Steel,wroughtiron

ULS

Cast iron

ULS

Concrete,masonry,timber Superimposed dead: Ballast *1, *2

*3

Track Fill Services

Live: The multiple components of Live Loading should be considered to act in accordance with Clause 2.3 Wind:

1.1

1.0

SLS ULS SLS

1.0 1.15 1.0

ULS SLS ULS SLS ULS SLS ULS SLS

1.75 1.2 1.2 1.0 1.2 1.0 1.25 1.0

ULS SLS

3

1.05

SLS

ULS SLS Temperature: Restraint to movement or due to frictional bearing restraint

Combination 2

1.4 *4 1.1

1.2 *4 1.0

1.2 *4 1.0

1.1 1.0

ULS SLS

1.3 1.0

Table 2.2 Values of Partial Factors (γfL) for Loads in Combinations *1

A value of

γ

of 1.35 at ULS and 1.1 at SLS may be adopted provided the depth of fL

ballast is controlled or dictated by the form of construction. Control measures may include datum plates or a Plimsoll line. *2

Ballast more than 300 mm below underside of sleepers may be considered as fill.

*3

Track includes rails, fixings and sleepers, but excludes ballast between sleepers.

*4

Subject to the approval of the Railtrack Director s Nominee a reduced value of 1.25 for combination 1 and 1.1 for combinations 2 and 3 may be adopted where the loading is of a controlled nature as follows:




Page 6 of 8

(a)

There is reliable control over the trains that can enter the route in question, and

(b)

For vehicles which comprise any of the following: Locomotives; Locomotive hauled passenger and/ or mail trains; Other passenger and/ or mail trains; Cranes and track plant not able to carry loads whilst in travelling mode; Freight wagons where loading is physically controlled, for example fluid fuel tank wagons, closed grain or closed cement wagons; Standard coal hopper or similar wagons where the load is weighed before dispatch.

Reduced values of γ fL can only be assumed for other vehicles where every vehicle after loading is weighed or is otherwise subject to proper assessment of weight, before details are submitted and accepted for such vehicles to cross the Bridge. These vehicles include freightliner container wagons, open top wagons for aggregates, spoil or waste and wagons for track infrastructure maintenance or renewal.

2.7 Assessment Load Effects

The assessment load effects, S *A , are obtained from the assessment loads by the relation:

= * SA =

S *A

γ f 3 ⋅ (effects of Q *A ) γ f 3 ⋅ (effects of γ fL ⋅ QK )

Note: For steel and wrought iron only (Section 5),

Equation 2.2A Equation 2.2B

γf 3

is applied within the

resistance R * (see Clause 2.8) such that:

= S =

S *A * A

effects of Q *A effects of

γ fL ⋅ QK

Equation 2.3A Equation 2.3B

where:

γf 3

is a factor that takes account of inaccurate assessment of the effects of loading, such as unforeseen stress distribution in the structure, inherent inaccuracies in the calculation model, and variations in the dimensional accuracy from measured values. The effects of the assessment loads should




be obtained by the use of analytical procedures applicable to the form of construction. The factor

γf 3

should normally be taken as 1.1 for ULS and 1.0 for SLS.

γf 3

may be taken as 1.0 for the ultimate limit state for members where the following conditions (a), (b) and (c) are all met: (a) members are either: (i)

Rail bearers or cross girders of steel, wrought iron or composite construction that are assumed to be simply supported, or;

(ii)

Main girders of steel, wrought iron or steel/concrete composite bridges with skew not greater than 25° (If main girders are continuous, any splices should be welded or made with HSFG bolts or rivets, and have cover plates to both flanges) or;

(iii)

Main beams of reinforced or prestressed concrete bridges with skew not greater than 25° that are assumed to be simply supported.

(b)

load effects are based upon static distribution within the structure;

(c)

geometric dimensions of the members are verified during inspection.

2.8 Assessment Resistance

The assessment resistance, R *A , of any structural element is the calculated resistance,

R * , of that element, making appropriate allowance for any deterioration identified. The calculated resistance, R * , determined from material strengths and measured section properties should be calculated from the following equation: *

R

=

function (f k

γm )

Equation 2.4

Except for steel and wrought iron structures only where (Section 5):

R*

=

where:

function (f k

(γ m ⋅ γ f 3 ))

Equation 2.5




fk

Page 8 of 8

is the characteristic (or nominal) strength of the material; is a partial factor for material strength.

γm

Values of f k and construction.

γm

are given in Sections 5, 7 and 8 according to the material of

For those materials where the calculated resistance is determined on a permissible stress basis, the following may be applied:

R*

=

function (f p ) where f p is the material permissible stress.

2.9 Verification of Structural Adequacy

Structures should be deemed to be capable of carrying a specified level of assessment loading when the following relationship is satisfied:

R *A

> S *A


The Structural Assessment of Underbridges Section 3 - Inspection for Assessment


CONTENTS

3. INSPECTION FOR ASSESSMENT ....................................................................................... 1 3.1 General ................................................................................................................................ 1 3.2 Requirements prior to Inspection..................................................................................2 3.3 ...................................................................................................... 2 3.4 Inspection Inspection for for Loading Resistance.................................................................................................3 3.4.1 General ........................................................................................................................ 3 3.4.2 Metal Bridges .............................................................................................................. 4 3.4.3 Masonry Arch Bridges .............................................................................................. 7 3.4.4 Reinforced and Prestressed Concrete Bridges .................................................13 3.4.5 Composite Bridges..................................................................................................15 3.4.6 Timber Bridges.........................................................................................................15 3.4.7 Substructures............................................................................................................16 3.4.8 Bearings......................................................................................................................17 3.5 Report on Inspection......................................................................................................18

3. INSPECTION FOR ASSESSMENT 3.1 General This Section gives recommendations for the inspection of underbridges, following the desk study of existing information. The purpose of the inspection is to obtain information required for the structural assessment and determination of safe load carrying capacities. The principles outlined below may be applied to all types of underbridge, and all materials of construction referred to in this Code of Practice.

Inspection for assessment is necessary to verify the form of construction, the dimensions of the structure and the nature and condition of the structural components. Inspection should cover not only the condition of individual components but also the condition of the structure as an entity, noting especially any signs of distress and possible causes. Should the inspection reveal a defect which is believed to seriously compromise the structure’s ability to carry load safely, the Railtrack Director’s Nominee is required to be advised urgently in order that consideration may be given to the appropriate emergency action to be instructed. Examples of defects that may require urgent action to maintain the safety of the Bridge would include cracks in metallic structures, or in the case of a masonry arch bridge if part of the arch is sagging. Where practicable, advantage should be taken of the presence of scaffolding for repairs/painting, the removal of ballast, longitudinal timbers, walkway boarding,




periods of low water etc. which may improve access for inspection of concealed and otherwise inaccessible parts. Where reasonably practicable Bridges should be observed under rail traffic and any signs of abnormal movement such as excessive deflection, working of connections, vibration or movement should be noted andbe considered as part of the assessment. Where possible, these observations should made under the passage of the heaviest rail traffic using the Bridge. When inspection is to be carried out in the hours of darkness the Bridge should first be observed in daylight. The skill, expertise and training of the person carrying out the inspection should be appropriate to the complexity of the structure being assessed. This person should be involved in the subsequent assessment process. Where the taking of samples is considered necessary to confirm material parameters or condition, the number, position and size of samples to be taken and any consequential making good is required to be agreed by the Railtrack Director’s Nominee. With regard to metallic structures, material testing should generally only be used to confirm the material types, allowing the adoption of typical material properties form Table A2 for assessment. Only in circumstances where this process shows the material to be untypical should additional testing be undertaken to confirm the yield stress and other appropriate material properties. Guidance on material identification, sampling and testing is included in Appendix F. 3.2 Requirements prior to Inspection

Prior to undertaking an inspection of a Bridge all existing information pertaining to the Bridge should be examined, including as-built drawings, soils data, past assessment and examination reports and details of mineral extraction, as appropriate. This examination may be useful in determining what further information should be obtained from the inspection and which items require special attention. Special attention should be paid to checking whether previously identified defects have worsened. Emergency reporting arrangements should be established and inspection personnel advised of these in advance of all site activities. 3.3 Inspection for Loading

The inspection should enable the material type and all dimensions necessary to calculate an accurate estimate of the dead and superimposed dead loads to be determined.




The position of tracks, rail joints (e.g. fish plated, welded and expansion) or switches and crossings (within 18 metres of the bridge bearings) relative to the Bridge and whether timber or concrete sleepers are installed should be recorded. Track radii, permissible speeds and any PSR or TSR should also be recorded wherecant, appropriate. The presence of longitudinal timbers, methods of fastening and positions of joints and notches in timbers should be recorded. Where the Bridge carries ballasted tracks, the overall ballast depth and depth to underside of sleepers should be determined. The extent and height of any ballast heaped on the bridge should also be noted, and the level relative to any control marks recorded. The location, number, size and type of services and service troughs should be recorded. 3.4 Inspection for Resistance 3.4.1 General The Bridge should be inspected to record all the parameters needed to determine:

•

the strength of elements and joints, including any observed defects, such as cracks, loss of section due to corrosion, settlement, defective materials, damage etc.;

•

the form of the structure to enable, in particular, assessment of dynamic effects (see Section 4).

This inspection should be carried out within touching distance. The inspection should supplement and provide confirmation of any information obtained from existing records, particularly:

•

dimensions of internal sections that may not be related to external features;

•

strengthening and repairs that may not appear on record drawings, as these elements may limit the load carrying capacity of the Bridge.

All constituent parts of the structure should be inspected in sufficient detail so that their respective strengths can be determined. In some cases sampling of materials may be required. Those parts not inspected should be recorded clearly and reasons given.




For buried members and those with hidden parts, excavation of trial holes etc., should be considered where there is doubt about the above parameters, especially where such parameters could be critical. Care should be exercised to ensure that there is no permanent damage caused to the structure by such excavations. 3.4.2 Metal Bridges 3.4.2.1 General

Prior to the inspection, a preliminary review of the structure, should be undertaken to identify and assess potentially fatigue prone components and details. The location, extent and remaining section of members where corrosion or other forms of deterioration has occurred should be recorded accurately (preferably in sketch form) to enable calculations to be made of section properties. The extent of corrosion should also be established where metal sections are in contact with timber decking or longitudinal timbers. The location, nature and extent of distortion of structural elements resulting from bridge strikes should be recorded. Samples should be taken where required for testing to determine yield stress or other material properties. Signs of poor quality and inferior metal should be noted and further tests carried out if appropriate. All cast iron members should be checked for the presence of cracks and blow holes especially in tensile areas. The location and extent of such defects should be recorded. Where suspension bolts support a live load carrying member, particularly where their failure could directly lead to collapse of the member, consideration should be given to removal of a bolt or plate for inspection purposes. The stability of the structure must be maintained after removal of these components. Evidence of water seepage which may have contributed to corrosion of parts that are not directly amenable to inspection should be noted. Exploration to establish the extent of any corrosion should be considered. Loose or missing bolts or rivets, rivets with severely corroded heads and any “working” or rust staining of any connections should be recorded. The dimensions and condition of free spanning longitudinal timbers should be recorded.




The verticality, and magnitude and direction of horizontal bow of top flanges of main girders as required by Clause 9.8.2A plus details of the end restraints including those for vehicle restraint should be recorded. Out of flatness of web panels should also be recorded. For half through type bridges with solid web or truss girders the presence of and condition of features which may be contributory to compression flange stability should be noted such as: (i)

cross girder to main girder connections including the relative locations of vertical stiffeners;

(ii)

signs of loose or working’ elements such as rivets, bolts or packings;

(iii)

presence of concrete or other haunching or infilling to the main girders;

(iv)

other connections between floor and main girders such as troughing, plate or timber floor, resting onto the bottom flange etc.;

(v)

trimmers or end cross girders and any infilling at or adjacent to the bearings;

(vi)

type of bearings and whether they or any infilling or haunching is providing torsional restraint main girders. A note should be made of any wear, cracking or spallingto of the bedstones;

(vii)

details and location of bearing stiffeners, end plates and other stiffening local to the bearings;

(viii)

verticality of the main girders at the bearings. Magnitude, shape and direction of horizontal bow of the main girder top flanges. A note should be made of any additional movements of the main girders under live loading.

3.4.2.2 Fatigue

Members particularly susceptible to fatigue should be closely examined for visible cracks so far as reasonably practicable. In particular close attention should be paid to the details shown in Figure 3.1 which are known to be fatigue susceptible. In addition to these, areas of severe and/or pitted corrosion around areas which have been subjected to mechanical damage and distortions, such as may arise from vehicle impact should be closely examined. Where visible cracks are found, their extent should be measured and recorded.




Page 6 of 18

LIABLE TO FATIGUE CRACK LIABLE TO FATIGUE CRACK NOTCHED CROSS GIRDER END

NOTCHED RAILBEARERS LIABLE TO WELD UNDERCUT

WELDED DOUBLER ENDS

FATIGUE CRACK CRACK RIVET HOLES IN TENSION AREAS

WELDED ATTACHMENTS AT FLANGE EDGES STRESS RAISER

TE N SIO N

C

O

N IO S S E R P M

STRESS RAISER

CRACK ENDS OF TRUSS MEMBERS

WELDED REPAIRS OR ATTACHMENTS TO RIVETED MEMBERS

ATTACHMENTS WELDED

WELDED REPAIR PATCH

Figure 3.1 Fatigue Susceptible details




Page 7 of 18

3.4.3 Masonry Arch Bridges 3.4.3.1 General

The external fabric should be inspected. The arch barrel should be inspected to ascertain all the information needed to determine the loading and resistance in accordance with Section 6. This information should be recorded on the arch data sheets on Figures 3.2 and 3.3. In particular the following information should be determined: (i)

thickness of the arch ring carrying rail traffic (this may not be the same as the number of rings visible on the face) and its shape;

(ii)

nature and condition of the brickwork, stonework and mortar, including the location and extent of any crushing, and the direction of bonding in the case of skew bridges;

(iii)

thickness of the joints and the depth of any mortar loss;

(iv)

presence of cracks, their width, length, position and number;

(v)

location and extent of any loss of section due to spalling or damage by vehicles from bridge strikes;

(vi) (vii)

location of any displaced voussoirs and displacement across cracks; deformation of the arch barrel from its srcinal shape;

(viii)

the presence and effectiveness of any previous strengthening such as saddling, stitching, grouting or strengthening rings;

(ix)

the presence and extent of any ring separation, which may be deemed to have occurred if the engineer has any reasons to believe that the ring is not acting integrally with the rest of the arch;

(x)

haunching over abutments and piers of multispan structures.

On site measurements should be made in imperial units and then converted to metric prior to commencement of assessment analysis. If part of the arch exhibits a significant change in profile from that described in previous reports, the bridge should not be assessed but the condition of the bridge reported to Railtrack immediately. Where there is uncertainty about the above information a site investigation should be considered, including trial holes where necessary. Probing into the construction should be carried out where the strength of the bridge is in doubt or if internal scour and leaching of the fill is suspected.




Page 8 of 18

The extent and location of water seepage should be recorded. The colour and nature of any leachates should be closely examined for signs of brick or stone slurry that may indicate internal movement. Parapets and spandrel walls should be inspected for evidence of any defects and their extent recorded on Figures 3.2 and 3.3, including, but not limited to:

• •

tilting, bulging or sagging; lateral movement of parapet or spandrel wall relative to the face of the arch barrel;

•

lateral movement of parapet or spandrel wall accompanied by longitudinal cracking of the arch barrel;

• • • •

weathering and lack of pointing; cracking, splitting and spalling; loosening of any coping stones; presence, location and details of ties, straps and patress plates.




Page 9 of 18

LINE OR BRANCH

MAP REFERENCE

NEAREST STATION

BRIDGENo.

M

C

ARCH PROFILE SPAN DIMENSION (SQUARE) SPAN DIMENSION (SKEW) NUMBER OF RINGS

LINEREFERENCE

SEMICIRCULAR SEGMENTAL ELLIPTICAL

PARABOLIC POINTED

A

C ARCH

=

=

= SPAN

A

ELEVATION LOOKING :

SECTION A-A The following information should be recorded above: A. SKETCH PROFILE OF SURFACE BALLAST AND TRACKS. B. DIMENSION FROM TOP OF PARAPET TO SOFFIT OF ARCH. C. DIMENSIONS FROM TOP OF PARAPET TO RAIL LEVEL. D. DIMENSIONS BETWEEN PARAPETS. E. POSITIONS OF TRACK ON STRUCTURE. F. TYPE OF SLEEPER AND TRACK

Figure 3.2 Arch Data Sheet 1

=




Page 10 of 18

MAP REFERENCE

LINE OR BRANCH BRIDGENo.

NEAREST STATION HARD STONE MEDIUM STONE ARCH RING SOFT STONE MATERIAL ENGINEERING BRICKS BUILDING BRICKS CONCRETE

M

C

LINEREFERENCE

OTHER (STATE) :

ARCH RING JOINTS MORTAR

LIME STONE

ARCH RING GOOD MATERIAL SOUND OR FRIABLE RANDOM TYPE OF LAYING SQUARED COURSED YES CORRECT BONDING REGULAR JOINTS

WIDTH OF JOINTS

DEPTH OF JOINTS

UP TO 6mm

0mm (FLUSH TO FACE)

6mm TO 12mm

UP TO 12mm

OVER 12mm

12mm TO 0.1 OF RING THICKNESS OVER 0.1 OF RING THICKNESS

GENERAL FAULTS YES DIAGONAL CRACKS FROM SPRINGING TO CENTRE LONGITUDINAL CRACKS IN SOFFIT TRANSVERSE CRACKS IN SOFFIT ARCH RADIAL DISPLACEMENT OF RING INDIVIDUAL STONE OR BRICKS PERMANENT DEFORMATION CONSTANTLY WET OR DAMP ABUTMENTS DIFFERENTIAL SETTLEMENT &/OR PIERS SPREAD SPANDREL CRACKS AT QUARTER POINTS WALLS BULGING WING CRACKS WALLS MOVEMENTS CONCRETE SLAB OR SADDLE FILLING GROUTED MATERIAL WELL COMPACTED MATERIALS

NO

IF`YES'GIVEDETAILS

WEAK MATERIALS EVIDENCED BY `TRACKING' OF SURFACE NOT KNOWN

Figure 3.3 Arch Data Sheet 2

NO




3.4.3.2 Cracking in M asonry Ar ches

The inspection should investigate all cracks in an effort to establish their size and depth, any associated displacement and their age. Old cracks, which probably occurred soon after the bridge was built, and are no longer propagating, may usually be ignored. Recent cracks,ofon the otherAlthough hand, usually showas clean faces, with possibly small and loose fragments masonry. appearing shear of the bricks or masonry, cracks normally follow an irregular line through the mortar. For this reason, care should be taken in checking that the defects are cracks and not deficiencies of the pointing material. Cracks in abutments may generally be ignored unless they are new or growing. If cracks in abutments are caused by subsidence they may have affected the arch ring. The possible causes of cracks in the arch are noted below:

•

longitudinal cracks outside the centre third of the arch between the spandrels and the arch ring may be caused by shear stresses generated by the spanwise deformation of the arch relative to the spandrels under the passage of live load (see Figure 3.4);

•

longitudinal cracks within the centre third of the bridge emanating from the abutments may be due to varying amounts of subsidence in different places along the length of the abutment, and are dangerous if large, because such cracks tend to indicate secondary breaking up;

•

longitudinal cracks along the centre of a twin track bridge, spreading outwards from the midspan area, may be caused by the stresses generated by the arrival on the bridge of trains travelling in opposite directions;

•

transverse cracks, usually found near the quarter points, due to permanent deformation of the arch, may be caused by partial collapse of the arch or movement at the abutments;

•

Diagonal cracks normally start near the sides of the arch at the springing and spread up towards the centre of the bridge at the crown may be due to a subsidence at the sides of the abutment. Diagonal cracks indicate that the bridge could be in a dangerous state. Where diagonal cracks meet or cross, there is a possibility that a portion at the joint could be punched out, as shown in Figure 3.5 below, and therefore, action should be taken as soon as possible to prevent this happening;

•

cracks in the corners and abutments of skewed arch bridges may be due to the differential resistance provided by the backfill.




Page 12 of 18 Spandrel walls

Face of abutment

(a)

(b) (c)

Figure 3.4 Plan on Arch showing Longitudinal Cracks a) Between arch ring and spandrels out with middle third b) From abutment within middle third c) Along centreline

Could be punched out

Diagonal cracks from arch springing

Figure 3.5 Diagonal Cracking in Arch




Page 13 of 18

3.4.3.3 I nspection of Jack Ar ches

The inspection of a jack arch deck should record the following information: (i)

the geometric configuration of the jack arches and their supporting members;

(ii)

the presence of arch ties, details of their size, spacing, condition and position

(iii)

within the height of the arch; rotations or horizontal displacement of a supporting member;

(iv)

transversely braced bottom flange of a supporting member;

(v)

inadequate support to springings, for example, corrosion of the bottom flange of supporting beam over a horizontal length or loss of bedding mortar;

(vi)

cracking at the crown of the arch due to spreading of springings;

(vii)

distortion and any associated cracking of the jack arch barrel;

(viii)

arch cracking associated with substructure cracking or distress.

3.4.4 Reinforced and Prestressed Concrete Bridges

A covermeter survey should be undertaken to check the cover and the location of reinforcing bars and prestressing tendons particularly in critical areas. If there are no drawings, if the available drawings do not give sufficient detail for assessment, or if there is evidence that the bridge is not as shown in the drawings, further investigation will be required. Other evidence may arise from records, from the covermeter survey, or from other findings of the inspection. Further investigation usually consists of a more comprehensive covermeter survey supplemented by local exposures of reinforcement to determine its size and confirm the position of critical bars. It will not normally be practical or desirable to expose sufficient reinforcing or prestressing steel to fully determine, its position, cross-sectional area and condition. When it is considered necessary to locally expose reinforcement, the extent and depth to be removed and method of making good is required to be agreed by the Railtrack Director’s Nominee. The worst credible strength of concrete should generally be derived from tests carried out on cores in accordance with BS 6089. Cores are destructive and cannot generally be taken at the critical locations of an element; hence interpretation or extrapolation is necessary to arrive at worst credible strengths in these locations. To assist in interpreting or extrapolating the results of core tests, an integrated programme of testing which may include destructive, semi-destructive (e.g. near surface) and non-destructive tests is necessary for each element. Care and judgement is required in selecting the locations and numbers of samples for such tests. The non-destructive tests can be used to give an indication of whether the area




of concrete from which the cores are taken is representative of the concrete in the critical areas. For reinforcement or prestressing tendons and bars, a worst credible strength should be obtained by testing samples taken from the element being assessed. It should be noted that Removal bars of different sizes aresteel likelyfor tosampling have significantly strengths. of prestressing will alterdifferent the stressyield distribution in the concrete section and the change should be allowed for in the assessment calculations. The extent and nature of spalling, corrosion of reinforcement, rust staining, crazing or soft or friable concrete should be recorded. Where cracking is present the following information should be obtained:

• • • •

details of position, extent and widths of significant or unusual cracks; details of any cracks showing evidence of rust staining; all cracks over approximately 0.2 mm wide; all flexural cracks in prestressed elements.

Consideration should be given to undertaking additional tests to determine constituents and condition of the concrete. The tests may include tests for chloride, half cell potential, sulphates, carbonation, alkali silica reaction or ettringite formation and cement content. However, in general these tests are not required unless there is other evidence of the associated forms of deterioration. For post-tensioned concrete structures the fundamental design and construction details should be established by a desk study, prior to the inspection for assessment, as outlined in BA 50/93: Post-tensioned Concrete Bridges. Planning, Organisation and Methods for Carrying Out Special Inspections. The inspection should follow the procedures for Special Inspections as described in BA 50/93, if the bridge may be at risk of sudden failure following tendon corrosion or if the integrity of transverse prestressing is to be assumed in the subsequent analysis of the adequacy of the structure. Any evidence of distress should be recorded, especially evidence of rust staining, spalling, cracking or water penetration at anchorage or tendon positions. In particular unexpected cracking and unexpected or changing deflection should be recorded. Further investigation is required whenever there is evidence that suggests tendon corrosion. Corrosion of tendons in post-tensioned members may not be visually manifest during inspection. For pretensioned concrete members, significant tendon corrosion usually causes visible rust staining and cracking of the cover concrete.




3.4.5 Composite Bridges

The steel and concrete elements of composite bridges or composite members known to have been designed for composite behaviour should be inspected in accordance with Clauses 3.4.2 and 3.4.4 as appropriate. For other bridges or members where concrete (or brick jack arches) is in contact or surrounds steel or wrought iron members then inspection should be used to decide whether composite behaviour can be assumed using Appendix A or Appendix C. It should be noted (see Clause 8.3.1C) that composite behaviour of cross girders is not to be assumed in Type A or other filler beam type decks less than 300 mm deep where there is no encasement above the top flange or below the bottom flange. For concrete slabs supported on steel or wrought iron beams the steel/ metal interfaces should be examined especially near the supports for signs of:

• • • • •

corrosion; fretting; relative longitudinal slip; vertical separation; cracking or spalling of concrete.

Any relative movement should be recorded including any under live loading. For cased beams the soffit (and other surfaces where practicable) should be examined for signs of:

• •

rust forcing or leakage; separation of or hollowness of the casing concrete.

For filler beams or concrete or brick arch decks the soffit (and other surfaces where practicable) should be examined for signs of:

• • •

corrosion;

•

cracking or spalling of concrete.

relative longitudinal slip; separation;

Where the infill consists of unreinforced concrete or brickwork or is unknown then probing should be undertaken to prove the presence of dense material in contact with the beam before composite behaviour can be assumed in assessment. For concrete infilled troughs, probing or other inspection should be undertaken to determine the depth and condition of concrete above the crests before composite behaviour can be assumed in assessment.




3.4.6 Timber Bridges

The timber should be inspected, noting the presence of any protective coatings and searching for the presence of rot or infestation (especially around metal fixings and where there is standing water round piers in estuary waters). Heart rot may only be detected by probing. The use of pressure sensitive, non-destructive drilling techniques should be considered. Inspection for splitting and rot should be carried out especially in areas of notching. Care should be taken not to transfer fungal spores to sound timber by the use of contaminated tools. Any holes drilled should be made good with sound timber dowels. Particular attention should be paid to timber in contact with metal. If site inspection indicates modifications to the structure, especially to primary members, and weak timber is suspected, or if the species is unknown, samples should be taken for identification of the timber. 3.4.7 Substructures

It is not normally possible to inspect the foundations, but where they are exposed, for example in tidal waters, their condition should be checked. Any defects present and their extent should be noted; defects may include cracking, erosion, disintegration or corrosion of reinforcement. Dimensional checks are required for preparing sketches for analysis or for confirmation of record drawings. The dimensional checks may require excavation or probing to determine depth and the extent of foundations. Care should be taken to ensure that exploratory work does not impair stability or damage underground services. In river beds and banks the removal of material by scouring, from around the base of piers or abutments may lead to undermining of the foundations, especially during flooding. Whilst assessment of the susceptibility of a substructure to scour is outside the scope of this standard, evidence of scour holes and approximate dimensions where possible should be recorded. The presence and type of scour protection should be recorded. Foundation deficiencies usually appear as movements which may be sufficiently large to cause tilting, cracking or excessive movements at joints or bearings. In arch bridge foundations movement or arch spreading is generally apparent from cracks showing distress in the arch rings and spandrels; diagonal cracking may be indicative of differential settlement of the foundations. All accessible parts of the substructures should be examined and any defects, including extent, and possible causes recorded.




Page 17 of 18

Some typical substructure defects are:

• •

tilting and rotation, in any direction;

• • •

cracking, splitting and spalling; erosion beneath water level;

• • • • •

vegetation intrusion;

rocking;

weathering and other material deterioration, including lack of pointing for masonry and brickwork;

lack of effective drainage; internal scour and leaking of fill; settlement of structure; settlement of fill.

Movement of substructures is likely to be caused by foundation movements. Differential foundation movements may be evident on abutment or pier walls in the form of vertical or inclined cracks. The effects that any observed substructure movement may have on the superstructure or deck should be investigated. For example, differential settlement will cause a twist in the deck; inspection may reveal dislocated bearings. Where continuous decks are encountered, substructure movements may be evident from signs of distortion or distress consistent with a sag’ over the settling support or hogging’ over intermediate adjacent supports. Movement of substructure may be related to the support of spans of unequal length or character. In arch bridges, predominantly horizontal cracks in piers or abutments may be the result of the arch spreading’. 3.4.8 Bearings Bearings if present, should be inspected so that the general condition and efficiency or operation of the bearings can be established. The following should be noted:

• •

general condition of bearings and their type and articulation;

•

condition of seating bedding and plinth;

any binding or jamming, looseness, or reaching limits of rotational or translational movement, or vertical movement under live load;




Page 18 of 18

•

whether correct operation of the bearings is prevented or impaired, such as by structural members built into abutment or pier;

•

for metal girder bridges, where applicable, whether the bearings are fulfilling their function of providing end torsional restraint.

In bridges without bearings or where the bearings have failed to function correctly, there may be local crushing or cracking, especially where supports are stone or brickwork. 3.5 Report on Inspection

A report containing all the relevant information obtained from the inspection should be produced. The report should include:

• •

a description of the structure including details of any services carried;

•

sketches, drawings or photographs identifying the nature, location and extent of any defects;

• •

sketches giving as measured’ dimensions; other photographs, including general views and specific details.

a description of the condition of the structure including any repairs and a discussion of the effect of any significant defects on the operational safety and assessment of the structure;


The Structural Assessment of Underbridges Section 4 - Loading for Assessment


CONTENTS

4. LOADING FOR ASSESSMENT.............................................................................................1 4.1 Dead Loads ......................................................................................................................... 1 4.2 Superimposed Dead Loads..............................................................................................2 4.2.1 Track Ballast............................................................................................................................ ........................................................................................................................... 2 2 4.2.2 4.2.3 Services ........................................................................................................................ 2 4.2.4 Miscellaneous..............................................................................................................2 4.3 Railway Live Load .............................................................................................................. 3 4.3.1 Vertical Static Loading .............................................................................................. 3 4.3.2 Dynamic Effects..........................................................................................................7 4.3.3 Dispersal of Railway Live Loading onto the Structure.....................................25 4.3.4 Nosing ........................................................................................................................28 4.3.5 Centrifugal Load.......................................................................................................29 4.3.6 Longitudinal Loads...................................................................................................30 4.3.7 Load Combinations .................................................................................................31 4.3.8 Elements Supporting More Than One Track.....................................................31 4.3.9 Structures Carrying Light Rail Systems...............................................................32 4.4 Other Live Loads.............................................................................................................32 4.4.1 Wind Loads...............................................................................................................32 4.4.2 Temperature.............................................................................................................32 4.5 Operational Safety Requirements................................................................................33 4.5.1 Track Twist ...............................................................................................................33 4.6 Accidental Loads from Vehicles....................................................................................33 4.6.1 Bridges over Highways ...........................................................................................33 4.6.2 Intersection Bridges ................................................................................................33 4.6.3 Train Derailments on Bridges ...............................................................................33

4. LOADING FOR ASSESSMENT 4.1 Dead Loads

The dead loads should, where possible, be based on dimensions verified during the inspection. For assessment Level 1 analysis the applicable values of unit weight given in Table 4.2 should be used. Where, however, the initial assessment shows inadequacies, or there is doubt about the nature of a particular material, tests should be carried out to determine actual densities.




Page 2 of 33

4.2 Superimposed Dead Loads 4.2.1 Ballast

The superimposed dead load due to ballast should be based on the measured depth with unit weight 1800 kg/m³. 4.2.2 Track

Where applicable, the superimposed dead loads due to track components given in Table 4.1 may be used. Where a different configuration of sleepers and rails has been identified during the inspection, the self weight to be used should be determined by measurement of dimensions of the configuration and by reference to data on weights of components produced by the manufacturer. Component Single Bullhead Rail SingleType113ARail Single UIC 60 Rail Conductor Rail

Mass 47.07 kg/m 56.22kg/m 60.22 kg/m 75.2 kg/m 1

Concrete Sleeper (Type F40 for use with 113A Rail) * *1 Concrete Sleeper (Type G44 for use with UIC 60 Rail) Timber Sleeper Chair for Bullhead Rail 1

*

300 kg 315 kg kg 94 21 kg

Includes shoulder, clips and rail pads. Table 4.1 Permanent Way Component Weights

4.2.3 Services

The superimposed dead load resulting from service cables and ducting should be determined, where possible, from examination and measurement during the inspection or from information provided by the service owner. Where this is not possible, any assumptions made regarding such equipment should be clearly stated in the assessment calculations. 4.2.4 Miscellaneous

Miscellaneous items such as walkways which are not deemed to be part of the structure should be considered as superimposed dead load. The nature and dimensions of such items should be established during the inspection, and the partial factor γ fL for dead load applicable to the material (given in Table 2.2) should be used.



The Structural Assessment of Underbridges Section 4 - Loading for Assessment Material1 Metal

Concrete Masonry

Timber2

Fill

Page 3 of 33

Aluminium Cast Iron Wrought Iron Steel

Unit Weights kg/m³ 2750 7200 7700 7850

Reinforced or Prestressed Plain Breeze Block Engineering Brickwork Other Brickwork Granite Sandstone Softwood Hardwoods generally Jarrah Greenheart Sand(dry) Sand (saturated) Hardcore

2400 2300 1400 2400 2100 2600 to 2930 2200 to 2400 640 typical (480 to 720) 640 to 1200 840 to 960 1040 to 1200 1600 2000 1920

CrushedStone Slag Rubble Packed Earth (dry, compact) Earth (moist, compact) Puddled Clay Asphalt Macadam

1440 2240 1600 1800 1920 2300 2560

1

Reference may also be made to BS 648 and BS 5268: Part 2: 1996.

2

Wide range of unit weights because of the variability of timber. For densities of specific timber types refer to BS 5268: Part 2: 1996 Table 4.2 Density of Materials used in Bridge Construction

4.3 Railway Live Load 4.3.1 Vertical Static Loading 4.3.1.1 Route Availabil ity (RA) N umber

The assessment of a Bridge should be determined in terms of its Route Availability (RA) number, that is its safe traffic load capacity. Route Availability numbers generally




Page 4 of 33

range from the lowest capacity RA0 to the highest at RA15 represented by 25 British Standard Units (BSUs) of Type RA1 loading respectively as shown by Table 4.3. R.A. NUMBER RANGE OF BSUs RANGE OF SINGLE AXLE IN GROUP WEIGHTS IN GROUP RA0 Upto10.99units Under13.96tonnes RA1 RA2 RA3 RA4 RA5 RA6 RA7 RA8 RA9 RA10 RA11 RA12 RA13 RA14 RA15

11.00 12.00to to11.99 12.99units units 13.00to13.99units 14.00to14.99units 15.00to15.99units 16.00to16.99units 17.00to17.99units 18.00to18.99units 19.00to19.99units 20.00to20.99units 21.00to21.99units 22.00to22.99units 23.00to23.99units 24.00to24.99units 25.00unitsandover

13.97 15.24to to15.23 16.50tonnes tonnes 16.51to17.77tonnes 17.78to19.04tonnes 19.05to20.31tonnes 20.32to21.58tonnes 21.59to22.85tonnes 22.86to24.12tonnes 24.13to25.39tonnes 25.40to26.66tonnes 26.67 t o27.93tonnes 27.94to29.20tonnes 29.21to30.47tonnes 30.48to31.74tonnes 31.75tonnesandover

Table 4.3 Route Availability Classification for Bridges

Type RA1 loading excludes dynamic effects which should be added in accordance with Clause 4.3.2 and are dependent upon train speed. RA numbers should therefore be determined according to a given train speed. In some cases it may be necessary to determine more than one RA number for a given Bridge, for example RA6 at 100 mph representing passenger trains (normally the permissible speed) and RA10 at 60 mph for freight trains. The number of units of Type RA1 loading that the Bridge can carry should be determined by calculating the live load capacity factor, C , as defined below: C

=

Live Load Capacity Effects of 20 units of Type RA1 loading

Capacity in terms of units of Type RA1 loading = 20 × C The RA number of the Bridge should be obtained from Table 4.3.

Equation 4.1




Page 5 of 33

Where the assessed RA number is below the RA of the line, the effects under static EUDLs for the “real” (actual) permitted vehicles and combinations, together with dynamic factors for their respective permitted speeds, may be considered acceptable. It should be noted that the RA effect of vehicles on a specific “span” (loaded length) is often lessspans. than the RA classification for the vehicle which has to allow for a full range of Bridge 4.3.1.2 RA1 Loading

The static loading used to determine the RA number is shown in Figure 4.1 for 20 units of Type RA1 loading. The Short Lengths configuration should be used when it produces more onerous effects than the axle and uniformly distributed load model. 4x200kN

4x150kN

4x200kN

4x150kN 65kN/m

2.4

1.5 1.5 1.5

2.7

1.8 1.8 1.8

4.0

1.5 1.5 1.5

2.7

1.8 1.8 1.8 1.5

2x250kN SHORT LENGTHS 1.8

Figure 4.1 20 Units of Type RA 1 Loading

Note 1:

20 units of Type RA1 loading is equivalent to Route Availability RA10 without allowance for dynamic effects.

4.3.1.3 Equivale nt U nif ormly Di stri buted L oading

For simply supported spans (with the exception of Masonry Arches, see Section 6), Type RA1 loading may be represented by an Equivalent Uniformly Distributed Load (EUDL). Table 4.4 gives EUDL and maximum end shear values for simply supported spans for 20 units of Type RA1 loading. The EUDL values equate with the maximum bending moment within the span that occurs under RA1 loading. For continuous spans the values in Table 4.4 are not strictly applicable, and loading should be as shown in Figure 4.1. This loading should be considered as a whole, but any parts of the loading that reduce the effects on the part of the element being considered should be omitted. 4.3.1.4 Appli cation of L oads

Type RA1 loading should be applied to each track and such as to produce the maximum effect in the part of the element being considered.




Page 6 of 33

(SPAN) (m)

EUDL (kN

END SHEAR (kN)

SPAN (m)

EUDL (kN)

END SHEAR (kN)

1.2 1.4 1.6

500 500 500

250 250 250

9.4 9.6 9.8

1105 1121 1137

640 650 659

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

500 500 500 500 500 500 500 513 532 554 574 594 618 643 667 689

250 269 291 308 322 335 346 356 364 372 378 384 390 395 401 417

10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 22.0 24.0 26.0 28.0 30.0

1152 1219 1282 1351 1411 1475 1547 1620 1687 1760 1837 1983 2126 2265 2415 2547

668 707 752 792 835 873 907 947 983 1017 1055 1146 1233 1319 1405 1488

5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2

709 728 745 764 784 810 834 858 879 899 919 937 954 971 986 1001 1015 1029 1042 1054 1070 1088

432 447 459 471 483 493 503 512 521 529 537 544 552 563 573 582 591 599 608 615 623 631

32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0 48.0 50.0 52.0 54.0 56.0 58.0 60.0 65.0 70.0 75.0 80.0 85.0 90.0 100.0

2702 2871 3039 3201 3358 3505 3651 3787 3921 4053 4186 4312 4437 4559 4677 4974 5260 5554 5846 6136 6427 6717

1569 1649 1726 1803 1878 1952 2026 2099 2171 2242 2313 2384 2454 2525 2594 2767 2939 3109 3279 3448 3616 3784

Table 4.4 EUDLs and End Shears for 20 Units of Type RA1 Loading (Static Load per Track)




Page 7 of 33

4.3.2 Dynamic Effects 4.3.2.1 General

The vertical static loading defined in Clause 4.3.1 should be multiplied by the applicable dynamic factor to allow for impact, oscillation and other dynamic effects, including those caused by track and wheel irregularities. 4.3.2.2 Dynamic F actor f or M ember s other than T r ansver se F loor M embers

The dynamic factor (1 + ϕ ) should be applied where the speed of trains is 5 mph or greater, up to 125 mph maximum, using the dynamic increment ϕ which should be taken as in Table 4.5. Dynamic Increment for Bending

ϕ

Dynamic Increment for Shear

ϕ

(ϕ1 + ϕ11 )

Normal track Permissible speed ≤ 100 mph



Track maintained for

1.3 ϕ1 +



Permissible speed 100 - 125 mph Fatigue calculations only

ϕ   2 

2

11

×ϕ

for Bending

3

ϕ + ϕ    2  

0.5

Permissible speed ≤ 125 mph

11

1

Table 4.5 - Dynamic Increment

ϕ

In Table 4.5:

ϕ1 =

k

(1− k + k 4 )

representing interaction of the structure

Equation 4.2

where: k=

v

but not greater than

4.47L φ n0 L   − Lφ  −  φ  Ln ϕ11 = α 56e  10  + 50 0 −1e  20    80  2

v

Equation 4.3

358 2

  

Equation 4.4

The dynamic factor as given in this Clause excludes the effects of any rail joints, or points and crossings. Equation 4.4 represents track irregularities where:




α = 0.0002v

Page 8 of 33

but not greater than 0.01

Equation 4.5

is the determinant length in metres obtained from Table 4.6, but not

Lφ

to be taken as less than 4 metres. Alternatively L φ may be defined as the length of the influence line for deflection of the element under consideration. In the case of floor members, 3 metres may be added to the length of the influence line as an allowance for load distribution through the track. L

is the effective span in metres.

n0

is the fundamental natural frequency in Hertz of the structural element, considered as a simply supported beam with span L loaded with a 20 kN uniformly distributed load in addition to dead loads. 17.75 Note: n0 may be assumed as where δ 0 is the maximum

δ0

deflection in millimetres of the structural element with a simply supported span and loading as defined for n . This expression is not 0

appropriate for elements of non simply supported form (for example continuous girders or braced spandrel arches). For such structures, the natural frequency under the loading defined for n0 should be determined by other means. is the speed in mph, normally taken as the permissible speed (or line speed) on the Bridge. For freight trains v may be taken as 75 mph or, if less, the permissible speed for freight trains on the Bridge.

v

4.3.2.3 Val ues of

ϕ

Figures 4.02 to 4.14 show values of ϕ for bending for different train speeds from 5 to 125 mph based on the formulae in Clause 4.3.2.2 for a range of natural frequencies (Hz) referred to as low frequency (LF) and high frequency (HF) and defined as follows: High frequencies (HF) n0

= 94.76L−

0.748

Low frequencies (LF)

Equation 4.6




Page 9 of 33

80 for 4 metres ≤ L ≤ 20 metres L

n0

=

n0

= 23.58L−

0.592

Equation 4.7

for 20 metres < L ≤100 metres

Element

DeterminantLengthL

Equation 4.8

φ

Steel and Wrought Iron

Deck Plate Discontinuous spanning longitudinally

Twice cross girder spacing plus 3 metres

Discontinuous spanning two ways

Three times cross girder spacing

Continuous over ribs or stringers

As for 4 span continuous beams

Rail Bearers Continuous

3timescrossgirderspacing

Simplysupported

Crossgirderspacing+3metres

Concrete Slabs & Other Elements

As part of box girders or upper flange of steel or concrete beams Spanning transversely

3 times beam or web spacing

Spanning longitudinally

3 times beam or web spacing or determinant length of main girders - whichever is the lesser

Edge cantilevers

For cantilever length e ’ from outer face or web of beam to centre of cess rail e ≤ 0.5 metres - 3 times beam or web spacing e > 0.5 metres - 4 metres

Continuous over cross girders

Twice cross girder spacing

For Half Through Bridges Spanning transverse to main beams

Twice main beam spacing

Spanning longitudinally

Twice cross girder spacing or determinant length of main girders - whichever is the lesser

End zones of transverse spanning elements, extending from supports for 1 4 of span of transverse element

4 metres

InvertedUUnits

Asforportalframe

Table 4.6 Determinant Length Lφ




Page 10 of 33

Table 4.6 (continued) Element

DeterminantLengthL

φ

Main Beams

Simplysupportedbeams

Centretocentreofsupports

Continuous beams

K x L , but not less than the maximum span m

where K is taken from the following table: n K

2 1.2

3 1.3

4 1.4

≥5 1.5

where L m is the average span n is the number of spans Web members of trusses

The length of span of the main beam which is loaded such as to produce the maximum force in the web member considered

Portalframes

Ascontinuousbeam,assuminglegsarespans

Arch, arch rib or stiffening girder of bowstring

Half the span

Multi-spanarches

Twicetheclearmaximumspan

Suspension bars to bowstring girders To bowstring having stiffening girder

4 times spacing of bars

Without stiffening girder

As for cross girders suspended from bars

Supports

Columns, trestles, cross heads, bearings, tension anchors.

The total length of superstructure that is supported by the element concerned.

4.3.2.4 Dynamic F actor f or T r ansverse F loor M ember s

The dynamic factor for cross girders and other discrete transverse floor members should be taken as (1+ I 4 ) , where I 4 is determined from Figure 4.15. For fatigue calculations only the value of I 4 should be taken as 50% of the value shown in Figure 4.15. 4.3.2.5 Reduced Dyn amic E ff ect

Where the depth of ballast or non-structural fill exceeds 1.0 metre, the dynamic increment ϕ may be reduced as follows:

Reduced dynamic increment

h − 1 = ϕ −    10 

Equation 4.9

where h is the depth in metres below underside of sleeper or track to top of arch crown or structural element.




Page 11 of 33

0.1

5 mph (8 kph, 2.2 m/s)

ϕ Fatigue HF

Normal Track HF

0.05 100mph+ Track LF

Normal Track LF

100mph+ Track HF

Fatigue LF

0 0

10

20

30

40

50

60

70

80

90

Span - metres

LF n0 HF

20

2 10

8

7

6

5

4

3

Figure 4.2 Dynamic Increment ϕ for Bending at 5 mph (2

3

ϕ

for shear)

100




Page 12 of 33

0.2

10 mph

0.15

(16 kph, 4.4 m/s) Normal Track HF

Normal Track LF

ϕ 0.1

100mph+ Track LF

Fatigue HF

100mph+ Track HF 0.05

Fatigue LF

0 0

10

20

30

40

50

LF n0 HF

60

70

80

90

Span - metres

20

2 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 13 of 33

0.5

0.4

20 mph (32 kph, 8.9 m/s)

0.3

ϕ Normal Track HF

Normal Track LF 0.2

100mph+ Track HF

100mph+ Track LF 0.1 Fatigue LF

Fatigue HF

0 0

10

20

30

40

50

60

70

80

90

Span - metres

LF n0 HF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 14 of 33

0.6

0.5

30 mph (48 kph, 13.3 m/s)

Normal Track HF

0.4

Normal Track LF

ϕ 0.3

100mph+ Track LF 0.2 100mph+ Track HF

Fatigue LF

0.1

Fatigue HF

0 0

10

20

30

40

50

60

70

80

90

Span - metres

LF n0 HF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 15 of 33

1.2

1.1

1

40 mph (64 kph, 17.8 m/s)

0.9

0.8

0.7

ϕ

Normal Track HF

0.6

0.5 Normal Track LF 0.4 100mph+ Track LF 0.3 100mph+ Track HF

Fatigue LF

0.2

Fatigue HF 0.1

0 0

10

20

30

40

50

LF n0 HF

60

70

80

90

Span - metres

20

2 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 16 of 33

1.2

1.1

1

50 mph (80 kph, 22.2 m/s)

0.9

0.8

Normal Track HF 0.7

ϕ 0.6 Normal Track LF

0.5

0.4 100mph+ Track LF

0.3

100mph+ Track HF

0.2 Fatigue HF Fatigue LF 0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 17 of 33

1.2

1.1

1

60 mph 0.9

(97 kph, 26.9 m/s)

0.8

Normal Track HF 0.7

ϕ 0.6 Normal Track LF

0.5

100mph+ Track LF 0.4


0.2 Fatigue HF Fatigue LF 0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 18 of 33

1.2

1.1

1

70 mph (113 kph, 31.4 m/s)

0.9

0.8 Normal Track HF

0.7

ϕ 0.6

Normal Track LF

0.5 100mph+ Track LF 0.4

0.3 100mph+ Track HF

0.2

Fatigue HF Fatigue LF

0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 19 of 33

1.2

1.1

1

80 mph (129 kph, 35.8 m/s)

0.9

0.8 Normal Track HF 0.7

ϕ Normal Track LF 0.6

0.5 100mph+ Track LF 0.4


Fatigue HF

0.2 Fatigue LF 0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 20 of 33

1.2

1.1

1

90 mph (145 kph, 40.3 m/s)

0.9

0.8 Normal Track HF 0.7

ϕ

Normal Track LF

0.6

0.5 100mph+ Track LF 0.4 100mph+ Track HF 0.3 Fatigue HF 0.2 Fatigue LF

0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 21 of 33

1.2

1.1

1

100 mph

Normal Track HF

(161 kph, 44.7 m/s)

0.9

0.8

Normal Track LF

0.7

ϕ 0.6

0.5

100mph+ Track LF

0.4

100mph+ Track HF

0.3

Fatigue HF

Fatigue LF

0.2

0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 22 of 33

1.2

1.1

1 Normal Track HF

110 mph (177 kph, 49.2 m/s)

0.9

0.8 Normal Track LF

0.7

ϕ 100mph+ Track LF

0.6

0.5 100mph+ Track HF 0.4 Fatigue HF 0.3 Fatigue LF 0.2

0.1

0 0

10

20

30

40

50

n0 HF

60

70

80

90

Span - metres

LF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 23 of 33

1.5

1.4

1.3

Normal Track HF

1.2

125 mph (200 kph, 55.8 m/s)

1.1 Normal Track LF 1

0.9

ϕ

100mph+ Track LF

0.8

0.7 100mph+ Track HF 0.6

0.5

0.4

Fatigue HF

0.3 Fatigue LF 0.2

0.1

0 0

10

20

30

40

50

60

70

80

90

Span - metres

LF n0 HF

2 20 10

8

7

6

5

4

3


3

ϕ

for shear)

100




Page 24 of 33

1.1

1

0.9

0.8

0.7

0.6 I4

0.5

0.4

0.3

0.2

0.1

0 0

10

20

30

40

50

60

70

80

90

100

110

120

Speed mph

Dynamic Factor

I4

Figure 4.15 for Transverse Floor Members

130

140




Page 25 of 33

4.3.3 Dispersal of Railway Live Loading onto the Structure Dispersal through the track onto Bridge floor elements may be applied as in Clauses 4.3.3.1 and 4.3.3.2 where: FEUDL

is a factor by which the unit EUDL loading is multiplied;

FA t

is a factor which is multiplied by the axle loading; is the depth of ballast between the underside of sleeper and the top of the member in mm;

L

is the effective span in metres of longitudinal members spanning between centres of cross girders or twice the spacing of cross girders in the case of continuous longitudinal members.

4.3.3.1 L ongitudi nal M ember s positi oned Di rectly Un der the Rails inclu ding Rail Bear er s, Tr ough i ng, Slabs, Plates, Ti mber D ecks etc.

For Longitudinal dispersal: To allow for longitudinal dispersal through track: To rail bearers, longitudinal

Longitudinal timbers only

troughing, plate or timber floor etc. L (m) FEUDL <0.5 0.5 L + 2.0 0.5 to 3.0

over cross girders L (m)

FEUDL

0 up to 2.4

L + 3 .6

>2.4

1.0

6

5

>3.0

0.60

1.0

Table 4.7 Live Load Factor For Dispersal Through Track - FEUDL

For Transverse distribution: (i)

The effective width of longitudinal troughing, slabs or similar, carrying one track load should be taken as shown in Table 4.8, but not greater than the actual widths. t ( mm) Up to 150 >150

Effective width (m) 3.0 3.6

Table 4.8 Effective Widths



(ii)


For longitudinal timber decks, barlow rails, old rails or similar, an effective width of (1.8 + 0.004 t ) metres with a maximum of 3.0 metres.

4.3.3.2 Tr ansver se M ember s

For Longitudinal dispersal: (i)

Cross girder with cross sleepers and ballast. For cross girders spaced at 1.8 metres or more, FA = 1.0. For cross girders spaced at less than 1.8 metres the axle load should be reduced in the ratio: FA =

(ii)

Cross girder centres in metres 1.8

Equation 4.10

Cross girders with longitudinal timbers. For cross girders spaced 1.5 metres or more, FA = 1.0 For cross girders spaced at less than 1.5 metres with longitudinal timbers equal to or greater than 225 mm deep FA =


Equation 4.11

For cross girders spaced at less than 1.5 metres, with longitudinal timbers less than 225 mm deep FA =


Equation 4.12

(iii)

Transverse reinforced concrete slabs, effective width = 1.8 metres or actual width if this is less.

(iv)

The effective width of transverse members should be taken from Table 4.9.



The Structural Assessment of Underbridges Section 4 - Loading for Assessment Member Type

Page 27 of 33 Effective Width (m)

Transverse Troughing Crosssleepertrack,sleepersintroughs

1.5

Cross sleeper track up to 150 mm depth of ballast below underside of sleeper to top of troughing Cross sleeper track more than 150 mm depth of ballast below underside of sleeper to top of troughing Longitudinal timber up to 150 mm deep directly on transverse troughing Longitudinal timber more than 150 mm deep directly on transverse troughing Transverse RC slabs

1.8 2.4 1.5 1.8

1.8

Transverse Timber decks Chairs directly on the deck

0.6

Cross sleeper track up to 150 mm depth of ballast below underside of sleeper to top of decking Cross sleeper track more than 150 mm depth of ballast below underside of sleeper to top of decking Longitudinal timber up to 150 mm deep directly on decking Longitudinal timber more than 150 mm deep directly on decking

1.2 1.8 1.5 1.8

Table 4.9 Effective Width of Transverse Members 4.3.3.3 Di sper sal thr ough B all asted Tr ack

For sleepered track 50% of a wheel load may be assumed to be transmitted to the sleeper beneath and 25% distributed to each of the sleepers on each side assuming a sleeper spacing ofuniformly 800 mm maximum. The load on theofsleeper may be to be distributed over the ballast at theacting underside the sleeper andassumed over a distance of 800 mm symmetrically about the centre line of the rail (or to twice the distance from the centre of rail to the nearer end of the sleeper if less). A sleeper width of 250 mm may normally be assumed. Dispersal through ballast or similar granular fill may be taken at 15 ° to the vertical. Where a flexible bridge floor such as flat or buckle plates is stiffened by rigid members such as rail bearers, the relative flexibility of the floor construction may be




Page 28 of 33

considered in the distribution of loading. For rail bearers a pressure, under nominal live loading (including dynamic factor) and dead load, of up to a maximum of 1000 kN/m² may be assumed to occur over a width of 200 mm (or the stiff bearing width of the rail bearer if greater). This pressure is reduced beneath the remainder of the loaded area, as shown in Figure 4.16. 800mm =

250mm

=

100% 50%

15°

25%

15°

1000 KN/m² Maximum 200

Figure 4.16 Dispersal through Ballasted Track onto a flexible floor with Rail Bearer 4.3.4 Nosing

An allowance should be made for lateral loads applied by trains to the track due to nosing which should be taken as two nominal loads spaced at 4.5 metres apart along the track. Each load N should be taken as: For all locomotives, passenger trains, and for freight vehicles where v does not exceed 40 mph: N=

0.72 v

Equation 4.13

For freight vehicles where v exceeds 40 mph: N=

28.8 + 2.56(v − 40 ) but not greater than 80

where: N v

is the value of each nosing force in kN; is defined in Clause 4.3.2.2.

Equation 4.14




Nosing should be considered as acting in either direction at right angles to the track at rail level and at a location so as to produce the maximum effect in the element under consideration. For elements supporting more than one track nosing should be applied to one track only. Nosing may be assumed applied wholly to one rail corresponding the beneath wheel flange. Transverse equally betweenwith the side railscontact may be from assumed sleepered track. distribution Other than on sleepered track, transverse distribution between members or longitudinal timbers may be considered provided these members are adequately connected. The vertical effects of nosing on elements supporting one rail only should be considered. It may be assumed that 25% of the nosing load will be transmitted longitudinally to each of the sleepers or track fastenings on each side assuming a sleeper or fastening spacing of 800 mm maximum. No addition for dynamic effects should be made to the nosing loads. 4.3.5 Centrifugal Load

Where the track on a Bridge is curved in plan, allowance for centrifugal action should be made in assessing the elements, all tracks on the structure being considered occupied. The nominal centrifugal load Fc in kN, per track acting radially at a height of 1.8 metres above rail Level should be calculated from the following formula: Fc

=

P (v + 6 )

50r

2

×f

Equation 4.15

where: P r v f

is the static axle load or equivalent uniformly distributed load for bending moment as applicable; is the radius of curvature of the track (in metres); is the speed in mph as defined in Clause 4.3.2.2; is a factor where for L less than 2.88 metres or v less than or equal to 75 mph: f

=

1. 0

and for L greater than 2.88 metres and v t over 120 km/h: f

=

 v − 75  510   v +1.751− 625     

1− 

t

2.88   L 

Equation 4.16




Page 30 of 33

where L is the loaded length of the element being considered. The vertical effect of centrifugal load on elements supporting one rail such as railbearers should be considered. This vertical effect may be reduced taking account of any track cant that is present. Centrifugal load may be dispersed using factor FEUDL or FA in Clause 4.3.3 as applicable. No addition for dynamic effects should be made to the centrifugal loads. 4.3.6 Longitudinal Loads

Allowance should be made for loads due to traction and braking as given in Table 4.10 which are equivalent to 20 BSUs. Loads for a different number of BSUs may be taken pro rata to these loads, using Equation 4.17, but not less than that applicable to 10 BSUs. Longitudinal loads should be considered as acting at rail Level in a direction parallel to the tracks. No addition for dynamic effects should be made to the longitudinal loads. Load Arising From

Loaded Length L (m)

Longitudinal Load (kN)

Traction(30%ofloadon

upto3

150

drivingwheels)

from3to5 from to 75 from to 725 over 25 upto3 from3to5 from to 75 over 7

225 300

Braking(25%ofloadon brakedwheels)

24 (

L-7) + 300 750

125 187 250 (20 L-7) + 250

Table 4.10 Nominal Longitudinal Loads (applicable to 20 BSUs of loading or RA10)

Longitudinal Load for X BSUs =

X × Value from Table 4.10 20

Equation 4.17

For Bridges supporting ballasted track, up to one third of the longitudinal loads may be assumed to be transmitted by the track to resistances outside the bridge structure, provided that no expansion switches or similar rail discontinuities are located on, or within, 18 metres of either end of the bridge. Bridges and elements carrying single tracks should be assessed for the greater of the two loads produced by traction and braking in either direction.




Page 31 of 33

Where a Bridge or element carries two tracks, both tracks should be considered as being occupied simultaneously. Where the tracks carry traffic travelling predominantly in opposite directions, the load due to braking should be applied to one track and the load due to traction to the other. Bridges and elements carrying two tracks in the same should be Consideration subjected to braking both tracks, whichever gives direction the greater effect. shouldor betraction given toon braking and traction acting in opposite directions producing rotational effects. Where elements carry more than two tracks, longitudinal loads should be considered as applied simultaneously to two tracks only. Longitudinal loads may be reduced in accordance with Clause 4.3.8. 4.3.7 Load Combinations

All loads from rail traffic, including vertical loading with dynamic effects, nosing, centrifugal and longitudinal loads should be considered to act simultaneously. Railway live loads should be combined in accordance with Table 2.1 to produce the most onerous effect in the element under consideration, except that nosing need not be assumed to act simultaneously with centrifugal load on the same track. 4.3.8 Elements Supporting More Than One Track

Where an element supports more than one track, all tracks should be considered to be loaded simultaneously. The track producing the most severe effect at the point under consideration should be considered to be fully loaded. The remaining tracks may be assumed to be loaded to 75% of the maximum value where this is specifically approved by the Railtrack Director’s Nominee.




Page 32 of 33

4.3.9 Structures Carrying Light Rail Systems

Where an element supporting tracks used by mainline rolling stock, also supports tracks used exclusively by light rail traffic, the Route Availability of the element should be determined assuming the latter tracks to be loaded with Type RL Loading as defined in BD 37/88: Loads for Highway Bridges. The dynamic factors appropriate to RL Loading given in BD 37/88 should be adopted. Values of load as given in Table 2.2 should be applied to all rail traffic.

γ fL

appropriate to live

It should be noted that the reduction factor for simultaneous loading of adjacent tracks defined in Clause 4.3.8 is not applicable to Type RL Loading. 4.4

Other Live Loads

Other live loads in addition to dead, superimposed and railway live load may need to be considered as required by Clauses 4.4.1 and 4.4.2. Loads and load combinations should be derived according to BD 37/88. 4.4.1 Wind Loads

Wind loads need not normally be considered in assessment except for the following:

•

Superstructures exceeding 40 metres span, excluding masonry or brick arches;

•

Piers exceeding 5 metres height in structural steel, wrought iron, cast iron or timber where pier height is taken as the distance from soffit of the superstructure at the pier to the base of the steel, wrought iron, cast iron or timber construction.

In using BD37/88 the following may be assumed: (i)

in BD 37/88 clause 5.3.2.1 it may be assumed that vc = 35 m/s;

(ii)

in BD 37/88 clauses 5.3.3.1.2 and 5.3.3.1.4, (a) superstructure without live load need not normally be considered;

(iii)

in BD 37/88 clause 5.3.3 it may be assumed that q = 0.75 kN/m².

4.4.2 Temperature

Temperature changes should be considered where they result in load effects within the superstructure. Differences in temperature between the top surface and other levels in the superstructure need not be considered in assessment except for continuous or rigid framed spans where the track bearing floor is monolithic or composite with the primary members.




4.5 Operational Safety Requirements 4.5.1 Track Twist

The deformation of the structure is required to be such that the twist of the track does not exceed 0.0025 radians within any 3 metre gauge length. This criterion should be considered under the passage of the most onerous real train for the assessed RA number of the structure. This check should be carried out using a value of γ fL = 1.0. 4.6 Accidental Loads from Vehicles 4.6.1 Bridges over Highways

Where agreed or instructed, railway Bridge supports should be assessed in accordance with Highways Agency Standard BD60/94: The Design of Highway Bridges for Vehicle Collision Loads for impact from errant road vehicles. 4.6.2 Intersection Bridges

Where it is necessary to consider impact on supports from train derailments beneath a Bridge, applicable criteria should be agreed with the Railtrack Director’s Nominee. 4.6.3 Train Derailments on Bridges

Assessment for the effects of train derailments is not required unless agreed with or instructed by the Railtrack Director’s Nominee.


The Structural Assessment Of Underbridges Section 5 - Steel & Wrought Iron


CONTENTS

5. STEEL & WROUGHT IRON.................................................................................................1 5.1 General ................................................................................................................................ 1 5.1.1 Applicability.................................................................................................................1 UseAnalysis...................................................................................................................2 of this Section and Appendix A ...................................................................... 1 5.25.1.2 Global 5.3 Material Properties............................................................................................................2 5.4 Partial Factors.....................................................................................................................2 5.5 Serviceability Limit State .................................................................................................. 2 5.6 Structural Form..................................................................................................................3 5.7 Assessment Process..........................................................................................................3 5.8 Measured Deviations ........................................................................................................ 3 5.9 Fatigue..................................................................................................................................3

5. STEEL & WROUGHT IRON 5.1 General 5.1.1 Applicability This Section provides recommendations for the assessment of structural steel and wrought iron superstructures. Where applicable this Section should be used for structural steel and wrought iron elements within superstructures of different materials, including composite bridges as covered by Section 8. This Section may be used for structural steel and wrought iron elements within substructures where required. 5.1.2 Use of this Section and Appendix A This Section should be used in conjunction with Appendix A of this Code of Practice for the detailed theoretical assessment of steel and wrought iron superstructures. Unless noted otherwise references to Appendices A, B, C, D, E or F shall be taken to mean Appendices A, B, C, D, E or F of this Code.

Appendix A is a set of additions and amendments to BS 5400: Part 3 (1982) incorporating amendments numbers 1 and 2 with which it should be read as a supplement. Clause numbers in Appendix A relate directly to those in BS 5400: Part 3. If a Clause does not appear in Appendix A then the srcinal Clause, where applicable, should be used. Clauses that appear in Appendix A but not in BS 5400: Part 3 are additional Clauses applicable to assessment only. The amendments enable strength aspects to be assessed that are not properly covered by BS 5400: Part 3 which was written as a design code. When assessing steel or wrought iron underbridges built before about 1960 using outmoded forms,




Page 2 of 2

assessing engineers should also consult the guidance in Appendix F which refers to this Section 5 and Appendix A by specific cross reference. 5.2 Global Analysis Plastic or yield line analysis is permitted at ultimate limit state for beams and for flat plates provided the components are compact. This is relevant for stiffened or

unstiffened floor plates supporting ballasted track. Wrought iron elements are not considered suitable for plastic global analysis. 5.3 Material Properties Properties of materials should be determined from specified values, tests of the material, or from mill test certificates. In the absence of this information then worst credible strengths should be assessed as given by Appendix A. 5.4 Partial Factors

γ fL should be obtained from Section 2. Values of the γ m , should be obtained from Appendix A of this Code of

Values of the partial load factor partial material factor Practice.

It is important to note that in Appendix A the factor of the safety format as in BS 5400: Part 3 thus:

γ fL ⋅ QK ≤ γf 3

is used in the resistance side

fK

Equation 5.1

γf 3 ⋅ γm

is normally to be taken as 1.1 for the ultimate limit state (and 1.0 for the

serviceability limit state). all met. (a)

γf 3

γf 3

may be taken as 1.0 where the following conditions are

Members are either: (i)

Rail bearers or cross girders that are assumed to be simply supported;

(ii)

Main girders of bridges with maximum skew of 25 °, and if continuous any splices are welded, or made with HSFG bolts or rivets with cover plates to both flanges.

(b)

Global analysis is based upon static distribution within the structure.

(c)

Geometric dimensions and properties of members are verified during the inspection for assessment.

5.5 Serviceability Limit State The Serviceability Limit State (SLS) need not normally be checked, except where relevant for:

•

stiffened floor construction that forms part of a girder flange;




• •

Page 3 of 3

certain trusses; HSFG joints in continuous structures.

Where particular serviceability problems have been experienced or have been reported, the serviceability limit state should be checked. Examples of serviceability problems could include excessive:

• • •

deflection or twist; vibrations; movement at joints.

5.6 Structural Form General guidance on common structural forms of steel and wrought iron rail underbridges, especially on outmoded forms built before about 1960 is given in Appendix F. Information is given on the approach for features of construction that are not covered specifically by Appendix A, such as buckle plates.

The guidance in Appendix F is applicable to most types of underbridges. However, there will be instances where a bridge may incorporate features of construction or behaviour which are not covered. In these cases judgement may be required as to the assessment procedure. In general it would is expected that assessment engineers would seek guidance, but that the assessment would use the basis of this Code of Practice and, in particular, Section 5 and Appendix A for the individual elements of steel and wrought iron. 5.7 Assessment Process The Levels of assessment are defined in Section 1. For Levels 1 & 2 the recommended steps to be followed when carrying out assessment of the different member types such as floor plates, railbearers, cross girders and main girders in forms including plate girders or box girders whether riveted or welded is included in Appendix F. At Level 3 the process should be similar to Level 2 results of actual material or load tests are used and real train loadings are assumed. 5.8 Measured Deviations Appropriate measured deviations from intended geometry should be taken into account in the assessment. In particular, the bow in main girders and verticality of webs and supports should be taken into account. 5.9 Fatigue

Fatigue should be considered in accordance with Appendices A and D.


The Structural Assessment of Underbridges Section 6 - Masonry Arches


CONTENTS

6. MASONRY ARCHES...............................................................................................................2 6.1 General ................................................................................................................................ 2 6.1.1 Applicability.................................................................................................................2 6.1.2 Levels of Analysis ....................................................................................................... 2 6.1.3 Assumptions................................................................................................................2 6.1.4 Loading and Load Distribution................................................................................2 6.1.5 Material Properties....................................................................................................5 6.1.6 Skewed Arches...........................................................................................................5 6.1.7 Permissible Capacity ................................................................................................. 6 6.2 Single Span Structures ...................................................................................................... 9 6.2.1 General ........................................................................................................................ 9 6.2.2 The MEXE Method of Assessment.......................................................................10 6.2.3 Other Methods of Analysis....................................................................................20 6.2.4 Advanced Analysis Methods..................................................................................22 6.3 Multispan Structures .......................................................................................................22 6.3.1 Modes of Failure.......................................................................................................22 6.3.2 Analysis for Failure Mode (i)..................................................................................23 6.3.3 Analysis for Failure Mode (ii).................................................................................23 6.3.4 Assessment................................................................................................................24 6.3.5 Advanced Assessment Methods ...........................................................................25 6.4 Spandrel Walls..................................................................................................................25 6.5 Jack Arches........................................................................................................................25




6. MASONRY ARCHES 6.1 General 6.1.1 Applicability

This Section should be used for the assessment of single and multiple span brick and/or stone arch superstructures. The methods described are also applicable to mass concrete arches. Recommendations for the assessment of the arch barrel, the piers of multi-span structures, and spandrel walls are included. Whilst it is recognised that spandrel walls can enhance the capacity of the arch barrel, the techniques presented other than three dimensional finite element analysis are not able to take this enhancement into account. Although considered on a purely qualitative basis, spandrel walls may affect the decision governing the permissible capacity/speed for the Bridge. Abutments, which are defined as those parts of the end supports below springing level, should be assessed on a qualitative basis in accordance with Section 11. 6.1.2 Levels of Analysis

For both single and multispan structures, analysis methods applicable to the three Levels of assessment identified in Section 1 are presented. Assessment should be carried out at the lowest Level possible. Higher Levels should be adopted only where, on assessment at a lower Level, the Bridge does not meet the required load carrying criterion. 6.1.3 Assumptions

Various assumptions are implicit in the methods of analysis presented. In addition the assessing engineer may need to make further assumptions regarding the nature of the structure being considered. The validity of all assumptions should be properly considered, and the assessing engineer should be aware of the sensitivity of the assessed capacity to all assumed parameters (such as the extent of backing in the haunch area at supports; the level of passive pressure exerted on the arch by the fill). 6.1.4 Loading and Load Distribution The assessment loading should be a single axle or group of axles based on the Type RA1 load train defined in Section 4, or representing specific trains as directed by the Railtrack Director’s Nominee. Loads should be distributed from the base of the sleeper through the fill onto the arch barrel at a slope of 1 horizontally to 2 vertically in the longitudinal direction (see Figure 6.1), and 1 horizontally to 1 vertically in the transverse direction (see Figure 6.2). Alternative distribution models may be adopted subject to approval in accordance with Railtrack’s Technical Approval Procedures.




Page 3 of 25

In determining the width of the arch barrel that is effective in carrying the load, due account should be taken of significant longitudinal cracks. Due account should also be taken of the vertical effects of nosing and centrifugal action in (on determined accordance with Section The effects may result ancurved uneventrack), distribution of loadinon the arch barrel. In such4.instances, loads should be distributed on to the arch barrel in accordance with Figure 6.3, and the analysis carried out considering the portion of the barrel associated with the more heavily loaded rail. Nosing may be assumed to be shared by the two adjacent sleepers, as detailed in Clause 4.3.4.

P

Rail

Sleeper 1 2

P/4

P/2

P/4

Ballast/fill

Arch barrel

Loads may be distributed from the underside of each sleeper, through the fill onto the arch barrel at a slope of 1 horizontal to 2 vertical

Figure 6.1 Longitudinal Distribution of Live Load Through Fill




C

Rail

C

Rail

C

Rail

400 400

Page 4 of 25

C

Rail

400 400

Ballast/fill

Spandrel

1

Angle of 1 dispersal Arch barrel

Effective width

a) No longitudinal cracks in arch barrel.

C

Rail

C

Rail

C

Rail

400 400

C

Rail

400 400

Ballast/fill

Spandrel

1


Longitudinal cracks

Effective width

b) Arch barrel cracked.

Figure 6.2 Transverse Distribution of Live Load Through Fill




P2

Page 5 of 25

P1 Spandrel wall

1


L2

L1

Effective widths

Note: P 1> P2 due to ce ntrifu gal a ction. Assessment to be based on a strip of width L 1 subject to a force P 1. Figure 6.3 Transverse Distribution of Live Load Through Fill Considering Centrifugal Action 6.1.5 Material Properties

For methods other than the Military Engineering Experimental Establishment (MEXE) method it may be necessary to make an assessment of the strength of the masonry from which the arch barrel is constructed. Figures 6.4 and 6.5 give typical compressive and characteristic strengths to be expected for various types of masonry by brick or stone type and mortar. Strength tests, if considered necessary, should be carried out on the individual components. BS 5628 and TRRL Contractor Report 244: Masonry Properties for Assessing Arch Bridges give information on suitable tests and strengths. 6.1.6 Skewed Arches

The assessment of skewed arches may be undertaken using a two dimensional analysis for skews up to and including 35°. The span length should be taken as the clear distance between abutment faces measured on the skew (see Figure 6.6). For skew angles greater than 35° a three dimensional analysis should be undertaken. Where, however, the applied live loads are located at a significant distance from the edges of the Bridge (for example tunnels) the assessment may be based on a two dimensional analysis of the square span.




Page 6 of 25

For skewed multispan structures, the analysis should be based on the skew spans and pier widths. Such structures may exhibit defects resulting from out of balance lateral thrusts at supporting piers (“racking” effects) and torsional effects. As it is difficult to assess these effects quantitatively, skewed multispan arches should also be subject to a qualitative assessment, which should consider the following:

•

whether the capacity of the structure is at risk owing to unknown implications of the defects;

•

whether monitoring of the structure would assist assurance of the safety of the structure under rail traffic.

The implications of both the quantitative and qualitative assessments should be considered in determining the safe load capacity of the structure. 6.1.7 Permissible Capacity When using the MEXE method, the safe load capacity should be determined by comparing the assessed permissible axle capacity with the single axle weight in the group indicated in Table 4.3. It should be noted that this can only be taken as a guide to the RA number of any arch since the assumptions made in the development of the capacity formulae apply to axle spacings of 2.0 metres.

When using methods other than MEXE to determine the permissible capacity from the Provisional Axlecarried (Ultimate) the partial factors used. Assessments out Capacity, by these methods should benoted based below on theshould actual be thickness of the arch barrel (that is a Geometric Factor of 1). Dead and Superimposed Dead Loads Arch Fill Live Loads

γ fL γ fL

γ fL

- Refer to Table 2.2.

= 1.2 = 1.4

γ f 3 = 1.35 γ m = 1.0 A dynamic factor of 1.8 should be applied to a single axle load or to the critical axle of a train of loads. Where the depth of ballast and fill from the underside of sleeper to the crown of the arch ring is greater than 600 mm, consideration may be given to reducing the value of the dynamic factor. Any alternative value proposed should be agreed in accordance with Railtrack’s Technical Approval Procedures. In certain circumstances permanent loads on the arch barrel may generate relieving effects. To ensure that the assessment does not produce an unconservative result, the analysis should also be undertaken with γfL = 1.0 for all such permanent effects, and the RA number determined from the lower calculated capacity.




Page 7 of 25

12

10

1:2:8 Mortar Designation (iv)

8

fK 2

(N/mm ) 6

1:3 Lime Mortar 4

2

0 0

10

20

30

40

50

60

70

80

2

Compressive Strength of Unit (N/mm )

(Material Designations as defined in BS 5628)

Figure 6.4 Characteristic Strength of Normal Brick Masonry fk

90

100




Page 8 of 25

20

Ashlar

15

Squared Rubble with 1:2:8 Mortar

fk 2

(N/mm ) 10

Random Rubble with Lime Mortar

5

0 20

40

60

80

100

120

140

160

180

2

Compressive Strength of Unit (N/mm )

(Material Designations as defined in BS 5628)

Figure 6.5 Characteristic Strength of Normal Stone Masonry fk

200

220




Page 9 of 25

In determining the load carrying capacity of the Bridge by any of the methods presented, due account should be taken of known defects and any deterioration identified during inspection.

C

cr

Spandrel wall

sed os

Skew angle of Bridge <35°

Face of abutment

C

Bridge

Span length to be adopted for Assessment

Figure 6.6 Definition of Span Length for the Assessment of Skewed Arches with Skew ≤ 35° 6.2 Single Span Structures 6.2.1 General

Assessment of single span structures may be carried out using the MEXE method described in Clause 6.2.2, or by one of the alternative methods given in Clause 6.2.3. The MEXE method is approximate and may only be adopted where: (i)

the clear span is less than 19.8 metres (see Figure 6.6 for skew arches);

(ii) (iii)

the arch barrel is not severely deformed; there is no evidence of significant ring separation for those rings under consideration;

(iv)

the arch does not support internal spandrel walls with vaulted construction.




Page 10 of 25

6.2.2 The MEXE Method of Assessment 6.2.2.1 General

The MEXE method of assessment assumes that the loading on arches is represented by that from the most common heavy freight wagons, such as 100 ton wagons with 2.0 metre bogie axle spacing, or two axle type wagons. Figures 6.7 to 6.12 are enveloping curves giving the provisional axle capacities of arched bridges. The envelope curves have been based on the application of single axles, bogie axles and adjacent bogies of coupled vehicles to the spans, and should be assumed to allow for dynamic effects. For this reason the values read from the curves may in some cases vary from calculated values by ± 2%. For each arch the permissible axle capacity in tonnes should be determined from the provisional axle capacity by modifying it using the factors given in Clause 6.2.2.3. 6.2.2.2 Provi sion al Ax le Capacity (QP)

Provisional axle capacity (QP) for arches of different spans, ring thickness and depths of fill may be obtained from Figures 6.7 to 6.12, where h is the depth of fill beneath the sleeper soffit and arch ring. The provisional axle capacity obtained using this method is generally less than the actual capacity. It should not be taken as more than an indication that the arch may be suitable to carry rail traffic for the following reasons:

• • •

minimum longitudinal distribution of loading has been assumed; only that part of the arch under the track has been assumed to carry load; the adjustment factors for condition are subjective.

In marginal cases, justification of a higher capacity may be possible through further consideration of the above factors, or by means of other, more refined methods of assessment. Where the characteristics of an arch are such that graph readings require interpolation, it may be more convenient to calculate the provisional axle capacity directly, using the formulae given in Appendix F. For an arch carrying more than one track, the axle capacity for the portion under each track should be assessed as if it were a separate arch. 6.2.2.3 M odifyin g Factors for Provis ional A xle Ca pacity

The provisional axle capacity should be adjusted to give the permissible axle capacity by taking into account various characteristics of the arch such as shape and condition, using the factors defined below.




Page 11 of 25

The actual conditions of the bridge being assessed may not exactly align with conditions defined below. The selected values of the factors should be those that approximate nearest to the conditions of the arch being assessed. The factors allocated to the various characteristics are as follows:

•

Profile Factor (KP) This factor is obtained from Figure 6.13.

•

Shape Factor (KS)

For parabolic arches KS = 1.0. For all other arch shapes this factor is obtained from Figure 6.14.

•

Material Factor (KM) Soft brick and soft stone Hard brick Mass concrete Masonry

1.0 1.2 1.2 1.5

It is permissible to interpolate for particular types of brick and stone. In allocating material factors the possibility of previous relining should be considered.

•

Condition Factor (KV) (a) Brick Good condition - No spalling Fair condition, Slight spalling (between 0% and 25% of arch surface) and no bricks missing Poor condition, Significant spalling (over 25% of arch surface) and/or bricks missing

1.0 0.9 0.8

If a large number of bricks are missing (over 10% of the arch surface) or the joints are only partly filled with mortar, or the jointing material is in a very poor and deteriorated condition, the arch should be treated as having one less ring when obtaining the provisional capacity (QP). (b)

Stone

Good condition - No spalling Fair condition, Slight spalling (between 0% and 25% of arch surface) Poor condition, Significant spalling (over 25% of arch surface) but no stones loose or missing Some stones loose or missing, severe loss of jointing material in undressed stone arches

1.0 0.9 0.8 0.75




Page 12 of 25

•

Crack Factor (KC)

a)

Nocracks

1.0

b)

Longitudinal cracks: outside the centre third of the arch, less than one tenth of the span in length

0.95

outside the centre third of the arch, longer than one tenth of the span in length

0.90

within the centre third of the arch, less than one tenth of the span in length

0.90

within the centre third of the arch longer than one tenth of the span in length

0.85

c)

Lateral and diagonal cracks: up to three small lateral or diagonal cracks less than 3 mm in width and less than one tenth of the arch width

numerous small cracks as above in the centre third of the arch

•

0.90 0.60

Deformation Factor (KD) Deformation of the arch may be due to partial failure of the arch ring or movement of the abutments. It should not be mistaken for irregularity due to inaccurate falsework centres being used during construction. If the deformation has resulted in a flat section, the final axle capacity of the bridge should be reduced, using the Deformation Factor (KD), in the proportion of rise at centre of flattened portion to srcinal rise at this point. KD = Rise at centre of flattened portion Original rise at the same point

6.2.2.4 Per mi ssibl e axl e capaci ty (CF )

The permissible axle capacity should be obtained by multiplying QP by all the above factors as applicable. Permissible axle capacity = QP x KP x KS x KM x KV x KC x KD Any number of axles up to the permissible capacity may be permitted.




Page 13 of 25

70

60

50

QP (t)

40

h=900 mm 30 h=750 mm

h=600 mm

h=450 mm 20

h=300 mm

h=150 mm 10 2

3

4

5

6

7

8

9

10

Span L (m)

Figure 6.7 Arch Ring Thickness d at the Crown = 300 mm




Page 14 of 25

70

60

50

QP (t)

40

h=900 mm

h=750 mm 30 h=600 mm

h=450 mm

20 h=300 mm

h=150 mm

10 3

4

5

6

7

8

9

10

11

Span L (m)





Page 15 of 25

70

60

50

h=900 mm QP (t)

40

h=750 mm

h=600 mm

30

h=450 mm

h=300 mm 20

h=150 mm

10 4

5

6

7

8

9

10

11

12

13

Span L (m)





Page 16 of 25

70

60

50 h=900 mm

h=750 mm QP (t)

40 h=600 mm

h=450 mm

30 h=300 mm

h=150 mm 20

10 6

7

8

9

10

11

12

13

14

15

16

Span L (m)





Page 17 of 25

80

70

60

h=900 mm

h=750 mm QP (t)

50 h=600 mm

h=450 mm 40

h=300 mm

30

h=150 mm

20 7

8

9

10

11

12

13

14

15

16

17

18

Span L (m)





Page 18 of 25

80

70 h=900 mm

h=750 mm 60

h=600 mm

h=450 mm QP (t)

50

h=300 mm 40

h=150 mm 30

20 8

9

10

11

12

13

14

15

16

17

18

19

20

Span L (m)




1.0

0.9

KP 0.8

0.7

0.6 345678 Span Rise Ratio L/R C

Figure 6.13 Profile Factor KP





Page 20 of 25

1.0

KS 0.5

0.0 0.75

0.80

0.85

0.90

0.95

1.00

¼ Point / Centreline Rise Ratio qR /Rc

Figure 6.14 Shape Factor 6.2.3 Other Methods of Analysis 6.2.3.1 General

The following methods of analysis may be adopted where, for the reasons given in Clause 6.2.1, the MEXE method is considered inappropriate: (i)

Mechanism methods which consider the failure of the arch by the formation of hinges (generally four - see Figure 6.15) close to the extremities of the arch ring. An iterative process is required to determine the critical position for hinge formation and the associated minimum load, which is considered to be the ultimate capacity of the arch.

(ii)

Elastic methods which, by the incremental application of loads, allow the area of the arch ring to be modified as tension develops and masonry yields (see Figure 6.16). The application of load is continued until, ultimately, sufficient hinges form to cause collapse as a mechanism. This form of analysis may be carried out using classic elastic theory or by a finite element approach.

Computer software for assessment by any method other than MEXE should be approved by the Professional Head of Structures prior to its use.




Page 21 of 25

P

Denotes hinge

Figure 6.15 Single Span Failure Involving the Formation of Four Hinges

Tension zone

Load applied incrementally

Crushed zone

The arch ring is modified after each iteration to take account of crushed and tension zones.

Figure 6.16 Thinning of Arch Ring During Elastic Analysis




6.2.3.2 Limitations

Where the span/rise ratio of the arch is greater than 6 and the clear span exceeds 15 metres, the failure mode may be a snap through’ involving the formation of a three, rather than four, hinged mechanism. As this form of failure involves the gross deformation of the arch in the vicinity of the load, mechanism methods, and elastic methods which do not take account of geometric non-linearity should not be used. Care should be exercised in modelling the soil/structure interaction of deep arches, where it may be possible to mobilise sufficient passive pressure to eliminate the horizontal thrust at the springing furthest from the point of loading. 6.2.4 Advanced Analysis Methods

The following methods of analysis may be applicable when a more detailed investigation is required: (i)

plane strain, (two dimensional finite element analysis);

(ii)

full three dimensional finite element analysis using curved shell or other appropriate elements.

Method (i) allows the analysis of problems such as ring separation to be investigated and may allow some advanced soil models to be adopted. Method (ii) should only be used where specific structural problems warrant such complex analytical techniques, and when approved by Professional Head of Structures Engineering in accordance with Railtrack’s Technical Approval Procedures. 6.3 Multispan Structures 6.3.1 Modes of Failure

In considering the ultimate capacity of multispan arches two distinct modes of failure should be examined: (i)

the collapse of a single span which is not modified by the presence of an adjacent span;

(ii)

the interaction of adjacent spans leading to partial or total collapse of the

structure. Mode (i) predominates where intermediate piers are sufficiently stocky to ensure that the structure can be considered as a series of independent spans, and may be considered to apply when: H t

≤

2

Equation 6.1




Mode (ii) may be considered to apply when: H t

>2

Equation 6.2

where h and t are as defined below and shown in Figure 6.17: t H

is the pier thickness; is the height of the pier above its foundation level.

t H

Figure 6.17 Definition of Parameters h and t 6.3.2 Analysis for Failure Mode (i)

Where the minimum actual pier thickness is greater than H 2 , each span of a multispan structure may be analysed using an applicable single span method. 6.3.3 Analysis for Failure Mode (ii)

Interaction of adjacent spans should be considered where the pier thickness is less than or equal to H 2 . Analysis may be carried out in these cases on the basis of: (i)

An equilibrium approach, in which a zone of thrust is traced throughout the structure is in equilibrium withdirection the applied Redundancy may at be dealt with which by specifying the location, andforces. magnitude of the thrust the highest point of each arch;

(ii)

A mechanism analysis, which for a multispan structure, is generally the formation of seven hinges within two adjacent spans and the intermediate pier as shown in Figure 6.18. Where span lengths and pier heights vary, the analysis should consider successive pairs of adjacent spans.




Denotes hinge position

Figure 6.18 Twin Span Failure Involving the formation of Seven Hinges

Notwithstanding the limit of pier height to thickness in Clause 6.3.1, the capacity of an individual span can still govern the overall capacity of the structure, and therefore, single span analysis should also be undertaken. The safe load capacity of the structure should be determined from the lowest capacity calculated from single and multispan assessment. 6.3.4 Assessment

In assessing a multispan arch structure using either of the analysis methods outlined in Clause 6.3.3 due account should be taken of the following features: (a)

Haunching The capacity of multispan arches can be greatly influenced by the extent of haunching (or backing) above the intermediate piers. In the absence of definite information the minimum level of haunching should be taken to be the level where the extrados crosses the vertical through the intrados at the springing point, unless there are features or other structural evidence to the contrary. No minimum haunching level at abutments should be assumed without some evidence of its presence;

(b)

Voided Elements of the Structure Allowance should be made for reduced weight over or within the piers which may arise due to the presence of voids such as internal spandrel walls with vaulted construction shell piers, or piers with cutouts;

(c)

Piers with Cut Outs




Consideration should be given to the increased width of thrust zone that may arise due to the variation in cross sectional area throughout the height of the pier; (d)

Foundations In the absence of definitive information, the top of each pier foundation may normally be assumed to be 0.5 metres below ground level;

(e)

Lateral Loading on Piers Lateral loading resulting from soil or surcharge pressure acting on the face of the piers, such as may arise from a significant difference in ground levels on opposite sides of a pier, should be considered in the analysis.

6.3.5 Advanced Assessment Methods

For multispan structures where the methods outlined in Clauses 6.3.2 and 6.3.3 are not applicable, structural modelling using plane (two dimensional) strain finite element analysis may be considered. 6.4 Spandrel Walls

Although spandrel walls affect the load carrying capacity of arch bridges, current analytical techniques do not satisfactorily model their interaction with the arch barrel. Consequently, these elements should be assessed qualitatively by considering their condition and the significance of any defects. 6.5 Jack Arches

Jack arches, which span between supporting members to form the deck of metallic beam bridges, should be assessed qualitatively. Details of an empirical approach which may be appropriate are contained in Bridgeguard 3 Current Information Sheet 22. Prior to the use of this method, the assessing engineer must satisfy himself as to it’s appropriateness to the structure under consideration.


The Structural Assessment of Underbridges Section 7 - Concrete Structures


CONTENTS

7. CONCRETE STRUCTURES..................................................................................................1 7.1 Introduction........................................................................................................................1 7.1.1 Scope ............................................................................................................................ 1 Background and Origins...........................................................................................1 7.27.1.2 Assessment of Strength....................................................................................................2 7.2.1 General ........................................................................................................................ 2 7.2.2 Material Strengths......................................................................................................2 7.2.3 Damaged and Deteriorating Structures................................................................4 7.3 Serviceability and Other Considerations......................................................................5

7. CONCRETE STRUCTURES 7.1 Introduction 7.1.1 Scope This Section should be read in conjunction with Appendix B for the assessment of concrete rail underbridges and elements of rail underbridges. It covers elements of

plain, reinforced and prestressed concrete construction, including both pre- and posttensioned prestressed concrete construction with internal bonded tendons. It does not cover in-fill joist type structures such as the decks of Z-type bridges that are considered to be composite, for which Section 8 should be used. It does not cover elements with external or unbonded tendons. 7.1.2 Background and Origins Appendix B is based on BD 44/95: The Assessment of Concrete Highway Bridges and Structures. Acknowledgement is made to the Highways Agency for allowing Standard BD 44 to be used. This document was being drafted at the same time that a revision to BD 44 was being prepared, there are inevitably some differences that are not directly related to the differences between bridges carrying roads and railways.

The Highways Agency Advice Note BA 44: The Assessment of Concrete Highway Bridges and Structures provides guidance on the use of BD 44/95 much of which is also relevant in this Section. However, where it gives alternative approaches which will frequently be used in the assessment of railway underbridges, the relevant clauses have been included here.




Page 2 of 5

7.2 Assessment of Strength 7.2.1 General The objective of Section 7 is to provide recommendations in order that a realistic assessment of the strength of concrete elements may be obtained. This is in part achieved by using measured values of parameters which at the design stage could have only been predicted. Section 7 enables the following aspects to be considered:

(a)

actual strength of steel and concrete;

(b)

details and designs not complying with the recommendations of BS 5400: Part 4;

(c)

less conservative assessments of some aspects compared to the interpretation used in design codes, or where better information has become available since these design standards were written.

In addition, equations which were srcinally written in true design’ format, giving required steel area as a function of required design strength, have been rearranged in assessment format, giving strength as a function of steel area. 7.2.2 Material Strengths 7.2.2.1 Wor st Cr edibl e Str ength

The term worst credible strength has been used to allow a conservative estimate of the actual material strength of structures and structural elements to be used for assessment. Worst credible strength can be defined as the worst value of that strength which the engineer, based on experience and knowledge of the material, realistically believes could be present in the structure or element being considered. This value may be greater or less than the characteristic strength of the material assumed at the design stage. Since this value eliminates some of the uncertainties associated with the use of characteristic strengths, reductions may be made in the partial factor for material γ m . The use of worst credible strengths should be considered in the following circumstances: (i)

when an initial assessment using characteristic, assumed or recorded values has not shown the element to be capable of carrying the full assessment loading to Section 4;

(ii)

if a structure has suffered damage or deterioration in such a way that the actual strengths are, or are thought to be, less than assumed characteristic values;

(iii)

if there is reason to believe the original materials were deficient and below intended characteristic values;

(iv)

where no information exists on the values used in design or recorded in construction.




The worst credible value should generally be taken as the lower bound value of the estimated in situ strength for the element under consideration. Advice on the assessment of worst credible strength is given in BA 44. 7.2.2.2 Assum ed, Specif ied and R ecor ded Str engths

It is usually desirable to undertake initial assessments at Level 1 without undertaking the material tests required to determine worst credible strengths. Guidance on material which may be used are given below at (a), (b) and (c). The strengths taken should be treated as characteristic strengths and used with the applicable γ m as defined in Clause 7.2.2.3. Where assessment using these values does not give the required element strength, consideration should be given to taking samples and using worst credible values instead. However, before undertaking the tests, the sensitivity of the assessed strength to assumed material strength should be investigated. In particular, the strength of lightly reinforced members is often very insensitive to the strength of the concrete. (a)

Concrete. Pre-1939 concrete may be assumed to have a characteristic strength not greater than 20 N/mm². The strength of more modern concrete is often specified on the record drawings. Where a characteristic strength is specified, this value may be used directly. Where concrete strength has been defined in terms of a 28 day minimum cube strength, this value may be assumed to be equivalent to characteristic cube strength. It may be noted, however, that for statistical reasons this is theoretically a conservative assumption. Less modern concretes also tended to gain strength with age more than modern concrete. Scope for obtaining a higher strength in accordance with Clause 7.2.2.1 may be considerable. Nominal mixes prescribed by constituents and grades designated by standard letters were frequently used. Information on some of these is given in Appendix B1. Where concrete strengths from Appendix B1 are used, they should be treated as characteristic strengths. The mix design of prestressed elements, particularly pre-tensioned ones, is often controlled by the requirement to achieve transfer at an early age. This also often resulted in 28-day strengths well above specified values. If construction records include 28-day cube results for concrete which is representative of the critical areas, these values may be used. They should be processed in the same way as core results are processed to obtain a representative value in accordance with BA 44. Alternatively, if the number of test results available exceeds 6, the actual standard deviation of the test results may be calculated and the representative value taken as the mean minus 1.65 times the standard deviation. Because of the difference between British Standard wet cured cubes and in situ concrete, the reduction in uncertainty is less than for in situ core tests so the representative value




Page 4 of 5

obtained from records should therefore be treated as a characteristic, rather than a worst credible value. Judgement is needed in using the results of non-standard cube tests. Older age tests and tests on cubes stored with the structure may be used. However, because the early age strength gain of cements varies significantly, extrapolation of early age cube results is not normally reliable. (b)

Reinforcement and Prestressing Steel - In the absence of definite information, a characteristic reinforcing steel strength of 230 N/mm² may be assumed for steel produced before 1961. Strengths of more recent reinforcement and prestressing steels can be obtained from contemporary standards. If construction records include actual test results for representative steel, the test results may be used. It should be noted that tests on one size of bar are unlikely to be representative of the strength of bars of another size. Where test results are used, they can be processed in the same way as tests on samples taken from the structure to obtain a representative value in accordance with BA 44. This value may be taken as the worst credible value.

7.2.2.3 Parti al F actor for M ateri als, γ m

The values of γ m for concrete and reinforcement or prestressing steel should be as given in Appendix B. The values for use with characteristic and worst credible strengths are different. To enable the correct value of γ m to be used, all limiting criteria are expressed in Appendix B as formulae with γ m stated explicitly, rather than as tabulated values. 7.2.3 Damaged and Deteriorating Structures The strength prediction methods in Appendix B assume that the structure is in the as-built’ condition. It will often be necessary to assess damaged or deteriorated structures. Some general principles for taking damage and/or deterioration into account are outlined below.

It is not normally appropriate to apply overall condition factors’ (such as are used with the MEXE’ method for masonry arches) to concrete structures. It is preferable to attempt to quantify the damage/deterioration in particular areas and allow for it in the assessment calculations. Where the deficiency takes the form of weak materials, the use of worst credible strength based on tests on samples from the affected areas automatically takes account of this and further allowance is not required. The commonest form of damage to concrete structures is caused by corrosion of the embedded steel. Where local corrosion is suspected, an estimate of the loss of steel section is required. For reinforced concrete structures, the reduced area of steel can be used directly in assessment. If the loss of area in a specific bar is taken to be




greater than 30%, consideration should be given to the resulting reduced ductility of the bar as well as its reduced strength. Bonded prestressing wires are likely to break due to prestress load when the loss of section reaches approximately 40%. Hence, when it is estimated that the amount of loss has reached 30% of the area, the relevant wires should be ignored in the assessment calculations. For post-tensioned tendons, provided the grout is in good condition, even whole tendons that have been broken are able to re-anchor in lengths of the order of one metre. The position of individual breaks as well as their number is therefore important. In assessing the strength of a structure allowing for damage to tendons, it is very important to be aware that the assumed loss can only ever be very approximate and to allow for upper and lower bounds to the actual loss. The bounds may be very far apart. For the more easily detected and common form of corrosion of reinforcement, the loss of steel area is not normally the dominant factor. The corrosion products (rust) occupy a greater volume than the parent steel causing cracking and eventually spalling of the cover concrete. A loss of bond strength occurs when the cover concrete cracks. The significance of the loss should be considered where reinforcement details are sensitive to bond. Where the cover concrete has seriously delaminated or spalled off over the whole length of a particular reinforcing bar, that bar should be disregarded in assessment. However, delaminating and spalling in the critical flexural area is less significant if the bar is anchored at either end of the damaged area. Anchorage may be present due to the local nature of the damage or because the ends of the bars are bent up or hooked. In such cases, the bar may be considered to contribute to the element strength with due allowance for loss of section or incomplete anchorage as appropriate. 7.3 Serviceability and Other Considerations Assessment is normally carried out for the ultimate limit state only. Appendix B does not give criteria for serviceability and deflection. Where checks for serviceability are required by the Railtrack Director’s Nominee, the criteria should be agreed with him. The initial approach is usually to take criteria from BS 5400: Part 4. However, calculated stresses and cracks widths that exceed these criteria frequently do not require immediate action and the requirements should be agreed with the Railtrack Director’s Nominee.



The Structural Assessment of Underbridges Section 8 - Composite (Steel or Wrought Iron with Concrete)

Page 1 of 1

CONTENTS

8. COMPOSITE (STEEL OR WROUGHT IRON WITH CONCRETE)...........................1 8.1 General ................................................................................................................................ 1 8.2 Use of Appendix C............................................................................................................1

8. COMPOSITE (STEEL OR WROUGHT IRON WITH CONCRETE) 8.1 General

This Section provides recommendations for the assessment of composite elements of rail underbridges involving components of steel or wrought iron interconnected with concrete. Assessment of the steelwork, wrought iron and concrete elements should be carried out using Sections 5 and 7 of this Code augmented by this Section where the materials act compositely. 8.2 Use of Appendix C

This Section should be used in conjunction with Section 5 and 7 of this Code of Practice. Appendix C contains relevant clauses and appendices as a set of additions and amendments to BS 5400: Part 5: 1979 incorporating amendment no 1 dated 31 May 1982, and should be read as though it is a supplement to BS 5400: Part 5. References within Appendix C to this standard” or that part” should therefore be taken as referring to Appendix C. Where there is no addition to a clause in BS 5400: Part 5, the existing design clause should be used. It should be noted that some clauses in BS 5400: Part 5, for example those dealing with construction aspects or recommended forms are generally not applicable to assessment and should be ignored.



The Structural Assessment of Underbridges Section 9 - Cast Iron

Page 1 of 6

CONTENTS

9. CAST IRON...............................................................................................................................1 9.1 General ................................................................................................................................1 9.2 Material Properties............................................................................................................1 9.3 Strength................................................................................................................................1 9.3.1 Permissible Stresses...................................................................................................1 9.3.2 Beams Continuously Restrained by Surrounding Fill..........................................2 9.3.3 Beams with Intermediate Lateral Restraints ........................................................2 9.3.4 Compression Members ............................................................................................2 9.3.5 Members subject to Bending and Axial Compression .......................................3 9.3.6 Restraints to Elements in Compression................................................................3 9.4 Fatigue ..................................................................................................................................4

9. CAST IRON 9.1 General

This Section provides recommendations for the assessment of Bridges that contain cast iron elements such as beams, columns forming part of a support framework, and arch ribs. The strength of all elements should be assessed on a permissible stress basis using the values given in Clause 9.3. 9.2 Material Properties

The following properties may be adopted for assessment purposes: Unit weight

7200 kg/m³

Modulusofelasticity

114000 N/mm²

Coefficient of linear thermal expansion

10.2 x 10

-6 o

C-1

9.3 Strength

9.3.1 Permissible Stresses

The stresses in cast iron under the combined effects of permanent and live loads should not exceed the following limits: Compression Tension Shear

154N/mm² 46 N/mm² N/mm² 46




9.3.2 Beams Continuously Restrained by Surrounding Fill

The section modulus of cast iron girders may be increased for live loading by the factor D/d, where D is the overall depth of the deck less an appropriate allowance for ballast and loose fill, and d is the depth of the bare girder at midspan, provided the following conditions are present: (i)

the girders are known to be firmly embedded in well consolidated filling material, other than pure sand or pure clay;

(ii)

there are no services in the fill that would decrease the support which it renders, such as stoneware pipes or large diameter water or gas mains.

The factor D/d should not be applied to longitudinal girders consisting of cast iron troughs. The maximum value for D/d which may be applied to the section modulus of cast iron sections for live load, should not exceed 2.0. If openings are made in the deck after an assessment that used the D/d factor, the opening should be back filled with concrete, or the assessment reconsidered. 9.3.3 Beams with Intermediate Lateral Restraints

Members subject to bending which have discrete, rather than continuous, restraint to their compression flange may be susceptible to lateral torsional buckling. In these cases a reduced value of permissible compressive stress due to bending, pbc , should be adopted, which takes due account of this phenomenon. The procedure for determining pbc should be agreed in accordance with Railtrack’s Technical Approval Procedures. 9.3.4 Compression Members

Cast iron struts that are adequately braced in accordance with Clause 9.3.6 should be assessed by the Gordon-Rankine equation as follows:

P

=

(2 x10

−4

  f A ) FaL 1+ K  cy

s 2

r

2

     

Equation 9.1

where: P f cy

is the safe load (kN); is the compressive yield stress which should be taken as 555 N/mm²;

A Ls

is the cross section area (mm²); is the length (mm);

Kr

is the least radius of gyration (mm);




Page 3 of 6

is the end fixity factor given in Table 9.1; 1 is the material factor, . 1600

F a

End Condition

F

Both endsfixed, pin jointed One end one end pin jointed Both ends rigidly fixed One end fixed, one end entirely free

1 0.5 0.25 4

Table 9.1 Values of End Fixity Factor ( F)

9.3.5 Members subject to Bending and Axial Compression

Members subject to both axial compression and bending, such as arch ribs, should satisfy the following condition at all points: fc pc

+

f bc pbc

≤ 1.0

Equation 9.2

where: fc

is the calculated average axial compressive stress;

f bc

is the compressive stress due to bending about the centroidal axis;

pbc

is the permissible compressive stress determined in accordance with

pc

Clause 9.3.3; is the allowable compressive stress in N/mm² determined in accordance with the following expression: pc =

0.2f cy

 FaL 1+  K

s 2

r

2

  

Equation 9.3

where f cy ,F ,a ,L s and K r are as defined in Clause 9.3.4. 9.3.6 Restraints to Elements in Compression

The load effects to be considered in assessing the adequacy of restraining members and their connections should be determined in accordance with Appendix A. The adequacy of members and their connections should be considered in accordance with the permissible stress requirements of this Section.




Page 4 of 6

9.4 Fatigue

In addition to the limits defined in Clause 9.3.1, the live load stresses, f L and qL , should not exceed the values defined in Equations 9.4 to 9.7. Equations 9.4 and 9.5 are represented graphically in Figure 9.1 and 9.2. In determining the live load effects, the dynamic factor applicable to fatigue as defined in Table 4.5 should be used. For tensile values of f L , f L should not exceed the greater of the values given by: fL = fL =

− 0.44f 19.6 − 0 .76f 24.6

d

d

N/mm², or N/mm²

Equation 9.4

where f d is the permanent load stress and tensile stresses are positive. For compressive values of f L , f L should not exceed the lesser of the values given by: fL = fL =

− 43.9 + 0.79f − 81.3 + 3.15f

d

N/mm², or

d

N/mm²

Equation 9.5

Where the live load shear stress qL acts in the same sense as the dead load shear stress qd : qL

≤ 24.6 − 0.44q

d

N/mm²

Equation 9.6

Where the live load shear stress qL acts in an opposite sense to the dead load shear stress qd : qL qL

≤ ≤

43.9 − 0.79 qd N/mm² when qL ≤ 2qd, or 24.6 + 0.44qd N/mm² when qL > 2qd

Equation 9.7

In Equations 9.6 and 9.7, the signs of the shears have been taken into account and only numerical values of qL and qd should be substituted.

RT/CE/C/025 Issue: 1 Date: February



Page 5 of 6

150 fL=

100 f=

Live Load Stress fL N/mm²

19.6-0.76 fd

-154

d

50

24.6-0.4 fd

fL=

46-fd

fL=

0

fd= fL=

46

-43.9+0.79 fd

-50 fL=

-154-fd

fL=

-81.3+31.5 fd

-100 -150 -200

-150

-100

-50

0

Stress due to Permanent Loads N/mm² Note Tensile Stresses are P ositive Figure 9.1 Permissible Stresses in Cast Iron

50

100

RT/CE/C/025 Issue: 1 Date: February



Page 6 of 6

40 fL=

19.6-0.76 fd

30 fL=

Live Load Stress fL N/mm²

24.6-0.4 fd

20 10 46-fd

fL=

0 fd= fL=

46

-43.9+0.79 fd

-10 -20 Note Tensile Stresses are P ositive

-10

0

10

20

30

Stress due to Permanent Loads N/mm² Figure 9.2 Permissible Stresses in Cast Iron

40

50


The Structural Assessment of Underbridges Section 10 - Timber


CONTENTS

10. TIMBER.....................................................................................................................................1 10.1 General..............................................................................................................................1 10.2 Assessment ....................................................................................................................... 1 10.3 .............................................................................................................................. 1 10.4 Loading Modifications to BS 5268: Part 2: 1996......................................................................1 10.4.1 Species ....................................................................................................................... 1 10.4.2 Dimensions and Geometrical Properties...........................................................2 10.4.3 Grades........................................................................................................................2 10.4.4 Grade Stresses for Strength Classes and Individual Species ..........................2 10.4.5 Duration of Loading................................................................................................2

10. TIMBER 10.1 General

This Section provides recommendations for the assessment of timber elements of underbridges. It covers timber elements of both superstructures and substructures. It also deals with timber decking and is applicable to longitudinal timbers. 10.2 Assessment

Timber Bridges and timber elements within Bridges should be assessed to provide a safe load carrying capacity using permissible stress principles in accordance with BS 5268: Part 2: 1996 unless modified by Clause 10.4 below. 10.3 Loading

For the purpose of assessment, the loading applied to the structure should be as defined in Section 4: Loading, except that, for decking timbers, no impact factor (1 + ϕ ) should be applied. 10.4 Modifications to BS 5268: Part 2: 1996 10.4.1 Species

Replace BS 5268: Part 2 Clause 2.3 with the following: Where available, the species and grading of timber should initially be taken as that shown on record drawings or other record documentation. In the absence of definite species identification, initial assessment calculations should assume the timber as Douglas Fir (Canada/North American), SS grade. Obtained



The Structural Assessment of Underbridges Section 10 - Timber

Page 2 of 2

samples should be identified if calculations show an inadequate assessment capacity relative to existing or future loading requirements. 10.4.2 Dimensions and Geometrical Properties

Replace BS 5268: Part 2 Clause 2.4 with the following: Member dimensions should be verified on site. Assessment should be based on the measured site dimensions. therefore taking production tolerances into account. Any loss of section area due to timber decay should be taken into account in the derivation of member capacity. 10.4.3 Grades

Replace BS 5268: Part 2 Clause 2.5 with the following: If record drawings or other information do not show the grade of timber, on-site visual stress grading by a specialist consultant should be carried out. The timber consultant should be qualified by a suitable certification body. 10.4.4 Grade Stresses for Strength Classes and Individual Species

Replace BS 5268: Part 2 Clause 2.6 with the following: Timber employed in railway underbridges should be considered as service class 3 (wet exposure). The grade stresses for service classes 1 and 2 given in BS 5268: Part 2, Tables 7 to 12 should be multiplied by the applicable modification factor given in Table 13. 10.4.5 Duration of Loading

Replace BS 5268: Part 2 Clause 2.8 with the following: The modification factor for duration of loading applicable to the assessment of underbridges, excepting decking timbers, should be taken as 1.5 as applicable to short term duration of loading. For decking timbers the modification factor for duration of load should be taken as 1.0.


The Structural Assessment of Underbridges Section 11 - Substructures


CONTENTS

11. SUBSTRUCTURES.................................................................................................................1 11.1 General..............................................................................................................................1 11.2 Assessment ....................................................................................................................... 1

11. SUBSTRUCTURES 11.1 General

This Section provides recommendations for the assessment of substructures and foundations for all types of bridges. Substructures and foundations are taken to include all elements of the bridge beneath the soffit of the deck, excluding bearings, but including piers, bank seats, abutments, wing walls and foundations, including piles. For arch bridges the substructure and foundations include the springings and all elements beneath the ground. For substructures and foundations, any failure is likely to be progressive and there will usually be some warning signs (such as movement, settlement, rotation and/or cracking) well before final collapse is imminent. For this reason the history of the observed defects should be determined if possible. For example, movements or rotations may have occurred early in the life of the structure and subsequently a state of equilibrium has been achieved. Most substructure and foundation elements, especially structures of brick or stone, are not amenable to assessment by calculation. Where increased capacity is not required, adequacy may be determined by a qualitative assessment of the structure, including the significance of any defects. The capacity may be considered sufficient if the substructure has performed satisfactorily over a long period of time and it is in adequate condition and shows no signs of distress or undue settlement. Where increased capacity is required, structural and/or geotechnical analysis should be carried out. 11.2 Assessment

The assessment of substructures and foundations should be based on the results of their detailed inspection, reference to record drawings and other available information.



The Structural Assessment of Underbridges Section 11 - Substructures

Page 2 of 2

If a foundation or substructure shows no signs of significant distress, if there is no evidence of scour either externally or internally, and if no significant increases in load are envisaged, the foundation or substructure may be assumed to be adequate and no further assessment is necessary. Where substructures however include elements amenable to assessment by calculation such as, metal trestles or concrete columns, the relevant Section of this Code should be used. In assessing elements which fall outside the scope of this Code, reference may be made to design standards. These documents are likely to contain conservative requirements which if unmodified, especially for serviceability aspects, are likely to produce unduly pessimistic estimates of load carrying capacity. Useful guidance can be found in the BA 55/94: The Assessment of Bridge Substructures and Foundations, Retaining Walls and Buried Structures. It suggests for example, that where wing walls are integral with abutment stems, and in good condition, consideration may be given to assessment of the substructure as a whole, rather than, on a unit width basis. A substructure that shows signs of significant defects should be quantitatively assessed taking the defects into account. For an analytical approach, realistic parameters (such as earth pressure coefficients) should be used. Detailed soil surveys should be carried out if such information is likely to improve the reliability of the assessment. Where it is necessary to take account of railway traffic surcharge loading in the assessment of abutments and other soil-retaining substructure elements, the values given in Table 11.1 should be adopted. The tabulated values may be deemed to take into account dynamic effects. Substructures and foundations should be considered inadequate in relation to current (not increased) capacity only if they exhibit signs of distress and do not meet the acceptance criteria of the numerical assessment. RA NUMBER RA0 RA1 RA2

SURCHARGE LOAD (kN/m²) 22 24 26

RA NUMBER RA8 RA9 RA10

SURCHARGE LOAD (kN/m²) 38 40 42

RA3 RA4 RA5 RA6 RA7

28 30 32 34 36

RA11 RA12 RA13 RA14 RA15

44 46 48 50 52

Table 11.1 Nominal Railway Traffic Surcharge Loading


The Structural Assessment of Underbridges Section 12 - Bearings


CONTENTS

12. BEARINGS...............................................................................................................................1 12.1 General..............................................................................................................................1 12.2 Assessment ....................................................................................................................... 1

12. BEARINGS 12.1 General

This Section provides recommendations for the assessment of bearings, where present, for all types of Bridges. The condition of a bearing and its seating is an important indicator, not only of the bearing itself, but sometimes of some defect in the structure. Bearings are located where movement is intended to take place. If they do not function adequately, the structure may suffer excessive stress. 12.2 Assessment

The assessment of bearings should be based on the results of their detailed inspection and reference to record drawings and other available information. For proprietary bearings, reference to manufacturer s information should be made if possible. If a bearing, or its seating, shows no signs of distress, if movements including rotations are free to take place, and if no significant increases in load are envisaged, the bearing may be assumed to be adequate and no further assessment is necessary. Bearings that show signs of significant defects should be assessed using the design principles of BS 5400: Part 9.1 and where applicable using the manufacturer s design recommendations. Any references within BS 5400: Part 9.1 to BS 5400: Part 3 should be read as Appendix A of this Code.


The Structural Assessment of Underbridges Appendix A - Assessment of Steel and Wrought Iron


CONTENTS Modifications and additions to BS 5400: Part 3 Clauses:

3.2.2A Main Symbols .......................................................................................................... 3 3.2.3A Subscripts ................................................................................................................ 3 4.2.2A Serviceability Limit State ...................................................................................... 4 4.2.3.A Fatigue.....................................................................................................................4 4.3.3A Values of Partial Factors....................................................................................... 5 6.1A Performance ................................................................................................................... 6 6.4A Ductility ........................................................................................................................... 7 6.6A Properties of Steel and Wrought Iron...................................................................... 8 7A. GLOBAL ANALYSIS FOR LOAD EFFECTS .................................................................. 8 7.1A General............................................................................................................................8 8A. STRESS ANALYSIS...............................................................................................................8 8.5.1A Imperfections Allowed For.................................................................................. 8 8.8A Originally Unintended Composite Action ............................................................... 9 8.8.1A General .................................................................................................................... 9 8.8.2A Cased Beams or Filler Beams or Jack Arch Decks .........................................9 8.8.3A Concrete Slab and Steel/Wrought Iron Beam Decks ..................................10 9A. ASSESSMENT OF BEAMS ................................................................................................10 9.3A Shape Limitations.........................................................................................................11 9.3.1A General ..................................................................................................................11 9.3.5A Flanges Curved in Elevation...............................................................................13 9.3.6A Circular Hollow Sections...................................................................................14 9.3.7A Compact Sections................................................................................................14 9.3.7.5A Flat Plates ...........................................................................................................15 9.4.2A Effective Section...................................................................................................15 9.6A Effective Length for Lateral Torsional Buckling ....................................................15 9.6.1A General ..................................................................................................................15 9.6.2A Beams with intermediate lateral restraints ....................................................16 9.6.3A Beams (other than cantilevers) without Intermediate Lateral Restraints16 9.6.5A Beams with Intermediate U-Frame Restraints ..............................................19 9.7A Slenderness...................................................................................................................21 9.7.1A General ..................................................................................................................21 9.7.2A Uniform I, Channel, Tee or Angle Sections ...................................................23 9.7.3A Other Uniform Sections ....................................................................................25 9.7.4A Varying Sections...................................................................................................26 9.7.5A Other Cases and Alternative Methods ...........................................................26 9.8A Limiting Moment of Resistance.................................................................................27 9.8.1A General ..................................................................................................................27 9.8.2A Allowance for flange straightness imperfection ............................................28 9.9A Beams Without Longitudinal Stiffeners ..................................................................31




9.9.1A Bending Resistance ..............................................................................................31 9.9.2A Shear Resistance ..................................................................................................31 9.10A Flanges in Beams with Longitudinal Stiffeners in the Cross Section...............34 9.12A Restraints to Compression Flanges.......................................................................34 9.12.1A General................................................................................................................34 9.12.2A Continuous IntermediateRestraint U-FrameProvided Restraints 9.12.3A by..................................................................35 Deck .....................................................36 9.12.4A Restraint at Supports........................................................................................37 9.14.4A Strength of Bearing Stiffeners .........................................................................42 10A. DESIGN OF COMPRESSION MEMBERS...................................................................42 10.3.1A Unstiffened Outstand .......................................................................................42 10.3.3A Circular Hollow Section ..................................................................................42 10.3.4A Assessment of Sections not Complying with Shape Limitations .............43 10.7.2A Evaluation of Stresses .......................................................................................43 11A. TENSION MEMBERS ......................................................................................................43 11.1A General........................................................................................................................44 11.3.2A Effective Area .....................................................................................................44 11.3.5A Pin Connected Members .................................................................................44 11.4A Thickness at Pin Holes .............................................................................................44 12A. TRUSSES ............................................................................................................................45 12.1A General........................................................................................................................45 12.4.1A General................................................................................................................45 12.4.2A Lateral Restraint by Deck to Compression Chord....................................45 12.5.1A Effective Length..................................................................................................45 12.5.2A Restraints to Compression Chords ..............................................................50 12.6.1A General................................................................................................................50 12.6.2A Forces on Bracing..............................................................................................50 12.6.3A Lateral Bracing not providing Adequate Restraint .....................................50 12.7A Curved Members ......................................................................................................51 14A. CONNECTIONS.............................................................................................................51 14.3.5A Connection of Restraints to Parts in Compression...................................51 14.4A Splices ..........................................................................................................................52 14.4.5A Obsolete Splicing Methods..............................................................................52 14.5A Connections made with Bolts, Rivets or Pins .....................................................53 14.5.2A Edge and End Distance .....................................................................................54 14.6A Welded Connections ...............................................................................................55 14.6.1A General................................................................................................................55 BS 5400: PART 3 APPENDIX B..............................................................................................60 BS 5400: PART 3 APPENDIX D.............................................................................................64 BS 5400: PART 3 APPENDIX E ..............................................................................................65 BS 5400: PART 3 APPENDIX G .............................................................................................66




Page 3 of 66

AMENDMENTS TO BS 5400: PART 3 (1982) 3.2.2A Main Symbols

Add to the list given in BS 5400: Part 3 Clause 3.2.2 the following symbols:

FR FS

ΨR

Restricted shear lag factor; Lateral restraint force on compression flange; Lateral force at support;

lw

Half wave length of buckling;

lR

U-frame spacing; Unit force related displacement at end support;

δe δi δt δr θL

Unit force related displacement at internal supports; Lateral displacement at end torsional restraint for unit force to each flange; Lateral displacement at intermediate restraints (other than internal supports); Slope (radians).

Replace the existing definitions of k and the following: k

β

β

given in BS 5400: Part 3 Clause 3.2.2 with

Buckling co-efficient; ratio of principal stresses; Slope of web to vertical; factor.

3.2.3A Subscripts

Add the following subscripts to the list given in BS 5400: Part 3 Clause 3.2.3: c e f i L min max s T ult

compressive flange; end; flange; internal; longitudinal; minimum; maximum; at support; transverse; ultimate condition.




Page 4 of 66

4.2.2A Serviceability Limit State

Delete the existing BS 5400: Part 3 Clause 4.2.2 and substitute the following: For underbridges serviceability need not be checked except where relevant for BS 5400: Part 3 Clauses: 9.10.3.3

In stiffened steel plate decking construction that forms part of a girder flange.

12.2.3

In trusses as defined in BS 5400: Part 3 Clause 12.2.3.

14.5.4.1.2

In HSFG connections where slip would cause redistribution in continuous structures.

4.2.3.A Fatigue

Delete the existing BS 5400: Part 3 Clause 4.2.3 and substitute the following: 4.2.3.1A G ener al

The fatigue endurance of steelwork and wrought iron underbridges should be checked in accordance with BS 5400: Part 10 as amended by Appendix D of this Code. 4.2.3.2A Br idges r equi r ing Assessment

Assessment of fatigue life of steelwork and wrought iron underbridges is required when one or more of the following conditions apply: (i)

there are visible cracks in components;

(ii)

the design is unable to be confirmed as having been carried out to BS 5400: Part 10;

(iii)

the construction is unable to be confirmed as having been carried out in accordance with BS 5400: Part 6;

(iv)

visual inspection has revealed, or records show, that the bridge has been subject to structural modification during or since construction. Note that this may include remaining temporary works features, new fixtures or repair or damage on the structural members, such as welded attachments, flame cut holes, strengthening etc.;

(v)

the bridge contains details not specifically covered by the scope of BS 5400: Part 10, in particular orthotropic steel decks. Standard Box girder decks in accordance with Railtrack’s standard bridge design may be considered as covered by BS 5400: Part 10;

(vi)

there has been evidence of traffic resonance in any of the structural members resulting in cracking of components or loosening of connections or supports.




4.3.3A Values of Partial Factors

Delete the existing BS 5400: Part 3 Clause 4.3.3 and substitute the following: The values of partial factors are as follows:

γ fL

the values of

γ fL

are given in Section 2 for each type and combination of

loading;

γf 3

the factor

γf 3

in BS 5400: Part 3 should be taken as 1.1 for the ultimate limit

state and 1.0 for the serviceability limit state.

γf 3

may be taken as 1.0 for the ultimate limit state where the following

conditions are all met: (a)

members are either: (i)

rail bearers or cross girders that are assumed to be simply supported;

(ii)

main girders of bridges with maximum skew of 25°. If continuous any splices are welded, or made with HSFG bolts or rivets with cover plates to both flanges.

(b)

global analysis is based upon static distribution within the structure;

(c)

geometric dimensions of the members are verified during inspection.

For the sake of simplicity, the expressions for strength in BS 5400: Part 3 contain a single factor γ m = γ m1 ⋅ γ m 2 . Values of the factor to be used where γ m is explicitly shown in the design strength equations in BS 5400: Part 3 are given in Table 2. Where explicitly expressed for Ultimate Limit State in a strength requirement in BS 5400: Part 3, γ m should be taken as 1.05, except where the applicable value of is tabulated in Table A1 for particular Clauses.

γm



The Structural Assessment of Underbridges Appendix A - Assessment of Steel and Wrought Iron Structural Component and Behaviour

Clauses

Buckling resistance of stiffeners

9.10.2.3(a) and (b), 9.11.5.2, 9.13.5.3A,

Fastenersintension Fastenersinshear FrictioncapacityofHSFGbolts Welds

Compressionmembers

Page 6 of 66

γm

9.13.6, 9.14.4.3, 1.20 9.17.6.7, 9.17.7.3.2, 9.17.8 14.5.3.2,14.5.3.3A, 1.20 14.5.3.5 14.5.3.4 1.10excl.rivets 1.33 rivets 14.5.4.2 1.30 14.6.3.11.1A, 1.20 14.6.3.11.2A, 14.6.3.11.3A 1.8 10.6.1.1A, 0.95 + ≤1.05 10.6.3  L +5 r  

Table A1 Partial Factors γ m for ULS

Where explicitly expressed for Serviceability Limit State in a strength requirement in BS 5400: Part 3, γ m should be taken as 1.00, except when considering friction capacity of HSFG bolts in accordance with Clause 14.5.4.2A, in which case γ m should be taken as 1.20. Note: Any other Clause making cross-reference to any of the above Clauses contained in Table A1 should incorporate the applicable γm value given in Table A1 above. 6.1A Performance

Delete the existing BS 5400: Part 3 Clause 6.1 and substitute the following: The mechanical properties of materials should be determined from specified values, tests of the material or from available mill test certificates. In the absence of this information, minimum ultimate tensile strengths may be taken for materials produced before BS 4360:1962 as given in Table A2.



The Structural Assessment of Underbridges Appendix A - Assessment of Steel and Wrought Iron σy

Material

N/mm²

Pre 1905 BS 15:1906

205 230

BS 15:1948 Wrought Iron Steel Rivets * Pre 1905 After 1905 WroughtIronRivets*

245 190 335 300 275

Steel

Page 7 of 66

Minimum Ultimate Tensile N/mm² 370 430

430 285 450 385 300

Table A2 Mechanical Properties

*

The yield and ultimate tensile stress of rivets is taken as the mean strength from test results.

6.4A Ductility

Delete the existing BS 5400: Part 3 Clause 6.4 and substitute the following: Steel or wrought iron should have a ductility not less than that corresponding to an elongation of 15%. Where plastic global analysis is used for steel under Clause 7.1A the ductility should not be less than that corresponding to an elongation of 19%. Elongation should be based on the standard proportional gauge length of 5.65 So where So is the cross sectional area of the test piece. If a different gauge length is used the percentage elongation value should be converted to the value for the standard gauge length in accordance with BS 3894.




Page 8 of 66

6.6A Properties of Steel and Wrought Iron

Delete the existing BS 5400: Part 3 Clause 6.6 and substitute the following: The properties given in Table A3 should be assumed:

Modulus of elasticity E( N/mm²) Shear Modulus G Poisson’s ratio, ν Coefficient of thermal expansion/oC

Steel 205000 0.4E 0.3 12x106

WroughtIron 190000 0.4E 0.3 12x106

Table A3 Properties of Steel and Wrought Iron 7A. GLOBAL ANALYSIS FOR LOAD EFFECTS 7.1A General

Add to end of existing BS 5400: Part 3 Clause 7.1: Plastic (or yield line) global analysis at ULS is permitted for beams and flat plates provided the components are compact sections under Clause 9.3.7A. Plastic global or yield line analysis is not permitted for wrought iron. The plastic modulus may be used for stress analysis of wrought iron see BS 5400: Part 3 Clause 9.4.2. 8A. STRESS ANALYSIS

8.5.1A Imperfections Allowed For

Add to end of existing BS 5400: Part 3 Clause 8.5.1: For bridges that are not known to have been constructed to the specification requirements of BS 5400: Parts 6 and 9, bearing misalignment, errors in level, bearing inclination, and imperfections in flatness and straightness should, where relevant, be taken into account in assessments. The assessment of half-through girders should take account of the measured bows of girders and verticality at supports. The strength of web panels should take account of measured out-of-flatness where significant distortion is evident from inspection. For elements of bridges known to have measured imperfections outside the tolerances required by BS 5400: Parts 6 and 9, the magnitude of these imperfections




should be taken into account in strength assessment. Imperfections less than the tolerances may be taken into account when this is beneficial. The imperfections should be assumed to be 1.2 times the measured imperfections to allow for inaccuracies. A factor of 1.2 is already contained in the relevant expressions of BS 5400: Part 3 and modifications by this Appendix are so that actual measured values may be used. 8.5.2.2A Column

Add to end of existing BS 5400: Part 3 Clause 8.5.2.2: Eccentricities exceeding 10 mm of bearings at top or bottom to the axes of columns should be taken into account. 8.8A Originally Unintended Composite Action

Add the following additional Clause 8.8A to BS 5400: Part 3. 8.8.1A General

Stiffness and strengths calculated for sections not srcinally intended as acting compositely can be enhanced by consideration of composite action with adjacent or surrounding structure using Appendix C of this Code of Practice where conditions are as given in Clauses 8.8.2A or 8.8.3A. 8.8.2A Cased Beams or Filler Beams or Jack Arch Decks

For cased beams and concrete filler beams, the stress analysis should be based on composite properties to Section 8 where there is no evidence of excessive corrosion, fretting action or cracking sufficient to adversely affect the composite action. Sections can be assumed to be compact where the compression flange and webs are cased on both sides. Where the requirements for resistance to longitudinal shear are not met, the beams should be assessed on the basis of the properties of the steel section only, which may be assumed to be compact carrying the entire load. Alternatively where attachments to the beams are sufficient to prevent relative longitudinal slip (such as rivet or bolt heads or other transverse elements) as demonstrated by push-out tests or by relevant evidence, these attachments may be assumed to transmit the longitudinal shear forces. For dense brickwork filler beam or jack arch decks, global and stress analysis should be based on composite properties provided that the bending resistance of the composite section is not taken as greater than 30% in excess of the calculated resistance of the steel section alone.




Page 10 of 66

8.8.3A Concrete Slab and Steel/Wrought Iron Beam Decks

For concrete slab and steel beam decks, global and stress analysis using composite properties can be used provided both (a) and (b) or (c) below apply: (a)

there is no significant evidence of corrosion, fretting, relative longitudinal slip separation or cracking at the steel/concrete interfaces sufficient to adversely affect the required composite action;

(b)

attachments to the beams at the interface (such as rivet or bolt heads or other transverse elements) are sufficient to prevent relative slip;

(c)

site testing is carried out to demonstrate that the live load: stiffness relationship is supportive of the composite action achieved, where the amount of composite action is required to increase the assessment strength by more than 30%. Normally test loading approximately equivalent to the nominal calculated live load capacity of the steel section only should be applied.

9A. ASSESSMENT OF BEAMS 9.2.1.2A Ef fects to be Consider ed

Delete the existing BS 5400: Part 3 Clause 9.2.1.2 and substitute the following: The effects at the ultimate limit state should be obtained for the relevant combinations of: (a)

flexure, shear, torsion (and, for box girders, distortion) due to any loads transverse to the longitudinal axis of the member;

(b)

the effects of axial load;

(c)

creep, shrinkage and differential temperature (see Section 8 for composite structures);

(d)

settlement of supports.

9.2.1.3A Ef fects th at may be Negl ected

Delete the existing BS 5400: Part 3 Clause 9.2.1.3 and substitute the following:

Effects that may be neglected for the ultimate limit states are: (i) shear lag; (ii)

restraint of torsional warping;

(iii)

items (c) and (d) of Clause 9.2.1.2A provided that: (a)

the section is compact through the span being considered in accordance with the provisions of Clause 9.3.7A, and




(b)

Page 11 of 66

the member is not prone to lateral instability which may be deemed to be satisfied when the slenderness parameter, λ LT is less than: 30

355 Mpe σ y Mult

Expression A1

where:

λ LT σy

is defined in Clause 9.7.1A; is defined in Clause 9.7.1A;

Mpe

is defined in Clause 9.7.1A;

Mult

is defined in Clause 9.8.1A.

9.3A Shape Limitations

9.3.1A General

Delete the existing BS 5400: Part 3 Clause 9.3.1 and substitute the following: BS 5400: Part 3 Figure 1 and Figure A1 in this Appendix set out the geometric notation used in this Appendix A. Where the proportions of flanges, stiffeners or hollow sections comply with the requirements of Clause 9.3.2.1A, BS 5400: Part 3 Clauses 9.3.4, or Clause 9.3.5A and 9.3.6A, taking σys or σy as the yield stress of the material as defined in Clause 6.1A and BS 5400: Part 3 Clause 6.2, the strengths of sections should be determined as specified in the applicable Clauses of this Appendix where:

σ ys σy

relates to the stiffener; relates to the flange, the web or the circular hollow sections, as applicable.

Where the proportions do not thus comply, a lower value or

σ ys

or

σy

should be

determined such that compliance with Clause 9.3.2.1A, BS 5400: Part 3 Clause 9.3.4, or Clauses 9.3.5A or 9.3.6A as applicable is achieved. This lower value of σ ys or σ y should be used in all subsequent assessments of strength. For riveted construction the outstand dimension bfo or hs may be taken from the edge of the head of rivets where the heads are consistently intact. A rivet head diameter may be considered to be 1.6 x nominal rivet diameter.




Page 12 of 66

b fo 0.8 x nominal rivet diameter

For individual flanges t fo t fo

b fo 0.8 x nominal rivet diameter C

dwe

rivet

b fo

0.8 x nominal rivet diameter Bf b fo

0.8xnominal rivet diameter

tw

0.8 x nominal rivet diameter

tw

dwe =

Bf b fo

0.8xnominal t fo rivet diameter

=

t fo tw

tw =

b fo

dwe

=

t fo

t fo ts ts hs

tw

hs tw

=

=

32 t w

0.8 x nominal rivet diameter

a n

e e

o n x

a d e v

=

tw

=

32 t w

Total 32 t w

b fo

b fo tf

t fo tf

tf

dw = d we

Figure A1 Geometric Notation for Sections that may be encountered in Assessment




Page 13 of 66

9.3.2.1A Ou tstands in Compression

In the existing BS 5400: Part 3 Clause 9.3.2.1, in the third and fourth lines, delete “or 16 whichever is the lesser”. Add to the end of the existing BS 5400: Part 3 Clause 9.3.2.1: Refer to Clause 9.3.1A for assessment of non-complying outstands in compression. 9.3.2.2A Ou tstands i n Tension

For assessment BS 5400: Part 3 Clause 9.3.2.2 may be ignored. 9.3.4.1.1A G ener al

Add to end of existing BS 5400: Part 3 Clause 9.3.4.1.1: Refer to Clause 9.3.1A for the assessment of non-complying stiffener configurations. Other shapes of stiffeners other than those specified should be assessed on the basis of the nearest standard shape. Reference may also be made to Appendix S of Advice Note BA 56/96: The Assessment of Steel Highway Bridges and Structures. 9.3.4.1.3A Bul b F lat Stif feners

For assessment BS 5400: Part 3 Clause 9.3.4.1.3 may be ignored. 9.3.4.1.4A An gle Sti ff eners

For assessment BS 5400: Part 3 Clause 9.3.4.1.4 may be ignored. 9.3.4.1.5 Tee Stif f ener s

In BS 5400: Part 3 Clause 9.3.4.1.5 item (c) (2), delete

σ

ys

355

and substitute

σy 355

.

9.3.5A Flanges Curved in Elevation

In the existing BS 5400: Part 3 Clause 9.3.5 delete the expressions in (a) and (b) and substitute: Flanges curved in elevation should be such that:

a)

b)

bfo t fo b tf

σ ys

R ≤ F 355 6bfo

σ ys 355

≤

RF 2b




Page 14 of 66

where: RF b tf

is the radius of curvature of the flange; is the distance between adjacent longitudinal stiffeners and/or webs; is the thickness of the flange in the panel such longitudinal stiffeners and/or

webs; bfo , t fo are defined in BS 5400: Part 3 Clause 9.3.2. 9.3.6A Circular Hollow Sections

In BS 5400: Part 3 Clause 9.36 delete the expression and substitute 60

355

σ

.

y

Add to end of existing BS 5400: Part 3 Clause 9.3.6: Refer to Clause 9.3.1A and BA 56: Appendix S for the assessment of non-complying sections. 9.3.7A Compact Sections 9.3.7.2.3A Al ter nati ve M ethod

Add additional Clause 9.3.7.2.3 to BS 5400: Part 3 Clauses 9.3.7.2.1 and 9.3.7.2.2 the depth of the web should not exceed: 34 t w m

355

σ yw

when is less m than 0.5

Expression A2

or 374 t w 355 when is greater m than 0.5 13m − 1 σ yw

Expression A3

where:

tw

is the ratio of the depth of the web plate that is on the compressive side of the plastic neutral axis of the beam to the depth of the web plate. The depth of the web referred to in this Clause should be measured in its plane and taken clear of root fillets for rolled sections and welds or flange angles for fabricated sections; is the thickness of the web plate;

σ yw

is the nominal yield stress of the web material or any other lower stress

m

assumed in the assessment.




Page 15 of 66

9.3.7.3.3A Composite Compr ession F l anges

In BS 5400: Part 3 Clause 9.3.7.3.3 delete the existing expressions and substitute the following expressions respectively: 355

30 t f

15t f 22 t f

;

σ yf 355

σ

;

yf

355

σ yf

.

9.3.7.4A Cir cular H oll ow Sections

In BS 5400: Part 3 Clause 9.3.7.4 delete the existing expression and substitute 46

355

σy

.

9.3.7.5A Flat Plates

Add additional Clause 9.3.7.5 to BS 5400: Part 3. Flat steel or wrought iron plates having a width at least equal to their thickness may be assumed as compact for bending applied about an axis parallel to their width. 9.4.2A Effective Section 9.4.2.2A Deduction for H oles

In BS 5400: Part 3 Clause 9.4.2.2 add 11.3.2 and’ before 11.3.3’. 9.6A Effective Length for Lateral Torsional Buckling

9.6.1A General

Delete the existing BS 5400: Part 3 Clause 9.6.1 and substitute the following: For all beams there should be restraint against rotation about the longitudinal axis in accordance with Clause 9.12.4A at or adjacent to the supports. A restraint or restraints within the following distance from a support may be considered as a support restraint: Half Through

l e3

BeamandDeckingType

le

but not greater than L 5 57.

but not greater than L 5




where L is the span of the beam. In all cases the effective length for lateral torsional buckling

le

Page 16 of 66

should be determined

in accordance with Clauses 9.6.2A, 9.6.3A, BS 5400: Part 3 Clauses 9.6.4 and Clause 9.6.5A as applicable. However, if the second moment of area of a cross section about the axis of bending is smaller than that about an axis perpendicular to it, the cross-section as a whole is stable against overall lateral torsional buckling, and the effective length le may be taken as zero. Where the resistance of the restraining systems is less than required to resist the force FS in accordance with Clause 9.12.4.1A, the slenderness parameter λ LT applicable to the length

le

at the support under consideration, should be modified as

follows:

λ = LT '

λ

LT

1  5FRD + 3   8  FS 

Equation A4

where:

λ

LT '

FS

is a modified value of λ LT as defined in Clause 9.7.1A; is as defined in Clause 9.12.4A; is the available resistance which is less than FS excluding the effects of wind, frictional and other applied forces.

FRD

9.6.2A Beams with intermediate lateral restraints

Amend the reference to Clause 9.12.1 to 9.12.1A. 9.6.3A Beams (other than cantilevers) without Intermediate Lateral Restraints

Delete the existing BS 5400: Part 3 Clause 9.6.3 and substitute the following: This Clause applies to half-through bridges which do not have effective intermediate U-frames. When there is no intermediate lateral restraint to a compression flange le should be taken as the greater of the values calculated in accordance with (a) and (b) below as applicable: (a) for single-span or continuous beams: le

=

where:

k1k2 ke L

Equation A5




L k1

Page 17 of 66

is the span of the beam between lateral restraints at supports; may conservatively be taken as: i)

1.0 where the compression flange is free to rotate in plan at the points of support; or

ii)

0.85 where the compression flange is partially against rotation in plan at the points of support or if itrestrained is fully restrained against rotation in plan at one support, and free to rotate in plan at the other; or

iii)

0.7 where the compression flange is fully restrained against rotation in plan at the points of support.

A more accurate value of k1 allowing for the degree of restraint in plan, may be obtained from Figure A7(a). k2

should be taken as 1.0 unless the load is applied to the top flange and both the flange and the load are free to move laterally in which case 1.2 should be used. 1

ke =

     60Et βδ  1−   L     ν    r   f max

Equation A6

t

3

4

y

where: t f max is the maximum thickness of the compression flange in the span;

δt

is the relative lateral deflection of the centroid of one flange of the

β

beam with respect to the centroid of the other flange which would occur when equal and opposite unit forces act laterally on the end torsional restraint only at the same levels as shown in Figure A8; should be taken as 1.0 for a simply supported beam, or for an internal support to a continuous beam with restraint in plan to the compression flange at the support, or as should be taken as 2.0 for an internal support to a continuous beam

υ

with restraint in plan to that support. is the value of υ calculated in accordance with Clause 9.7.2A which may derived using ke = 1.0 in calculating λF.

ke should be taken as the greater of the values obtained for either support. The restraint should be such that the denominator has a positive sign. (b) for continuous beams only:




le

=

k1k 2 ∑ L

1

  2(∑ L ) 1+   π 4 EI  δ + δ 2   3

e

c

i

Page 18 of 66

Equation A7

     

where:

∑L

is the sum of the adjacent spans; is the second moment of area of the compression flange about its Y-Y

Ic

axis, as defined in BS 5400: Part 3 Figure 1, at the point of maximum bending moment; is the value of δ t for an internal support;

δi δe

is the value of

δt

for the end or internal supports at the opposite ends

of the adjacent spans.

1.0

1.0

0.9

0.9

0.8

0.8

1=1.0

0.9 0.8 0.7

1

0.7

3

0.7 0.6

0.6

0.6

0.5

0.5

0.4

5

15

10

20

25

0.4

0.5

0.2

0.4

o

µ

0.6

0.8

1.0

o

c

(a) Effect of rotational end restraint Note 1: k o is the smaller value, at either end, of the rotational stiffness to lateral bending of the compression flange, chord or strut.

(b) Effect of bending restraint L is the span of the beam or truss or length between the ends of a compression member effectively held in

I c is as defined in Clause 9.6.5A or 12.5.1A or

position. lo is the value of le obtained from

BS 5400: Part 3 Clause 10.4.1 as applicable.

Clause 9.6.3A or 12.5.1A but calculated with k1 = 1.0 NOTE 2: For basis of curves, see BS 5400: Part 3 Clause G6.

Figure A7 Influence on Effective Length of Compression Flange Restraint




Page 19 of 66

Structural connection

9.6.5.2

(9.6.6.) t

t unit force

Figure A8 Restraint of Compression Flange by U-Frames on Deck End Torsional Restraint (Note: Clause number in brackets refers to BS 5400: Part 3)

9.6.5A Beams with Intermediate U-Frame Restraints

Delete the existing BS 5400: Part 3 Clause 9.6.5 and substitute the following: Where restraint to a compression flange is provided by U-frames in accordance with Clause 9.12.2A, l e should be taken as: le

=

k2 k3k5 l1 but not less than k3 l R and not greater than the value given by Clause 9.6.3A.

Equation A8

where: is as defined in Clause 9.6.3A; may be taken as 1.0 but, where the compression flange is restrained against

k2 k3

rotation in plan at supports, a lower value of k 3 may be obtained from Figure A7(b); k5

=

2.22 + where:

0.69 X + 0.5

Equation A9




Page 20 of 66

3

l1

X=

l1

=

Equation A10

2EI c δ t

(EI c l R δ R )

0.25

Equation A11

where: Ic

is as defined in Clause 9.6.3A;

lR

is equal to the U-frame spacing; is equal to the lateral deflection which would occur in any intermediate U-frame at the level of the centroid of the flange being considered, when a unit force acts laterally to the U-frame only at this point and simultaneously at each corresponding point on the other flange or flanges connecting to the same U-frame. The direction of each unit force should be such as to produce the maximum aggregate value of δ R . The U-frame should be taken as fixed in position at each point of intersection between the cross member and a vertical, and as free and unconnected at all other points;

δR

δ

t

is as defined in Clause 9.6.3A.

Provided that

δ +δ R

t

2 for U-frame restraints adjacent to an end support, and that

2 δ R for U-frame restraints not adjacent to an end support, are both not greater than lR

3

20EIc , the restraints may be taken as fully effective and k5 l1 may be taken as

lR .

Where the lateral stiffness of the decking system does not comply with Clause 9.12.2.2A then the intermediate U-frames should be ignored in deriving the effective length. In cases of symmetrical U-frames, where cross members and verticals are each of constant moment of inertia throughout their own length, as shown in Figure 8, it may be assumed that: 3

R

δ =

d1 uBd 2 3EI1 + EI 2

2 2

+ fd

2

Equation A12

where: d1

is the distance from the centroid of the compression flange to the nearer face of the cross member of the U-frame, as shown in Figure 8;



d2 I1

I2

u B f


is the distance from the centroid of the compression flange to the centroidal axis of the cross member of the U-frame, as shown in Figure 8; is the second moment of area of the effective section of the vertical about its axis of bending perpendicular to the plane of the U-frames. A width of web plate of up to 16 times the web thickness may be included on each side of the centreline of its connection when determining the effective section of the vertical; is the second moment of area of the cross member of the U-frame about an axis perpendicular to the plane of the U-frame. A width of deck on either side of the U-frame equal to B 8 or lR 2 whichever is less, may be taken as the effective cross member when no other discrete member is present, or may be taken together with a cross member if structurally connected to it. In calculating the transformed area of a concrete deck, the gross area of concrete within this effective width may be considered; is 0.5 for an outer beam, and 0.33 for an inner beam if there are three or more beams interconnected by U-frames; is the distance between centres of parallel beams, or where the beams are not parallel the maximum distance within the mid-third of the span; is the flexibility of the joint between the cross member and the verticals of the U-frame, expressed in radians per unit moment. Values of f may be: • taken from BS 5400: Part 3 Figure A42;

•

determined from test results (which should cover the required ultimate capacity of the joint);

•

determined from theoretical methods as described in BS ENV 1993: Part 1 Annex J.

9.7A Slenderness

9.7.1A General

Delete the existing BS 5400: Part 3 Clause 9.7.1 and substitute the following: The slenderness parameter λ LT required for the calculation of the limiting moment of resistance (see Clause 9.8A) should be determined for all beams in accordance with Clauses 9.7.2A to 9.7.5A applicable to the type of beam, using the effective length for lateral torsional buckling obtained from Clause 9.6A. The half wavelength of buckling of the compression flange should, if required be taken as: (a)

the distance between supports for beams without intermediate restraints;




(b)

Page 22 of 66

the distance between effective discrete intermediate restraints (Clause 9.6.2A).

The derivation of the moduli of the cross sections should be based on the following requirements: Z pe

is the plastic modulus of the effective section derived in accordance with BS 5400: Part 3 Clause 9.4.2, and is defined as Mpe σ ye , where: Mpe

is the plastic moment of resistance of the effective cross section derived in accordance with BS 5400: Part 3 Clause 9.4.2 and based on rectangular stress blocks of intensity equal to the strength of the elements. For elements in structural steel, the strength should be taken as the nominal yield stress of the elements, as defined in Clause 6.1A and Clause 6.2A. For concrete flanges in compression, the area of reinforcement should be ignored and the strength should be taken as 0.4 f cu γ m . For concrete flanges in tension the area of concrete should be ignored and the strength of the reinforcement taken as 0.87 f y γ m ; where: f cu

is the concrete cube strength in accordance with Section 7;

fy

is the characteristic strength of the reinforcement in

γm

accordance with Section 7; is taken from Clause 4.3.3A.

σ ye

is the nominal yield stress value, as defined in Clause 9.3.1A for the

Z xc

compression flange; is the elastic section modulus with respect to the extreme compression fibre, based on the effective section derived in accordance with BS 5400: Part 3

Z xt

Clause 9.4.2;section modulus with respect to the extreme tension fibre, based is the elastic

Z xw

on the effective section derived in accordance with BS 5400: Part 3 Clause 9.4.2; is the minimum elastic modulus of the section with respect to the web, based on the effective section derived in accordance with BS 5400: Part 3 Clause 9.4.2.




Page 23 of 66

For composite sections Z xc , Z xt and Z xw should be based on the transformed section. The transformed area of the concrete compression flange should be obtained using the short term modular ratio of the concrete. Concrete in tension should be ignored but the area of reinforcement in concrete subject to tension should be included. Where the web is discontinuous at the location considered, its area should be ignored in the calculation of Mpe, Zxc Zxt, but with the neutral axis location calculated assuming that the area of web is present.

9.7.2A Uniform I , Channel, Tee or Angle Sections

Delete the existing BS 5400: Part 3 Clause 9.7.2 and substitute the following: The value of λ LT for overall lateral buckling of a beam of I, channel, tee or angle section, uniform within the half-wavelength of buckling of the compression flange, and bending about its X-X axis, as defined in BS 5400: Part 3 Figure 1, should be taken as:

λ LT =

le

k 4 ηυ

Equation A13

r

y

where: le

is the effective length determined in accordance with Clause 9.6A;

ry

is the radius of gyration of the gross cross-section of beam about its Y-Y axis

k4

(see BS 5400: Part 3 Figure 1); should be taken as 0.9 for rolled I or channel section beams in accordance with BS 4: Part 1 or any I section symmetrical about both axes with t f not

η

υ

greater than twice the web thickness, or 1.0 for all other beams; should be taken as 1.0, but where the bending moment varies substantially within the half-wavelength of buckling of the compression flange, advantage may be obtained by using a value of η, from BS 5400: Part 3 Figure 9(a), if the loading is substantially of the halfwavelength or from BSconcentrated 5400: Part 3 within Figure the 9(b),middle-fifth for other loading patterns; is dependent on the shape of the beam, and may be obtained from BS 5400: Part 3 Table 9, using the parameters:

λF = where:

le

ry

 tf    D

and i

=

Ic Ic

+ It

Equations A14(a) and (b)




Page 24 of 66

tf

is the mean thickness of the two flanges of an I or channel section

D Ic

including any horizontal legs of connections angles, or the mean thickness of the table of a tee or leg of an angle section; is the depth of the cross section (see BS 5400: Part 3 Figure 1); is the second moment of area of the compression flange, about its Y-Y

It

axis, as defined in BS 5400: Part 3 Figure 1, at the section being checked; is the second moment of area of the tension flange, about its Y-Y axis, as defined in BS 5400: Part 3 Figure 1, at the section being checked.

For beams with Ic

≥ Ior with λ F ≥ 8 t

,

λ LT

may conservatively be taken as

When in accordance with BS 5400: Part 3 Clauses 9.6.4.1.3 or 9.6.4.2, than l R ,

λ LT

should be taken as

le

le

le

ry .

is greater

ryc where ryc is the radius of gyration about the

Y-Y axis of the gross cross-section of the compression flange plus one third of the height of the web. Where a flange is common to two or more (n number) beams (for example in a girder bridge with a composite deck) the properties ry , I y , Ic ,or It may be calculated by attributing a fraction n −1 of the lateral second moment of area and of the area of the common flange to the section of each beam. For beams restrained by U-frames the lateral stiffness of the decking system may be taken into account whether or not its lateral stiffness complies with the minimum value given by Clause 9.12.2.2A. Where the decking is assumed to act compositely as part of the beams it may be treated as a flange common to the beams as above but with any concrete in tension ignored. Where the decking is not assumed to act compositely then its lateral inertia may be proportioned between the beams and added algebraically to the value of I t or otherwise combined to take account of the plan stiffness through the cross members. Concrete decking may be assumed as uncracked provided it is continuously reinforced in the longitudinal direction. In calculating t f , Ic and I t for composite beams, the equivalent thickness of the composite flange in compression should be based on the short term elastic modulus for concrete. Concrete in tension should be ignored and the equivalent thickness of tension reinforcement should be taken as the area of reinforcement divided by the flange width over which it is placed. The calculation of ry should include the web even when the web is discontinuous at the location considered.




Page 25 of 66

9.7.3A Other Uniform Sections

Delete the existing BS 5400: Part 3 Clause 9.7.3 and substitute the following Clauses 9.7.3.1A and 9.7.3.2A: 9.7.3.1A Un if orm Rectangul ar or Trapez oidal Box S ections

The value of λ LT for overall lateral buckling of a beam of rectangular or trapezoidal box section, uniform within the half-wavelength of buckling of the compression flange, should be taken as:

λ LT =

2.25ηξ

Z pe le

EquationA15

ry AJ

where:

η, ry

are as defined in Clause 9.7.2A;

Z pe

is as defined in Clause 9.7.1A. For non-compact sections, Z pe need not be calculated explicitly since it may be replaced by Mpe

σ yc

and in the

subsequent application of Clause 9.8A, Mpe will cancel; le

is as determined in accordance with Clause 9.6A;

A J

is the area of the gross cross section; 2 (B t ) ; is the torsional constant 4 A0

∑

where: is the area enclosed by the median line of the perimeter material of the

A0

section; are the width and thickness, respectively, of each wall of the section forming the closed perimeter. In the case of a wall made from concrete it should be taken as the actual thickness multiplied by the ratio of the short term modulus of elasticity of the concrete to the E of the section.

B,t

ξ=

 (I − I )(I − 0.385 J )    I   x

y

x

2

0.25

Equation A16

x

where I x and I y are the second moments of area of the gross cross section about axes through the centroid normal to the plane of bending, and in the plane of bending respectively.




Page 26 of 66

9.7.3.2A Un if orm Soli d Rectangul ar Sections

The value of λ LT for overall lateral buckling of a beam of homogeneous solid rectangular section, which is uniform within the half-wavelength of buckling of the compression flange, should be taken as:

λ LT =

2.8 η

l eD

EquationA17

B

where:

η le

is as defined in Clause 9.7.2A; is determined in accordance with Clause 9.6A;

D B

is the depth of the section in the place of bending; is the width of the section.

9.7.4A Varying Sections

Delete the existing BS 5400: Part 3 Clause 9.7.4 and substitute the following: The value of

λ LT

for overall lateral buckling of a beam of varying section within the

half-wavelength of buckling should be taken as (1.5 − 0.5ρf ) times the value obtained from Clause 9.7.2A or 9.7.3A using the values of ry and υ applicable to the section where the limiting compressive stress is to be derived. where:

ρ = f

lw

minimum total area of two flanges at any section in lw total area of two flanges at section being considered is the half wavelength of buckling as defined in Clause 9.12.2A.

This Clause is not applicable to beams with U-frame restraints. 9.7.5A Other Cases and Alternative Methods

Delete the existing BS 5400: Part 3 Clause 9.7.5 and substitute the following: For cases not covered by Clauses 9.7.2A, 9.7.3A or 9.7.4A or as an alternative, for overall lateral buckling may be taken as:

λ LT =

π 2 EZ pe Mcr

Equation A18

λ LT




Page 27 of 66

where: Z pe

is as defined in Clause 9.7.1A. For non-compact sections, Z pe need not be calculated explicitly since it may be replaced by Mpe

Mcr

σ yc

and in the

subsequent application of Clause 9.8A, Mpe will cancel; is the bending moment at which, under the given pattern of loading, the beam reaches its theoretical elastic buckling condition as determined by an elastic analysis.

9.8A Limiting Moment of Resistance

Delete the existing BS 5400: Part 3 Clause 9.8 and substitute the following: 9.8.1A General

The limiting moment of resistance, MR , should be obtained from Figure A10 or A11 as applicable, according to the value of:

 σ  M   355  M  yc

β =

λ

LT

ult

Equation A19

pe

where:

λ LT Mult

is obtained from Clause 9.7A; is the moment of resistance of the cross section if lateral-torsional buckling is prevented, defined as: Mult = Mpe for compact sections; or Mult =

Z xc σ yc for non-compact sections.

MR is determined as: a) for compact sections

 MR  M   pe  Mult 

b) for non-compact sections the least of

 M M M  R

ult

,or Z xt σ yt

ult

where

 MR     Mult 

is taken from Figure A10 or A11 as applicable;

Mpe , Z xc , Z xt are as defined in Clause 9.7.1A;

σ yt

is the nominal yield stress of the tension flange material;




σ yc

Page 28 of 66

is the nominal yield stress of the compressive flange material.

For Level 1 assessment or in the absence of measured values of flange straightness the curve for k = 0 should be used where k is defined in 9.8.2A, corresponding to the tolerances given by BS 5400: Part 6. Where measured values of flange straightness are obtained such values should be used to derive the applicable value of k when using Figure A10 or A11. Figure A10 should be used for unwelded sections; it should be used for rolled and riveted sections and for sections having welding limited to vertical stiffeners and local gussets etc. Figure A11 should be used for sections fabricated by welding. For the basis of the curves to Figures A10 and A11 see to BS 5400: Part 3 Appendix G7 as modified by this Appendix. 9.8.2A Allowance for flange straightness imperfection

Where allowance is to be made for measured flange straightness imperfections, ∆ F should be measured in accordance with BS 5400: Part 6 Table 5. MR Mult should be obtained from Figures A10 and A11, corresponding to the value of the imperfection factor k given by: y k= (∆ F − 0.001g ) 2 ry

Equation A20

where:

∆F g y ry

is the greater of the values measured in accordance with 4(a) and 4(b) respectively of BS 5400: Part 6 Table 6 but not less than 3 mm over a gauge length normally equal to the span of the beams between points of support; Gauge length used to measure ∆ F ; is the distance in the x-direction from the Y-Y axis to the extreme fibre of the compression flange (see BS 5400: Part 3 Figure 1); is the radius of gyration of the gross cross section about its Y-Y axis.

The curve k = 0 represents ∆ F = 0.001g as specified by BS 5400: Part 6. Positive values of k = 0.5, 1, 2, 3 and 4 represent bows greater than 0.001 g. The curves for negative values of k (-0.0029 β , for Figure A10 and -0.0067 β for Figure A11) represent bows of less than 0.001g and allow for enhancement of MR Mult up to the maximum possible, equivalent to

η =0 where η is defined in Clause G.7A.

Measured bow ∆ F and gauge length g should be applicable to the shape of bow which exists. For example a double curvature bow may require additional measurements.




Page 29 of 66

1

0.9

0.8

0.7

0.6

MR/Mult 0.5

0.4

k -0.0029β

0.3 0.5 2 4

0

1

3

0.2

0.1

0 0

50

100

150

200

250

β

Figure A10 Limiting Moment of Resistance MR for Unwelded Sections

300




Page 30 of 66

1

0.9

0.8

0.7

0.6

MR/Mult 0.5

0.4

k

-0.0067

0.3 0.5 2 4

0

1

3

0.2

0.1

0 0

50

100

150

200

250

β

Figure A11 Limiting Moment of Resistance MR for Welded Sections

300




Page 31 of 66

9.9A Beams Without Longitudinal Stiffeners

9.9.1A Bending Resistance

Delete the existing BS 5400: Part 3 Clause 9.9.1 and substitute the following: 9.9.1.1A G ener al

Beams should be checked for bending in accordance with Clause 9.9.1.2A. Beams that are known to have been constructed in several stages during which the loading and section properties changed should be assessed in accordance with BS 5400: Part 3 Clause 9.9.5. Effects due to differential temperature where required by Section 4 and concrete shrinkage where required by Appendix C should be taken into account in accordance with BS 5400: Part 3 Clause 9.9.7. 9.9.1.2A Al l Sections

The bending resistance MD of all sections should be taken as: MD =

MR

γ mγ f 3

Equation A21

where MR is the limiting moment of resistance derived in Clause 9.8A. 9.9.1.3A Non- compact secti ons

For Assessment BS 5400: Part 3 Clause 9.91.3 may be ignored. 9.9.2A Shear Resistance 9.9.2.1A G ener al

Delete the existing BS 5400: Part 3 Clause 9.9.2.1 and substitute the following: The shear resistance of a web of a beam with transverse stiffeners at supports and with or without intermediate transverse stiffeners should be determined in accordance with BS 5400: Part 3 Clause 9.9.2.2. provided that: (a)

there are no longitudinal stiffeners on the web which are assumed effective in resisting bending or shear resistance of the beam;

(b)

the web panel considered has no openings other than those within the limits set out in BS 5400: Part 3 Clause 9.3.3.2 (a) (b) and (c);

(c)

the provisions of BS 5400: Part 3 Clauses 9.9.4 and 10.6 are met if the beam is subjected to axial load;



(d)


flange or flanges are straight and parallel in elevation where the beam is other than simply supported.

Web panels which do not meet these conditions should be checked in accordance with BS 5400: Part 3 Clause 9.11. For simply supported hog back beams with intermediate transverse web stiffeners or beams with sloped bottom flange, the shear resistance should be checked to Clause 9.9.2.3A. 9.9.2.3A Shear Resistance of Si mpl y Suppor ted H og Back Beams or Sloped Bottom Flange

Add additional Clause 9.9.2.3 to BS 5400: Part 3. For simply supported hog-back beams (top flange sloped at the supports so that the beam depth increases towards the span centre) having intermediate transverse web stiffeners, a contribution to the vertical shear may be assumed to be carried by the sloping top flange. Each web panel may be considered as part of a truss mechanism with the web acting as a tie connecting opposite corners of the panel as shown in Figure A17. The connection between the sloping flange and the bearing stiffeners including any portion of web extending beyond the bearing stiffeners should be checked for resistance to horizontal shear. In this Clause d w should be taken at the shallowest end and d we at the mid length of each web panel. The shear resistance of the web should be taken as in BS 5400: Part 3 Clause 9.9.2.2. For simply supported beams with sloped bottom flange, that is with flange that is sloped at the supports, so that the beam increases in depth along the span, a contribution to the vertical shear may be assumed to be carried by the sloping bottom flange provided λ does not exceed 50. The web may be assumed as divided into panels between vertical stiffeners (if any), each panel not exceeding 1.2 d we in length. Each web panel may be considered as part of a truss mechanism with the web acting as both struts and ties as shown in Figure A17.




Page 33 of 66

d we

>30 Vertical stiffeners Slope of top flange

Reaction Contribution of bottom flange

Contribution of top flange

Reaction

HOG-BACK

SLOPED BOTTOM FLANGE

Figure A17 Shear on a Hog-Back Beam and a Beam with Sloped Bottom Flange 9.9.3.1A Webs wit h I ntermediate Tr ansver se Sti ff eners

In BS 5400: Part 3 Clause 9.9.3.1, replace: “ MR ” with “ MF ” (7 times). Definitions of

σf

is

σ yt

σf

with:

(for the tension flange) the nominal yield stress, as defined in Clause

6.1A, or BS 5400: Part 3 Clause 6.2, or (for the compression flange) is the lesser of σ yc the nominal yield stress value, as defined in Clause 9.3.1A and MR Z xc MR Z xc

is defined in Clause 9.8.1A; is defined in Clause 9.7.1A.




Page 34 of 66

9.10A Flanges in Beams with Longitudinal Stiffeners in the Cross Section 9.10.1.1A F lan ges Str aigh t in El evation

In BS 5400: Part 3 Clause 9.10.1.1 in expression (a) replace “

σ γ γ

lc

m

” with “

f3

MR ” Z xc γ m γ f 3

Delete the definition for σ c and add the following definitions: MR is as defined in Clause 9.8.1A; Z xc is as defined in Clause 9.7.1A. l

9.12A Restraints to Compression Flanges

9.12.1A General

Delete the existing BS 5400: Part 3 Clause 9.12.1 and substitute the following: The load effects on restraints to a compression flange may be determined by nonlinear analysis of representative structural models of beams and the restraint system under ultimate limit state factored loads with imperfections in geometry of the beams and their supports corresponding to 1.2 times the relevant tolerances given in BS 5400: Part 6. Alternatively, where the compression flange is provided with discrete elastic restraints connected to the end supports by a system of plan bracing, the system may be considered to be effective provided that it complies with the following requirements. The arrangement and proportions of bracing members should be able to resist, at all transverse sections of the beam, a restraining lateral shear force F having a value equal to: F=

∑P

f

plus the direct shear arising from wind and other laterally applied 80 forces, Equation A22

or F=

∑40P

f

when the effects of wind and other laterally applied forces are not

included. where

∑P

f

Equation A23

is the sum of the greatest forces in two of the compression flanges of

the beams connected by the bracing at the section under consideration.




Page 35 of 66

The bracing system should also be checked such that the lateral shear force F as calculated and applied at the position of maximum flange forces can be transmitted to the restraints at supports. 9.12.2A Intermediate U-Frame Restraints 9.12.2.2A Strength

Delete the existing BS 5400: Part 3 Clauses 9.12.2.1 and 9.12.2.2 and substitute the following: The decking system in combination with the tension flange of the beams should have a lateral stiffness such that the deflection which would occur when a unit force acts laterally is no greater than

le

40EI c

where

le

and I c are defined in Clause 9.6.5A.

Where the effective length is determined in accordance with Clause 9.6.5.A, each intermediate U-frame and its connections should be checked for, in addition to the effects of wind and other applied forces, the effect of horizontal forces FR acting normal to the compression flange at the level of its centroid given by: FR

=

lw  σ σ−fcσ  667  ci fc  δ R

but not greater than

 σ σ−fcσ 16.EI7 lc 2  ci fc  R

Equation A24

where: is the half wavelength of buckling, and is determined by taking L

lw

next integer below L le

le

lw

as the

but not less than unity;

, δ R ,I c ,lR are defined in Clause 9.6.5A;

σ fc σ ci =

is the maximum compressive stress in the flange;

π 2 ES λ LT 2

Equation A25

where: S

= Z pe Z xc ;

Z pe

is the plastic modulus of the section defined in Clause 9.7.1A;

Equation A26

Z xc

is the elastic modulus of the section with respect to the extreme

λ LT

compression fibre defined in Clause 9.7.1A; is as derived in Clause 9.7A.




Page 36 of 66

When there are several interconnected beams, two such forces FR should be applied, in the same or opposite directions, in such a way as to produce the most severe effect in the part being considered. 9.12.2.3A U- F r ames with Cr oss M ember s subj ected to Verti cal L oading

Delete the existing BS 5400: Part 3 Clause 9.12.2.3 and substitute the following: The following additional effects should be included for U-frames with cross members subjected to vertical loading: (a)

Additional force FC applied to the U-frame, in the same manner as FR in Clause 9.12.2.2A, and resulting from the interaction between the bending of the cross members and verticals may be taken as: θd 2 FC = Equation A27 3 l

1.5δ R + R 12 EI c where: lR

θ

(b)

, d 2 , δ R and I c are as defined in Clause 9.6.5A; is the difference in rotation under the coincident loading between the cross member of the U-frame under consideration, and the mean of the rotations of the cross members of the adjacent U-frames on either side. The rotations are calculated in radians under the loading, at the junction of the relevant cross member with the main beam under consideration, assuming that the cross member is simply supported.

The lateral flexure of a compression flange due to loading on a cross member should be taken into account. A conservative method of determining the resulting transverse moment, and of combining it with other effects, may be obtained from BS 5400: Part 3 Appendix E as amended by this Appendix.

Alternatively, the effects in (a) and (b) may be evaluated using an analysis that takes account of the lateral flexure of the compression flange such as an upstand grillage. Alternatively for (b), the value of M given in Clause 9.8A may be reduced by 5% and R the effect of lateral flexure disregarded. 9.12.3A Continuous Restraint Provided by Deck 9.12.3.2A Deck not at Compression F l ange L evel

In BS 5400: Part 3 Clause 9.12.3.2(b) replace expression for f c by:




Page 37 of 66

θd 2 Equation A28 1.5δ where δ and d 2 are as defined in BS 5400: Part 3 Clause 9.6.6.2. fc

=

9.12.4A Restraint at Supports 9.12.4.1A Restr ain in g F orces

Delete the existing BS 5400: Part 3 Clause 9.12.4.1 and substitute the following: All beams, including cantilever beams assessed in accordance with Clause 9.6A, should be restrained against rotation about their own axes at or adjacent to each support in accordance with this Clause and Clause 9.12.4.2A as applicable. The restraint may be assumed to be shared between support restraints in proportion to their relative stiffnesses where more than one restraint has been assumed as part of the support restraint under Clause 9.6.1A. The restraining system should be capable of resisting, in addition to the co-existent effects of wind, frictional and other applied forces, two equal and opposite forces Fs applied normal to the beam and in the planes of its two flanges. Where several beams are restrained by a or common lateral member, two of suchthe forces should beeffect taken, in the same opposite directions, such aspairs to produce most severe in the part under consideration. Where the resistance is less than required to carry the force Fs the slenderness parameter

λ

LT

should be modified as under Clause 9.6.1A.

The value of each force F s in a direction normal to the longitudinal axis of the beam should be taken as follows: Fs =

Fs1 + Fs 2 + Fs 3 + Fs 4

Equation A32

where: F s1

=

0.004

1000 ∆    g  D1−  σ     σ     M

F

fc

Equation A33

2

ci

which represents the force on a support due to the end torque on the beam resulting from the initial bow of the compression flange. If ∆ F is not measured, ∆ F should be taken as g 1000 ;




Fs 2 =

1.2β ∆σ fc

(σ ci − σ fc )δ t

Page 38 of 66 Equation A34

which represents the force on the support resulting from the compressive force in the flange magnified due to the initial departure from verticality of the supports; Fs 3 =

Rd 1.2 ∆ + θL tanα  D  D 

Equation A35

which represents the force at a support due to the eccentricity of the lateral location of the centre of the applied loading relative to the centre of bearing reaction resulting from departure from verticality of the support; Fs 4 =

2βθ tanα  δ + DL     D GJ  L

Equation A36

t

which represents the force at a support due to any twisting of the beam caused by interconnection with adjacent beams at a skewed support. Force Fs 4 may alternatively be derived from a two dimensional grillage analysis of the structure. where:

∆ ∆ L

F

,g

are defined in Clause 9.8.2A; is the departure from verticality of the web at the support as measured but not less than 3 mm. For Level 1 assessment or in the absence of measurement a value of D 300 should be assumed; is the span of a simply supported beam or is the greater of the spans on each side of an internal support when deriving Fs 2 , or the mean of the 2 spans on either side of a support in a continuous beam when deriving Fs 4 ;

d

G J Ic

is the vertical distance between the levels beams of the bearing support the loads applied load respectively. For composite the levels of the and applied should be taken as the top of the steel beam. For cambered beams d should be taken as the sum of the individual applied loads multiplied by their relevant values of d , divided by the total applied loads; is the shear modulus; is the St. Venant torsion constant for the beam; is defined in Clause 9.6.3A;




Page 39 of 66

M

is the largest bending moment occurring either at the support or within length lw of it, whether sagging or hogging;

σ

is the maximum compressive stress in the flange averaged over the whole

fc

flange width, either at the support under consideration or in the span either side of it;

σ

D R

β θ

L

α δ

t

2EI c δ t for simply L supported beams or as Clause 9.6.3A(b) for continuous beams; is the overall depth of the beam at the support; is the bearing reaction; is 1.0 for an end support, or 2.0 for an internal support in a continuous beam; is the change due to live loads in the longitudinal slope of the beam adjacent to the support; is the angle of skew defined as the angle between the axis normal to the longitudinal axis of the beam and the axis of the support in plan for a skewed bridge; are defined in Clause 9.6.3A. is as defined in Clause 9.12.2.2A taking

ci

, k2

le

=

πk

2

9.12.4.2A Addi ti onal F orces due to Cros s member s subj ected to ver tical l oading i n beams with U-F rame res trai nts

When the compression flange of a beam is restrained laterally between points of support by a system of U-frames with cross members subject to vertical loading, a force FL should be added to Fs as defined in Clause 9.12.4.1A. where: FL =

θd δ l 2.5δ + + 2 t

R

θd δ

R

2 where:

lR

3

3EI c

for an end support to a beam with several internal U-frames; Equation A37 for a support to a beam with a single internal U-frame; Equation A38 for an internal support.

2

+δ + t

3EI c

2

2δ R + δ t + FL =

R

2

θd

FL =

3

lR

3

12EI c

Equation A39



θ


is defined in Clause 9.12.2.3A and is to be calculated for the most adverse distribution on cross beams as follows: For beams with multiple U-frames in a span, θ is the difference between the rotation of the U-frame adjacent to an end support and the mean of the rotations at the end support and the second U-frame from the support respectively. For beams with only one internal U-frames in a span, θ is the difference between the rotation of the internal frame and the mean of the rotations at the supports at each end of the span. For an internal support, θ is the difference between the rotation at the support and the mean of the coincident rotations at the U-frames on each side of the support.

d 2 ,δ R ,lR and I c are defined in Clause 9.6.5A;

δ

t


When a beam is continuously restrained by the deck, so that its effective length is determined in accordance with BS 5400: Part 3 Clause 9.6.6, FL may be taken as: FL =

f c (lw1 + lw 2 ) 2

Equation A40

where: fc

is as derived in Clause 9.12.3.2A;

lw1 , lw 2

are the half wavelengths of buckling of the beam on each side of the support under consideration derived in accordance with Clause 9.12.2.2A.

U-frames other than those at the supports assumed to be part of the support Uframe under Clause 9.6.1A, should be checked against force Fc in accordance with Clause 9.12.2.3A together with the appropriate proportion of force FL under this Clause. 9.12.4.3A Stiffness

Delete the existing BS 5400: Part 3 Clause 9.12.4.2 and substitute the following: The assumed stiffness of restraints including bearing stiffeners as limited by the stability of the beam against overturning at supports against rotation about the longitudinal axis should be compatible with the assumptions for effective length in Clause 9.6A.




Page 41 of 66

9.13.5.3A Bu ckli ng of Ef f ective Sti ff ener Section

In BS 5400: Part 3 Clause 9.13.5.3: Replace “ σ ys ” with “ σ y ”in the first expression. Delete the definitions for σ ls , Z x and σ ls is taken as equal to the value of

σ ys σc

and substitute the following definitions: determined from:

(a)

for double sided stiffeners, curve D of BS 5400: Part 3 Figure 37; or

(b)

for single sided stiffeners, curve B of BS 5400: Part 3 Figure 37 when the web plate is in compression or curve D when the stiffener outstand is in compression:

where:

λ=

ls

σy

rse

355

;

Zx

is the elastic section modulus of the effective section about the

σy

centroidal axis parallel to the web with reference to the extreme fibres under maximum compressive stress; is the nominal yield stress value, as defined in Clause 9.3.1A for the web plate,

σ yw

or for the stiffener,

σ ys , whichever is the lesser.

Add to end of existing BS 5400: Part 3 Clause 9.13.5.3: When assessing the adequacy of a transverse web stiffener allowance should be made for initial departures from straightness, ∆ sx , measured in accordance with BS 5400: Part 6 over a gauge length taken as g as defined in Clause 9.8A,

σs l

should be

calculated from the equations in BS 5400: Part 3 Appendix G12 with

η =

λ −15 [1.2 ∆ − 0.0016a] y  r  λ 

0.0083(λ −15 ) + 

sx

se

2

η taken as:

but not less than zero Equation A41

where: y

∆ sx

is the distance from the neutral axis of the effective stiffener to the extreme fibre under consideration; is taken as positive when the bowing is in a direction away from the extreme fibre under consideration.




Page 42 of 66

9.14.4A Strength of Bearing Stiffeners 9.14.4.4A Bu ckli ng Resistance of U nstif fened Webs

The buckling resistance PD of an unstiffened web over a bearing should be taken as:

σ c beff t w γ mγ f 3

PD =

Equation A42

where : σc is the ultimate compressive stress about an axis along the centre line of the web obtained from

σc σ y

in accordance with BS 5400: Part 3 Figure 37

Curve C. In using Figure 37, beff

le

should be determined taking account of the

lateral and rotational restraint of the flange; is the effective breadth of web obtained from: beff

=

(d 2 + s 2 ) but not greater than the width available (refer to BS 5400: Part 3 Figure 27);

d

is the overall depth of the beam;

s

is the bearing length.

10A. DESIGN OF COMPRESSION MEMBERS

Where specified recommendations for the geometry of members including battened and laced compression members given in BS 5400: Part 3 Clause 10 are not complied with, reference may be made to BD 56/96: The Assessment of Steel Highway Bridges and Structures or other recognised literature or research for consideration as a departure from standards in the application for Approval in Principle. 10.3.1A Unstiffened Outstand

Delete the existing definition for

σ y′

σy′

and substitute following definition:

is the lesser of the nominal yield stress of the material or such lower value of yield stress as would be necessary to meet the strength criteria of the subsequent Clauses.

10.3.3A Circular Hollow Section

Delete the expression and substitute: 60

355

σ y′

Expression A43




10.3.4A Assessment of Sections not Complying with Shape Limitations

Outstands not complying with Clause 10.3.1A or BS 5400: Part 3 Clause 10.3.2 should be assessed in accordance with BS 5400: Part 3 Clause 9.3.2. Circular hollow sections not complying with Clause 10.3.3A should be assessed in accordance with Clause 9.3.6A. As a result a lower value of yield stress should be determined such that compliance with the strength criteria of BS 5400: Part 3 Clause 10.6 and Clause 10.3.1A, BS 5400: Part 3 Clause 10.3.2 or Clause 10.3.3A as applicable is achieved. This lower value of yield stress should be used in all subsequent assessment of strength in accordance with Clause 9.3.1A. 10.6.1.1A Strength

Add to end of existing BS 5400: Part 3 Clause 10.6.1.1: Where in assessing the adequacy of a compression member allowance is made for initial departures from straightness, ∆ s , measured in accordance with BS 5400: Part 6, over a gauge length g equal to the clear length of the compression member, σ c should be calculated from the equation in BS 5400: Part 6 Appendix G16 with η taken as:

η=

α(λ −15 )+  λ −λ15 [1.2∆ s − 0.00012G ] ry2

but not taken less than zero Equation A44

10.7.2A Evaluation of Stresses

Add to end of existing BS 5400: Part 3 Clause 10.7.2(c): In assessment of the adequacy of a compression member with longitudinal stiffeners where allowance is to be made for measured initial departures from straightness, ∆i should be determined separately for the X-X and Y-Y axes using the expression:

∆i =

1.2 ∆ s

where

∆s

Equation A45

is the departure from straightness measured in accordance with

BS 5400: Part 6 over a gauge length g equal to the distance between applicable points of restraint. 11A. TENSION MEMBERS

Where specified recommendations for the geometry of members including those such as battens are not complied with, reference may be made to BD 56/96 or other recognised literature or research for consideration as a departure from standards in the application for Approval in Principle.




11.1A General

Add to end of existing BS 5400: Part 3 Clause 11.1: Nominally straight members subjected to axial tension or to combined tension and bending should be assessed as described below:

•

Where acting in compression the member should be assessed in accordance with Clause 10 unless sufficient redundancy or an alternative load path exists. For these cases such compression may be ignored.

•

Where the requirements for battens, lacing, perforated plates, back to back members are not met, the ability of the members and their relevant fixings to resist the load effects to which the members are subjected should be assessed.

11.3.2A Effective Area

Add to end of existing BS 5400: Part 3 Clause 11.3.2: For assessment the value of k 2 may be taken as follows: 1.2 1.15 1.1 1.0

where the member is BS 4360 grade 43, BS 15: 1906, BS 15: 1948 or pre 1905 steel where the member is wrought iron where the member is BS 4360 grade 50 or BS 968 steel where the member is BS 4360 grade 55 or Thirty Oak steel

Alternatively where the member is steel or wrought iron not complying with BS 4360, BS 15: 1906, BS 15: 1948 or BS 968 and σ y and σ ult are the nominal yield stress and ultimate stress derived in accordance with BS 5400: Part 3 Clauses 6.2 and 6.3 respectively, k 2 may be determined as: k2 =

 σ ult  −1.2 but not taken greater than 1.2. σ  y 

1.0 + 0.5

Equation A46

11.3.5A Pin Connected Members

Add to end of existing BS 5400: Part 3 Clause 11.3.5: Where this recommendation is not met, a check should be made that tearing will not occur beyond the pin hole. 11.4A Thickness at Pin Holes

Add to end of existing BS 5400: Part 3 Clause 11.4:




Where this recommendation is not met a check should be made that local buckling will not occur beyond the pin hole. 12A. TRUSSES 12.1A General

Add to end of existing BS 5400: Part 3 Clause 12.1: Bending effects may be ignored in BS 5400: Part 3 Clauses 12.2.2 and 12.2.3 where: (i)

the truss is fully triangulated;

(ii)

the centroids of the members intersect at the joint being considered;

(iii)

all significant loadings excluding the self weight of the members are applied at the joints;

(iv)

the joints use untensioned bolts or rivets such that any secondary bending can be relieved by joint movement.

12.4.1A General

Add to end of existing BS 5400: Part 3 Clause 12.4.1: A compression chord may be considered to be effectively braced provided that the restraint system complies with the recommendations of Clause 9.12.1A with the chord treated as a flange. 12.4.2A Lateral Restraint by Deck to Compression Chord

For assessment BS 5400: Part 3 Clause 12.4.2 may be ignored. 12.5.1A Effective Length

Delete the existing BS 5400: Part 3 Clause 12.5.1 and substitute the following: Where there is no intermediate lateral restraint to a compression chord, the effective length may be derived as for a beam in accordance with Clause 9.6.3A with k e =1. Where the lateral restraint to a chord is provided by U-frames comprising cross members and web members (see Figure A41) the effective length may be determined in accordance with Clause 9.6.5A with δ R taken as follows:

δR =

1 1

δv

+∑

1

δi

Equation A47




where:

δv

is the deflection for a U-frame component with a vertical truss web member, given by: 3

d1

δi

uBd 2

2

fd Equation A48 3EI1 + EI 2 + 2 is the deflection for a U-frame component with an inclined truss web member, given by: 3 2 d3 uBd δi = + 2 + fd 2 2 + θs Equation A49 3EI 3 EI 2

δv =

where: d3

is the length of the diagonals measured as the distance sloping from the

I3

centroid of the chord to the top face of the cross member of the U-frame as shown in Figure A41; is the second moment of area of the diagonals forming an arm of a

f

U-frame about its axis perpendicular to the plane of the U-frame; is the flexibility of the joints between the cross member and the truss

θ

member, expressed in radians per unit moment. Values of f may be taken from Figure A42; may be taken as zero when the bottom truss chords are fully restrained against lateral deflection throughout their lengths by an integral deck. Otherwise, θ may conservatively be taken as: sB θ = foranenddiagonal; EquationA50 2nEI 4 sB θ = for an intermediate diagonal. Equation A51 nEI 4 where: I4 is the second moment of area of the cross member about its vertical axis; m may be taken as: a) b)

n

s

1.5 for a Warren or a Pratt Truss; 4.0 for a modified Warren Truss type (1) or (2) as shown in Figure A41 having a cross member at each bottom chord joint with web members;

should be taken as 1.0 when the connection between the chord and cross member is rigid in plan or zero when the connection between chord and cross member is flexible in plan; is the spacing of cross members forming effective U-frames.




d3

Page 47 of 66

d3

Warrentruss

ModifiedWarrentruss(1)

d3 d3

ModifiedWarrentruss(2)

Pratttruss

Y

Y R

R

FR

FR

d3 DIAGONAL LENGTH

d1 d 2

I

1

I2 B Y

Y

Figure A41 Truss Types and Lateral Restraint by U-Frame




Page 48 of 66

Connection through top or bottom flange

(Or top cleat unstiffened)

f -4 x10 RAD/kNm.

0.5 Vertical stiffener

Extended end plate

Connection to both flanges

0.2

Connection to bottom flange Gusset

Gusset Weld to gussets

0.1

Connected

Troughing

Bars through stiffeners

All crests cleated and troughs bolted

0.5

Connect crests to vertical stiffeners <2 bolts.

0.2

Stiffener

0.8 Cross Girders not at stiffeners

0.4 Cross Girders cleated to stiffener

Figure A42 U-Frame Joints and f’ Values




Rebar through stiffeners

Page 49 of 66

f -4 x10 RAD/kNm.

0.5

Cleats to web

Stiffeners not at crests

0.95 Fabricated troughs

if decking plate not connected to vertical stiffener

Floor plate NOT connected to vertical stiffeners

0.3 0.2*

* connected Floor plate

H.S.F.G. bolted connection


0

Welded



Figure A42 (continued) U-Frame Joints and f’ Values

A U-frame restraint should be taken into account at each connection of a web member with the compression chord. At any restraint position, more than one web member may be connected, and the members may be diagonal or vertical. The Uframe restraint assumed may include all the web members at each position, or may conservatively neglect the more flexible web members, such as the tension diagonals




in a Pratt truss. Web members should be included only when the members are adequately connected to the cross members either directly or by stiffening. Where a cross member acts with components of more than one U-frame the moment of area I 2 should be proportioned between the components concerned. Where more than one type of intermediate U-frame occurs alternately, such as in a modified Warren truss, (1) in Figure A41, when all the web members are taken into account, the average value of δ R may be assumed. 12.5.2A Restraints to Compression Chords

Delete the existing BS 5400: Part 3 Clause 12.5.2 and substitute the following: Restraints to compression chords should comply with the relevant recommendations of Clause 9.12A with chords treated as flanges. In calculating FR in accordance with Clause 9.12.2.2A, S may be taken as 1.0 and λ LT = le ry where ry is the radius of gyration of the chord about the Y-Y axis (see Figure A41). 12.6.1A General

In BS 5400: Part 3 Clause 12.6.1: In the second paragraph, line 3, substitute “12.6.2” with the relevant recommendations of Clause 9.12.1A, with the chords treated as flanges”. In the last line replace BS 5400: Part 3 Clause 12.5.3 with Clause 12.5.2A. Delete the last sentence beginning with “U-frames.....12.5.3”. 12.6.2A Forces on Bracing

For assessment BS 5400: Part 3 Clause 12.6.2 may be ignored. 12.6.3A Lateral Bracing not providing Adequate Restraint

Add additional Clause 12.6.3 to BS 5400: Part 3. Where any of the provisions of Clause 12.6.1A are not met, one of the following options should be used to assess the Bridge: (i)

lateral bracing should be ignored and assumed to provide no restraint with the member capacities reduced accordingly;




Page 51 of 66

(ii)

the structure capacity should be reduced such that the provisions of Clause 12.6.1A are met;

(iii)

a rigorous non-linear analysis should be carried out of the complete system to verify the capacity of the members and adequacy of the bracings.

12.7A Curved Members

Add to end of existing BS 5400: Part 3 Clause 12.7: Where members do not comply with recommendations (a) to (d) consideration should be given to: (1)

the forces and stresses according to BS 5400: Part 3 Clause 9.5.7;

(2)

the effects of the change in neutral axis position due to curvature;

(3)

the buckling resistance of the section if it does not satisfy the criteria for a compact section;

(4)

the adequacy of flanges to resist the radial component of the flange force. Assuming the axial force in the flange is distributed uniformly across the width, the line load radial force per unit width across the flange per unit length of the flange may be expressed as:

σ f t fo Rf

σf tf Rf where

in a flange outstand, or

in a plate panel between longitudinal stiffeners and/or webs

σ f , t f , t fo and R f

are as defined in BS 5400: Part 3 Clause 9.5.7.1.

14A. CONNECTIONS

14.3.5A Connection of Restraints to Parts in Compression

Add to end of existing BS 5400: Part 3 Clause 14.3.5: Where the connection cannot resist the forces in (a) and (b) above, the intermediate restraint should be ignored. Alternatively, the system may be checked making due allowance for the maximum restraint that can be provided, see Clause 12.6.3A.




Page 52 of 66

14.4A Splices 14.4.1.1A G ener al

Add to end of existing BS 5400: Part 3 Clause 14.4.1.1: The following assumptions may be made for assessment: (a)

where both surfaces of the spliced parts are provided with covers, axial stresses only should be assumed.

(b)

where only one surface is provided with a cover, bending effects should be considered at the serviceability limit state, but may be ignored at the ultimate limit state. For the calculation of bending effects the line of action of the axial force in the splice may be assumed located along the interface between the parent material and the cover. The effects of eccentricity should be ignored when bending is effectively prevented by: (i) (ii)

the presence of surrounding or adjacent concrete or other solid infill; or the presence of an element which prevents bending of the parent material or the cover. Such an element should be within a distance of 12t from the furthest fastener, where t is the thickness of the parent material to which the cover plate is attached.

14.4.5A Obsolete Splicing Methods

Add additional Clause 14.4.5 to BS 5400: Part 3: When assessing splices where several plates were required to build up the required section thickness, consideration should be given to the load path through the joint to ensure no single component is overloaded, see Figures A63 and A64.

C

P

P

Figure A63 Force Flow in Typical Triple Plate Shingle Joint




P

P Filler plate (a) Loose Fillers

P

P Filler plate

(b) Tight Fillers Figure A64 Types of Filler Plate 14.5A Connections made with Bolts, Rivets or Pins 14.5.1.5A Assessment of Non Complyi ng Ar r angements

Add additional Clause 14.5.1.5 to BS 5400: Part 3. Where any of the limits in BS 5400: Part 3 Clauses 14.5.1.1, 14.5.1.2, 14.5.1.3 or 14.5.1.4 are not complied with, allowance should be made for a reduced strength of the fasteners or plate where there is evidence of plate bulging, distortion near or to fasteners, or excessive rust forcing. Reductions in strength should also be applied in the following cases: (a)

Where the parts joined are in compression and the distance, in the direction of stress, S a between centres of adjacent rivets or bolts exceeds Ss the maximum distance specified according to the requirements of BS 5400: Part 3

 Ss    Sa 

Clause 14.5.1, the yield stress of the outer plies should be reduced by 

(b)

2

.

The gauge limit in BS 5400: Part 3 Clause 14.5.1.3 may be increased to 80 mm in determining the specified maximum spacings under BS 5400: Part 3 Clause 14.5.1.3.




14.5.2A Edge and End Distance

Add to end of existing BS 5400: Part 3 Clause 14.5.2: Where any of the above limits are not complied with, the strength of the fastener or plate should be reduced as follows: (a)

For fasteners away from an edge when the spacing between two fasteners is less than 2.5d, the strength of each should be reduced linearly to zero when the spacing reaches 1.5d. Where a number of fasteners are close to each other, the reduction factors should be multiplied together;

(b)

For fasteners adjacent to an edge parallel to the direction of force - the value of k 2 in BS 5400: Part 3 Clause 14.5.3.6 should be linearly reduced from the value 2.5 when the edge distance is 1.2d to a value of zero when the edge distance is 0.8d;

(c)

For fasteners adjacent to an end, loaded by a force away from the edge of the part - no reduction should be made, subject to a minimum end distance of 0.8d. For a lesser distance, the fastener should be ignored;

(d)

For fasteners adjacent to an end loaded by a force toward the edge of the part - the value of k 2 in BS 5400: Part 3 Clause 14.5.3.6 should be reduced linearly from the value of 1.2 when the edge distance is 1.2d to a value of zero when the edge distance is 0.9d.

(e)

For HSFG bolts - when the spacing between fasteners is less than 2.5 d, the friction capacity should be reduced in linear proportion from a value of 100% of the maximum capacity at 2.5d to 80% of the maximum capacity at 2.0d. When the spacing is less than 2.0d, the fasteners should be ignored. When the edge distance is less than 1.5d, the friction capacity should be reduced linearly to a value of zero when the distance is 1.0d. When the edge distance is less than 1.0d the fastener should be ignored.

14.5.3.3A Ri vets Subj ect to Ax i al Tension

Add to end of existing BS 5400: Part 3 Clause 14.5.3.3: The tensile capacity should be reduced where there is significant loss of material from rivet heads. Where rivets are subject to tension due to live loads, σ f should be reduced to that for countersunk rivets where the remaining effective head diameter is 1.3 times the nominal diameter and to zero if less than 1.3 times the nominal diameter remains.




Page 55 of 66

14.5.3.4A F astener s Subject to Shear Onl y

In BS 5400: Part 3 Clause 14.5.3.4 replace definition of

σq

σq

by:

σ y for all fasteners except black bolts and rivets for which 0.85 σ y or 0.9 σ ult respectively may be assumed where σ ult is the ultimate tensile is

strength of rivet material as given in Clause 6.1A. 14.6A Welded Connections

14.6.1A General

Add to end of existing BS 5400: Part 3 Clause 14.6.1: For bridges known to have been welded in accordance with BS 5400 Part 6 or BS 5135: 1974 (or 1984), the strength of the welds should be determined in accordance with BS 5400: Part 3 Clauses 14.6.2.3 and 14.6.3.11. For bridges not known to have been welded in accordance with BS 5400: Part 6 or with BS 5135: 1974 (or 1984) the strengths of the welds should be derived in accordance with (a) to (d) below: (a)

For butt welds in compression and butt welds in tension or shear demonstrated to comply with BS 5135: Table 18, quality A, the strengths may be taken as defined in BS 5400: Part 3 Clause 14.6.2.3;

(b)

For butt welds in tension or shear free from surface cracks but not known to comply with BS 5135: Table 18, quality A, the strengths may be taken as 85% of those derived from BS 5400: Part 3 Clause 14.6.2.3;

(c)

For fillet welds in bridges constructed to BS 153: Part 1(1958 or 1972) and free from surface cracks, the weld strengths should be taken as 90% of those derived from BS 5400: Part 3 Clause 14.6.3.11 in the absence of demonstration of their compliance with BS 5135: Table 19 quality A or equal to those strengths when such compliance has been demonstrated;

(d)

For other fillet welds free from visible surface cracks, the strengths should be calculated in accordance with BS 5400: Part 3 Clause 14.6.3.11, but replacing the equation σ w = 0.5(σ y + 455 ) by either: (i)

0.4 (400 + σ y min ) in the absence of demonstration of their compliance with BS 5135: Table 19 quality A, or

(ii)

0.5(400 + σ y min ) when such compliance has been demonstrated:

where

σ y min

welds.

is the yield stress of the weaker of the parts connected by the




Where any of the general or specific recommendations of this or any of the following sub-clauses are not met, due allowance should be made in the assessment of the strength of the welds. Further information is given in Appendix F of this Code. 14.6.2.1A I ntermittent Bu tt Welds

Delete the existing BS 5400: Part 3 Clause 14.6.2.1 and substitute the following: For intermittent butt welds no contribution to strength of the weld should be assumed for a weld length equal to three times the throat thickness at each end of any intermittent length. 14.6.2.2A Parti al Penetrati on Bu tt Welds

Add to end of existing BS 5400: Part 3 Clause 14.6.2.2: The strength of partial penetration butt welds should be calculated as for fillet welds. The throat thickness should be taken as the depth of the web preparation less 3 mm, or as measured consistently at site. Where the weld is unsymmetrical relative to the parts being jointed the resulting eccentricity should be allowed for when calculating the maximum stresses, under all loadings other than those which act along the axis of the weld. 14.6.3.11.1A Welds Subj ect to Lon gitu din al Shear Onl y i.e., shear in th e dir ection of i ts length ( see F igu r e 55(a))

Delete the existing BS 5400: Part 3 Clause 14.6.3.11.1 and substitute the following: The stress in a weld, calculated as the longitudinal shear force per unit length PL divided by the effective throat, g , should not exceed:

σw γ f 3γ m

3

Expression A52

where:

σw

is the yield stress of the deposited weld metal and may be taken as 0.5(σ y + 455 ) N/mm²;

σy

is the lesser nominal yield stress value, as defined in Clause 9.3.1A or Clause 10.3.1A of the two parts joined.

14.6.3.11.2A Weld Subj ect to Tr ansver se F orce Only (f orce at r igh t angles to its l ength see Fi gur e 55(b))

Delete the existing BS 5400: Part 3: Clause 14.6.3.11.2 and substitute the following:




Page 57 of 66

The stress in a weld, calculated as the transverse force per unit length P T1 (or PT2) shown in Figure 55(b) divided by the effective throat g, should not exceed: Kσ w Expression A53 γ f 3γ m 3 where:

σw

is as defined in Clause 14.6.3.11.1A;

K

depends on the angle θ between the direction of the resultant transverse force and the throat and is given by: 3 but may not be greater than 1.4 1+ 2cos 2 θ

K=

For equal fillets between components at right angles

θ = 45°

and K =1.225 .

14.6.3.11.3A Welds Subj ect to F orces in Both T r ansver se and L ongi tudi nal Directions

Add additional Clause 14.6.3.11.3 to BS 5400: Part 3: The following condition should be satisfied: 2

P 1 2 PL + T 2 g K

≤

σw γ f 3γ m

3

Equation A54

where:

σw

is the longitudinal shear force per unit length of the weld; is the resultant of transverse forces per unit length of the weld (see Figure 55(c)); is the effective throat of the weld; is the angle between the direction of the resultant transverse force and the throat; is as defined in Clause 14.6.3.11.1A;

K

is as defined in Clause 14.6.3.11.2A

PL PT g

θ




Page 58 of 66

p

L

p

L

Figure 55(a) Weld subjected to longitudinal shear

Throat of the weld

2P T2

PT1 PT1

PT2

Figure 55(b) Weld subjected to transverse force

PT2


The Structural Assessment of Underbridges Appendix A - Assessment of Steel and Wrought Iron PT

Throat of the weld

Figure 55(c) Resultant transverse force at weld





Page 60 of 66

BS 5400: PART 3 APPENDIX B DISTORTION AND WARPING STRESSES IN BOX GIRDERS B.1A GENERAL

In BS 5400: Part 3 Appendix B Clause B.1 delete “highway” from the first sentence. B.2A RESTRAINT OF TORSIONAL WARPING

In BS 5400: Part 3 Appendix B Clause B.2 in the definition for J, delete “B” and replace by “W”. In BS 5400: Part 3 Appendix B Clause B.2 in the definition for B, delete “B” and replace by “W”. B.3.2A CORNER STRESS In BS 5400: Part 3 Appendix B Clause B.3.2:

In Clause (b) replace “knife-edge load” with “concentrated load”. In Clause (c) replace “HA loading, the effects of the uniformly distributed and knifeedge load” with “RU loading, the effects of the uniformly distributed and axle loads”. Add the following at the end of the definition for R D : “or from 1+ RD =

BT BB

DYT d  2 BB   BB   B  D d    +  − VD  2 + B  YT +1 DYC BT  BT + BB   BT + BB   BT  DYC BT 

Equation A55

where: is as defined in Clause B.4.2A; are defined in Clause B.2A.

VD B ,B B

T

B.3.4.2A

STRENGTH

Delete the existing BS 5400: Part 3 Appendix B Clause B.3.4.2 and substitute the following: (a)

A plate diaphragm should be capable of resisting a shear stress

τD

given by:




T 2BDt d

τD =

Page 61 of 66

Equation A56

where: td

is the thickness of the diaphragm plate;

B

is as defined in BS 5400: Part 3 Clause 9.17.2.7, and is the average of the widths at the top and bottom flanges; is as defined in Clause B.2A; is the torque due to loads applied at the diaphragms and between the adjacent diaphragms or cross frames on either side. Any torque applied between the diaphragm and an adjacent diaphragm or cross frame may be apportioned by simple static analysis.

D T

B.3.4.3A

STIFFNESS

Delete the existing BS 5400: Part 3 Appendix B Clause B.3.4.3 and substitute the following: A cross frame or a diaphragm should have a dimensionless stiffness S not less than the value obtained from Table A17. where:

S=

Gt d L p

2

δb 2K

2 Ap L D

for a plated diaphragm;

2

S=

S=

EAb δ b K LD Lp EAb L p

2

δb 2K

4 LD L b

3

for a cross braced frame;

for a vee braced cross frame, irrespective of whether the centre of the V is at the top or bottom flange.

Alternatively, S=

KR

for an unbraced ring cross frame with constant section framing members;

KL D 2

S=

Pp δ b K

∆ pLD

for any type of cross frame including a ring cross frame.




Page 62 of 66

where:

∆p

is the change in length of the diagonal L p calculated to occur under the system of diagonal forces Pp as shown in the Figure A65 below. This method of

Lp

deriving stiffness may be used for any type of frame including those given below; is (D 2 + B 2 ) which is the length of the diagonal;

Lb

is the length of the brace;

Ap

is B × D which is the surface area of the plated diaphragm;

Ab

is the area of cross section of brace;

δb

is

B

is (BT + BB ) 2 which is the average width of the box girder;

L D ,K

are as defined in Clause B.3.2;

KR

is the value of K derived by taking DYT , DYB and DYC as the flexural rigidities of

4 BD which is a unit length flexibility; KBB L P

the effective framing members attached to the top and bottom flanges and webs respectively; BT , B D and D are as defined in Clause B.2A.

BT

Pp

Pp

B

D/ 2 D LW

Lp

Bn Pp Figure A65

D/ 2

Pp




Page 63 of 66

Cross Frame Parameters SINGLE TORQUE UNIFORMLY DISTRIBUTED TORQUE For σ DW For σ DB For σ DW For σ DB

βL D 3.0

0

0

0

5

2.0 0 5 0 1.5 2 10 10 1.0 20 50 100 0.8 50 100 200 0.5 500 1000 200 0.3 2000 10000 200 For intermediate values of β LD , values of S may be obtained by logarithmic

30 200 500 1000 10000 20000

interpolation. TABLE A17 DIAPHRAGM STIFFNESS S B.4.2A CORNER STRESS In BS 5400: Part 3 Appendix B Clause B.4.2:

In Clause (b) replace “knife-edge load” with “concentrated load”. In Clause (c) replace “HA loading” with “RU loading” and “knife-edge” with, “axle loads”. Add the following at the end of the definition for VD : or from

VD =

 DYT  D YC  BT +1 1+ 2  DYT     BB    DYC

d  BB  2+ BT  B T

 +1     2 3  d  B B  BB   DYT  BB   1+ +    +    BT  BT  BT   DYB  BT  

Equation A57




Page 64 of 66

BS 5400: PART 3 APPENDIX D PATCH LOADING ON WEBS

Delete the existing BS 5400: Part 3 Appendix D Clause D.1 and substitute the following: D.1A Beams without Longitudinal Stiffeners on Web

The limiting value of patch load P on each web in its plane should be taken as the lesser of: (a)

Web buckling criterion

=

P

0.5t w

(b)

2

Eσ yw

tf tw

 wt 1+ 3   d  t

  γ γ σ  1    1−     σ  γ γ 2

1.5

w f

f3

m

f

yw

m

Equation A58

f3

Web yielding criterion 2

γ γ σ  1 w ) 1−  σ  γ γ m

P

(2 t σ σ f

=

yf

yw

Bf t w

+σ

t

yw w

f3

yw

f

m

f3

Equation A59

where: tf

is the flange plate thickness;

tw

is the web plate thickness;

w

is the width of the patch load (see BS 5400: Part 3 Clause 9.5.6 and Figure 6) but to be taken not greater than 0.2d; is the width of the flange plate;

Bf d

σ ,σ yf

is the depth of the web in its plane; are the nominal yield stresses of the material of flange and web respectively, as yw

σf

defined in Clause 6.2A; is the longitudinal stress in the flange due to bending moment and/or axial

γ

force on the beam; is taken as 1.05 for the ultimate limit state.

m




Page 65 of 66

BS 5400: PART 3 APPENDIX E Transverse Moments in Compression Flanges: U-frame Restraints

In BS 5400: Part 3 Appendix E in line 2 replace “9.12.2.3(b) or 9.12.3.2(c)” by “BS 5400: Part 3 Clause 9.12.3.3(b) or Clause 12.5.2A”. Replace the definitions as follows: Ic

is as defined in BS 5400: Part 3 Clause 9.6.4.1.1.2;

d2

is as defined in BS 5400: Part 3 Clause 9.6.4.1.3; is as defined in BS 5400: Part 3 Clause 9.12.3.3(a) or Clause 9.12.4.2A as applicable; is the maximum compressive stress in the flange;

θ

σ fc σ ci ′

is taken as follows: (a)

if

lw

is less than three times the spacing of U-frames,

σ ci ′ = σ ci

as

defined in Clause 9.12.2.2A or 12.5.2A; (b)

if

lw

is more than four times the spacing of U-frames, or if

lc

has been

calculated in accordance with BS 5400: Part 3 Clause 9.6.6.2, σ'ci= 1.25σcior; (c)

for intermediate values of

le ,

σci, is obtained by linear interpolation

where: lw

is as defined in Clause 9.12.2.2A;

L

is as defined in BS 5400: Part 3 Clause 9.6.2”.

In the expression in item (b), replace “ σ f “ σb

σ co ” with “ σ b Z xc l

σ c ” with “ σ f Z xc l

MR ” and replace

Mult ”.

Delete the definitions for

σc l

and

σ co l

and add the following definitions:

MR and Mult are as defined in Clause 9.8.1A; Z xc is as defined in Clause 9.7.1A. Add the following to the end of BS 5400: Part 3 Appendix E: (c)

for chords in trusses see BS 5400: Part 3 Clause 10.6.2.




Page 66 of 66

BS 5400: PART 3 APPENDIX G Equations used for production of curves in Figures G.7A Figure 10 - Basic Limiting Stress

Delete the existing BS 5400: Part 3 Clause G.7 and substitute the following: MR Mult

=

 

0.51+ (1)+ η

2 1+ 1+ η 5700  − 22800  when β > 30 , or − ( )    β2   β2  β 2 

5700 

Equation A60 MR Mult

=

1.0 when

β ≤ 30

Equation A61

where:

 β k   β − 30 

η=

0.0035(β − 30 ) +1.2

η=

0.008(β − 30 ) +1.2 β − 30 k forFigureA11butnotlessthanzero;

β=

 σ  M  λ LT  yc  ult  ;  355  Mpe 

for Figure A10 but not less than zero; or

β

MR , Mult and Mpe

σy

are as defined in Clause 9.8.1A;



The Structural Assessment of Underbridges Appendix B – Assessment of Concrete Structures


CONTENTS

1B. SCOPE .................................................................................................................................... 5 2B. DEFINITIONS AND SYMBOLS ........................................................................................ 5 2.1B Definitions ....................................................................................................................... 5 2.1.1B Partial General.....................................................................................................................5 2.1.2B Factors ......................................................................................................... 5 2.1.3B Materials...................................................................................................................5 2.2B Symbols ............................................................................................................................ 5 3B. LIMIT STATE PHILOSOPHY ...........................................................................................10 3.1B General...........................................................................................................................10 3.2B Deflection ......................................................................................................................11 4B. GENERAL..............................................................................................................................11 4.1B Limit State Criteria ......................................................................................................11 4.1.1B Serviceability Limit States ...................................................................................11 4.1.2B Ultimate Limit States ...........................................................................................11 4.1.3B Other Considerations .............................................................................................12 4.2B Loads, Load Combinations and Partial Factors γfL and γf3 ....................................12 4.2.1B Loads.......................................................................................................................12 4.2.2B Serviceability Limit State.....................................................................................12 4.2.3B Ultimate Limit State.............................................................................................12 4.3B Properties of Materials ...............................................................................................12 4.3.1B General...................................................................................................................12 4.3.2B Material Properties ..............................................................................................13 4.3.3B Values of γm ............................................................................................................16 4.4B Analysis of Structure ...................................................................................................17 4.4.1B General...................................................................................................................17 4.4.2B Analysis for Serviceability Limit State ..............................................................17 4.4.3B Analysis for Ultimate Limit State ......................................................................18 4.5B Analysis of Section .......................................................................................................18 4.5.1B Serviceability Limit State.....................................................................................18 4.5.2B Ultimate Limit State.............................................................................................19 4.6B Deflection ......................................................................................................................19 4.7B Fatigue ............................................................................................................................19 4.8B Combined Global and Local Effects .........................................................................19 4.8.1B General...................................................................................................................19 4.8.2B Analysis of Structure ...........................................................................................19 4.8.3B Analysis of Section ...............................................................................................20 5B. REINFORCED CONCRETE .............................................................................................20 5.1B General...........................................................................................................................20 5.1.1B Introduction ..........................................................................................................20 5.1.2B Limit State Assessment of Reinforced Concrete ..........................................20




5.1.3B Loads.......................................................................................................................21 5.1.4B Strength of Materials ...........................................................................................21 5.2B Structures and Structural Frames.............................................................................21 5.2.1B Analysis of Structures..........................................................................................21 5.2.2B Redistribution of Moments ................................................................................21 5.3B Beams.........................................................................................................................22 5.3.1B General...................................................................................................................22 5.3.2B Resistance Moment of Beams............................................................................23 5.3.3B Shear Resistance of Beams.................................................................................26 5.3.4B Torsion...................................................................................................................30 5.3.5B Longitudinal Shear................................................................................................33 5.3.6B Vertical Deflection of Beams .............................................................................33 5.3.7B Crack Control in Beams.....................................................................................33 5.4B Slabs ................................................................................................................................33 5.4.1B Moments and Shear Forces in Slabs .................................................................33 5.4.2B Resistance Moments of Slabs.............................................................................33 5.4.3B Resistance to In-plane Forces............................................................................34 5.4.4B Shear Resistance of Slabs....................................................................................34 5.4.5B Deflection of Slabs ...............................................................................................37 5.4.6B Crack Control in Slabs........................................................................................38 5.4.7B Torsion in Slabs ....................................................................................................38 5.5B Columns.........................................................................................................................38 5.5.1B General...................................................................................................................38 5.5.2B Moments and Forces in Columns .....................................................................39 5.5.3B Short Columns Subject to Axial Load and Bending about the Minor Axis............................................................................................................40 5.5.4B Short Columns Subject to Axial Load and either Bending about the Major Axis or Bi-axial Bending.........................................................................42 5.5.5B Slender Columns ..................................................................................................43 5.5.6B Shear Resistance of Columns ............................................................................45 5.5.7B Crack Control in Columns ................................................................................46 5.6B Reinforced Concrete Walls.......................................................................................46 5.6.1B General...................................................................................................................46 5.6.2B Forces and Moments in Reinforced Concrete Walls ...................................46 5.6.3B Short Reinforced Walls Resisting Moments and Axial Forces ...................47 5.6.4B Slender Reinforced Walls...................................................................................47 5.6.5B Shear Resistance of Reinforced Walls .............................................................47 5.6.6B Deflection of Reinforced Walls.........................................................................48 5.6.7B Crack Control in Reinforced Walls .................................................................48 5.7B Bases ...............................................................................................................................48 5.7.1B General...................................................................................................................48 5.7.2B Moments and Forces in Bases ...........................................................................48




5.7.3B Assessment of Bases............................................................................................49 5.7.4B Deflection of Bases ..............................................................................................50 5.7.5B Crack Control in Bases.......................................................................................50 5.8B Considerations of Details...........................................................................................50 5.8.1B Constructional Details ........................................................................................50 5.8.2B Reinforcement: Concrete CoverGeneral to Reinforcement .................................................................50 5.8.3B Considerations ........................................................52 5.8.4B Minimum Areas of Reinforcement in Members.............................................53 5.8.5B Bond, Anchorage and Bearing ...........................................................................53 5.8.6B Curtailment and Anchorage of Reinforcement .............................................57 5.8.7B Maximum Distance between Bars in Tension................................................57 5.9B Additional Considerations for Lightweight Aggregate Concrete ......................60 5.9.1B General...................................................................................................................60 5.9.2B Strength of Concrete ..........................................................................................60 5.9.3B Shear Resistance of Beams.................................................................................60 5.9.4B Torsional Resistance of Beams..........................................................................60 5.9.5B Deflection of Beams ............................................................................................60 5.9.6B Shear Resistance of Slabs....................................................................................60 5.9.7B Deflection of Slabs ...............................................................................................60 5.9.8B Columns.................................................................................................................61 5.9.9B Local Bond, Anchorage Bond and Laps ...........................................................61 5.9.10B Bearing Stress inside Bends .............................................................................61 6B. PRESTRESSED CONCRETE ............................................................................................61 6.1B General...........................................................................................................................61 6.1.1B Introduction ..........................................................................................................61 6.1.2B Limit State Assessment of Prestressed Concrete .........................................62 6.1.3B Loads.......................................................................................................................62 6.1.4B Strength of Materials ...........................................................................................62 6.2B Structures and Structural Frames.............................................................................63 6.2.1B Analysis of Structures..........................................................................................63 6.2.2B Redistribution of Moments ................................................................................63 6.3B Beams .............................................................................................................................64 6.3.1B General...................................................................................................................64 6.3.2B Serviceability Limit State: Flexure....................................................................64 6.3.3B Ultimate Limit State: Flexure............................................................................64 6.3.4B Shear Resistance of Beams.................................................................................66 6.3.5B Torsional Resistance of Beams..........................................................................71 6.3.6B Longitudinal Shear................................................................................................72 6.3.7B Deflection of Beam ..............................................................................................72 6.4B Slabs ................................................................................................................................72 6.5B Columns.........................................................................................................................72 6.6B Tension Members ........................................................................................................72




6.7B Prestressing Requirements ........................................................................................73 6.7.1B Maximum Initial Prestress ..................................................................................73 6.7.2B Loss of Prestress other than Friction Losses .................................................73 6.7.3B Loss of Prestress due to Friction ......................................................................76 6.7.4B Transmission Length in Pre-tensioned Members ..........................................77 6.7.5B End Blocks..............................................................................................................79 6.8B Considerations of Details...........................................................................................80 6.8.1B General...................................................................................................................80 6.8.2B Cover to Prestressing Tendons ........................................................................80 6.8.3B Spacing of Prestressing Tendons.......................................................................81 6.8.4B Longitudinal Reinforcement in Prestressed Concrete Beams....................81 6.8.5B Links in Prestressed Concrete Beams .............................................................81 7B. PRECAST, COMPOSITE AND PLAIN CONCRETE CONSTRUCTION.............81 7.1B General...........................................................................................................................81 7.1.1B Introduction ..........................................................................................................81 7.1.2B Limit State Assessment .......................................................................................81 7.2B Precast Concrete Construction ...............................................................................82 7.2.1B Framed Structures and Continuous Beams....................................................82 7.2.2B Other Precast Members .....................................................................................82 7.2.3B Supports for Precast Members .........................................................................82 7.2.4B Joints between Precast Members .....................................................................84 7.3B Structural Connections between Units ..............................................................86 7.3.1B General...................................................................................................................86 7.3.2B Continuity of Reinforcement.............................................................................87 7.3.3B Other Types of Connection ..............................................................................88 7.4B Composite Concrete Construction ........................................................................88 7.4.1B General...................................................................................................................88 7.4.2B Ultimate Limit State.............................................................................................89 7.4.3B Serviceability Limit State.....................................................................................92 7.5B Plain Concrete Walls and Abutments .....................................................................92 7.5.1B General...................................................................................................................92 7.5.2B Moments and Forces in Walls and Abutments ..............................................92 7.5.3B Eccentricity in the Plane of the Wall or Abutment.......................................93 7.5.4B Eccentricity at Right-angles to Walls or Abutments.....................................93 7.5.5B Analysis of Section ...............................................................................................93 7.5.6B Shear .......................................................................................................................94 7.5.7B Bearing....................................................................................................................94 7.6B Mass Concrete Arches ...............................................................................................94 APPENDIX B1 HISTORICAL CONCRETE GRADES




1B. SCOPE

The scope is as given in Section 7. 2B. DEFINITIONS AND SYMBOLS

2.1B Definitions

2.1.1B General

The definitions given in Section 1 apply in addition to those specifically defined in this Appendix. 2.1.2B Partial Factors

The partial factors given in Section 2 apply. 2.1.3B Materials 2.1.3.1B Strength

Material strengths are expressed in terms of the cube strength of concrete, fcu , the yield or proof strength of the reinforcement, fy , or the breaking stress of a prestressing tendon, fpu . The material strengths that may be used are either: (a)

The characteristic strength, (the strength below which not more than 5% of all possible test results may be expected to fall); or

(b)

The worst credible strength, (the lowest value of the strength which the Engineer, based on experience and knowledge of the material, realistically believes could occur). The method of determining the worst credible strength should be agreed with the relevant Railtrack Director’s Nominee.

2.1.3.2B Ch ar acter isti c Stress

The characteristic stress is the value of stress at the assumed limit of linearity on the stress-strain curve for the material. 2.2B Symbols

Symbols that appear in this Appendix are as noted in this Clause. Additional clarification is also given where necessary in other Clauses. Ac Acon

area of concrete contact area



Ae Ai Ao Aps s A A's As1 As2 Asc AsL

Ast Asup Asv At a´ ab acr av b ba bc bcol bs bt bw c cnom Dc d d´ dc de ds


area of fully anchored reinforcement per unit length crossing the shear plane cross-section area of in-situ concrete area enclosed by the median wall line area of prestressing tendons in the tension zone area of of compression tension reinforcement area reinforcement area of compression reinforcement in the more highly compressed face area of reinforcement in other face area of longitudinal reinforcement (for columns) cross-sectional area of one bar of longitudinal reinforcement provided for torsion cross-sectional area of one leg of a closed link supporting area cross-sectional area of shear reinforcement at a particular cross section area of reinforcement in a particular direction distance from compression face to a point at which the crack width is being calculated centre-to-centre distance between bars or group of bars perpendicular to the plane of the bend distance from the point (crack) considered to the surface of nearest longitudinal bar distance from the section under consideration to the face of the supporting member width or breadth of section average breadth of section excluding the compression flange breadth of compression face width of column width of section containing effective reinforcement for punching shear breadth of section at level of the centroid of the tension steel breadth of member web or rib, or edge zone cover nominal cover density of lightweight aggregate concrete effective depth to tension reinforcement depth from the surface to the reinforcement in the more highly compressed face depth of concrete in compression effective depth for a solid slab or rectangular beam; otherwise the overall depth of the compression flange effective depth to tension steel in prestressed member



dt

d2 Ec c (El) e ec ex

Fbst Fbt f fcav fci fcp fcu fpb fpe fpt fpu fs fs2 ft fy fyc fyL fyv h hf hmax hmin hw hx hy I K

k kt kl


effective depth from the extreme compression fibre to either the longitudinal bars or the centroid of the tendons, around which the stirrups pass, whichever is the greater depth from the surface to the reinforcement in the other face static secant modulus of elasticity of concrete flexural rigidity of the column eccentricity, edge distance, or cross-section base of napierian loagarithms eccentricity to compression face resultant eccentricity of load at right-angles to plane of wall, or as defined in Figure B5 tensile bursting force tensile force due to ultimate loads in a bar or group of bars stress average compressive stress in the flexural compressive zone concrete strength at (initial) transfer compressive stress at the centroidal axis due to prestress characteristic or worst credible concrete cube strength tensile stress in tendons at failure effective prestress (in tendon) stress due to prestress characteristic or worst credible strength of prestressing tendons reinforcement stress stress in reinforcement in other face maximum principal tensile stress; tensile strength of reinforcement characteristic or worst credible strength of reinforcement assessment compressive strength of longitudinal steel characteristic, or worst credible, strength of the longitudinal reinforcement characteristic of worst credible strength of shear reinforcement overall depth (thickness) of section (in plane of bending or buckling) thickness of flange larger dimension of section smaller dimension of section wall thickness overall depth of the cross-section in the plane of bending Miy overall depth of the cross-section in the plane of bending Mix second moment of area a factor depending on the type of duct or sheath used, the nature of its inside surface, the method of forming it and the degree of vibration employed in placing the concrete a constant (with applicable subscripts) depends on the type of tendon depends on the concrete bond across the shear plane




Ls le lex ley lo

length of shear plane effective height of a column or wall effective height for bending about the major axis effective height for bending about the minor axis clear height of column between end restraints

LMt Mcr Mg Mi Mix

transmission length bending moment due to ultimate loads cracking moment at the section considered moment due to permanent loads maximum initial moment in a column due to ultimate loads initial moment about the major axis of a slender column due to ultimate loads initial moment about the minor axis of a slender column due to ultimate loads twisting moment per unit length in a slab adjacent to the edge zone referred to axes perpendicular (n) and parallel (t) to the edge moment due to live loads total moment about the major axis of a slender column due to ultimate loads total moment about the minor axis of a slender column due to ultimate loads ultimate resistance moment ultimate moment capacity in a short column assuming ultimate axial load and bending about the major axis only ultimate moment capacity in a short column assuming ultimate axial load and bending about the minor axis only moments about the major and minor axes of a short column due to ultimate loads moment necessary to produce zero stress in the concrete at the depth d smaller initial end moment due to ultimate loads (assumed negative if the column is bent in double curvature) larger initial end moment due to ultimate loads (assumed positive) ultimate axial load at section considered; number of bars in a group ultimate resistance axial load

Miy Mnt Mq Mtx Mty Mu Mux Muy Mx, My Mo M1 M2 N Nu Nuz nw Pf Ph Pk Po Px

axial loading capacity of a column ignoring all bending ultimate axial load per unit length of wall effective prestressing force after all losses horizontal component of the prestressing force after all losses basic load in tendon prestressing force in the tendon at the jacking end (or at tangent point near jacking end) prestressing force at distance x from jack




Page 9 of 100

QA Qk r rps SA

assessment load nominal load internal radius of bend radius of curvature of a tendon assessment load effects

ssL v T Tu V Vc Vco Vcr Vi V1 Vs Vt

vtu x x1 y

spacing of of shear longitudinal reinforcement spacing links along the member torque due to ultimate loads ultimate torsional strength shear force due to ultimate loads ultimate shear resistance of concrete ultimate shear resistance of a section uncracked in flexure ultimate shear resistance of a section cracked in flexure shear capacity of infill concrete longitudinal shear force due to ultimate load shear resistance of shear reinforcement flexural shear force per unit width at the edge acting on a vertical plane perpendicular to the edge ultimate shear resistance of section ultimate shear capacity of a section for the X-X axis ultimate shear capacity of a section for the Y-Y axis applied shear due to ultimate loads for the X-X axis applied shear due to ultimate loads for the Y-Y axis shear stress ultimate shear stress in concrete (Halving joint) ultimate shear stress in concrete ultimate longitudinal shear stress per unit area of contact surface torsional shear stress minimum ultimate torsional shear stress for which reinforcement is required ultimate torsional shear stress neutral axis depth smaller centre line dimension of a link distance of the fibre considered in the plane of bending from the centroid

yo ypo y1 z α αn

of the concrete section half the side of end block half the side of loaded area larger centre line dimension of a link lever arm inclination of shear reinforcement; factor to determine fpb coefficient as a function of column axial load

Vu Vux Vuy Vx Vy v vu vc v1 vt vtmin



α1 α2 β γf1,γf2,γf3 γγfL m γmb γmc γmcw γms γmv ε εm εs ε1 µ ξs ρ ρnet ΣAsv Σbd φ


angle between the normal to the axis of the applied moment and the direction of the tensile reinforcement angle of friction at the joint coefficient dependent on bar type partial factors product of γf1,γf2 partial factor for material strength partial factor for bond strength partial factor for concrete strength partial factor for plain concrete wall strength partial factor for steel strength partial factor applied to vc strain average strain, cracking strain strain in tension reinforcement strain at level considered coefficient of friction depth factor geometrical ratio of reinforcement equal to As/bd area of transverse reinforcement in the flange as a percentage of the minimum flange area area of shear reinforcement area of the critical section size (nominal diameter) of bar or tendon)

3B. LIMIT STATE PHILOSOPHY 3.1B General

Structures should be assessed using limit state principles for the ultimate limit state and, where required, the serviceability limit state. In general, where a Bridge has been inspected in accordance with Section 3 and no serviceability failures are apparent, it will not be necessary to assess cracking and stress limits at the Serviceability Limit State. Where damage is apparent, serviceability assessments should be considered in order to investigate the cause of observed damage. Serviceability assessment may be required where the Bridge is being assessed for a class of loading that it has not previously experienced. This requirement should be agreed with Railtrack Director’s Nominee.




However, if the structure does not meet serviceability requirements, it will not necessarily preclude the application of a proposed more severe loading. The likely consequences of the ‘failures’ should be considered. Where consideration is given to keeping prestressed structures in service that do not meet the Class 3 criteria of BS 5400: Part 4 Clause 6.3.2.4, the crack widths may be calculated using the requirements applied load. for reinforced concrete sections treating the estimated prestress as an 3.2B Deflection

The live load deflection should not be such as to significantly affect the trains using the Bridge. Where checks are required, criteria should be agreed with the Railtrack Director’s Nominee. 4B. GENERAL 4.1B Limit State Criteria

4.1.1B Serviceability Limit States

Under serviceability loads the Bridge should not suffer local damage that would shorten its intended life or incur excessive maintenance costs. When a serviceability limit state assessment is required, the predicted stresses and crack widths should be checked against the criteria given in BS 5400: Part 4 except that the characteristic stresses used may be those given in Clause 4.3.1B (a) of this Appendix. 4.1.2B Ultimate Limit States

The strength of the structure should be sufficient to withstand the assessment loads, so that collapse will not occur as a result of rupture of one or more critical sections, by overturning or by buckling caused by elastic or plastic instability, having due regard to the effects of sway when applicable. The effects of creep and shrinkage of concrete, temperature difference and differential settlement need not be considered at the ultimate limit state.




Page 12 of 100

4.1.3B Other Considerations

When other effects (such as deflection vibration, fatigue and durability) are to be considered, applicable criteria should be agreed with the Railtrack Director’s Nominee. 4.2B Loads, Load Combinations and Partial Factors γfL and γf3

4.2.1B Loads

The nominal values of loads and load combinations are given in Sections 2 and 4. 4.2.2B Serviceability Limit State

Where serviceability assessments are required applicable loading criteria should be agreed with the Railtrack Director’s Nominee. Partial factors, γfL, for the serviceability limit state from Section 2 should be used. 4.2.3B Ultimate Limit State

The values of the partial factor γfL are given in Section 2. In calculating the resistance of members to vertical shear and torsion, γfL for the prestressing force should be taken as 1.15 where it adversely affects the resistance and 0.87 in other cases. In calculating secondary effects in statically indeterminate structures, γ for the fL prestressing force may be taken as 1.0. The value of γf3 should be taken as 1.1 except as stated in Section 2. 4.3B Properties of Materials

4.3.1B General

Either the characteristic strength, or the worst credible strength may be used for a material strength. In general, in analysing a structure to determine load effects, the material properties applicable to the characteristic, or worst credible, strength should be used, irrespective of the limit state being considered. For the analysis of sections, the material properties to be used for the individual limit states should be as follows: (a)

Serviceability limit state and deflection - in th e absence of tests, the elastic modulus for concrete and steel given in Clauses 4.3.2.1B and 4.3.2.2B respectively. The characteristic stress of concrete in compression may be taken as 0.5fcu. Where tests are undertaken, the characteristic stress of reinforcement may be taken as the 0.2% proof stress. In the absence of tests, it may be taken as 0.75fy but not less than 225N/mm² where fy is greater than 250N/mm² and 0.9fy where fy is less than 250N/mm².



(b)


Ultimate limit state - the values given in Clause 4.3.2B.

4.3.2B Material Properties 4.3.2.1B Concrete

In assessing the strength of sections themay ultimate limitusing state, the stress-strain for normal weight concrete in FigureatB1 be taken the value of γmc for curve concrete given in Clause 4.3.3.3B. Guidance on strengths of historical concrete grades can be found in Appendix B1. The modulus of elasticity, Ec , to be used for elastic analysis should be applicable to the cube strength of the concrete, and, in the absence of test data, the short term value should be taken as (20 + 0.27 fcu) kN/mm² with fcu in N/mm² units. The effect of creep under long term loading may be allowed for by using half of the short term modulus of elasticity. For lightweight concrete having an air dry density between 1400 kg/m³ and 2300 kg/m³, the values given in the previous paragraph should be multiplied by (Dc/2300)² where Dc is the density of the lightweight aggregate concrete in kg/m³. Poisson's ratio may be taken as 0.2. The value for the coefficient of thermal expansion may be taken from BD 21/97: The Assessment of Highway Bridges and Structures, Table 4/3. 4.3.2.2B Rein f or cement and Prestr essing Steel

The stress-strain curves may be taken as follows: (i)

For reinforcement: Figure B2, using the value of γms given in Clause 4.3.3B and modulus of elasticity 200 kN/mm²;

For prestressing steel: Figure B3 or Figure B4, using the value of γms given in Clause 4.3.3B. and the modulus of elasticity from Figure B3 or Figure B4 as applicable for tangent modulus at zero load. Alternatively where the reinforcement or tendon type is known, the manufacturer’s (ii)

stress-strain curves may be used with the values of γms given in Clause 4.3.3B.




Page 14 of 100

0.67 × f cu

γm Parabolic curve

Stress f (N mm2 )

5.5

f cu

γm Strain

(k N mm 2 )

ε

0.0035 f cu

2.44 × 10 − 4

γm

NOTE 1

0.67 × f cu takes account of the ratio between the cube strength and the bending strength in a f lexural member.

NOTE 2

The equation for the parabolic curve between

ε = 0 and 2.44 ×10 −4

f cu

γm

maybetakenas

f

 50002  2  f  ε =  5000 cu ε −  γm   2.68  

Figure B1 Short Term Stress-Strain Curve for Normal Weight Concrete for Assessment fy

γm

0.8 × f y fy

γm

 f  γ m +  y   2000 

Stress f (N mm2 )

200 (k N mm2 ) 0.002

Strain

ε

Figure B2 Short Term Stress-Strain Curve for Reinforcement for Assessment




Page 15 of 100

f pu

γm 0.8 f pu

γm Stress f (N/mm²)

200 kN/mm² for wire and st rand to BS 5896 sections 2 and 3 165 kN/mm² for alloy bars to BS 4486 and 19-wire strand to BS 4757 section 3.

0.005

Strain e

Figure B3 Short Term Stress-Strain Curve for Normal and Low Relaxation Prestressing Steel for Assessment pu

γ

m

0.6f pu

γm

Stress f (N/mm²)

200 kN/mm² to BS 5896 175k N/mm² for 19-wire strand to BS 4757 section 2.

0.005

Strain e

Figure B4 Short Term Stress-Strain Curve for As Drawn’ Wire and As Spun’ Strand for Assessment




Page 16 of 100

4.3.3B Values of γm 4.3.3.1B G ener al

For the analysis of sections, the values of γm are given in Clauses 4.3.3.2B to 4.3.3.4B 4.3.3.2B Ser viceabil i ty L im it State

Where stress checks at serviceability limit state are required, the values for γm from Table B1 should be used with the characteristic stresses given in Clause 4.3.2B above. Type of Construction Material

TypeofStress

Reinforced Concrete

Prestressed Concrete

Concrete

Triangularorneartriangular compressive distribution (e.g. due to bending)

1.00

1.25

Uniform or nearuniform compressive

1.33

1.67

stress due todistribution axial loading(e.g. Tension

Reinforcement

Prestressingtendons

1.25 pre-tensioned 1.55 post-tensioned

Compression

1.00

Tension

1.00

Tension

Notrequired

Table B1 Values of

γm at the Serviceability Limit State

4.3.3.3B Ul timate L imi t State

For both reinforced concrete and prestressed concrete, the values of γm applied to either the characteristic strengths or worst credible strengths are summarised in Table B2.




Page 17 of 100 Value for use with

Application

Symbol

Characteristic Strength

Worst Credible Strength

Reinforcement and prestressing tendons

γms

1.15

1.10*

Concrete Shear in concrete

γmc γmv

1.50 1.25

1.20 1.15

Bond

γmb

1.4

1.25

Plain concrete wall

γmcw

2.25

1.80

Table B2 Values of

γm at the Ultimate Limit State

* May be reduced to 1.05 if measured steel depths are used in addition to the worst credible steel strength 4.3.3.4B F atigue

When applying Clause 4.7B, the values of γms applied to a reinforcement stress range should be 1.00. 4.4B Analysis of Structure

4.4.1B General

The requirements for methods of analysis applicable to the determination of the distribution of forces and deformations that should be used in ascertaining that the limit state criteria are satisfied are described in Sections 2 and 4. 4.4.2B Analysis for Serviceability Limit State 4.4.2.1B G ener al

Elastic methods of analysis should be used to determine internal forces and deformations. The flexural stiffness constants (second moment of area) for sections of discrete members or unit widths of slab elements may be based on any of the following: a) concrete section - the entire member cross-section, ignoring the presence of reinforcement; b)

gross transformed section - the entire member cross-section including the reinforcement, transformed on the basis of modular ratio;




c)

Page 18 of 100

net transformed section - the area of the cross-section that is in compression together with the tensile reinforcement, transformed on the basis of modular ratio.

A consistent approach should be used which reflects the different behaviour of various parts of the structure. It is, however, acceptable to use properties which are intermediate between (a) and (b) when it is considered this will give a more realistic representation of behaviour. Axial, torsional and shearing stiffness constants, when required by the method of analysis, should be basedon the concrete sectionand used with (a) or(b). Reduced torsional stiffnesses may be used when applicable in accordance with Clause 5.3.4.2B or to achieve compatibility with (c). Values of moduli of elasticity and shear moduli should be applicable to the characteristic, or worst credible strength of the concrete. 4.4.2.2B M ethods of An alysi s and their Requir ements

The method of analysis should take account of all significant aspects of behaviour that govern the structure’s response to loads and imposed deformations. 4.4.3B Analysis for Ultimate Limit State Elastic methods may be used to determine the distribution of forces and deformations throughout the structure. Stiffness constants may be based on any of those determined in accordance with Clause 4.4.2.1B. The torsional stiffness may be reduced when Clause 5.3.4.2B applies. Other constants may alsobe adjusted to give some allowance for redistribution where this will give a more realistic representation of behaviour. Non-linear and plastic methods of analysis (e.g. plastic hinge methods for beams, or yield line methods for slabs) may be used may be used with the agreement of the Railtrack Director’s Nominee. 4.5B Analysis of Section

4.5.1B Serviceability Limit State

An elastic analysis should be carried out. In-plane shear flexibility in concrete flanges (shear lag effects) should be taken into account, by taking an effective width of flange in accordance with Clause 5.3.1.2B.




4.5.2B Ultimate Limit State

The strength of critical sections, and whether the requirement of Clause 4.1.2B is satisfied, should be assessed in accordance with Clauses 5B, 6B or 7B. In-plane shear flexibility in concrete flanges (shear lag effects) may be ignored. 4.6B Deflection Deflection should be calculated for the most unfavourable distributions of loading for the member (or strip of slab) and may be derived from an elastic analysis of the structure. The material properties, stiffness constants and calculations of deflections may be based on the information given in Clause 4.3.2.1B or BS 5400: Part 4 Appendix A. 4.7B Fatigue

When required, the effect of repeated live loading on the fatigue strength of a Bridge should be assessed. For reinforcing bars that are known or suspected to have been subjected to welding, details of compliance criteria are given in BS 5400: Part 10 as implemented by BD 9/81: Implementation of BS 5400: Part 10 (1980) Code of Practice for Fatigue. For unwelded non-corroded reinforcement, the fatigue life should be determined in accordance with BS 5400: Part 10 as implemented by BD 9, using the following parameters for the σr - N relationship: bars ≤ 1 6 mm diameter; m = 9 , k2 = 0.75 x 1027 bars > 16 diameter; m = 9 , k2 = 0.07 x 1027 The fatigue life of corroded reinforcement should be determined in accordance with BA 38/93: Assessment of the Fatigue Life of Corroded or Damaged Reinforcing Bars. 4.8B Combined Global and Local Effects

4.8.1B General

In addition to the assessment of individual primary and secondary elements to resist loading applied directly to them, it is also necessary to consider the loading combination that produces the most adverse effects due to global and local loading where these coexist in an element. 4.8.2B Analysis of Structure

Analysis of the structure may be accomplished either by one overall analysis (e.g. using finite elements) or by separate analyses for global and local effects. In the latter case the forces and moments acting on the element from global and local effects should be combined as applicable.




4.8.3B Analysis of Section

Section analysis for combined global and local effects should be carried out in accordance with Clause 4.5B to satisfy the requirements of Clause 4.1B: a)

b)

Serviceability limit state (i)

For reinforced concrete elements, the total crack width due to combined global and local effects should be determined in accordance with Clause 5.8.7B.

(ii)

For prestressed concrete elements, co-existent stresses, acting in the direction of prestress, may be added algebraically in checking the stress limitations.

Ultimate limit state The resistance of the section to the combination of local and global effects should be checked using the assumptions given in Clauses 5.3.2.1B or 6.3.3.1B as applicable allowing for the axial force. For a deck slab, however the resistance to the combined global and local effects may be deemed to be satisfactory if the axial force from the global effects is checked separately from the resistance to moments.

5B. REINFORCED CONCRETE 5.1B General

5.1.1B Introduction

Methods of assessment are given below that will in general ensure that, for reinforced concrete structures, the criteria set out in Clauses 4.1.1B and 4.1.2B are met. Other methods may be used with the approval of Railtrack Director’s Nominee. In certain cases the assumptions made below may not be applicable and the Assessing Engineer should adopt a more suitable method having regard to the nature of the structure to be assessed. 5.1.2B Limit State Assessment of Reinforced Concrete 5.1.2.1B Basi s of Assessment

The limit state philosophy as set out in Clauses 3B and 4B should be adopted. For most structures assessment is needed only at the Ultimate Limit State. Assessment for Serviceability Limit State deflection or fatigue is required only when specifically requested by Railtrack Director’s Nominee.




5.1.2.2B Durability

In Clause 5.8.2B guidance is given on the nominal cover to reinforcement that is necessary to provide adequate durability. In an existing structure the actual cover may be more or less than recommended for design. Assessment for durability should be based on the actual conditions found during inspection. 5.1.3B Loads

The assessment load effects(see Section 2) for the ultimate and serviceability limit states are referred to as ‘ultimate loads’ and ‘service loads’ respectively. The values of the ‘ultimate loads’ and ‘service loads’ that should be used in assessment are derived from Section 4. When analysing sections, the terms ‘strength’, ‘resistance’ and ‘capacity’ are used to describe the assessment resistance of the section in accordance with Section 2. 5.1.4B Strength of Materials 5.1.4.1B Defin iti on of Strengths

The assessment strengths of concrete and reinforcement are given by fcu/γmc and fy/γms, respectively where γmc and γms are the partial factors for material strength given in Clause 4.3.3B. The applicable value of γmc or γms should be substituted in all equations in Clause 5B. 5.1.4.2B Str ength of Concrete

Assessment may be based on either the srcinal specified characteristic cube strength, or the worst credible cube strength assessed from the estimated in-situ cube strength in accordance with BA 44: The Assessment of Concrete Highway Bridges and Structures. 5.1.4.3B Str ength of Re in for cement

Assessment may be based on either the srcinal specified characteristic yield or proof stress, or the worst credible yield or proof stress assessed from tests on reinforcement samples extracted from the structure. 5.2B Structures and Structural Frames

5.2.1B Analysis of Structures

Structures should be analysed in accordance with the requirements of Clause 4.4B. 5.2.2B Redistribution of Moments

Redistribution of moments obtained by rigorous elastic analysis under the ultimate limit state may be carried out provided the following conditions are met:




(a)

Page 22 of 100

checks are made that adequate rotational capacity exists at sections where moments are reduced, making reference to applicable test data, and in the absence of a special investigation, the plastic rotation capacity may be taken as the lesser of: dc de ) ; 0.08 + 0.0035(0.5 −or 0.6φ but not less than 0 d − dc where: dc de

φ d

is the calculated depth of concrete in compression at the ultimate limit state; is the effective depth for a solid slab or rectangular beam, otherwise the overall depth of the compression flange; is the diameter of the smallest tensile reinforcing bar; is the effective depth to tension reinforcement.

(b)

proper account is taken of changes in transverse moments and transverse shears consequent on redistribution of longitudinal moments;

(c)

shears and reactions used are those calculated either prior to or after redistribution, whichever are the greater.

5.3B Beams 5.3.1B General 5.3.1.1B Ef fecti ve Span

The effective span of a simply supported member should be taken as the smaller of: (a)

the distance between the centres of bearings or other supports;

(b)

the clear distance between supports plus the effective depth;

(c)

for members resting directly on masonry, concrete or brick, the distance between the centroids of the bearing pressure diagrams. In this case, the bearing pressure diagrams should be determined by assuming that the reaction is distributed linearly from a maximum at the front edge of the support to zero at the back of the bearing area. The length of the bearing area should not be taken as greater than the depth of the beam where the




support is of soft stone, or one-quarter of the depth of the beam where the support is of hard stone such as granite or good quality concrete. The effective span of a member framing into supporting members should be taken as the distance between the shear centres of the supporting members. The effective span of a continuous member should be taken as the distance between centres of supports except where, in the case of beams on wide columns, the effect of column width is included in the analysis. The effective length of a cantilever should be taken as its length from the face of the support plus half its effective depth except where it is an extension of a continuous beam when the length to the centre of the support should be used. 5.3.1.2B Ef f ective Wi dth of F lan ged Beams

In analysing structures, the full width of flanges may be taken as effective. In analysing sections at the serviceability limit state, and in the absence of any more accurate determination (such as that given in Section 5 and BS 5400: Part 3), the effective flange width should be taken as the width of the web plus one-tenth of the distance between the points of zero moment (or the actual width of the outstand if this is less) on each side of the web. For a continuous beam the points of zero moment may be taken to be at a distance of 0.15 times the effective span from the support. 5.3.1.3B Sl endern ess L im i ts f or B eams

Adequate lateral stability of a simply supported or continuous beam is generally present where the beam is so proportioned that the clear distance between lateral restraints does not exceed 300 bc²/d, where d is the effective depth to tension reinforcement and bc is the breadth of the compression face of the beam midway between restraints. Similarly, for cantilevers with lateral restraint provided only at the support, the clear distance from the end of the cantilever to the face of the support should not exceed 150 bc²/d. 5.3.2B Resistance Moment of Beams 5.3.2.1B An al ysi s of Secti ons

When analysing a cross-section to determine its ultimate moment of resistance, the following assumptions should be made: (a)

The strain distribution in the concrete in compression and the compressive and tensile strains in the reinforcement are derived using the assumption that plane sections remain plane;



(b)


the stress-strain curve in Figure B1 with the applicable value of γmc given in Clause 4.3.3.3B applies for the derivation of the stresses in the concrete in compression or, for rectangular sections and flanged, ribbed and voided sections where the neutral axis lies within the flange. The compressive stress should be taken as equal to outermost 0.6 fcu/γmc over the wholefibre compression both cases the strain at the compression at failure zone. is takenInas 0.0035;

(c)

the tensile strength of concrete is ignored;

(d)

the stresses in the reinforcement are derived from either the stress-strain curves in Figure B2 or, when available, manufacturers' stress-strain curves. The values of γms are given in Clause 4.3.3.3B.

In the analysis of a cross-section of a beam that has to resist a small axial thrust, the effect of the ultimate axial force may be ignored if it does not exceed 0.1 fcu times the cross-sectional area. 5.3.2.2B Desi gn Char ts

For the analysis of beams reinforced in tension only or in tension and compression, the design charts that form CP 110: Parts 2 and 3 (based on Figure 1, Figure 2 and the assumptions of Clause 5.3.2.1B) may be used with applicable modifications for the value of γm, which is defined in Clause 4.3.3B. 5.3.2.3B Assessment F or mu l ae

The following formulae may be used to calculate the ultimate moment of resistance of a solid slab or rectangular beam, and for a flanged beam, ribbed slab or voided slab when the neutral axis lies within the flange. For sections without compression reinforcement the ultimate moment of resistance may be taken as the lesser of the values obtained from Equations B1 and B2. Equations B3 and B4 may be used for sections with compression reinforcement. A rectangular stress block of maximum depth 0.5d and a uniform compression stress of 0.6 fcu/γmc have been assumed. Mu =

(f y γms )As z

Mu =

(0.225f cu

Mu =

(0.60 f cu γ()mc bx) d −(0.5)x

Equation B1

γ mc )bd 2

Equation B2

+ f s ′ As′ d − d ′

Equation B3




Page 25 of 100

(f y γ ms )As = (0.6f cu γ mc )bx + f s′ + A′s

Equation B4

where Mu As

is the ultimate resistance moment; is the area of tension reinforcement;

As ′

is the area of compression reinforcement;

b d d′ fy

is the width of the section; is the effective depth to the tension reinforcement; is the depth to the compression reinforcement; is the characteristic, or worst credible strength, of the reinforcement; fy

f s′ = x z f cu

γms + f y

2000

is the depth of the neutral axis; is the lever arm; is the characteristic, or worst credible strength of the concrete;

γ mc , γ ms

are the material partial factors given in Clause 4.3.3.3B.

The lever arm, z, in Equation B1 may be calculated from Equation B5: z

=

1− 0.84(f y γms )As d  (f γ )bd  cu ms

butnotgreaterthan0.95

d

Equation B5

When using Equations B3 and B4 for sections with compression reinforcement, the neutral axis depth x should first be calculated from Equation B4. If d ′ ≤ 0.429 x the ultimate resistance moment should be determined from Equation B3. If d ′ > 0.429 x either the compression steel should be ignored and the section treated as singly reinforced or the ultimate resistance should be determined using Clauses 5.3.2.1B or 5.3.2.2B as applicable. The ultimate resistance moment of a flanged beam may be taken as the lesser of the values given by Equations B6 and B7, where hf is the thickness of the flange. Mu =

 

(f y γms )As  d −

hf 2

  

Equation B6




Mu =

(0.6f cu

Page 26 of 100

h γmc )bhf  d − f   2

Equation B7

5.3.3B Shear Resistance of Beams 5.3.3.1B Shear Stress

The shear stress, v in N/mm², at any cross-section should be calculated from Equation B8. v

=

V bw d

Equation B8

where: V bw

is the shear force due to ultimate loads; is the breadth of the section which, for a flanged beam, should be taken as the

rib width; is the effective depth to tension reinforcement. d In no case should v exceed 0.92 f cu γmc or 7 γmc (where

γmc

is the partial factor

for concrete given in Clause 4.3.3.3B), whichever is the lesser, whatever shear reinforcement is provided. 5.3.3.2B Shear Capacit y

Shear reinforcement in the form of vertical links, inclined links or bent-up bars should only be considered effective in resisting shear if the spacing of the legs of links in the direction of the span and at the right angles to it, does not exceed the effective depth, d, and where: Asv (sinα + cosα )(f yv

γ ) ≥ 0.2b ms

w

s v and α ≥ 30°

Equation B9

where: Asv

is the cross-sectional area of shear reinforcement at a particular cross-section;

sv bw

is the spacing of the shear reinforcement along the member; is the breadth of the section which, for a flanged beam, should be taken as the

α f yv

rib width; is the inclination of the shear reinforcement to the axis of the member; is the characteristic, or worst credible, strength of the shear reinforcement

γms

but not greater than 480 N/mm²; is the partial factor for material strength of steel given in Clause 4.3.3.3B.




In the absence of effective shear reinforcement, the ultimate shear resistance, Vu of a section is given by: Vu

=

ξ s v c bw d

Equation B10

Where effective vertical links are present, the ultimate shear resistance Vu of a section is given by: Vu

=

ξ s v c bw d + (f y γms )

d Asv sv

Equation B11

For vertical links to be effective, the tensile capacity of the longitudinal reinforcement at a section, As f y γms , should be greater than: M (V − ξ s v c b w d ) + z 2 M, V z

Expression B12

are the co-existent ultimate bending moment and shear force at the section under consideration; is the lever arm which may be taken as 0.9 d;

Within an individual sagging or hogging region however, the assessed tensile capacity need not exceed Mmax z , where Mmax is the maximum ultimate moment within that region. In the above equations/expressions:

ξs =

 550   d   

0.25

but m 0.7;

v c is the ultimate shear stress in concrete and should be calculated from: 1

vc

=

1 3 0.24  100 As  (f cu ) 3 γmv  bw d 

In the Equation B13 the term 100 than 3.0.

Equation B13

As bw d

should not be taken less than 0.15 or greater




γmv

Page 28 of 100

is a partial factor defined in Table B2.

The term As is that area of longitudinal tension reinforcement which continues at least a distance equal to the effective depth, d, beyond the section being considered. At supports however the full area of tensile reinforcement may be used provided, the requirements of Clause 5.8.6B are met. Where both top and bottom reinforcement are provided, the area of As used should be that which is in tension under the loading that produces the shear force being considered. Sections within a distance d from the support generally need not be assessed for shear providing the shear reinforcement calculated for the section at distance d is continued up to the support and the anchorage requirements of Clause 5.8.6B are met. Where, however, more than half the total shear force on the member is due to load applied within a distance d of the support, sections within d of the support should be assessed. Inclined links or bent up bars should be assumed to form the tension members of one or more single systems of lattice girders in which the concrete forms the compression members. The maximum stress in any link or bar should be taken as f yv . Bent-up bars should be checked for anchorage and bearing in accordance with

γ ms

Clause 5.8.5B. 5.3.3.3B En han ced Shear Strength of Secti ons Cl ose to Suppor t

If the main reinforcement continues to the support and is provided with an effective anchorage equivalent to 20 times one bar size, an enhancement of shear strength may be allowed for at sections within a distance a v ≤ 3d from the face of a support, the front edge of a rigid bearing or the centre line of a flexible bearing. This enhancement should take the form of an increase in the allowable shear stress, ξ s v c to ξ s v c 3d a v but should not exceed 0.92 f cu γmc or 7 γmc whicheveristhe lesser. Where the anchorage at a simply supported end is less than required above but not less than 2.5 bar diameters, the enhancement may still be used but with the effective steel area, As used to calculate v c taken as actual area times (anchorage length/20 bar diameters) for plain round bars or actual area times (anchorage length/12 diameters) for deformed bars. However, in this case, the actual steel area may be used in the formula for v c in Clause 5.3.3.2B even if less than 0.15%.




In the above, for cases where the simply supported end is resting directly on the abutment, the centre of the support may be taken as the centroid of the bearing pressure diagram considered in Clause 5.3.1.1B(c). Where this Clause gives a lower shear strength than Clauses 5.3.3.1B and 5.3.3.2B, the greater value should be used. 5.3.3.4B Bot tom L oaded Beams

Where vertical load is applied near the bottom of a section, sufficient vertical reinforcement to carry the load to the top of the section should be present in addition to any reinforcement required to resist shear. 5.3.3.5B Al ter nati ve Appr oach

As an alternative to the method given in Clause 5.3.3.1B and Clause 5.3.3.2B, sections with links may be assessed using the varying angle truss approach. The shear strength Vu is the lesser value obtained from Equations B14(a) and B14(b) for elements with vertical links. For elements with inclined links the shear strength Vu is the lesser value obtained from Expressions B14(c) and B14(d). 0.9(f

)(d s )A cot yv

γms

v

0.72bw dv (f cu (γ)mc 0.9(f yv

γms )(d )(sv

Expression B14(a)

sv

θ cot) θ + tanθ

Asv cot)θ + cotα sinα

0.72bw dv (f cu γ)(mc cotθ +)cotα (1+ cot 2 θ )

Expression B14(b) Expression B14(c) Expression B14(d)

where:

θ v

is the angle of the assumed concrete struts to the horizontal taken such that cotθ lies in the range 0.4 to 2.5 for members with constant reinforcement and 0.5 to 2.0 for members with curtailed reinforcement; is the effectiveness factor taken as: 0.7 −

f cu ≥ 0.5 250

Other symbols in Expressions B14(a) to B14(d) have the same meaning as in Clauses 5.3.3.1B and 5.3.3.2B except that Asv should not be taken as greater than:

(0.4bw sv vf cu sinα ) (f yv γmc )withverticallinksand;




[(0.4bw sv vf cusinα ) (f yv γmc )] sinα (1− cosα )

Page 30 of 100

with inclined links.

If this approach is used, the flexural reinforcement at any section is capable of resisting the bending moment at a distance 0.9d (cotθ − cotα ) 2 in the direction of increasing moment from the section considered provided effective anchorage is present in accordance with Clause 5.8.5.2B. However, at a simply supported end, the bond stress in the length of reinforcement immediately over the bearing may be taken as 1.5 times that given in Clause 5.8.5.2B. 5.3.4B Torsion 5.3.4.1B G ener al

In some members the maximum torsional moment does not occur under the same loading as the maximum flexural moment. In such circumstances reinforcement in excess of that required for flexure and other forces may be considered in the assessment of torsional resistance. 5.3.4.2B Tor sion l ess Systems

In general, where the torsional resistance or stiffness of members has not been taken into account in the analysis of the structure, no specific calculations for torsion are necessary. However, it is essential that sound engineering judgement has shown that torsion plays only a minor role in the behaviour of the structure, otherwise torsional stiffness should be used in analysis. 5.3.4.3B Stresses and Rein f or cement

Where torsion in a section substantially increases the shear stresses, the torsional shear stress should be calculated assuming a plastic stress distribution. Where the torsional shear stress, vt, exceeds the value vtmin, torsion reinforcement should be present, where: v t min = 0.082 f cu

γ mc

In no case should the sum of the shear stresses resulting from shear force and torsion (v + vt) exceed the value of the ultimate shear stress, v tu , nor, in the case of small sections ( y1 <550 mm), should the torsional shear stress, vt,exceed v tu y1 550 , where y1 is the larger centre line dimension of a link and, v tu =

0.92 f cu

γmc

but not greater than 7

γmc

.




Page 31 of 100

Torsion reinforcement should consist of rectangular closed links in accordance with Clause 5.8.5.4B together with longitudinal reinforcement. Only reinforcement in excess of that necessary to resist shear or bending should be considered as torsion reinforcement. Torsional capacity should be calculated that by thelongitudinal closed linksand form a thinwalled tube, the shear stresses in which assuming are balanced transverse forces provided by the resistance of the reinforcement. 5.3.4.4B Tr eatment of Var i ous Cr oss-sections

(a)

Box sections The ultimate torsional strength (Tu) should be taken as the greater of: Tu =

2 Ao

 AsL (f yL γ ms ) Ast (f yv γ ms )  2( x + y )   sv    1 1

Equation B15

and Tu =

2hw Ao v t min

Equation B16

where: Ao

is the area enclosed by the median wall line;

Ast

is the area of one leg of a closed link of a section;

AsL

is the area of one bar of longitudinal reinforcement;

f yv

is the characteristic, or worst credible, strength of the links;

f yL

is the characteristic, or worst credible, strength of the longitudinal

hw

reinforcement; is the thickness of the thinnest wall;

sv

is the spacing of the links along the member;

y1

is the larger centre line dimension of a link;

x1

is the smaller centre line dimension of a link.

In Equation B15, f yv and f yL should not be taken as greater than 480 N/mm². In addition, the limits given in Clause 5.3.4.3B should not be exceeded by the torsional shear stress calculated from:



vt

=

T 2 hw A o

RT/CE/C/025 Issue: 1 Date: February 2001 Page 32 of 100 Equation B17

where T is the torque due to ultimate loads. (b)

Rectangular sections The ultimate torsional resistance should be taken as the greater of the values calculated from Equation B15 (with Ao taken as 0.8 x1y1 , where x1 and y1 are defined above), and Tu =

hmin 2

2

 h − hmin v  max  min 3  

Equation B18

where hmin and hmax are, respectively, the smaller and larger dimensions of the section. In addition, the limits given in Clause 5.3.4.3B should not be exceeded by the torsional shear stress calculated from: vt

2T

= hmin

(c)

2

 h − hmin   max  3  

Equation B19

T, L and I sections - Such sections should be divided into component rectangles for purposes of torsional assessment. Any division into component rectangles may be chosen which is compatible with the torsional reinforcement present. The ultimate torsional resistance of each component rectangle should be determined using Clause 5.3.4.4B(b), and the section torsional resistance taken as the sum of the torsional resistances of the component rectangles. In addition, the torsional shear stress in each component rectangle should be calculated from Equation B19 and should not exceed the limits in Clause 5.3.4.3B. A component rectangle should be treated as reinforced for torsion only if its link reinforcement ties it to its adjacent rectangles.

5.3.4.5B Detail in g

A section should be treated as reinforced for torsion only if the pitch of the closed links is less than the smaller of ( x1 + y1 ) 4 or 16 longitudinal corner bar diameters and




Page 33 of 100

the diameter of the longitudinal corner bars are not less than the diameter of the links. In areas subjected to simultaneous flexural compressive stress, the value of

∑ A sL

of

reinforcement in the compressive zone used in Equation B15 may be notionally increased by [f cav (area of section subject to flexural compression )/ (f yL where γ ms )] f cav , is the average compressive stress in the flexural compressive zone. For beams, the depth of the compression zone used to calculate the area of section subject to flexural compression should be taken as twice the cover to the closed links. 5.3.5B Longitudinal Shear

For flanged beams, the longitudinal shear resistance at the horizontal flange/web junction and across vertical sections of the flange that may be critical should be checked in accordance with Clause 7.4.2.3B. 5.3.6B Vertical Deflection of Beams

If required by the Railtrack Director’s Nominee, deflections may be calculated in accordance with Clause 4.6B. 5.3.7B Crack Control in Beams

If required by the Railtrack Director’s Nominee, flexural crack widths in beams should be calculated in accordance with Clause 5.8.7B. 5.4B Slabs

5.4.1B Moments and Shear Forces in Slabs

Moments and shear forces in slab bridges and in the top slabs of beam and slab, voided slab and box beam bridges may be obtained from a general elastic analysis, or such particular elastic analyses as those due to Westergaard or Pucher. Non-linear methods may also be used. Alternatively, Johansen's yield line method may be used to obtain the slab strength directly. When using non-elastic methods, the agreement of Railtrack Director’s Nominee should be obtained. The effective spans should be in accordance with Clause 5.3.1.1B. 5.4.2B Resistance Moments of Slabs

The ultimate resistance moment in a reinforcement direction may be determined by the methods given in Clause 5.3.2B. In assessing whether the reinforcement can resist a combination of two bending moments and a twisting moment at a point in a slab, allowance should be made for the fact that the principal moment and




reinforcement directions do not generally coincide. The resistance in all directions should therefore be checked. In voided slabs, the transverse flexural strength should be calculated allowing for the effects of transverse shear, using an analysis based on the assumption that the transverse section acts as a Vierendeel frame. 5.4.3B Resistance to In-plane Forces When checking whether reinforcement can resist a combination of two in-plane direct forces and an in-plane shear force, allowance should be made for the fact that the principal stress and reinforcement directions do not generally coincide. The resistance in all directions should therefore be checked. 5.4.4B Shear Resistance of Slabs 5.4.4.1B Shear Str ess i n Sol id Slabs: General

The shear stress, v, at any cross-section in a solid slab, should be calculated from: v=

V bd

Equation B20

where: V b d

is the shear force due to ultimate loads; is the width of slab under consideration; is the effective depth to tension reinforcement.

V should not exceed the maximum value given in Clause 5.3.3.1B for beams. The shear capacity should be assessed in accordance with Clauses 5.3.3.2B and 5.3.3.3B, with the following amendments: (a)

bw should be replaced with b in all equations;

(b)

shear reinforcement should not b e considered as effective in slabs less than 200 mm thick.




(a)

(b)

Load at middle of slab

1.5dx

1.5dx

1.5dy

n a

by

o n o c e

1.5dy y

1.5dx

1.5dy

(ii) Shortest straight line which touches loaded area

bx

ex

ex

1.5dy b

by

by

1.5dy

ey

D

Loaded area

Critical area

Load at corner of cantilever slab

(i)

bx

1.5dx

x

(c)

Load at edge of slab

bx Critical section for calculating shear resistance Vc (Critical sections (a), (b) and (c) (i) are assumed to have squared corners for rectangular and circular loaded areas)

Page 35 of 100

Critical area

Unsupported edge

As

Critical area

Unsupported edges As

x

Unsupported edges

Critical area As

x

x

Idealized mode of failure (only tension reinforcement shown) As bs Parameters used to derive Vc from Table 8 for each portion of critical section

As

3dy

As

x

As

3dy

y

x

3dy

bs

bs

S jsVc bd for

3dx

4 critical portions

As

3dy

y As

y

As

y

x

x

bs

bs

x

bs y

As

3dx

e x<3dx

0.8 S jsVc bd for 3 critical portions

3dx

e x<3d x

0.6 S jsVc bd for 2 critical portions

y

x

Shortest straight line which touches loaded area

y As

y

As

y

x

e y<3d y

3dy 3dx

Shear resistance Vc at critical section

As

bs

As NOTE A s should include only tensile reinforcement which is effectively anchored.

y

x

As

x

bs

y

The ratio of reinforcement 100As bsd should be taken as the average of the two ratios of reinforcement in the two directions.

[(

j

sx + jsy ) /2

] vc b [ ( d x + d y ) /2 ]

Figure B5 Parameters for Shear in Solid Slabs under Concentrated Loads

y




Page 36 of 100

5.4.4.2B Shear Str esses i n Soli d Slabs un der Con cent r ated L oads

The dispersal of load allowed in Section 4 should be taken to the top surface of the concrete slab only and not down to the neutral axis. The critical section for calculating shear should be taken on a perimeter 1.5d from the boundary of the loaded area, as shown in Figure B5(a) where d is the effective depth to the flexural tension reinforcement. Where concentrated loads occur on a cantilever slab or near unsupported edges, the relevant portions of the critical section should be taken as the worst case from Figure B5(a), (b) or (c). For a group of concentrated loads, adjacent loaded areas should be considered singly and in combination using the preceding requirements. The ultimate punching shear capacity, Vu , is given by: Vu =

Vc + ∑ Asv sinα (f yv

γ ) m

Equation B21

where: f yv

is the characteristic, or worst credible, strength of the shear reinforcement but not greater than 480 N/mm²;

γ ms

is the material partial factor for material strength of steel given in Clause 4.3.3B; is the area of shear reinforcement within the area between the loaded area and the critical perimeter, except for case c(ii) of Figure B5 when it is the area of shear reinforcement within a distance from the load equal to the effective depth. Shear reinforcement should however be considered to be effective only if:

∑ A sv

∑A α Vc

sv

sinα(f yv

γ ) ≥ 0.2∑ bd ms

where ∑ bd is the area of the critical section; is the inclination of the shear reinforcement to the plane of the slab; is the shear resistance of the concrete.

Vc should be taken as the sum of the shear resistances of each portion of the critical perimeter (see Figure B5). The value of 100 A s (bd ) to be used, in the calculation of v c from Clause 5.3.3.2B, should be derived by considering the effectively anchored flexural tensile reinforcement associated with each portion as shown in Figure B5. The ultimate punching shear capacity should also be checked on perimeters progressively 0.75d out from the critical perimeter. The value of Asv used in Equation B21 is the area of shear reinforcement between the perimeter under consideration and a perimeter 1.5d within the perimeter under consideration.




Page 37 of 100

If a part of a perimeter cannot, physically, extend 1.5d from the boundary of the loaded area, the part perimeter should be taken as far from the loaded areas as is physically possible and the value of v c for that part may be increased by a factor, 1.5d av where a v is the distance from the boundary of the loaded area to the perimeter actually considered. When openings in slabs and footings (see Figure B6) are located at a distance less than 6d from the edge of a concentrated load or reaction, that part of the periphery of the critical section which is enclosed by radial projections of the openings to the centroid of the loaded area should be considered as ineffective. Where a hole is adjacent to the loaded area and its greatest width is less than onequarter of the side of the loaded area or one-half of the slab depth, whichever is the lesser, the presence of the hole may be ignored. 5.4.4.3B Shear in Voi ded Slabs

The longitudinal ribs between the voids should be assessed in accordance with Clause 5.3.3B as beams to resist the shear forces in the longitudinal direction including any shear due to torsional effects. The top andglobal bottom flanges, acting as solid slabs, should be capable of resisting part of the transverse shear force proportional toeach the flange thickness. The a top flange of a rectangular voided slab should be capable of resisting the punching effect due to concentrated loads (see Clause 5.4.4.2B). Where concentrated loads may punch through the slab as a whole, this should also be checked.

Openings <6 d Loaded area Critical section

Figure B6 Openings in Slabs

5.4.5B Deflection of Slabs If required by the Railtrack Director’s Nominee, deflections should be calculated in accordance with Clause 4.6B.




Page 38 of 100

5.4.6B Crack Control in Slabs If required by the Railtrack Director’s Nominee, flexural crack widths in slabs should be calculated in accordance with Clause 5.8.7B. 5.4.7B Torsion in Slabs 5.4.7.1B Sl ab I nteri or

The assessment of regions of slabs, other than edge zones, to resist twisting moments should be carried out in accordance with Clause 5.4.2B. 5.4.7.2B Slab Edges

An edge zone of width equal to the overall depth of the slab should be capable of resisting a total shear force of (Vt b e + Mnt ) when assessed in accordance with Clause 5.3.3B, where: be

is taken as the width of the edge zone which may be assumed to be equal to the slab overall depth (h).

Vt

is the flexural shear force per unit width at the edge acting on a vertical plane perpendicular to the edge

Mnt

is the twisting moment per unit length in the slab adjacent to the edge zone referred to axes perpendicular (n) and parallel (t) to the edge.

5.5B Columns

5.5.1B General 5.5.1.1B D efi ni tions

A reinforced concrete column is a compression member whose greater lateral dimension is less than or equal to four times its lesser lateral dimension, and in which the reinforcement is taken into account when considering its strength. A column should be considered as short if the ratio

le

h in each plane of buckling is

less than 12, where in the plane of buckling under consideration: le is the effective height of the column; h is the depth of the cross-section. Otherwise the column should be considered as slender.

5.5.1.2B Ef fective H eight of Colu mn

The effective height, le , in a given plane may be obtained from Table B3 where the clear height between end restraints.

lo

is




Page 39 of 100

The values given in Table B3 are based on the following assumptions: (a) (b)

rotational restraint is at least 4 EI c lo for cases 1,2 and 4 to 6 and 8EI c lo for case 7, where EI c is the flexural rigidity of the column cross-section; lateral and rotational rigidity of bearings are zero.

Case 4 from Table B3 may be used for columns that are restrained at the base and have roller bearings at the top, provided the rollers are equipped with racks to maintain them in position. Where a more accurate evaluation of the effective height is required or where the end stiffness values are less than those values given in (a), the effective heights should be derived from first principles. The accommodation of movements and the method of articulation influences the degree of restraint developed for columns. These factors should be assessed as accurately as possible using engineering principles based on elastic theory and taking into account all relevant factors such as foundation flexibility, type of bearings, articulation system, etc. 5.5.1.3B Assessment of Str ength

Clauses 5.5.2B to 5.5.7B give methods, for assessing the strength of columns at the ultimate limit state, and are based on a number of assumptions. These methods may be used provided the assumptions are valid for the case being considered and the effective height is determined accurately. In addition, for columns subject to applied bending moments the Railtrack Director’s Nominee, may, in accordance with Clause 4.1.1B require crack widths to be calculated at the serviceability limit state. 5.5.2B Moments and Forces in Columns The moments, shear forces and axial forces in a column should be determined in accordance with Clause 4.4B, except that if the column is slender the moments induced by deflection should be considered. An allowance for these additional moments is made in the assessment requirements for slender columns set out in Clause 5.5.5B. The bases or other members connected to the ends of such columns should also be capable of resisting these additional moments.

Generally in columns with end moments the maximum and minimum ratios of moment to axial load should be considered.




Page 40 of 100 Effective Height,

Restraints Case

Idealized column and buckling mode Location

1

Rotation

Top

Full

Full

Bottom

Full

Full

Top

Full

None

Bottom

Full

Full

Top

Full

None

Bottom

Full

None

Top

None

None

Bottom

Full

Full

Top

None

None

Bottom

Full

Full

Top

None

Full

Bottom

Full

Full

Top

None

None

Bottom

Full

Full

0.85 µo

µo

3

µo

4

Elastomeric bearing

µo

5

1.4 µo

1.5 µo

µo

µo

1.0 µo

1.3 µo

µo

6

µe

0.70 µo

µo

2

7

Position

or

2.3 µo

µo

Table B3 Effective Height le for Columns

5.5.3B Short Columns Subject to Axial Load and Bending about the Minor Axis 5.5.3.1B G ener al

A short column should be assessed in accordance with the following recommendations provided that the moment at any cross-section has been increased by an additional moment caused by the actual eccentricity of the (assumed) axial load arising from construction tolerances. If the actual eccentricity has not been determined, the eccentricity should be taken as equal to 0.05 times the overall depth of the cross-section in the plane of bending, but not more than 20 mm.




Page 41 of 100

5.5.3.2B An al ysi s of Secti ons

When analysing a column cross-section to determine its ultimate resistance to moment and axial load, the following assumptions should be made: (a)

The strain distribution in the concrete in compression and the compressive and tensile strains in the reinforcement are derived using the assumption that plane sections remain plane.

(b)

The stresses in the concrete in compression are either derived from the stress-strain curve in Figure B1 with the applicable value of γ mc from Clause 4.3.3.3B, or taken as equal to 0.6f cu γ mc over the whole compression zone where this is rectangular or circular. In both cases, the concrete strain at the outermost fibre at failure is taken as 0.0035.

(c)

The tensile strength of the concrete is ignored.

(d)

The stresses in the reinforcement are derived from either the stress-strain curves in Figure B2 or, when available, manufacturers' stress-strain curves. The values of γms are given in Clause 4.3.3.3B.

For rectangular and circular columns the following assessment methods, based on the preceding assumptions, may principles be used. For other column shapes, assessment methods should be derived from first using the preceding assumptions. 5.5.3.3B Design Char ts for Rectangul ar Column s

The design charts that form CP 110: Parts 2 and 3 include charts (based on Figure B1, Figure B2 and the assumptions in Clause 5.5.3.2B) which, with applicable modifications for the value of γ m , may be used for the analysis of rectangular and circular column sections having a symmetrical arrangement of reinforcement. 5.5.3.4B Assessment For mul ae for Rectangul ar Colu mns

The formulae given in Equations B22 and B23 (based on a concrete stress of 0.6f cu γ mc over the whole compression zone and the assumptions in Clause 5.5.3.2B) may be used for the analysis of a rectangular column having longitudinal reinforcement in the two faces parallel to the axis of bending, whether that reinforcement is symmetrical or not. Both the ultimate axial load, N, and the ultimate moment, M, should not exceed the values of N u and Mu given by Equations B22 and B23 for the applicable value of d c .

(0.6 f cu

Mu =

(0.3f cu γ) mc( bd ) c h − dc

where:

γ

)bd c + f yc As1′ + f s 2 As 2

Nu =

mc

+f

yv

Equation B22

As1′ 

h

2

− d ′  − f 

s2

As 2 

h

2

− d   2

Equation B23



N M Nu Mu f cu b dc f yc

γ ms + f y

As 2

h d′ d2 fy

Page 42 of 100

is the ultimate axial load applied on the section considered; is the moment applied about the axis considered due to ultimate loads including the allowance for construction tolerance (see Clause 5.5.3B); is the ultimate axial load capacity of the section for the particular value of dc assumed; is the ultimate bending capacity of the section for the particular value of dc assumed; is the characteristic, or worst credible, cube strength of the concrete; is the breadth of the section; is the depth of concrete in compression assumed subject to a minimum value of 2d ′ ; is the assessment compressive strength of the reinforcement (in N/mm²) taken as: fy

As1′ f s2


2000

Expression B24

is the area of compression reinforcement in the more highly compressed face; is the stress in the reinforcement in the other face, derived from Figure B2 and taken as negative if tensile; is the area of reinforcement in the other face which as the resultant eccentricity of load increases and dc decreases from h to 2d´, may be considered as being in compression, inactive, or in tension; is the overall depth of the section in the plane of bending; is the depth from the surface to the reinforcement in the more highly compressed face; is the depth from the surface to the reinforcement in the other face; is the characteristic or worst credible strength of reinforcement.

5.5.4B Short Columns Subject to Axial Load and either Bending about the Major Axis or Bi-axial Bending The moment about each axis due to ultimate loads should be increased by that moment caused by the actual eccentricity, such as that arising from construction tolerances, of the (assumed) axial load. If the actual eccentricity has not been

determined, the construction tolerance eccentricity should be taken as equal to 0.03 times the overall depth of the cross-section in the applicable plane of bending, but not more than 20 mm. For square and rectangular columns having a symmetrical arrangement of reinforcement about each axis, the section may be analysed for axial load and bending about each axis in accordance with any one of the methods of assessment given in Clauses 5.5.3.2B, 5.5.3.3B or 5.5.3.4B. The following relationship should be satisfied:




αn

α  M  +  M  ≤ 1.0  M   M    n

x

y

ux

uy

Equation B25

where: M x , My are the moments about the major X-X axis and minor Y-Y axis respectively due to ultimate loads including the allowance for construction tolerances (see preceding paragraph); Mux is the ultimate moment capacity about the major X-X axis assuming an ultimate axial load capacity, Nu , not less than the value of the ultimate axial load, N ; Muy is the ultimate moment capacity about the minor Y-Y axis assuming an

αn

ultimate axial load capacity, Nu , not less than the value of the ultimate axial load, N ; is given by 0.67 +1.66 Nu Nuz but not less than 1.0 and greater than 2.0 where N uz is the axial loading capacity of a column ignoring all bending, taken as: N uz =

(0.675 f cu

γ mc )Ac + f yc Asc

Equation B26

where: f cu and f yc are as defined in Clause 5.5.3.4B; Ac is the area of concrete; Asc is the area of longitudinal reinforcement. For other column sections, assessment should be in accordance with Clause 5.5.3.2B. 5.5.5B Slender Columns 5.5.5.1B G ener al

A cross-section of a slender column may be assessed by the methods given in Clauses 5.5.3B and 5.5.4B for a short column but, in addition, account should be taken of the additional moments induced in the column by its deflection. For slender columns of constant rectangular or circular cross-section having a symmetrical arrangement of reinforcement, the column should be able to resist the ultimate axial load, N , together with the moments Mtx and Mty derived in accordance with Clause 5.5.5.4B. Alternatively, the simplified formulae given in Clauses 5.5.5.2B and 5.5.5.3B may be used where applicable in this case the moment due to ultimate loads need not be increased by the eccentricity given in Clause 5.5.3B. The minimum value of moment should be not less than the allowance given in Clause 5.5.3B.




5.5.5.2B Slender Colu mns Bent a bout a M in or Ax is

A slender column of constant cross-section bent about the minor Y-Y axis should be assessed for its ultimate axial load, N, together with the moment Mty given by: Mty =

Miy +

2

 l  1 − 0.0035l     h 1750  h    Nhx

e

e

x

Equation B27

x

where: is the initial moment due to ultimate loads, but not less than that corresponding to the allowance for eccentricity as given in Clause 5.5.3B; is the overall depth of the cross-section in the plane of bending Miy ;

Miy hx

is the effective height either in the plane of bending or in the plane at rightangles, whichever is greater.

le

For a column fixed in position at both ends where no transverse loads occur in its height the value of Miy may be reduced to: Miy

=

0.4 M1 + 0.6 M2

Equation B28

where: M1 M2

is the smaller initial end moment due to ultimate loads (assumed negative if the column is bent in double curvature); is the larger initial end moment due to ultimate loads (assumed positive).

In no case should Miy be taken as less than 0.4 M2 or such that Mty is less than M2 . 5.5.5.3B Slender Column s Bent ab out a M ajor A xi s

When the overall depth of the cross-section, hy , is less than three times the width, hx, a slender column bent about the major X-X axis should be assessed for its ultimate axial load, N, together with the moment Mtx given by: Mtx =

Mix +

2

 l  1− 0.0035l     h 1750  h    Nhy

e

x

e

Equation B29

x

where

le

Mix

is the initial moment due to ultimate loads, but not less than that corresponding to the allowance for eccentricity as given in Clause 5.5.3B; is the overall depth of the cross-section in the plane of bending Mix .

hy

and h x are as defined in Clause 5.5.5.2B;




Page 45 of 100

Where hy is equal to or greater than three times h x the column should be considered as biaxially loaded with the moment about the minor axis equal to that due to eccentricity in accordance with Clause 5.5.3B. 5.5.5.4B Slender Col umn s Bent about bo th Ax es

A slender column bent about both axes should be assessed for its ultimate axial load, N , together with the moments Mtx about its major X-X axis and Mty about its minor Y-Y axis, given by:

=

Nhy  lex  Mix + 1750  hy

  

2

Mtx

=

Nh x  ley Miy +  1750  h x

  

2

Mty

 0.0035lex 1− hy 

  

Equation B30

 0.0035ley 1− hx 

  

Equation B31

where: h x and hy are as defined in Clauses 5.5.5.2B and 5.5.5.3B respectively; Mix Miy lex ley

is the initial moment due to ultimate loads about the major X-X axis, including the allowance for eccentricity in accordance with Clause 5.5.4B; is the initial moment due to ultimate loads about the minor Y-Y axis, including the allowance for eccentricity in accordance with Clause 5.5.4B; is the effective height of the column in respect of bending about the major axis; is the effective height of the column in respect of bending about the minor axis.

5.5.6B Shear Resistance of Columns A column subject to uni-axial shear due to ultimate loads should be assessed in accordance with Clause 5.5.3B except that the ultimate shear stress, ξ s v c , may be multiplied by the enhancement factor given by:

1+

0.07 N Ac

Expression B32

where: N Ac

is the ultimate axial load (in Newtons) is the area of the entire concrete section (in mm²)

A column subjected to biaxial shear due to ultimate loads should satisfy the expression:



Vx Vux

+

Vy Vuy

≤ 1.0


Equation B33

where: are the applied shears due to ultimate loads for the major X-X axis and minor Y-Y axis respectively, Vux ,Vuy are the corresponding ultimate shear capacities of the concrete and link reinforcement for the major X-X and minor Y-Y axis respectively derived allowing for the enhancement factor. Vx ,Vy

In calculating the ultimate shear capacity of a circular column, the area of longitudinal reinforcement As to be used to calculate v c should be taken as the area of reinforcement that is in the half of the column opposite the extreme compression fibre. The effective depth should be taken as the distance from the extreme fibre with maximum compression to the centroid of this reinforcement. The web width should be taken as the column diameter. 5.5.7B Crack Control in Columns

If required by the Railtrack Director’s Nominee, for the purpose of calculating flexural crack widths, a column subjected to bending should be considered as a beam in accordance with Clause 5.8.7B. 5.6B Reinforced Concrete Walls

5.6.1B General A reinforced wall is a vertical load-bearing concrete member whose greater lateral dimension is more than four times its lesser lateral dimension, and in which the reinforcement is taken into account when considering its strength.

Retaining walls, wing walls, abutments, piers and other similar elements subjected principally to bending moments, and where the ultimate axial load is less than 0.1f cu Ac may be treated as cantilever slabs and assessed in accordance with Clause 5.4B. In other cases, this Clause applies. A reinforced wall may be considered as either short or slender. In a similar manner to columns, a wall should be considered as short where the ratio of the effective height to thickness does not exceed 12. Otherwise the wall should be considered as slender. 5.6.2B Forces and Moments in Reinforced Concrete Walls Forces and moments should be calculated in accordance with Clause 4.4B except that, if the wall is slender, the moments induced by deflection should also be considered. The distribution of axial and horizontal forces along a wall from the loads




on the superstructure should be determined by analysis and their points of application decided by the nature and location of the bearings. For walls fixed to the deck, the moments should similarly be determined by elastic analysis. Unless the actual eccentricity of load is determined, the moment per unit length in the direction at right-angles to a wall should be taken as not less than 0.05nw h ,where nw is the ultimate axial load per unit length and h is the thickness of the wall. Moments in the plane of a wall may be calculated from statics for the most severe positioning of the relevant loads. Where the axial load is non-uniform, consideration should be given to deep beam effects and the distribution of axial loads per unit length of wall. It will generally be necessary to consider the maximum and minimum ratios of moment to axial load in assessing a wall. 5.6.3B Short Reinforced Walls Resisting Moments and Axial Forces Each cross-section of the wall should be capable of resisting the applicable ultimate axial load and the transverse moment per unit length calculated in accordance with Clause 5.6.2B. The assumptions given in Clause 5.3.2.1B for the analysis of beam sections apply under axial load and bending and also when the wall is subject to

significant bending only in the plane of the wall. When the wall is subjected to significant bending both in the plane of the wall and at right-angles to it, consideration should be given first to bending in the plane of the wall in order to establish a distribution of tension and compression along the length of the wall. The resulting tension and compression should be combined with the compression due to the ultimate axial load to determine the combined axial load per unit length of wall. This may be achieved by an elastic analysis assuming a linear distribution along the wall. The bending moment at right-angles to the wall should be considered and the section checked for this moment and the resulting compression or tension per unit length at various points along the wall length, using the assumptions given in Clause 5.3.2.1B. 5.6.4B Slender Reinforced Walls The distribution of axial load along a slender reinforced wall should be determined as

for a short wall. The critical portion of wall should be considered as a slender column of unit width and assessed as such in accordance with Clause 5.5.5B. 5.6.5B Shear Resistance of Reinforced Walls A wall subject to uni-axial shear due to ultimate loads should be assessed in accordance with Clause 5.4.4.1B except that the ultimate shear stress, ξsvc, may be multiplied by the enhancement factors given by:



1+

0.07 N Ac

RT/CE/C/025 Issue: 1 Date: February 2000 Page 48 of 100 Expression B34

where: is the ultimate load (in Newtons); is the area of entire concrete section (in mm²).

N Ac

A wall subject to biaxial shear due to ultimate loads should satisfy the expression: Vx Vux

+

Vy Vuy

≤ 1.0

Equation B35

where: are the applied shears due to ultimate loads for the X-X axis and Y-Y axis respectively; Vux ,Vuy are the corresponding ultimate shear capacities of the concrete and link reinforcement for the X-X axis and Y-Y axis respectively, derived allowing for the enhancement factor given in this Clause. Vx ,Vy

5.6.6B Deflection of Reinforced Walls Deflections of walls need not be calculated. 5.6.7B Crack Control in Reinforced Walls If required by Railtrack Director’s Nominee, flexural crack widths in walls subject to bending should be calculated in accordance with Clause 5.8.7B. 5.7B Bases

5.7.1B General Where pockets have been left for precast members, allowance should be made when calculating the flexural and shear strength of base sections, for the effects of these pockets unless they have been grouted up using a cement mortar of compressive strength not less than that of the concrete in the base. 5.7.2B Moments and Forces in Bases Except where the reactions to the applied loads and moments are derived by more accurate methods, such as an elastic analysis of a pile group or the application of established principles of soil mechanics, the following assumptions should be made:

(a)

Where the base is axially loaded, the reactions to ultimate loads are uniformly distributed per unit area or per pile;



(b)


Where the base is eccentrically loaded, the reactions vary linearly across the base. For columns and walls restrained in direction at the base, the moment transferred to the base should be obtained from Clause 5.5B.

The critical section in the assessment of an isolated base may be taken as the face of the column or wall. The moment at any vertical section passing completely across a base should be taken as that due to all external ultimate loads and reactions on one side of that section. No redistribution of moments should be made. 5.7.3B Assessment of Bases 5.7.3.1B Resistance to Bendi ng

Bases should be assessed in accordance with Clause 5.4B, and should be capable of resisting the total moments and shears at the sections considered. Where the width of the section considered is less than or equal to 1.5(b col + 3d ) , where bcol is the width of the column and d is the effective depth to the tension reinforcement of the base, all reinforcement crossing the section may be considered to be effective in resisting bending. For greater widths, all reinforcement within a band of width (b col + 3d ) centred on the column may be considered to be effective and the area of effective reinforcement outside this band should be taken as the lesser of: (a)

the actual area of reinforcement outside the band, and

(b)

50% of the area of reinforcement within the band.

Pile caps may be assessed either by bending theory or by truss analogy taking the apex of the truss at the centre of the loaded area and the corners of the base of the truss at the intersections of the centre lines of the piles with the tensile reinforcement. In pile caps assessed by truss analogy, the effective area of reinforcement at a section should be taken as the lesser of: (a)

the total area at the section, and

(b)

1.25 times the area of reinforcement in the strips linking the pile heads.

Pile caps only be assessed as beams if the reinforcement is uniformly distributed across themay section under consideration. 5.7.3.2B Shear

The assessment shear force is the algebraic sum of all ultimate vertical loads acting on one side of or outside the periphery of the critical section. The shear strength of bases in the vicinity of concentrated loads is governed by the more severe of the following two conditions.




(a)

Shear along a vertical section extending across the full width of the base, at a distance equal to the effective depth from the face of the loaded area assessed in accordance with Clause 5.4.4.1B.

(b)

Punching shear around the loaded area assessed in accordance with Clause 5.4.4.2B.

The shear strength of pile caps is governed by the more severe of the following two conditions. (a)

Shear along any vertical section extending across the full width of the cap. The recommendation of Clause 5.4.4.1B apply, except that the enhancement of the shear resistance in accordance with Clause 5.3.3.3B for sections close to supports should be applied only to strips of width not greater than twice the pile diameter centred on each pile. Where av is taken as the distance between the face of the column or wall and the nearer edge of the piles it should be increased by 20% of the pile diameter. In applying Clause 5.4.4.1B the allowable ultimate shear stress should be taken as the average over the whole section;

(b)

Punching shear around loaded areas, where the requirements of Clause 5.4.4.2B apply. When considering case (c)(ii) of Figure B5, the allowable ultimate shear stress may be enhanced, in accordance with Clause 5.3.3.3B, over a width not greater than twice the pile diameter centred on the corner pile.

5.7.3.3B Bond and An chor age

The recommendations of Clause 5.8.6B apply to reinforcement in bases. 5.7.4B Deflection of Bases The deflection of bases need not be considered. 5.7.5B Crack Control in Bases If required by the Railtrack Director’s Nominee, crack widths may be calculated in accordance with Clause 5.8.7B taking into account the type of base and method of assessment, in accordance with Clause 5.7.3.1B. 5.8B Considerations of Details

5.8.1B Constructional Details When the reduced partial factor for material strength of steel of 1.05, given in Clause 4.3.3.3B, is adopted, measured reinforcement covers and effective depths should be used. 5.8.2B Concrete Cover to Reinforcement 'Nominal' cover is that dimension generally used in design and indicated on the drawings. However, design to BS 5400: Part 4 recognises that the actual cover may be up to 5 mm less than the nominal cover.




In the absence of drawings or other information, nominal cover may be determined using the conditions below for Bridges known to be designed to BS 5400: Part 4. The nominal cover should be taken as not less than either the size of the bar plus 5 mm or maximum aggregate size plus 5 mm. For a bundle of bars the nominal cover should be assumed equal to or greater than the size of a single bar of equivalent area plus 5 mm. Where not measured the nominal cover of dense natural aggregate concrete to all reinforcement, including links, may be taken as not less than the value given in Table B4 for particular grades of concrete and conditions of exposure. Where surface treatment such as bush hammering has cut into the face of the concrete, the depth of treated concrete should not be considered as contributing to the cover. For bridges that have not been or are not known to have been designed to BS 5400: Part 4, the cover assumed in assessment calculations should be determined on the basis of measurements made on site.




Page 52 of 100 Nominal Cover (mm)

Environment

Extreme Concrete surfaces exposed to: abrasive action by sea water or water with a pH ≤ 4.5

Examples

ConcreteGrade

25

30

40

50+

+

+

65 55

+

§

50 40

+

45 35

45

35

Marine Structures Parts of structure in contact with moorland water

Very severe Concrete surfaces directly affected by: de-icing salts or sea water spray

Walls and structure supports adjacent to a highway Parapet edge beams adjacent to a highway Concrete adjacent to the sea

Severe Concrete surfaces exposed to: driving rain or alternative wetting and drying

Wall and structure supports remote from a highway Bridge deck soffits Buried parts of structure

Moderate Concrete surfaces above ground level and fully sheltered against all of the following: rain, de-icing salts, sea water spray Concrete surfaces permanently saturated by water with a pH >4.5

Surface protected by bridge deck water-proofing or by permanent formwork Interior surface of pedestrian subways, voided superstructures or cellular abutments Concrete permanently under water

30

30

25

Table B4 Nominal Cover to Reinforcement under Particular Conditions of Exposure used in Design to BS 5400: Part 4 +

§

Concrete grade could cause inadequate durability. Concrete should have been air entrained where the surface is liable to freezing whilst wet and this cover was used. For parapet beams only, grade 30 concrete should have been air entrained and the nominal cover should have been 60 mm if grade 30 concrete was used.

5.8.3B Reinforcement: General Considerations 5.8.3.1B Gr oups of Bars

Subject to the reductions in bond stress, bars arranged as pairs in contact or in groups of three or four bars bundled in contact should be considered as effective only if the following conditions are satisfied: (a)

the bundle is restrained by links;




(b)

the bars in a bundle terminate at different points spaced at least 40 times the bar size apart except for bundles stopping at a support;

(a)

bars in pairs or bundles of three are lapped one bar at a time, but the laps staggered so that in any cross-section there are no more than four bars in a bundle.

5.8.4B Minimum Areas of Reinforcement in Members There is no minimum area of reinforcement requirement. 5.8.5B Bond, Anchorage and Bearing 5.8.5.1B Geometr ical Classif i cation of De for med Bar s

For the purposes of this Appendix there are two types of deformed bars, as follows: Type 1 A plain square twisted bar or a plain chamfered square twisted bar, each with a pitch of twist not greater than 18 times the nominal size of the bar. Type 2 A bar with transverse ribs with a substantial uniform spacing not greater than 0.8 φ (and continuous helical ribs where present), having a mean area of ribs (per unit length) above the core of the bar projected on a plane normal to the axis of the bar, of not less than 0.15 mm²/mm where

φ

is the size (nominal diameter) of the bar.

Other bars may be classified as Types 1 or 2 from the results of the performance tests described in BS 5400: Part 7. 5.8.5.2B An chor age Bon d

Bond failure is prevented provided the tension or compression in any bar at any section due to ultimate loads is developed on each side of the section by the embedment length or other end anchorage. The anchorage bond stress, assumed to be constant over this effective anchorage length, is taken as the force in the bar divided by the product of the effective anchorage length and the effective perimeter of the bar or group of bars in accordance with Clause 5.8.5.3B. The anchorage bond stress should not exceed the value of:

β

f cu

γmb

Expression B36

where:

β f cu

γmb

is a coefficient dependent on bar type given in table B5; is the characteristic, or worst credible concrete cube strength; is a partial factor equal to 1.4, unless the worst credible concrete strength is used, in which case it is equal to 1.25.




Page 54 of 100

β Bar Type

Plain bars Type 1:deformed bars Type 2:deformed bars Fabric

Bars in Tension

Bars in Compression

0.39 0.56 0.70 0.91

0.49 0.70 0.88 1.13

Table B5 Values of Bond Coefficient

β

5.8.5.3B Ef fective Peri meter of a Bar or Gr oup of Bars

The effective perimeter of a single bar may be taken as 3.14 times its nominal size. The effective perimeter of a group of bars should be taken as the sum of the effective perimeters of the individual bars multiplied by the applicable reduction factor given in Table B6. Number of Bars in a group 2 3

Reduction Factor

4

0.4

0.8 0.6

Table B6 Reduction factor for Effective Perimeter of a Group of Bars 5.8.5.4B An chorage of Li nks

A link may be considered to be fully anchored if records show that it passes round another bar through an angle of 90 ° and continues beyond for a minimum length of eight times its own size, or through 180° and continues for a minimum length of four times its own size. If the radius of any bend in the link is less than twice the radius of the test bend defined in BS 4449, or BS 785 prior to 1969, it should not be considered to be fully anchored. Where full anchorage of links is not achieved, its effective size should be taken as the equivalent bar diameter that the anchorage provides. 5.8.5.5B L aps and Joi nts

Continuity of reinforcement may be assumed if connection has been made using any of the following jointing methods: (a)

lapping bars, see Clause 5.8.5.6B;

(b)

butt welding, see Clause 7.3.2.4B;

(c)

sleeving complying with Clause 7.3.2.2B;

(d)

threading of bars complying with Clause 7.3.2.3B.




The strength of joints made using the methods given in (c) and (d) and any other method not listed should be verified by test evidence. 5.8.5.6B L ap L ength s

When bars are lapped, the length of the lap should at least equal the anchorage length (derived from Clause 5.8.5.2B) required to develop the stress in the smaller of the two bars lapped. The required minimum lap length calculated as above should have been increased for bars in tension by a factor of 1.4 if any of the following conditions apply: (a)

the cover to the lapped bars from the top of the section as cast is less than twice the bar size;

(b)

the clear distance between the lap and another pair of lapped bars is less than 150 mm;

(c)

a corner bar is lapped and the cover to either face is less than twice the bar size.

Where conditions (a) and (b) or conditions (a) and (c) apply the required minimum lap length should have been increased by a factor of 2.0. The required minimum lap length for bar reinforcement under any condition should not be less than 15 times the size of the smaller of the two bars lapped. Where the minimum lap length is not present the effective size of the smaller bar at the section should be determined as being L/15 where L is the lap length provided. 5.8.5.7B H ooks and Bends

Hooks, bends and other reinforcement anchorages should have been provided in such form, dimension and arrangement as to avoid overstressing the concrete. Hooks and bends can be considered fully effective if in accordance with BS 4466. The effective anchorage length of a hook or bend should be measured from the start of the bend to a point four times the bar size beyond the end of the bend, and may be taken as the lesser of 24 times the bar size or: (a)

for a hook, eight times the internal radius of the hook;

(b)

fora90

° bend, four times the internal radius of the bend.

In no case should the radius of the bend be less than twice the radius of the test bend defined in BS 4449, or BS 785 prior to 1969. However, bends should be of sufficient size to ensure that the bearing stress at the mid-point of the curve does not exceed the value given in Clause 5.8.5.8B.




For a hooked bar to be effective at a support, the beginning of the hook should be at least four times the bar size inside the face of the support. The effective anchorage length of a hook or bend that does not satisfy Paragraphs 1,3 and 4 of this Clause should be taken as not greater than the actual length of bar from the start of the bend to a point four times the bar size beyond the end of the bend. 5.8.5.8B Bear ing Str ess i nside Bends

The bearing stress inside a bend, in a bar that does not extend or is not assumed to be stressed beyond a point four times the bar size past the end of the bend, need not be checked. The bearing stress inside a bend in any other bar should be calculated from: Bearing Stress

=

Fbt rφ

Equation B37

where: Fbt r

φ

is the tensile force due to ultimate loads in a bar or group of bars; is the internal radius of the bend; is the size of the bar or, in a bundle, the size of a bar of equivalent area.

The bearing stress should not exceed; 5.63

γmc

L  ab f cu  L1  φ

  

1 3

Expression B38

where: ab

ab

φ

is the centre to centre distance between bars or group of bars perpendicular to the plane of the bend. For a bar or group of bars adjacent to the face of the member ab is taken as the cover plus φ ; should not exceed 8;

L1

is the length of the bar measured inside the bend and bearing on to the

L

concrete; is the thickness of concrete member in the plane of the bend, but not greater than 3L1 ;

Values of the partial factor,

γ mc , are given in Clause 4.3.3.3B.




5.8.6B Curtailment and Anchorage of Reinforcement Curtailment lengths and anchorages of bars should be assessed either by rigorous analysis at the curtailment or anchorage point for the worst load case in accordance with Clause 5.8.5.2B, or by use of the recommendations below.

Bars should be considered effective at a distance from their end equal to the effective depth of the member or 12 times the size of the bar, whichever is the greater. In addition, where reinforcement is stopped in a tension zone, one of the following conditions should be satisfied: (a)

the bars extend an anchorage length applicable to their assessment strength f y γms from the point at which they are no longer required to resist bending; or

(b)

the shear capacity at the section where the reinforcement stops is greater than 1.5 times the shear force actually present; or

(c)

the continuing bars at the section where the reinforcement stops provide double the area required to resist the moment at that section.

At a simply supported end of a member, a tension bar should only be considered fully effective if anchored by one of the following: (a) an effective anchorage equivalent to 12 times the bar size beyond the centre line of the support - no bend or hook should begin before the centre of the support, or (b)

an effective anchorage equivalent to 12 times the bar size plus d 2 from the face of the support, where d is the effective depth to tension reinforcement of the member - no bend should begin before d 2 from the face of the support.

Where these conditions are not met, the effective size of the tension bar at the support may be taken as 1/12 of the effective anchorage present beyond the centre line of the support. Where the simply supported end is resting directly on the abutment and/or where the short shear span enhancement is being used, the effective reinforcement area used may be taken as given in Clause 5.3.3.3B. 5.8.7B Maximum Distance between Bars in Tension When required by the Railtrack Director’s Nominee, crack widths, should be calculated as follows: (a) For solid rectangular sections, stems of T-beams and other solid sections shaped without re-entrant angles, the crack widths at the surface (or at a distance c nom from the outermost bar) should be calculated from:




Crack width

=

3acr ε m 1+ 2(acr −)(c h) − d c

Page 58 of 100 Equation B39

where: a cr

c nom c

dc h

εm

is the distance from the point (crack) considered to the surface of the nearest bar that controls the crack width; is the required nominal cover to the outermost reinforcement given in Table B4; is the effective cover to the reinforcement that controls the width of the cracks under consideration and should be taken as the lesser of (i)

actual cover to this reinforcement; and

(ii)

perpendicular distance from this reinforcement to a surface at a distance c nom from the outermost bars.

is the depth of the concrete in compression (if d c is zero crack widths should be calculated using Equation B42); is the overall depth of the section; is the calculated strain at the level where cracking is being considered, allowing for the stiffening effect of the concrete in the tension zone; a negative value of

The value of equation:

εm =

εm

εm

indicates that the section should be uncracked.

should be not greater than

3.8b h(a′ − d c )   Mq  −9  ε1 −  t 1− 10   ε s As (h − d c )   Mg  

ε1 and be obtained from the

Equation B40

where:

ε1

Mg

is the calculated strain at the level where cracking is being considered, ignoring the stiffening effect of the concrete in the tension zone; is the width of the section at the level of the centroid of the tension steel; is the distance from the compression face to the point at which the crack width is being calculated; is the moment at the section considered due to permanent loads;

Mq

is the moment at the section considered due to live loads;

εs

is the calculated strain at the centroid of reinforcement, ignoring the stiffening effect of the concrete in the tension zone; is the area of tension reinforcement. Where the axis of the moment and the direction of the tensile reinforcement resisting that moment

bt a′

As




are not normal to each other (such as in a skew slab), As should be taken as:

∑ ( A cos α ) 4

Expression B41

1

t

where: is the area of reinforcement in a particular direction; is the angle between the normal to the axis of the moment and the direction of the tensile reinforcement, At, resisting that moment.

At

α

1

(b)

For flanges in overall tension, including tensile zones of box beams, rectangular voided slabs and, when subjected to longitudinal bending, circular voided slabs, the crack width at the surface (or at a distance c nom from the outermost bar) should be calculated from: crack width where

εm

=

3acr ε m

Equation B42

is obtained from equation B40.

For flanges of circular voided slabs subjected to transverse bending, the crack width at the surface (or at a distance c nom from the outermost bar) should be calculated from the following equation: crack width

=

1.2ε m (hf

ρ net )

c

φ

Equation B43

where:

(c)

hf

is the minimum flange thickness;

σ net φ

is the area of transverse reinforcement in the flange as a percentage of the minimum flange area; is the diameter of the outermost transverse bar;

εm =

 3.8bt hf   Mq  −9  ε1 −  ε s As  1− Mg 10 

Equation B44

Where global and local effects are calculated separately, in accordance with Clause 4.8.3B the value of ε m may be obtained by algebraic addition of the strains calculated separately. The crack width should be calculated in accordance with (b) but may, in the case of deck slab where a global compression is being combined with a local moment, be obtained using (a), and calculating d c on the basis of the local moment only.




5.9B Additional Considerations for Lightweight Aggregate Concrete

5.9.1B General Lightweight aggregate concrete may generally be assessed in accordance with the requirements of Clauses 4B and 5.1B to 5.8B inclusive. Clauses 5.9.2B to 5.9.11B below relate specifically to reinforced lightweight aggregate concrete of strength 25 N/mm² or above. Only the requirements of Clause 7.5B (plain concrete walls) apply to concretes below a strength of 25 N/mm².

For lightweight aggregate concrete, the properties for any particular type of aggregate should be to be established far more accurately than for most naturally occurring materials and, when the aggregate type can be identified, specific data should be obtained from the aggregate producer or other source. All the properties of lightweight aggregate concrete to be used should be supported by applicable test data. 5.9.2B Strength of Concrete Clause 5.1.4.2B applies. 5.9.3B Shear Resistance of Beams

The shear resistance of lightweight concrete beams should accordance with Clauses 5.3.3.1B toaggregate 5.3.3.3B except that the value of be v c established calculated in from the expression given in Clause 5.3.3.2B should be multiplied by 0.9 and the maximum allowable value of v referred to in Clauses 5.3.3.1B and 5.3.3.3B should be multiplied by 0.8. 5.9.4B Torsional Resistance of Beams The torsional resistance of lightweight aggregate concrete beams should be established in accordance with Clause 5.3.4B except that the values of v t min and v tu calculated from the expressions given in Clause 5.3.4.3B should be multiplied by 0.8. 5.9.5B Deflection of Beams Deflection of lightweight aggregate concrete beams may be calculated using a value of Ec as described in Clause 4.3.2.1B. 5.9.6B Shear Resistance of Slabsaggregate concrete slabs should be established in The shear resistance of lightweight accordance with Clause 5.4.4B, except that v c and the maximum allowable value of v should be modified in accordance with Clause 5.9.3B. 5.9.7B Deflection of Slabs Deflection of lightweight aggregate concrete slabs may be calculated using a value of Ec as described in Clause 4.3.2.1B.




5.9.8B Columns 5.9.8.1B G ener al

The requirements of Clause 5.5B apply to lightweight aggregate concrete columns subject to the conditions in Clauses 5.9.8.2B and 5.9.8.3B. 5.9.8.2B Shor t Colu mns

The ratio of effective height, µ e, to thickness, h, for a short column should not exceed 10. 5.9.8.3B Slender Colu mns

The divisor 1750 in Equations B27, B29, B30 and B31 in Clause 5.5.5B should be replaced by 1200. 5.9.9B Local Bond, Anchorage Bond and Laps Anchorage bond stresses and laps lengths in reinforcement for lightweight aggregate concrete members should be assessed in accordance with Clause 5.8.6B except that the bond stresses should not exceed 80% of those given in Clause 5.8.5.2B.

For concrete with foamed slag or similar aggregates, bond stresses should be less than the maximum values in the preceding paragraph for reinforcement that was known be to obtained have beenfrom in a horizontal should test data. position during casting. Acceptable bond stresses 5.9.10B Bearing Stress inside Bends The requirements of Clause 5.8.5.8B apply to lightweight aggregate concrete, except that the bearing stress should not exceed two-thirds of the allowable value given by the expression in Clause 5.8.5.8B. 6B. PRESTRESSED CONCRETE 6.1B General

6.1.1B Introduction Methods of assessment for prestressed concrete construction for compliance with the recommendations set out in Clause 4B are given below. Other methods may be used provided such methods can be shown to be satisfactory for the type of structure

or member considered. In certain cases the assumptions included below may be inapplicable and the Engineer should adopt a more suitable method having regard to the nature of the structure in question. Assessment of prestressed concrete construction where any of the following are incorporated in the structure is not included: (a)

unbonded tendons, where prestress is solely transmitted through compression of the ends of the member and these tendons are not grouted within the ducts;




(b)

external tendons (a tendon is considered external if, after it was stressed and incorporated in the permanent works but before protection, it was outside the concrete section);

(c)

lightweight aggregate concrete.

6.1.2B Limit State Assessment of Prestressed Concrete 6.1.2.1B Basi s of Assessment

Usually a structure needs to be assessed only at the ultimate limit state and for deflection under working loads. A serviceability limit state assessment for stresses and cracking is required only when specifically requested by the Railtrack Director’s Nominee. 6.1.2.2B Durability

Clause 6.8.2B gives recommendations for the nominal cover to prestressing tendons that in design are assessed to provide adequate durability. 6.1.2.3B Other L im i t States and Consider ation s

The recommendations of Clause 4.1.3B apply. 6.1.3B Loads

The assessment load effects(see Section 2) for the ultimate and serviceability limit states are referred to as ‘ultimate loads’ and ‘service loads’ respectively. The values of the ‘ultimate loads’ and ‘service loads’ that should be used in assessment are derived from Section 4. When analysing sections, the terms ‘strength’, ‘resistance’ and ‘capacity’ are used to describe the assessment resistance of the section, see Section 2. Consideration should be given to the construction sequence and to the secondary effects due to prestress, particularly for a serviceability limit state assessment. 6.1.4B Strength of Materials 6.1.4.1B Defin iti on of Strengths

The symbol f cu represents either the characteristic or the worst credible cube strength of the concrete. The symbol f pu represents either the characteristic or the worst credible tendon strength. The assessment strengths of concrete and prestressing tendons are given by f cu γ mc and f pu γ ms respectively, where γ mc and γ ms are the applicable partial factors for material strength given in Clause 4.3.3.3B. The applicable value of γ mc or be substituted in all equations/ expressions contained within Clause 6B.

γ ms

should




6.1.4.2B Str ength of Concrete

Assessment may be based on either the specified characteristic cube strength, or the worst credible cube strength assessed as the lower bound to the estimated in-situ cube strength determined in accordance with BS 6089. 6.1.4.3B Strength of Prestr essing Tendon s

Assessment may be based on either the specified characteristic strength, or the worst credible strength assessed from tests on tendon samples extracted from the structure. 6.2B Structures and Structural Frames

6.2.1B Analysis of Structures Complete structures and complete structural frames may be analysed in accordance with the recommendations of Clause 4.4B but, when applicable the methods given in Clause 6.3B may be used for the assessment of individual members.

The relative stiffness of members should generally be based on the assumptions for the concrete section as described in Clause 4.4.2.1B. 6.2.2B Redistribution of Moments

Redistribution obtained rigorous elastic analysis under the ultimate limit state may of bemoments used provided the by following conditions are met: (a)

Checks are made to ensure that adequate rotational capacity exists at sections where moments are reduced, making reference to applicable test data. In the absence of a special investigation, the plastic rotation capacity may be taken as the lesser of: 0.008

d + 0.035 0.5 − c  de  

Expression B45

or 10 d − dc but not less than 0 where: dc de d

is the calculated depth of concrete in compression at the ultimate limit state (in mm); is the effective depth for a solid slab or rectangular beam, otherwise the overall depth of the compression flange (in mm); is the effective depth to tension reinforcement (in mm).




Page 64 of 100

(b)

Account is taken of changes in transverse moments and transverse shears consequent on redistribution of longitudinal moments.

(c)

Shears and reactions used in assessment are taken as those calculated either prior to or after redistribution, whichever are the greater.

6.3B Beams

6.3.1B General 6.3.1.1B D efi ni tions

The definitions and limitations of the geometric properties for prestressed beams are as given for reinforced concrete beams in Clause 5.3.1B. 6.3.2B Serviceability Limit State: Flexure When a serviceability limit state assessment is required, the necessary criteria should be agreed with the Railtrack Director’s Nominee. 6.3.3B Ultimate Limit State: Flexure 6.3.3.1B An al ysi s of Secti ons

When analysing a cross-section assumptions should be made: to determine its ultimate strength the following (a)

The strain distribution in the concrete in compression is derived from the assumption that plane sections remain plane.

(b)

The stress-strain curve in Figure B1 with the applicable value of γmc given in Clause 4.3.3.3B applies for the derivation of the stresses in the concrete in compression or, for rectangular sections and flanged, ribbed and voided sections where the neutral axis lies within the flange, the compressive stress is taken as equal to 0.6 fcu/γmc over the whole compression zone. In both cases the strain at the outermost compression fibre at failure is taken as 0.0035.

(c)

The tensile strength of concrete is ignored.

(d)

The strains in bonded prestressing tendons and in any additional reinforcement, in tension or compression, are derived from should the assumption thatwhether plane sections remain plane. In addition, the tendon have an initial strain due to prestress after all losses.




(e)

Page 65 of 100

The stresses in bonded prestressing tendons, whether initially tensioned or un-tensioned, and in additional reinforcement, are derived either from the applicable stress-strain curve in Figures B2, B3 and B4 or, when available, manufacturers' stress-strain curves. The values of γ ms are given in Clause 4.3.3.3B. An empirical approach for obtaining the stress in the tendons at failure is given in Clause 6.3.3.3B.

6.3.3.2B Desi gn Char ts

The design charts in CP 110: Part 3 include charts, based on Figures 1, 3 and 4, and the assumptions given in Clause 6.3.3.1B, which, with applicable modifications for γ ms which is defined in Clause 4.3.3B, may be used for the assessment of rectangular prestressed beams. 6.3.3.3B Assessment F or mu la

In the absence of an analysis based on the assumptions given in Clause 6.3.3.1B, the resistance moment of a rectangular beam, or of a flanged beam in which the neutral axis lies within the flange, may be obtained from: Mu

=

f pb Aps (d − 0.5 x )

Equation B46

where: is the ultimate moment of resistance of the section; is the tensile stress in the tendons at failure; is the neutral axis depth; is the effective depth to tension reinforcement; is the area of the prestressing tendons in the tension zone.

Mu f pb x d Aps

The tensile stress, fpb, should not be greater than 1.0 N/mm² and may be calculated from: f pb

(f pu γ ms )

f =  α −  f

pu

Aps 

cu

bd

 

Equation B47

where:

α γ ms

may be taken as 1.3 for pre-tensioning, and 1.15 for post-tensioning with effective bond; is the partial factor for the tendons given in Clause 4.3.3.3B.

The neutral axis depth, x , may be calculated from: x

=

f pb Aps γ mc 0.6f cu b

Equation B48




where

γ mc

Page 66 of 100

is the partial factor for concrete given in Clause 4.3.3.3B.

Prestressing tendons and additional reinforcement in the compression zone should be ignored in strength calculations when using this method. 6.3.3.4B Non -r ectangul ar Section s

Non-rectangular beams should be analysed using the assumptions given in Clause 6.3.3.1B. 6.3.4B Shear Resistance of Beams 6.3.4.1B G ener al

Calculations for shear are only required for the ultimate limit state. In a haunched box beam, the component of the flange forces perpendicular to the longitudinal axis of the beam calculated from an elastic section analysis under the relevant load case may be subtracted algebraically from the applied shear force. At any section, the ultimate shear resistance is the sum of the resistance of the concrete alone, Vc , calculated in accordance with Clauses 6.3.4.2B and 6.3.4.3B, and of the shear reinforcement, Vs, calculated in accordance with Clause 6.3.4.4B. For vertical links to be effective, the tensile capacity of the longitudinal steel at a section should be greater than: M (V − ξ s v c bw d ) + z 2

Expression B49

where: M, V

are the co-existent ultimate bending moment and shear force at the section under consideration; z is the lever arm which may be taken as 0.9 d; ξ s ,v c ,bw and d are as defined in Clause 5.3.3.2B. The tensile capacity of the longitudinal steel is: () t As ()t f pu

+( )As( )u f yL u γms

Expression B50

where As )( t , )(f pu ()t , As u and f yL u are as defined in Clause 6.3.4.3B. However within an () individual sagging or hogging region, the tensile capacity need not exceed Mmax z where Mmax is the maximum ultimate moment within that region. At a section where the applied moment, M, does not exceed the cracking moment, Mcr , calculated in accordance with Clause 6.3.4.2B, Vc may be taken as equal to the




uncracked value, Vco , in accordance with Clause 6.3.4.2B. In all other cases Vc should be taken as the lesser of the uncracked value, Vco , and the cracked value, Vcr , calculated in accordance with Clause 6.3.4.3B. For a cracked section the conditions of maximum shear with co-existent bending moment and maximum bending moment with co-existent shear should both be considered. Within the transmission length of pre-tensioned members, in accordance with Clause 6.7.4B, the shear resistance of a section should be taken as the greater of the values calculated from: (a)

Clause 5.3.3B except that in determining the area As , the area of tendons should be ignored unless the tendons are rigid bars; and

(b)

Clauses 6.3.4.2B to 6.3.4.4B, using the applicable value of prestress at the section considered, assuming a linear variation of prestress over the transmission length.

6.3.4.2B Sections Un cracked i n Fl exu r e

A section may be assumed to be uncracked in flexure if the applied moment does not exceed the cracking moment, Mcr given by: Mcr = (0.49 f cu γmc + f pt ) I y

Equation B51

where: f pt is the stress due to prestress only at the tensile fibre distance y from the centroid of the concrete section that has a second moment of area I. The value of f pt should be derived from the prestressing force after all losses have occurred, multiplied by the applicable value of γfL , given in Clause 4.2.3B. Values of the partial factor γmc are given in Clause 4.3.3.3B. It may be assumed that the ultimate shear resistance of a section uncracked in flexure, Vco , corresponds to the occurrence of the maximum principal tensile stress f , at the centroidal axis of the section, given by: t

ft

=

0.32 f cu

γmc

Equation B52

In the calculation of Vco , the value of f cp should be derived from the prestressing force after all losses have occurred, multiplied by the applicable value of Clause 4.2.3B. The value of Vco is given by:

γ fL

given in



Vco

=

0.67 bh f t

2

+ f cp f t

RT/CE/C/025 Issue: 1 Date: February 2000 Page 68 of 100 Equation B53

where:

γmc

ft

is 0.32 f cu

f cp

is the compressive stress at the centroidal axis due to prestress, taken as positive; is the breadth of the member which for T, I and L beams, should be replaced by the breadth of the rib, bw. Where the position of a duct coincides with the position of maximum principle tensile stress, such as at or near the junction of flange and web near a support, the value of b should be reduced by the full diameter of the duct if ungrouted and by two-thirds of the diameter if grouted; is the overall depth of the member.

b

h

and istaken aspositive;

In flanged members, where the centroidal axis occurs in the flange, the principal tensile stress should be limited to f t at the intersection of the flange and web. For such members, the algebraic sum of the stress due to the bending moment under ultimate loads and the stress due to prestress at this intersection should be used in calculating Vco . For a section with inclined tendons, the component of prestressing force (multiplied by the applicable value of γ fL ) normal to the longitudinal axis of the member should be algebraically added to Vco . This component should be taken as positive where the shear resistance of the section is increased. For flanged sections the actual maximum principal tensile stress may be less than given by Equation B53. The shear strength, Vco in such cases should be taken as equal to that under which the actual maximum principal tensile stress is equal to0.32 f cu

γmc

2.

6.3.4.3B Sections Cr acked in F l exu r e

The ultimate shear resistance of a section cracked in flexure, Vcr , should be calculated using Equation B54 when the factored effective prestress, f pe , exceeds 0.6 f pu . When f pe is less than 0.6 f pu , the shear strength may still be taken as given by Equation B54. Alternatively, the value given by Equation B55 may be used if this is greater. Vcr =

0.045bd f cu

γmc +

Mcr

 M−d   V 2  

but not less than 0.12bd f cu

γmc

Equation B54



Vcr =

(1− 0.55 f pe f pu ) v c bd s +

Mo

 M−d  V 2  


Equation B55

s

but not less than 0.12bd s f cu

γ mc

where: is the distance from the extreme compression fibre to the centroid of the tendons at the section considered but not less than 0.625h; is the partial factor for concrete given in Clause 4.3.3.3B; is the cracking moment defined in Equation B51; are the shear force and bending moment (both taken as positive) at the section considered due to ultimate loads; is the moment necessary to produce zero stress in the concrete at the depth d as given by:

d

γmc Mcr V ,M Mo

Mo =

fpt I/y

fpt is of d, distance in which thethe stress due to prestress the depth the centroid concrete section thatonly has aatsecond moment of areay I.from The value of fpt should be derived from the prestressing forces after all losses have occurred, multiplied by the applicable value of γfL given in Clause 4.2.3B. Mo should not be taken as greater than Mcr ; is the factored effective prestress that is equal to the effective prestress after

f pe

all losses have occurred, multiplied by the applicable value of γfL , given in Clause 4.2.3B; is obtained from Clause 5.3.3.2B; (required to obtain vc) should be taken as the actual area of steel in the tension zone, irrespective of its characteristic strength; is the distance from the compression face to the centroid of the steel area, As .

vc As ds

For cases where both tensioned and untensioned steel are contained in As , f pe f pu may be given by: f pe f pu

=

where:

Pf As ()t f pu () t

+( )As( )u f yL u

Equation B56




Pf

is the effective prestressing force after all losses;

As ( t )

is the area of tensioned steel;

As ( u ) f pu ( t )

is the area of untensioned steel;

f yL ( u )

Page 70 of 100

is the characteristic strength or the worst credible strength of the tensioned steel; is the characteristic strength or the worst credible strength of the untensioned steel.

For sections cracked in flexure and with inclined tendons, the component of prestressing force normal to the longitudinal axis of the member should be ignored. However, in a haunched section the component of prestress normal to the (inclined) longitudinal axis of the member may be considered. This component should not be taken as greater than it would be if the tendons were parallel to the flange, that is, the tension flange ignoring the effect of prestress. 6.3.4.4B Shear Rein for cement

Types of shear reinforcement and the criterion for the amount of shear reinforcement required to be present for it to be considered effective are defined in Clause 5.3.3.2B. In addition links should be considered as effective only if their spacing both along a beam and laterally does not exceed dt, nor four times the web thickness flanged beams. When thefor above criteria are met, the shear resistance of vertical links is given by: Vs

=

Asv (f yv

γ )d ms

t

sv

Equation B57

where dt is the depth from the extreme compression fibre either to the centroid of the tendons or to the longitudinal bars, tendons, or groups of tendons in the tension zone around which the links are anchored in accordance with Clause 5.8.6.5B, whichever is greater. All other terms in the equation for Vs are defined in Clause 5.3.3.2B. Sections within a distance d from the support need not be assessed for shear providing the shear reinforcement calculated for the section at a distance d is continued up to the support. Inclined links or bent up bars should be assumed to form the tension members of lattice girders as described in Clause 5.3.3.2B. 6.3.4.5B M aximu m Shear F orce

The shear force, V , due to ultimate loads, should not exceed the stress 0.36 (0.7 − f cu 250 ) γmcmultipliedby bd s , where b is as defined in Clause 6.3.4.2B and d s is defined in Clause 6.3.4.3B. Where a section is uncracked in flexure according




to Clause 6.3.4.2B d s = d t which is defined in Clause 6.3.4.4B. factor for concrete given in Clause 4.3.3.3B.

Page 71 of 100

γmc

is the partial

6.3.4.6B Segmental Con str ucti on

In post-tensioned segmental construction, the shear force due to ultimate loads should not be greater than: 0.7 γfL Ph tanα 2

Equation B58

where:

γfL

is the partial factor for the prestressing force, taken as 0.87;

Ph

is the horizontal component of the prestressing force after all losses; is the angle of friction at the joint. Tan α 2 may be taken as 0.7 for a smooth interface and 1.4 for a roughened or castellated interface. If there is any doubt regarding the type of interface, tan α 2 should be taken as 0.7.

α2

The method of assessment of match cast joints with shear keys should be agreed with the Railtrack Director’s Nominee. 6.3.4.7B Al ter nati ve Appr oach

As an alternative to the method given in Clauses 6.3.4.1B to 6.3.4.5B, beams may be assessed using the varying angle truss approach described in Clause 5.3.3.5B in which case the vertical component of prestress may be deducted algebraically from the applied shear force. 6.3.5B Torsional Resistance of Beams 6.3.5.1B G ener al

In some members, the maximum torsional moment does not occur under the same loading as the maximum flexural moment. In such circumstances reinforcement and prestress in excess of that required for flexure and shear may have been used to resist torsion. 6.3.5.2B Stresses and Rein f or cement

Calculations of torsion are only required for the ultimate limit state. The torsional shear stresses should be calculated assuming a plastic shear stress distribution. Calculations for torsion should be in accordance with Clause 5.3.4B with the following modifications: (i)

when prestressing steel is used as transverse torsional steel, or as longitudinal torsional steel, the stress assumed in assessment should be the lesser of f pe + 460 γms or f pu γms , where γms is the partial factor for material strength of steel given in Clause 4.3.3.3B;




Page 72 of 100

(ii)

the compressive stress in the concrete due to prestress should be taken into account separately in accordance with Clause 5.3.4.5B;

(iii)

in calculating (v + vt) for comparison with vtu (in accordance with Clause 5.3.4.3B), v should be calculated from Equation B8 regardless of whether Clause 6.3.4.2B or Clause 6.3.4.3B is critical in shear.

6.3.5.3B Oth er Assessment M eth ods

Alternative methods of assessing members subjected to combined bending, shear and torsion may be used provided that the method used can be shown to satisfy the ultimate limit state requirements. 6.3.6B Longitudinal Shear For flanged beams, the longitudinal shear resistance across vertical sections of the flange that may be critical should be checked in accordance with Clause 7.4.2.3B. 6.3.7B Deflection of Beam If required by the Railtrack Director’s Nominee, deflections may be calculated by a method applicable to the level of prestress in the member and the level of loading. 6.4B Slabs The analysis of prestressed slabs should be in accordance with Clause 5.4.1B provided that due allowance is made for moments due to prestress. The assessment should be in accordance with Clause 6.3B.

The assessment of shear should be in accordance with Clause 6.3.4B. For assessment of shear stresses under concentrated loads, the ultimate shear resistance of a section uncracked in flexure, Vco , may be taken as corresponding to the occurrence of a maximum principal tensile stress of f t = 0.32 f cu γ mcat the centroidal axis around the critical section that is assumed at a perimeter h/2 from the loaded area. The shear resistance of any shear reinforcement, Vs , should be assessed in accordance with Clause 6.3.4.4B. 6.5B Columns

Prestressed concrete columns, where the mean stress in the concrete section imposed by the tendons is less than 2.5 N/mm², may be assessed as reinforced columns in accordance with Clause 5.5B. In all other cases the full effects of the prestress should be considered. 6.6B Tension Members The tensile strength of tension members should be based on the assessment strength f pu γms of the prestressing tendons and the strength developed by any additional

reinforcement. The additional reinforcement may usually be assumed to be acting at




its assessment stress f y γ ms . In special cases it may be necessary to check the stress in the reinforcement using strain compatibility. 6.7B Prestressing Requirements

6.7.1B Maximum Initial Prestress The initial prestress should be assessed from record drawings, available site data or srcinal design calculations. In the absence of such information, the likely nominal value of the initial prestress should be assessed from the standards current at the time of the design. 6.7.2B Loss of Prestress other than Friction Losses 6.7.2.1B G ener al

Allowance should be made, when calculating the forces in tendons, for losses of prestress resulting from: (a) (b) (c) (d)

relaxation of the steel comprising the tendons; the elastic deformation and subsequent shrinkage and creep of the concrete; slip or movement of tendons at anchorages during anchoring; other causes in special circumstances, such as when steam curing has been used with pre-tensioning.

If experimental evidence on performance is not available, account should be taken of the properties of the steel and the concrete when calculating the losses of prestress from these Clauses. For a wide range of structures, the recommendations given in this Clause should be used. It should be recognised, however, that these requirements are necessarily general and approximate. 6.7.2.2B L oss of Pr estr ess due to Rel axati on of Steel

The loss of force in the tendon allowed for in the assessment should be the maximum relaxation after 1000 hours duration, for a jacking force equal to that which is estimated was imposed at transfer, as given by the applicable British Standard or manufacturer's data. In special cases, such as tendons at high temperature or subjected to large lateral loads as in deflected tendons, greater relaxation losses may be present and specialist literature should be consulted. 6.7.2.3B L oss of Pr estr ess due to Elastic Defor mation of th e Concr ete

Calculation of the immediate loss of force in the tendons due to elastic deformation of the concrete at transfer may be based on the values for the modulus of elasticity of the concrete given in Clause 4.3.2.1B. The modulus of elasticity of the tendons may be obtained from Clause 4.3.2.2B.




Page 74 of 100

For pre-tensioning, the loss of prestress in the tendons at transfer should be calculated on a modular ratio basis using the stress in the adjacent concrete. For members with post-tensioning tendons that were not stressed simultaneously, there would have been a progressive loss of prestress during transfer due to the gradual application of the prestressing force. The resulting loss of prestress in the tendons should be calculated on the basis of half the product of the modular ratio and the stress in the concrete adjacent to the tendons, averaged along their length. Alternatively, the loss of prestress may be computed exactly based on the sequence of tensioning if that is known. In making these calculations, it may usually be assumed that the tendons are located at their centroid. 6.7.2.4B L oss of Pr estr ess due to Shr ink age of th e Concr ete

The loss of prestress in the tendons due to shrinkage of the concrete may be calculated from the modulus of elasticity for the tendons given in Clause 4.3.2.2B, assuming the values for shrinkage per unit length given in Table B7. Shrinkage per Unit Length System

HumiE dxposure (90% rh)

Normal Exposure (70% rh)

Pre-tensioning: transfer at between 3 and 5 days after concreting

100 x 10 -6

300 x 10-6

Post-tensioning: transfer at between 7 and 14 days after concreting

70 x 10 -6

200 x 10-6

Table B7 Shrinkage of Concrete

For other ages of concrete at transfer, for other conditions of exposure, or for massive structures, some adjustment to these values is necessary. Reference should be made to BS 5400: Part 4 Appendix C or to specialist literature, details of which are given in Appendix F. 6.7.2.5B L oss of Pr estr ess due to Creep of th e Concr ete

The loss ofon prestress in the tendons due is toproportional creep of the to concrete should be calculated the assumption that creep stress in the concrete for stress of up to one-third of the cube strength at transfer. The loss of prestress is obtained from the product of the modulus of elasticity of the tendons, given in Clause 4.3.2.2B and the creep of the concrete adjacent to the tendons. Usually it is sufficient to assume, in calculating this loss, that the tendons are located at their centroid. For pre-tensioning at between 3 days and 5 days after concreting and, for humid or dry conditions of exposure, where the cube strength at transfer was greater than




40 N/mm², the creep of the concrete per unit length should be taken as 48 x 10 -6 per N/mm². For lower values of cube strength at transfer the creep per unit length should be taken as 48 x 10-6 x40/fci per N/mm². For post-tensioning at between 7 days and 14 days after concreting, and for humid or dry conditions of exposure, where the cube strength at transfer was greater than 40N/mm², the creep of the concrete per unit length should be taken as 36 x 10 -6 per N/mm². For lower values of cube strength at transfer, the creep per unit length shall be taken as 36 x 10-6 x 40/fci per N/mm². The concrete strength at transfer, f ci , (in N/mm²) which should be taken from contract record drawings, available site data or srcinal design calculations. In the absence of such information, the likely nominal value should be assessed from the standards current at the time of the design. Where the maximum stress anywhere in the section at transfer exceeded one-third of the cube strength of the concrete at transfer, the value of the creep per unit length used in calculations may be increased. When the maximum stress at transfer was half the cube strength at transfer, the values for creep should be taken as 1.25 times those given above. At intermediate stresses, the values should be interpolated linearly. In applying these requirements, which are necessarily general, reference should be made to BS 5400: Part 4 Appendix C or specialist literature, details of which are given in Appendix F, for more detailed information on the factors affecting creep. 6.7.2.6B L oss of Pr estr ess dur in g Anchor age

In post-tensioning systems, allowance should be made for any movement of the tendon at the anchorage that would have occurred when the prestressing force was transferred from the tensioning equipment to the anchorage. 6.7.2.7B L osses of Pr estr ess due to Steam Cur ing

Where steam curing was employed in the manufacture of prestressed concrete units, changes in the behaviour of the material at higher than normal temperatures needs to be considered. In addition, where the ‘long-line' method of pre-tensioning was used there may be additional losses as a result of bond developed between the tendon and the concrete when the tendon was hot and relaxed. Since the actual losses of prestress due to steam curing are a function of the techniques used by the various manufacturers, specialist advice should be sought.




6.7.3B Loss of Prestress due to Friction 6.7.3.1B G ener al

In post-tensioning systems there will have been movement of the greater part of the tendon relative to the surrounding duct during the tensioning operation. If the tendons were in contact with either the duct or any spacers provided, friction would have caused a reduction in the prestressing force as the distance from the jack increased. In addition, friction would have developed in the jack itself and in the anchorage through which the tendon passed. In the absence of site data, the stress variation likely to be expected along the tendon profile should be assessed in accordance with Clauses 6.7.3.2B to 6.7.3.5B in order to obtain the prestressing force at the critical sections considered in assessment. 6.7.3.2B F r iction i n the Du ct due to Unin tenti onal Var iation f r om the Specifi ed Profile

Whether the desired duct profile was straight or curved or a combination of both, there will have been slight variations in the actual line of the duct, which may have caused additional points of contact between the tendon and the sides of the duct, and so produced friction. The prestressing force, Px ,at any distance x from the jack may be calculated from: − Kx

Px =

P0 e

Equation B59

where: P0 e K

is the prestressing force in the tendon at the jacking end; is the base of Napierian logarithms (2.718); is the constant depending on the type of duct, or sheath employed, the nature of its inside surface, the method of forming it and the degree of vibration employed in placing the concrete.

When Kx ≤ 0.2 , e − Kx may be taken as ( 1− Kx ) The value of K per metre length in Equation B59 should generally be taken as not less than 33 x 10-4, except where strong rigid sheaths or duct formers were used closely supported to prevent displacement during the concreting operation, In which case the value of K may be taken as 17 x 10-4. Other values may be used provided they have been established by tests to the satisfaction of the Engineer at time of construction and their use for assessment is agreed by the Railtrack Director’s Nominee. 6.7.3.3B F r iction i n the Duct due to Curvature of the Tendo n

When a tendon is curved, the loss of tension due to friction is dependent on the angle turned through and on the coefficient of friction between the tendon and its supports. The prestressing force, Px , at any distance x along the curve from the tangent point may be calculated from:




Px =

P0 e

−µx

rps

Page 77 of 100

Equation B60

where: P o

rps

is the prestressing force in the tendon at the tangent point near the jacking end; is the radius of curvature

When µx rps ≤ 0.2, e

−µx

rps

maybetakenas

(1− µx rps ) .

When the combined effects of Clause 67.3.2B and this Clause result in (Kx + µx rps )≤ 0.2, e − Kx +µx rps maybetakenas [1− (Kx + µx rps )]. Values of 0.55 0.30 0.25

µ

may be taken as:

for steel moving on concrete; for steel moving on steel; for steel moving on lead.

The value of µ may be reduced where special precautions were taken during construction and where results are available to justify the value assumed. For example, a value of µ = 1.0 has been observed for strand moving on rigid steel spacers coated with molybdenum disulphide. Such reduced values should be used only if construction records confirm that precautions were used. 6.7.3.4B F ri ction in Ci rcular Cons truction

Where circumferential tendons have been tensioned by means of jacks, the losses due to friction may be calculated from the equation in Clause 6.7.3.3B, but the value of µ may be taken as: 0.45 0.25 0.10

for steel moving in smooth concrete; for steel moving on steel bearers fixed to the concrete; for steel moving on steel rollers.

6.7.3.5B L ubr icants

Where lubricants were specified and lower values of µ than those given in Clauses 6.7.3.4B and 6.7.3.5B were obtained by trials prior to construction, the lower values may be used for assessment. 6.7.4B Transmission Length in Pre-tensioned Members The transmission length is defined as the length over which a tendon is bonded to concrete to transmit the initial prestressing force in a tendon to the concrete.




The transmission length depends on a number of variables, the most important being: (a) (b) (c) (d)

the degree of compaction of the concrete; the strength of the concrete; the size and type of tendon; the deformation (such as crimp) of the tendon;

(e) (f)

the stress in the tendon; and the surface condition of the tendon.

The transmission lengths of the tendons towards the top of a unit may be greater than those at the bottom. A sudden release of tendons may also cause a considerable increase in the transmission lengths. Where the initial prestressing force was not greater than 75% of the characteristic strength of the tendon and where the concrete strength at transfer was not less than 30 N/mm², the transmission length, Lt , may be taken as follows: kt φ Lt = Equation B61 f ci where: f ci

Lt

φ kt

is the concrete strength at transfer (in N/mm²) which should be assessed from record drawings, available site data or srcinal design calculations. In the absence of such information, the likely nominal value should be assessed from the standards current at the time of the design: is the transmission length (in mm); is the nominal diameter of the tendon (in mm); is a coefficient dependent on the type of tendon, to be taken as: 600 for plain, indented and crimped wire with a total wave height less than 0.15φ; 400 for crimped wire with a total wave height greater than or equal to 0.15 φ; 240 for 7-wire standard and super strand; 360 for 7-wire drawn or compacted strand.

The development of stress from the end of the unit to the point of maximum stress should be assumed to be linear over the transmission length. If the tendons have been prevented from bonding to the concrete near the ends of the unit by the use of sleeves or tape, the transmission lengths should be taken from the ends of the de-bonded portions.




6.7.5B End Blocks The end block (also known as the anchor block or end zone) is defined as the highly stressed zone of concrete around the termination points of a pre-tensioned or posttensioned tendon. It extends from the point of application of prestress (that is the end of the bonded part of the tendon in pre-tensioned construction) or the anchorage in post-tensioned construction to that section of the member at which linear distribution of stress is assumed to occur over the whole cross-section.

The following aspects should be considered in assessing the strength of end blocks: (a) (b) (c)

bursting forces around individual anchorages; overall equilibrium of the end block; spalling of the concrete from the loaded face around anchorages.

In considering each of these aspects, particular attention should be given to factors such as: (a) (b) (c) (d) (e) (f)

shape, dimensions and position of anchor plates relative to the crosssection of the end block; the magnitude of the prestressing forces and the sequence of prestressing; shape of the end block relative to the general shape of the member; layout of anchorages including asymmetry, group effects and edge distances; influence of the support reaction; forces due to curved or divergent tendons.

The following recommendations are applicable to a circular, square or rectangular anchor plate, symmetrically positioned on the end face of a square or rectangular post-tensioned member. The bursting tensile forces in the end blocks, or end regions of post-tensioned members should be assessed on the basis of the load in the tendon at the ultimate limit state. The bursting tensile force, Fbst , existing in an individual square end block loaded by a symmetrically placed square anchorage or bearing plate, may be derived from: Fbst =

Pk 0.32 − 0.3 y po y o

where: Pk yo y po

is the load in the tendon; is half the side of end block; is half the side of loaded area.

Equation B62




The force, Fbst , is distributed in a region extending from 0.2 y o to 2 y o fromtheloaded face of the end block. Reinforcement present may be assumed to sustain the bursting tensile force working up to a stress of f y γms . bst In rectangular end blocks, the bursting should be assessed from Equation B62. tensile force F in the two principle directions

When circular anchorage or bearing plates are present, the side of the equivalent square area should be derived. Where groups of anchorages or bearing plates occur, the end blocks should be divided into a series of symmetrically loaded prisms and each prism treated as above. When assessing the end block as a whole, it is necessary to check that the groups of anchorages are appropriately tied together by reinforcement. Special attention should be paid to end blocks having a cross-section different in shape from that of the general cross-section of the beam. Reference should be made to the specialist literature. Compliance with the preceding requirements will generally ensure that the bursting tensile forces along the load axis can be sustained. Alternative methods of assessment which use higher values of Fbst PK and allow for the tensile strength of concrete may be more applicable in some cases, particularly where large concentrated tendon forces are involved. Consideration should also be given to the spalling tensile stresses that occur in end blocks where the anchorage or bearing plates are highly eccentric. These stresses reach a maximum at the loaded face. 6.8B Considerations of Details

6.8.1B General The considerations in Clauses 6.8.2B to 6.8.5B are intended to supplement those for reinforced concrete given in Clause 5.8B. 6.8.2B Cover to Prestressing Tendons 6.8.2.1B G ener al

The covers given in Clauses 6.8.2.2B and 6.8.2.3B, other than those for curved ducts, are those that are currently considered to be necessary to provide adequate durability.




Page 81 of 100

6.8.2.2B Pr e-tension ed Tendon s

The requirements of Clause 5.8.2B concerning cover to reinforcement may be taken to be applicable. The ends of individual pre-tensioned tendons do not normally require concrete cover. 6.8.2.3B Tendons in D ucts

The cover to any duct should be not less than 50 mm. Requirements for the cover to curved tendons in ducts are given in BS 5400: Part 4 Appendix D. 6.8.3B Spacing of Prestressing Tendons 6.8.3.1B Tendons in D ucts

Recommendations for the spacing of curved tendons in ducts are given in BS 5400: Part 4 Appendix D. 6.8.4B Longitudinal Reinforcement in Prestressed Concrete Beams Reinforcement in prestressed concrete members may be considered to enhance the strength of the sections. 6.8.5B Links in Prestressed Concrete Beams Links present in a beam may be considered as shear reinforcement, in accordance with Clause 6.3B or to resist bursting tensile stresses in the end zones of posttensioned members, in accordance with Clauses 6.3.4B and 6.7.4B. 7B. PRECAST, COMPOSITE AND PLAIN CONCRETE CONSTRUCTION 7.1B General

7.1.1B Introduction Additional considerations that arise in assessment when precast members or precast components are incorporated into a structure or when a structure in its entirety is of precast concrete construction are detailed below. The assessment of plain concrete walls and abutments is also described. 7.1.2B Limit State Assessment 7.1.2.1B Basi s of Assessment

The limit state philosophy set out in Clause 4B applies equally to precast and in situ construction. In general, therefore, the relevant methods of assessment for reinforced concrete given in Clause 5B and those for prestressed concrete given in Clause 6B apply also to precast and composite construction. Sub-clauses in Clause 5B or 6B that do not apply are either specifically worded for in situ construction or are modified below.




Page 82 of 100

7.1.2.2B Connections and Joi nts

The strength of connections is of fundamental importance in precast construction and should be carefully considered in assessment. In the assessment of beam and slab ends on corbels and at supports, particular attention should be given to the detailing of overlaps and anchorages and all reinforcement adjacent to the contact faces. Reinforcement detailing should be assessed in accordance with Clause 5.8.7B. 7.2B Precast Concrete Construction

7.2.1B Framed Structures and Continuous Beams When the continuity of reinforcement or tendons through the connections and/or the interaction between members is such that the structure behaves as a frame, or other rigidly interconnected system, the analysis, re-distribution of moments and assessment of individual members, may all be in accordance with Clause 5B or 6B, as applicable. 7.2.2B Other Precast Members All other precast concrete members should be assessed in accordance with the applicable requirements of Clauses 5B, 6B or 7.5B. Connections should be assessed

in accordance with Clause 7.3B. 7.2.3B Supports for Precast Members 7.2.3.1B Concr ete Cor bels

A corbel is a short cantilever beam in which the principal load is applied such that the distance av between the line of action of the load and the face of the supporting member does not exceed the effective depth and the depth, at the outer edge of the bearing is not less than one-half of the depth at the face of the supporting member. The shear capacity at the face of the supporting member should be assessed in accordance with Clause 5.3.3.3B, but using the modified definition of av given above. The adequacy of the main tension reinforcement in a corbel should be assessed on the assumption that the corbel behaves as a simple strut and tie system and making due allowance for horizontal forces. The tensile force which the main reinforcement can develop may be limited by any one of the following: • the yield of the reinforcement;

• •

the anchorage of the reinforcement in the supporting member; the anchorage at the front face of the corbel.

Any part of the area of the bearing that projects beyond the straight portion of the bars forming the main tension reinforcement should be ignored when proportioning the strut and tie system, and when checking bearing stresses in accordance with Clause 7.2.3.3B.




Page 83 of 100

7.2.3.2B Wi dth of Suppor ts f or Precast Uni ts

The width of supports for precast units should be sufficient to provide proper anchorage of tension reinforcement in accordance with Clause 5.8.6B. 7.2.3.3B Bear i ng Str esses

The compressive stress in the contact area should not exceed 0.6 fcu/γmc under the ultimate loads. When the members are made of concretes of different strengths, the lower concrete strength is applicable. Higher bearing stresses are acceptable where suitable measures have been taken to prevent splitting or spalling of the concrete, such as the provision of well-defined bearing areas and additional binding reinforcement in the ends of the members. Bearing stresses due to ultimate loads should be limited to: 3(f cu

γmc )

Expression B63

1+ 2 Acon Asup where: Acon Asup

is the contact area; is the supporting area.

For rectangular bearings (see Figure B7): Asup =

(b x + 2 x )(by + 2 y ) and x ≤ b x , and y ≤ by

x Supporting area bx Contact area

x

y

b

y

y

Figure B7 Bearing Areas

where: b x , by

are the dimensions of the bearing in the x, y directions respectively;



x ,y


are the dimensions from the boundary of the contact area to the boundary of the support area.

For lightweight aggregate concrete the bearing stresses due to ultimate loads should be limited to two-thirds of those for normal weight aggregate concrete given by the above formula. Higher bearing stresses due to ultimate loads should be used only where justified by tests. 7.2.3.4B H ori zontal F orces or Rotations at Beari ngs

The presence of significant horizontal forces at bearings can reduce the load-carrying capacity of the supporting and supported members considerably by causing premature splitting or shearing. These forces may be due to creep, shrinkage and temperature effects or result from misalignment, lack of plumb or other causes. When these forces are likely to be significant, it is necessary to check that either: (a) (b) (c)

sliding bearings are present; or suitable lateral reinforcement is present in the top of the supporting member; and continuity reinforcement is present to tie together the ends of the supported members.

Where, owing to large spans or other reasons, large rotations are likely to occur at the end supports of flexural members, suitable bearings capable of accommodating these rotations should be present. In the absence of such bearings, bearing stresses could be increased due to concentration of the reaction towards one edge of a bearing and/or flexure of the supported member could result, depending on the type of bearing actually present. 7.2.4B Joints between Precast Members 7.2.4.1B G ener al

The critical sections of members close to joints should be assessed under the worst combinations of shear, axial force and bending effects caused by the co-existing ultimate vertical and horizontal forces. The evaluation of the effects should take due account of any fixity imposed by the joints. 7.2.4.2B H alvin g Join t

For the type of joint shown in Figure B8(a), the maximum vertical ultimate load, Fv , should not exceed v u bd o . where: vu b

is the lesser of 0.92 f cu γor mc is the breadth of the beam;

7

γ mc

inN/mm²;




do

Page 85 of 100

is the depth to the horizontal reinforcement in the halving joint.

The capacity of a halving joint may be determined by considering the two following strut and tie systems and summing the capacities of the two systems, and in accordance with the recommendations of BA 39/93: Assessment of Reinforced Concrete Half-joints. The system, shown in Figure B8(b), involves the inclined reinforcement which intersects the line of action of Fv . The inclined reinforcement may take the form of bent-up bars or links. For bent-up bars, the bearing stresses inside the bends should be checked to determine whether the stress in the bars should be limited to less than f y γms in accordance with Clause 5.8.5.8B. For links, their anchorage in the compression face of the beam should be in accordance with Clause 5.8.5.4B, whilst in the tension face the horizontal component, Fh , of the link force is transferred to the main reinforcement. The links may be considered to be fully anchored in the tension face if the anchorage bond stress of the main reinforcement due to the force Fh does not exceed twice the anchorage bond stresses given in Clause 5.8.5.2B. The strut and tie system shown in Figure B8(c) involves the vertical reinforcement in the full depth section adjacent to the halving joint, and requires the horizontal reinforcement in the halving joint to be in excess of that required to resist the horizontal ultimate load.




Inclined links

Page 86 of 100

Vertical links

d0

Horizontal reinforcement

Main tension reinforcement

(a)

Compressive strut Reinforceme

(b)

nt tie

(c) Figure B8 Halving Joint

7.3B Structural Connections between Units 7.3.1B General 7.3.1.1B Str uctur al Requi r ements of Connec ti ons

When assessing the connections across joints between precast members the overall stability of the structure should be considered.




Page 87 of 100

7.3.1.2B Assessment M ethod

Connections should, where possible, be assessed in accordance with the generally accepted methods applicable to reinforced concrete (see Clause 5B), prestressed concrete (see Clause 6B) or structural steel. Where, by the nature of the construction or material used, such methods are not applicable, the adequacy of the connection should be proved by tests. 7.3.2B Continuity of Reinforcement 7.3.2.1B G ener al

The assumptions made in analysing the structure and assessing critical sections should reflect the degree of continuity of reinforcement through a connection. The following methods are capable of achieving continuity of reinforcement: (a) (b) (c) (d)

lapping bars; butt welding; sleeving; parallel threading of bars and tapered threads.

The strength of the joints in (c) and (d) and any other method not listed should be assessed on the basis of test evidence. 7.3.2.2B Sleevin g

The following three principal types of sleeve jointing may be found: (a) (b) (c)

grout or resin filled sleeves; sleeves that mechanically align the square-sawn ends of two bars to allow the transmission of compressive forces only; sleeves that are mechanically swaged to the bars.

7.3.2.3B Threading

The following methods for joining threaded bars may be found: (a) (b) (c) (d)

parallel threaded ends of bars are joined by a coupler having left - and right hand threads; one set of bars is welded to a steel plate that is drilled to receive the threaded ends of the second set of bars, which are fixed to the plate by means of nuts; threaded anchors cast into a precast unit to receive the threaded ends of reinforcement; taper threaded bars joined by the use of internally taper threaded couplers.




The structural assessment of special threaded connections should refer to testing in accordance with BS 5400: Part 1, (including behaviour under fatigue conditions where relevant) which may have been carried out when the structure was designed. Where tests have shown the tensile strength of the threaded connection to be greater than or equal to the characteristic strength of the parent bars, the strength of the joint may be based on the specified characteristic strength of the joined bars divided in each case by the applicable γms partial factor. 7.3.2.4B Weldi ng of Bar s

Where bars are known or suspected to be welded the fatigue strength of the bars should be assessed in accordance with Clause 4.7B. 7.3.3B Other Types of Connection The load carrying capacity of any other type of connection should have been justified by test evidence when the Bridge was designed. For resisting shear and flexure suitable connections are those types which are made by prestressing across the joint. 7.4B Composite Concrete Construction

7.4.1B General The requirements of this Clause apply to flexural members consisting of precast concrete units acting in conjunction with added concrete where the contact surface is capable of transmitting longitudinal shear. The precast units may be of either reinforced or prestressed concrete.

In general, the analysis and assessment of composite concrete structures and members should be in accordance with Clause 5B or 6B, modified where applicable by Clauses 7.4.2B and 7.4.3B. Particular attention should be given in the assessment to the effect of the method of construction and whether or not props were used. The relative stiffness of members should be based on the concrete, gross transformed or net transformed section properties as described in Clause 4.4.2.1B. If the concrete strengths in the two components of the composite member differ by more than 10 N/mm², allowance for this difference should be made in assessing stiffness and stresses. Differential shrinkage between the added concrete and precast concrete members should be considered in analysing composite members for the serviceability limit statesstate. in accordance with Clause 7.4.3B. It need not be considered for the ultimate limit Precast units, incorporating pre-tensioned tendons, assessed as continuous members with continuity obtained with reinforced concrete cast in situ over the supports, the compressive stresses due to prestress in the ends of the units may be assumed to vary linearly over the transmission length of the tendons in assessing the strength of sections.




Page 89 of 100

7.4.2B Ultimate Limit State 7.4.2.1B G ener al

Where the cross-section of composite members and the applied loading increased during construction in stages (such as when a precast prestressed unit initially supporting self-weight and the weight of added concrete and subsequently acting compositely for live loading), the entire load may be assumed to act on the final cross-section. 7.4.2.2B Ver ti cal Shear

The assessment of the resistance of composite sections to vertical shear should be in accordance with Clause 5.3.3B for reinforced concrete (except that in determining the area As , the area of tendons within the transmission length should be ignored) and Clause 6.3.4B for prestressed concrete, modified where applicable as follows: (a)

(b)

For I, M, T, U and box beam precast prestressed concrete units with an in situ reinforced concrete top slab cast over the precast units (including pseudo box construction), the shear resistance should be based on either of the following: (i)

the precast unit acting alone assessed in accordance with Clause 6.3.4B;

(ii)

the composite section assessed in accordance with Clause 6.3.4B. In this case, section properties should be based on those of the composite section, with due allowance for the different grades of concrete where applicable.

For inverted T beam precast prestressed concrete units with transverse reinforcement placed through holes in the bottom of the webs of the units, completely infilled with concrete placed between and over the units to form a solid deck slab, the shear resistance should be taken as the sum of Vi and Vp where: Vi

is the shear capacity of the precast prestressed section assessed in accordance with Clause 6.3.4B with the breadth taken as the web thickness and the depth as the depth of the precast unit. In applying Clause 6.3.4.4B, dt should be derived for the composite section. Vp

(c)

is the shear capacity of the infill concrete assessed in accordance with Clause 5.3.3B with the breadth taken as the distance between adjacent precast webs and the depth as the mean depth of infill concrete, or the mean effective depth to the longitudinal reinforcement where this is provided in the infill section.




Page 90 of 100

7.4.2.3B L ongi tudi nal Shear

The longitudinal shear force, V1 , per unit length of a composite member, whether simply supported or continuous, should be calculated at the interface of the precast unit and the in situ concrete and at any vertical planes that may be critical in longitudinal shear (as shown by planes 2-2 or 2´-2´ in Figure B9). An elastic method may be adopted using properties of the composite concrete section, in accordance with Clause 4.4.2.1B, with due allowance for different grades of concrete where applicable.

2

2' In-situ concrete

1

1

2

2'

Precast beam

Figure B9 Potential Planes

Vi

should not exceed the lesser of the following:

(a)

( k1f cu

(b) (

)v1(

γmc )L s

γ mv

L s +) 0.8 Ae f s

Expression B64

γms

Expression B65

where: k1

f cu

γmc γms Ls v1

is a constant depending on the concrete bond across the shear plane under consideration, which may be taken as 0.24 for monolithic construction or surface type 1, or 0.14 for surface type 2. These values should be reduced by 25% for lightweight aggregate concrete construction; is the characteristic, or worst credible, strength of the weaker of the two concretes each side of the shear plane but should not be taken as >45 N/mm². is the partial factor for concrete given in Clause 4.3.3.3B; is the partial factor for steel given in Clause 4.3.3.3B; is the breadth of the shear plane under consideration; is the longitudinal shear stress in the concrete for the plane under consideration, and should be taken as:



for monolithic construction:

γmv Ae

fs


0.05 f cu but not less than 1.13 N/mm² and not greater than 1.56 N/mm²;

forsurfaceType1:

0.04 f cu but not less than 0.8 N/mm² and not greater than 1.28 N/mm²;

forsurfaceType2:

0.019 f cu but not less than 0.38 N/mm² and not greater than 0.63 N/mm².

All values should be reduced by 25% for lightweight aggregate concrete construction; is the partial factor for material strength for shear given in Clause 4.3.3.3B; is the area of reinforcement per unit length crossing the shear plane under consideration; reinforcement assumed to resist co-existent bending and vertical shear, in accordance with Clause 7.4.2.2B, may be included; is the stress at the ultimate limit state in the steel reinforcement of area Ae . The stress may be assumed to be the characteristic, or worst credible strength, f y , if the reinforcement Ae is fully anchored, in accordance with Clause 5.8.6B. Otherwise f s should be taken as a fraction of f y in proportion to the ratio of the anchorage available to that required by Clause 5.8.6B. The value of f s should be such that Ae f s b is not greater than 10 N/mm² where b is the width of the interface under consideration.

For composite beam and slab construction, reinforcement crossing the shear plane should be considered as effective only if its spacing does not exceed the lesser of the following: (a)

six times the minimum thickness of the in situ concrete flange and;

(b)

900mm.

The types of surface are defined as follows: Type 1 - The contact surface of the concrete in the precast members was prepared as described in either (1) or (2) as applicable: (1)

when the concrete had set but not hardened the surface was sprayed with a fine spray of water or brushed with a stiff brush, just sufficient to remove the outer mortar skin and expose the larger aggregate without disturbing it;

(2)

the surface skin and laitance were removed by sand blasting or the use of a needle gun, but not by hacking.




Type 2 - The contact surface of the concrete in the precast member was jetted with air and/or water to remove laitance and all loose material. (This type of surface is known as ‘rough as cast'.) The type of surface should be assessed from record drawings, available site data or srcinal design calculations. In the absence of such information, surface Type 2 should be assumed. For inverted T-beams defined in Clause 7.4.2.2B(b) no longitudinal shear strength check is required. As an alternative to the use of conventional elastic analysis for checking longitudinal shear, the following approach may be used provided the effective area of steel crossing the interface exceeds 0.15 times 460/fs % of the concrete area: The force required according to the ultimate flexural analysis in the part of the critical section in flexure which is outside the interface should be calculated. This force should be not greater than the total interface shear strength available over the length of beam between the critical section in flexure and the point of contraflexure under the relevant load case. This approach is more consistent with the flexural analysis and gives higher strengths when the interface is locally inadequate to the conventional check. 7.4.3B Serviceability Limit State When a serviceability limit state assessment is required, the method of checking should generally be as given in BS 5400: Part 4. However, where flexural tensile stresses in the in situ concrete exceed the permitted values, stresses in the precast concrete may be calculated using section properties determined ignoring the in situ concrete that is in tension. 7.5B Plain Concrete Walls and Abutments

7.5.1B General A plain concrete wall or abutment is a vertical load bearing concrete member whose greatest lateral dimension is more than four times its least lateral dimension and which is assumed to be without reinforcement when considering strength.

The requirements given in Clauses 7.5.2B to 7.5.10B refer to the assessment of a plain concrete wall that has a height not exceeding five times its average thickness. 7.5.2B Moments and Forces in Walls and Abutments Moments, shear forces and axial forces in a wall should be determined in accordance with Clause 4.4B.




The axial force may be calculated on the assumption that the beams and slabs transmitting forces into it are simply supported. The resultant axial force in a member may act eccentrically due to vertical loads not being applied at the centre of the member or due to the action of horizontal forces. Such eccentricities should be treated as indicated in Clauses 7.5.3B and 7.5.4B. The minimum moment in a direction at right-angles to the wall should be taken as not less than that produced by considering the ultimate axial load per unit length acting at an eccentricity of 0.05 times the thickness of the wall. 7.5.3B Eccentricity in the Plane of the Wall or Abutment For a single member, the eccentricity may be calculated from statics alone. Where a horizontal force is resisted by several members, the amount allocated to each member should be in proportion to its relative stiffness provided the resultant eccentricity in any individual member is not greater than one-third of the length of the member. Where a shear connection between vertical edges of adjacent members can withstand the calculated forces, an applicable elastic analysis may be used. 7.5.4B Eccentricity at Right-angles to Walls or Abutments The load transmitted to a wall by a concrete deck may be assumed to act at one-third the depth of the bearing area from the loaded face. Where insitu concrete decks span onto either side of the member, the common bearing area may be assumed to be shared equally by each deck.

The resultant eccentricity of the total load on a member unrestrained in position at any level should be calculated making full allowance for the eccentricity of all vertical loads and the overturning moments produced by any lateral forces above that level. The resultant eccentricity of the total load on a member restrained in position at any level may be calculated assuming that immediately above a lateral support the resultant eccentricity of all the vertical loads above that level is zero. 7.5.5B Analysis of Section Loads of a purely local nature, for example, bearings or column bases, may be assumed to be immediately dispersed, provided the local stress under the load does not exceed that given in Clause 7.5.7B. Where the resultant of all the axial loads acts

eccentrically in the plane of the member, theanultimate axial load per unitalength wall, nw , should be assessed on the basis of elastic analysis, assuming linear of distribution of load along the length of the member, and a tensile resistance of concrete of: 0.12

f cu

γmc

Expression B66




Consideration should first be given to the axial force and bending in the plane of the wall to determine the distribution of tension and compression along the wall. The bending moment at right-angles to the wall should be considered and the section assessed for this moment and the compression or tension per unit length at various positions along the wall. Where the eccentricity of load in the plane of the member is zero, a uniform distribution of nw may be assumed. For members restrained in position, the axial load per unit length of member, nw , due to ultimate loads should be such that: nw

≤

h) − 2e x (0.675 f cu γ)( mcw

Equation B67

where: nw h ex f cu

γmcw

is the maximum axial load per unit length of member due to ultimate loads; is the overall thickness of the section; is the resultant eccentricity of load at right-angles to the plane of the member, see Clause 7.5.2B, (minimum value 0.50h); is the characteristic, or worst credible, concrete strength; is a material partial factor defined in Clause 43.3.3B.

7.5.6B Shear The resistance to shear forces in the plane of the member may be assumed to be adequate, provided the horizontal shear force due to ultimate loads is less than either one-quarter of the vertical load, or the force to produce an average shear stress of 0.45 N/mm² over the whole cross-section of the member where f cu is at least 25 N/mm². Where f cu is less than 25 N/mm², a figure of 0.3 N/mm² should be used. 7.5.7B Bearing Bearing stresses due to ultimate loads of a purely local nature, as at girder bearings, should be limited in accordance with Clause 7.2.3.3B. 7.6B Mass Concrete Arches Mass concrete arches may be analysed according to the rules for masonry arches given in Section 6. Where methods other than MEXE are used, the compressive strength of the concrete should be taken as 0.6 f cu .




APPENDIX B1 : HISTORICAL CONCRETE GRADES

This Appendix summarises historical data relating to concrete strengths that has been gathered during various assessment projects. The data are for advice and guidance only. It should be noted that depending on the particular Standard or Specification, the strength of concrete is not always the same for a given classification or concrete mix. Furthermore, depending on the particular Specification, as the class increases from A to C, the concrete strength may increase or decrease. Caution should therefore be exercised when using this data. It is always preferable to use the actual Specification for the particular structure where it is identified and available.




Page 96 of 100

The Engineer's Year Book, 1923, edited by H R Kempe and W Hanneford Smith 1923 443 Pg 28-day Cube Strength Re f Nom in al M i x Works Test lb/in2 N/mm2 (lb.:c.ft.:c.ft.) 12 :4 : 1800 12.4

Concrete Construction, Charles E Reynolds, 2nd Edition, 1945, Pg. 257 Table XIX Reinforced Concrete Designers' Handbook, Chas E Reynolds, 3rd Edition, Table 1945, No. 23 1945 Ministry of Transport Ordinary Grade Concrete 28-day Cube Strength Re f Nom in al M i x Works Test 2 2 (lb.:c.ft.:c.ft.) lb/in N/mm IV 42 :1 : 2250 15.5 2

III I

1

1 : 1 /3 : 3 /3 2580 17.8 11½ : 3 : 2700 18.6 21 :1 : 3600 24.8

LCC By-laws Ordinary Grade Concrete 28-day Cube Strength Re f

Nom in al M i x

Works Test lb/in2 N/mm2

(lb.:c.ft.:c.ft.) III

1 2: 4

2250 2

1

1 : 1 /3 : 3 /3 II

11½ : 3:

I

21 :1 :

15.5 2450

16.9

2550

17.6

2925

20.2

LCC By-laws High Grade Concrete 28-day Cube Strength Re f

Nom in al M i x

Works Test lb/in2 N/mm2

(lb.:c.ft.:c.ft.) IIIA

12 :4 : 2850 2 1 1 : 1 /3 : 3 /3 3155

IIA

11½ : 3 :

IA

21 :1 :

3300

19.7 21.8 22.8

3750

25.9

Ministry of War Transport Memorandum No. 577 1945(Reprinted 1949) Section6 28-day Cube Strength Re f Nom in al M i x Works Test lb/in2 N/mm2 (lb.:c.ft.:c.ft.) A 150 : 2 : 4 3600 24.8 B 1202 :4 : 3300 22.8 C 90 4 2: 2850 19.7

Approximate Equivalent Nominal Mix by volume 3

assuming 90 lb/ft cement 1 : 1.2 : 2.4 1 : 1½ : 3 1:2:4

Note: ond to mixes II, III, IV for High Grade he Code of Practice for Reinforced Concrete.

Design and Construction of Reinforced Concrete Bridges, A W Legat, G Dunn, W A Fairhurst, 1948 Specification for a Typical Bridge Contract 1948

Pg. 467, Table A 28-day Cube Strength

Re f A

Nom in al M i x 1:2:3

Works Test 2 2 lb/in N/mm 3150

3300

21.7

B

1 1½ : 3:

C

21 :1 :

3750

25.9

22.8

D

53 :1 :

1800

12.4

First Report on Prestressed Concrete, Institution of Structural Engineers 1951

Clause 4.(f)(i)




Page 97 of 100

Design and Construction of Reinforced Concrete Bridges, A W Legat, G Dunn, W A Fairhurst, 1948 Specification for a Typical Bridge Contract 1948

Pg. 467, Table A 28-day Cube Strength

Ref

N omina l Mix

Wor ks Test 2

A

1:2: 3

lb/in

N/mm

3150

21.7

B C

11½ : 3 : 21 :1 :

3300 3750

22.8 25.9

D

53 :1 :

1800

12.4

2

First Report on Prestressed Concrete, Institution of Structural Engineers 1951

Clause 4.(f)(i) 28-day Cube Strength Works Test 2

Pre-tensioned steel

2

lb/in

N/mm

6000 min.

41.4 min.

generally

CP 1 1 4 - R e inf o r c e d C onc r e t e in B ui ld i ng s 1957 Table 1

N o m i n a lM i x

2 8 - d ayC u b eS t r e n g t h W o r k sT e s t 2

1:1:2 1:1½:3 1:2:4

lb/in 4500 3750 3000

N/mm

2

N/mm

41.4min. 31.0min.

2

2:1:1 1½ 3:1 : 4:2:1

2 8 - d ayC u b eS t r e n g t h W o r kTse s t lb/in 6000min. 4500min.

Table 1

(with amendments up to 1973) 2 8- d ayC u b eS t r e n g t h N o m i n a lM i x W o r k sT e s t

2

31.0 25.9 20.7

C P 1 1 5 - P r es t r es s e d C o n cr e t e in B u i ld i n g s 1959 Clause 207

Pre-tensioned Post-tensioned

C P 1 14 : Pa r t 2 1969

lb/in 30.0 25.5 21.0

2

N/mm

C P 11 5 1969 Clause 207 (with amendments up to 1977) 2 8- d ayC u b eS t r e n g t h W o r kTse s t 2

2

lb/in Pre-tensioned Post-tensioned

2

N/mm 40.0min. 30.0min.




Page 98 of 100

British Rail Midland Region - Drawing Office Handbook Section 5.2 1960's 28-day Cube Strength Ref N omina l Mix Wor ks Test 2

A B C D

lb/in 1 : 1½ : 3 4000 42 :1 : 3000 63 :1 : 1500 1:4:8 Notspecified

N/mm

2

27.6 20.7 10.3

British Rail Midland Region - Specification 1960's 28-day Cube Strength Ref N omina l Mix Wor ks Test 2

lb/in 1 : 1½ : 3 4000 4:2:1 3000 6:3:1 1500 1:4:8 Notspecified

A B C D

Prestressed Concrete A1

7000

N/mm

2

27.6 20.7 10.3

48.3

Standard Beam Sections for Pretressed Concrete Bridges, Prestressed Concrete Development Group (C&CA) 1961 28-day Cube Strength Works Test 2

Pre-tensioned steel

lb/in 6000 min. generally

N/mm

2

41.4 min.

M i ni s t r y o f T r a n sp or t

[S o u r c e: C o n c r e t e B r i d g e D e si g n , R E R o w e , 1 9 6 2 ]

Memorandum No. 785 1961 Ref

Nom ina l Mix 28 da y Cu be St re ngt h (l b . : c . f t . : c . f t . ) Wor ks Test 2

A B C

lb/in 150 : 2 : 4 4200 120:2:4 3750 90 2 4 :: 3000

N/mm

29.0 25.9 20.7

2

Approximate Equivalent Nominal Mix by volume 3

assuming 90 lb/ft cement 2.4 1.2 :: 1 3 1½ : : 1 4 : 2 : 1




Page 99 of 100

Ministry of Transport Specification for Road & Bridge Works 3rd Edition 1963 Table A & Table B 28-day Cube Strength Class Wor k s Te s t 2

A¾ A 1½ ¾B 1½ B ¾C 1½ C 1½ D 3 D E 1½ E 3

lb/in N/mm 29.0 4200 29.0 4200 25.9 3750 25.9 3750 20.7 3000 20.7 3000 16.5 2400 16.5 2400 Not specified Not specified

¾X

7500

51.7

3

7500 6000

51.7 41.4

3

6000 4200 4200

41.4 29.0 29.0

X /8 ¾Y Y /8 ¾ Z 1½ Z

C P1 1 6-P r e c a s tC o n c r e t e 1965

2

Table 1 2 8 - d a yC u b eSt r e n g th W o r kTse s t

Gr a de

C P1 1 6 1969 Table 1 (with amendments up to 1977) 2 8 - d a yC u b eSt r e n g th Gr a de W o r kTse s t

2

2 lb/in N/mm 20.7 3000 25.9 3750 31.0 4500 41.4 6000 51.7 7500 * Lowest grade for post-tensioned steel ** Lowest grade for pre-tensioned steel

A B C* D** E

2

lb/in A B C* D** E (Clause 208)

General Specification for Concrete, British Rail Civil Engineering Handbook No. 21, 1965 [Source: Notes for Designers on the Use of the General Specification for Concrete, BRB Civil Engineering Department, Technical Note No.3, February 1966] 1965 StandardMixes Table1&Appendix2 28-day Cube Strength Class Wor k s Te s t 2

3

SAH /8 SAH ¾ SAH 1½ 3

SBH /8 SBH ¾ SBH 1½ SCH ¾ SCH 1½

lb/in

N/mm

3000 3000 3000

20.7 20.7 20.7

3750

25.9

3750 3750 4500 4500

25.9 25.9 31.0 31.0

2

Notes: S = Standard mix Y = Designed mix A-E = Strength grade H = High workability

2

N/mm 21.0 25.5 30.0 40.0 50.0



The Structural Assessment of Underbridges Appendix B – Assessment of Concrete Structures DesignedMixes Class

Table2&Appendix2 28-day Cube Strength Wor k s Te s t 2

Page 100 of 100

A p p l ic a t i on

2

lb/in

N/mm

3

3000

20.7

Reinforced concrete

3

3750

25.9

Reinforced concrete

3

4500

31.0

Reinforced concrete

41.4

RC and infill concrete to precast beams

51.7

RC, precast T beams, precast rectangular hog back beams and jointing to transversely stressed beams

YA /8 - 1½ YB /8 - 1½ YC /8 - 1½ YD ¾ 1½ -

6000

YE 3/8 - 1½

7500

55.2

Precast rectangular beams

YAP

3000

20.7

Bored piles

YCP

4500

31.0

Bored piles (sulphate resisting)

YF ¾ 1½ -

8000

British Rail Southern Region - Contract Specification for Norwood High St Bridge 1968 S = Standard mix Y = Contractor designed mix A-E = Concrete grades to CP116 (see above) LWT = Lightweight concrete RH = Rapid harde ning

Technical Memorandum BE20 Prestressed Concrete for Highway Structures 1969 Prestressed concrete normally: Class 7500 Prestressed concrete exceptionally:

52.5 N/mm2

Class6000

45N /m m2

Class9000

60 N/m m2

Interim Memor andum IM8 (superseded BE20 + I M3) Prestressed Concrete for Highway Structures 1970 Prestressed concrete normally: 52.5 N/mm2 45 N/mm2

Technical Memora ndum BE10 (superseded Memo 57 7/2) Prestressed Concrete for Highway Structures 1970 Reinforcedconcretenormally: 30N /m m2


The Structural Assessment of Underbridges Appendix C Assessment of Composite Structures


CONTENTS

APPENDIX C 1C. SCOPE....................................................................................................................................2 4.3C Limit State Recommendations.................................................................................... 2 4.3.1C General....................................................................................................................2 4.3.2C Serviceability Limit State ...................................................................................... 2 5C. ANALYSIS OF STRUCTURE - SERVICEABILITY LIMIT STATE............................... 3 5.1.1C Distribution of Bending Moments and Vertical Shear Forces...................... 3 5.2.3C Effective Breadth of Concrete Flange ............................................................... 3 5.2.6C Control of Cracking in Concrete ...................................................................... 4 5.3C Longitudinal Shear......................................................................................................... 4 5.3.1C General....................................................................................................................4 5.4C Temperature Effects and Shrinkage Modified by Creep .......................................6 5.4.1C General....................................................................................................................6 6.1.2C Deck Slabs Forming the Flanges of Composite Beams..................................6 6.2C Analysis of Sections....................................................................................................... 6 6.2.1C General....................................................................................................................6 6.2.2C Bending Resistance of Compact Sections.........................................................7 6.2.3C Bending Resistance of Non Compact Sections ............................................... 7 6.2.4C Analysis of Slender Cross Sections.................................................................... 7 6.3.4C Shear Connectors ................................................................................................. 7 7.3C Composite Box Girders Effective Breadth .............................................................. 7 7.7C Composite Plate ............................................................................................................ 7 8.0C CASED BEAMS AND FILLER BEAM CONSTRUCTION ........................................8 8.2C Limit State Recommendations.................................................................................... 8 8.3.1C Transverse Moments in Filler Beam Deck ....................................................... 8 8.4C Analysis of Sections....................................................................................................... 8 8.4.1C Serviceability Limit State ...................................................................................... 8 8.4.2C Ultimate Limit State .............................................................................................. 8 8.5C Longitudinal Shear......................................................................................................... 9 8.5.1C General....................................................................................................................9 8.5.2C Cased Beams .......................................................................................................... 9 8.5.3C Filler Beams...........................................................................................................10 8.6C Temperature and Shrinkage Effects.........................................................................11 14C. CONCRETE INFILLED TROUGH CONSTRUCTION .........................................11




Page 2 of 12

1C. SCOPE Delete the existing BS 5400: Part 5 Clause 1 and substitute the following:

This Appendix augments Section 5 and Section 7 when steel or wrought iron components of bridges are interconnected with concrete bridge components, and act compositely. All references to structural steelwork should be read as also applicable to wrought iron except where stated otherwise. 4.3C Limit State Recommendations 4.3.1C General Delete the existing BS 5400: Part 5 Clause 4.3.1 and substitute the following: 4.3.1.1C Steelwor k and Wr ought I r on

All structural steelwork and wrought iron in composite beams should be assessed in accordance with Section 5 and Appendix A in relation to the ultimate limit state and in addition the serviceability limit state where required by Appendix A. The effects of temperature and shrinkage modified by creep should be assessed in accordance Clause 5.4C. 4.3.1.2C Concr ete and Rein f or cement

The concrete and reinforcement should be assessed to the ultimate limit state in accordance with Section 7 and Appendix B. Slabs, which are part of a composite beam, should also be assessed to the ultimate limit state in accordance with Section 7 and Appendix B. The serviceability limit state should be checked when coexistent stresses occur under local and global effects in accordance with BS 5400: Part 5 Clause 5.2.4.1. Where EUDL live loading is being considered, coexistent stresses are only likely to be critical in the vicinity of a transverse member supporting the slab. 4.3.1.3C Shear Conn ecti on

The shear connection should be assessed for the ultimate limit state in accordance with Clause 6.3.4C and additionally where relevant to Appendix D for fatigue. Assessment forconnectors the serviceability limit is only necessary thePart spacing between shear exceeds thestate recommendations of where BS 5400: 5 Clause 5.3.3.1. 4.3.2C Serviceability Limit State

Delete the existing BS 5400: Part 5 Clause 4.3.2 and substitute the following:




A serviceability limit state is reached when any of the following conditions occur: (a)

The stress in structural steel reaches the applicable limit given in Section 5. For most underbridges this is not likely to govern, see Appendix A;

(b)

Where stresses in concrete or reinforcement reach the applicable limits given in Section 7. This will only be critical where local and global effects are added together in accordance with BS 5400: Part 5 Clause 5.2.4.1;

(c)

Spacing of connectors exceeds the recommendations of BS 5400: Part 5 Clause 5.3.3.1, in which case a check on the serviceability recommendations for the shear connection is required.

5C. ANALYSIS OF STRUCTURE - SERVICEABILITY LIMIT STATE Recommendations given in the following Clauses are only required for serviceability limit state checks as required by Clause 4.3.1C. 5.1.1C Distribution of Bending Moments and Vertical Shear Forces Delete the existing BS 5400: Part 5 Clause 5.1.1.1 to 5.1.1.3 and substitute the following:

Analysis should be carried out to the same assumptions used for the analysis for the ultimate limit state in accordance with BS 5400: Part 5 Clause 6.1.4. 5.2.3C Effective Breadth of Concrete Flange Delete the existing BS 5400: Part 5 Clauses 5.2.3.1 to 5.2.3.8 and Tables 2, 3, and 4 and replace by Clauses 5.2.3.1C to 5.2.3.3C: 5.2.3.1C G ener al

In calculating the stresses in a flange, and in the absence of rigorous analysis, the effect of in-plane shear flexibility (shear lag) should be allowed for by assuming an effective breadth of flange in accordance with BS 5400: Part 3 Clause 8.2 or Clause 9.15.2.2 as applicable. 5.2.3.2C Ef fective Br eadth of Cr acked F lan ge

For a concrete in tension that is assumed to be5.2.3.1C cracked should the mean breadth ratio ψflange obtained in accordance with Clause be effective modified by adding (1− ψ ) 3 . 5.2.3.3C Wi dth Over Wh ich Slab Rei nf orcement is Ef fective

Only reinforcement within the effective breadth of the concrete slab parallel to or within 20o of parallel to the span of the steel beam should be assumed to be effective in analysing cross sections.




Page 4 of 12

5.2.4.3C Co-existent Stresses

Delete the existing BS 5400: Part 5 Clause 5.2.4.3 and substitute the following: In calculating combined stresses in a deck slab, which also forms the flange of a composite beam, the global longitudinal bending stress across the deck width may be calculated in accordance with BS 5400: Part 3 Appendix A Clause A.6. 5.2.5.1C Steel Secti on - General

Delete the existing BS 5400: Part 5 Clause 5.2.5.1 and substitute the following: The serviceability limit state should be checked in accordance with Appendix A. In carrying out serviceability limit state checks consideration should be given to the effects detailed in BS 5400: Part 5 Clauses 5.2.5.2 to 5.2.5.3 and Clause 5.2.5.4C. 5.2.5.4C Slab Cast i n Specif ied Sequence

Delete the existing BS 5400: Part 5 Clause 5.2.5.4 and substitute the following: Where the deck slab is known to have been cast in a specified sequence, the dead load stresses may be calculated on the composite section in accordance with BS 5400: Part 5 Clause 12.1 using the effective breadth calculated from BS 5400: Part 3 Clause 8.2. Where the sequence of casting is unknown, the slab should be assumed to have been cast in one operation. 5.2.6C Control of Cracking in Concrete For assessment BS 5400: Part 5 Clauses 5.2.6.1 to 5.2.6.4 including Tables 5 and 6 may be ignored. 5.3C Longitudinal Shear 5.3.1C General Delete the existing BS 5400: Part 5 Clause 5.3.1 and substitute the following:

Longitudinal shear per unit length q, of a composite beam, whether simply supported or continuous, should be calculated on the basis of elastic theory using the properties of the transformed composite cross section calculated assuming the concrete flange to be uncracked and unreinforced. The effective breadth of the concrete flange may be assumed to be constant over any span and may be taken as the quarterspan value for uniformly distributed loading given in BS 5400: Part 3 Clause 8.2. Where the second moment of area of the composite section varies significantly along the length of any span, account should be taken of the variation of stiffness in calculating the longitudinal shear flow. Calculation of horizontal shear for the serviceability limit state is only required for the assessment of fatigue or when the longitudinal spacing of connectors exceeds the recommendations of BS 5400: Part 5 Clause 5.3.3.1.




Page 5 of 12

5.3.2.1C Nomi nal Str ength s of Shear Conn ectors Embedded in N orm al D ensity Concrete

Add the following paragraph to that below BS 5400: Part 5 Table 7: Nominal static strengths of bolt and rivet heads may be calculated from Equation C1. Means of preventing separation of the concrete should be present such as by encasement or other mechanical devices, unless tests are carried out to demonstrate that adequate means of preventing separation are present. Nominal Static Strength

=

2.0 A1f cu

γmc γb

Equation C1

where:

γb

may be taken as 1.25 for bolts/bolt heads or other connectors with predominantly vertical surfaces resisting the horizontal shear, and as 2.0 for rivet heads or for other connections with predominantly inclined surfaces resisting the horizontal shear;

A1

is the face area of the connector in the direction of horizontal shear.

5.3.3.4C Upl if t on Shear Conn ectors

In BS 5400: Part 5 Clause 5.3.3.4(b), insert the following after “bracing”: “or from the forces generated at the corners when the slab acts as part of a Uframe”. Delete the remainder of BS 5400: Part 5 Clause 5.3.3.4 from “the effect of axial tension .......” and insert “ where a stud connector is subject to both shear Q and tension due to uplift Tu the equivalent shear Q max for checking the connector should be taken as: 2

Q max =

Q2 +

Tu 3

Equation C2

5.3.3.5C Design Pr ocedur e: Gener al

Delete the existing BS 5400: Part 5 Clause 5.3.3.5 and substitute the following: Shear connectors need not be checked for the serviceability limit state except for fatigue in accordance with Appendix D or where the longitudinal spacing exceeds the recommendations of BS 5400: Part 5 Clause 5.3.3.1.




Page 6 of 12

5.4C Temperature Effects and Shrinkage Modified by Creep 5.4.1C General Delete the existing BS 5400: Part 5 Clause 5.4.1 paragraph 1, lines 3 to 9 from “serviceability limit state in composite beams ............” to “ the serviceability limit state.”, and insert:

“for the beam section when serviceability checks are to be carried out in accordance with Appendix A. In such checks account should be taken of the longitudinal shear forces arising from these effects.”. 5.4.2.1C Temper atur e Ef fects to be considered

Delete the existing BS 5400: Part 5 Clause 5.4.2.1 paragraph 2, lines 7 and 8 from “and assuming the concrete slab to be of effective breadth as given in Table 8” in and insert: “No account need be taken of shear lag. Concrete may be assumed to be uncracked, except that, for calculating longitudinal bending stresses due to the secondary effects in (c) above, the concrete in tension may be ignored”. Delete Table 8. 6.1.2C Deck Slabs Forming the Flanges of Composite Beams Delete the existing BS 5400: Part 5 Clause 6.1.2 second and third paragraphs and replace with:

“Local and global effects should be considered separately and should not be combined for the ultimate limit state except for a deck slab spanning longitudinally between a series of cross beams and which is composite with the longitudinal members and where the dispersed width of the local loading is greater than 75% of the effective width of the slab forming part of the composite longitudinal members.” 6.2C Analysis of Sections 6.2.1C General Delete the existing BS 5400: Part 5 Clause 6.2.1 and substitute the following:

The strength of composite sections should be assessed using a plastic or an elastic modulus as specified in Appendix A for calculating the bending resistance of the beam. If a plastic modulus is used, the transformed width of the concrete flange should be obtained from:

 0.4f cu    σ yc γm 

( width of concrete flange ) where:

Expression C3



f cu

σ yc γm


is the characteristic concrete cube strength (refer to Section 7); is the nominal yield strength of the compression flange of the steel member (refer to Section 5); is the partial factor for material strength for the steel member (refer to Section 5).

If an elastic modulus is used, the stresses in the concrete and reinforcement should not exceed the limits given in Section 7 and Appendix B. 6.2.2C Bending Resistance of Compact Sections For assessment BS 5400: Part 5 Clause 6.2.2 may be ignored. 6.2.3C Bending Resistance of Non Compact Sections For assessment BS 5400: Part 5 Clause 6.2.3 may be ignored. 6.2.4C Analysis of Slender Cross Sections For assessment BS 5400: Part 5 Clause 6.2.4 may be ignored. 6.3.3.3C I nteraction be tween L ongi tudi nal Shear & Tr ansver se Bendin g

In BS 5400: Part 5 Clause 6.3.3.3(b), paragraph 2, line 6 - delete “6.3.2(a)”, insert “6.3.3.2(a)”. 6.3.4C Shear Connectors Delete the existing BS 5400: Part 5 Clause 6.3.4 and substitute the following:

Assessment of shear connectors should be considered at the ultimate limit state in accordance with BS 5400: Part 5 Clause 6.3.1. The static strength per connector 0.8Pu should be taken as where Pu is the nominal strength (as defined in Clause 5.3.2.1C).

γm

γm

should be taken as 1.10.

7.3C Composite Box Girders Effective Breadth In BS 5400: Part 5 Clause 7.3 delete references to “Clauses 5 and 6 as appropriate”, and insert “BS 5400: Part 3 Clause 8.2.”. 7.7C Composite Plate Add to the end of the existing BS 5400: Part 5 Clause 7.7 the following new paragraph: The longitudinal shear forces due to local loading in the regions of a composite plate supported by cross members may be determined by considering the plate as an equivalent simply supported beam spanning between these cross frames; the width of the equivalent beam, b, supporting the load should be taken as:




b

=

4x +u 3

Page 8 of 12 Equation C4

where:

x u

is the distance from centroid of the loaded area to the nearest cross frame; is the length of the loaded area, which is parallel to cross frame.

8.0C CASED BEAMS AND FILLER BEAM CONSTRUCTION 8.2C Limit State Recommendations Delete the existing BS 5400: Part 5 Clause 8.2 and substitute the following:

Except where specific recommendations are given in BS 5400: Part 5 Clause 8, cased beams and filler beam decks should be assessed for the ultimate limit state only. 8.3.1C Transverse Moments in Filler Beam Deck Delete the existing BS 5400: Part 5 Clause 8.3.1 and substitute the following:

Transverse moments should be calculated using the load distribution as follows: (a)

the decks spans simply supported filler beams taking a static of railway distribution of loads between the between filler beams including for dispersal live loading through track and ballast in accordance with Section 4.

(b)

Alternatively, or where the assessment does not meet the recommendations on the basis of static distribution in (a) above, an elastic distribution analysis should be carried out of the filler beam construction assuming that the concrete is uncracked and unreinforced with the filler beams assumed composite. Where filler beam decks occur in half through bridges Appendix A Clauses 9.6.5A, 9.12.2A and BS 5400: Part 3 Clauses 9.6.6 and 9.12.3 should be used to calculate U-frame forces.

For filler beam decks less than 300 mm in depth where there is no encasement above the top flange nor below the bottom flange (as in former type A’ decks) only static distribution as in (a) should be assumed. No composite behaviour should be taken, but the steel beam may be assumed to be compact. 8.4C Analysis of Sections 8.4.1C Serviceability Limit State For assessment BS 5400: Part 5 Clause 8.4.1 may be ignored. 8.4.2C Ultimate Limit State Delete the existing BS 5400: Part 5 Clause 8.4.2 and substitute the following:




Page 9 of 12

The moments of resistance of cased and filler beams should be assessed at the ultimate limit state in accordance with Clause 6.2C. The effects of shear lag in filler beam decks may be neglected. The steel beam may be considered as compact. Vertical shear should be assumed to be resisted by the steel section, and in addition by the concrete if reinforced by vertical stirrups and assessed in accordance with Section 7. Alternatively, the vertical shear may be assumed to be resisted by the steel section with the addition of the shear resistance of the section of reinforced concrete between adjacent filler beam flange outstands, provided the proportions of shear can be resisted at the end connections. 8.5C Longitudinal Shear 8.5.1C General Delete the existing BS 5400: Part 5 Clause 8.5.1 and substitute the following:

The longitudinal shear force per unit length between the concrete and steel beam should be calculated by elastic theory in accordance with Clause 5.3.1C, except that in positive (sagging) moment region of cased beams and in filler beams, concrete in tension should be neglected. Shear lag effects may be neglected in filler beam decks. The shear force to be transferred should be that applicable to the area of concrete and steel reinforcement in compression. 8.5.2C Cased Beams Delete the existing BS 5400: Part 5 Clause 8.5.2 and substitute the following:

The longitudinal shear resistance should be taken as the lesser of Expressions C6 to C8 below:

f b Lb

Expression C6

k1sL s + 0.7 Ae f ry

Expression C7

k 2 L s f cu

Expression C8

where: is the local bond stress taken as 0.7 N/mm²; f b Lb is the bond perimeter; k1 , k 2 , s , L s , Ae , f ry and f cu are defined in BS 5400: Part 5 Clause 6.3.3.2;

Ls Lb

is taken as the length of plane 5-5 in BS 5400: Part 5 Figure 6(d); is given by: Fully encased 2d w + btf bbf ; Soffit exposed

2d w + b tf ;




Page 10 of 12

where:

dw btf

is the web depth; is the top flange width;

bbf

is the bottom flange width.

Alternatively, the steel section, assumed compact, may be assumed to carry the entire loading. 8.5.3C Filler Beams Delete the existing BS 5400: Part 5 Clause 8.5.3 and substitute the following:

The longitudinal shear resistance should be taken as the lesser Expressions C9 to C12 below:

f b Lb +1.4 Aw f ry f b Lb +

2t w d b σ y

sb

Expression C9

Expression C10

k1sL s + 0.7 Ae f ry

Expression C11

k 2 L s f cu

Expression C12

where:

fb is the local bond stress taken as 1.0 N/mm²; Lb is as defined in Clause 8.5.2C; k1 , k 2 , s , L s , f ry and f cu are defined in BS 5400: Part 5 Clause 6.3.3.2; Aw tw db sb

σy

is the area of any reinforcement passing through the beam web; is the web thickness of the beam; is the diameter of any reinforcing bars passing through the beam; is the spacing of reinforcement bars passing through the beam; is the yield strength of the beam web;

Ls

is taken as the total length of planes 6-6 in Figure 6(e) of this Appendix;

Ae =

At + Ab + Aw

Equation C13

where At and Ab are defined in BS 5400: Part 5 Clause 6.3.3.1. Alternatively, the steel section, assumed compact, may be assumed to carry the entire loading.




Page 11 of 12

8.6C Temperature and Shrinkage Effects Delete the existing BS 5400: Part 5 Clause 8.6 and substitute the following:

Temperature and shrinkage effects need not be considered in filler beam construction.

6

6

Lb

6

At

6

Lb

At

Aw 6

6

Aw

sb

w

6

6 w

Figure 6 (e) Shear Plane and Transverse Reinforcement for Filler Beams

14C. CONCRETE INFILLED TROUGH CONSTRUCTION Add the following Clause to BS 5400: Part 5:

Dispersal of live loading to troughing should be allowed in accordance with Section 4. Troughing may generally be assumed to satisfy the compact section criteria in accordance with Appendix A, allowing use of the plastic modulus. The elastic modulus should be used for U-frame calculations where transverse troughing forms cross girders. Composite action with concrete infill may be assumed where: (i)

Troughs are filled up to at least 75 mm above crests, as shown in Figure C10, with concrete known to be dense and without significant evidence of slip or separation; and

(ii)

Rivets occur along at least every alternate crest.




The strength or rigidity determined should not exceed the calculated strength or rigidity of the troughs by more than 30% unless justified by applicable testing.

75mm MINIMUM.

Figure C10 Trough Construction



The Structural Assessment of Underbridges Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 1 of 12

CONTENTS

APPENDIX D 1D SCOPE.....................................................................................................................................1 1.1D General............................................................................................................................1 3D. DEFINITIONS AND SYMBOLS ....................................................................................... 2 3.1.17D Element...................................................................................................................2 3.1.18D Element Identification..........................................................................................2 3.2D Symbols ........................................................................................................................... 2 4D GENERAL GUIDANCE ...................................................................................................... 2 4.1D Residual Life.................................................................................................................... 2 4.4D Methods of Assessment...............................................................................................3 Stage A Fatigue Assessment - Identify Fatigue Criticality by Inspection and CutOff Stress ................................................................................................................ 3 Stage B Fatigue Assessment - Damage Calculation to Standard Spectrum...........5 Stage C Fatigue Assessment – Damage Calculation to Particular Spectrum........5 Stage D Fatigue Assessment – Assessment using Measured Strains.........................6 5.4.D Steel Decks.................................................................................................................... 6 5.5.D Classification for Wrought Iron Elements.............................................................. 6 6.1.4.D Calculation of Stresses ........................................................................................ 6 9.1.3D Load Factors ............................................................................................................... 6 9.2.D Stage B Assessment - Damage Calculation to Standard Spectrum ................6 9.2.1D General....................................................................................................................6 9.2.2.D Procedure .............................................................................................................. 7 9.3.D Stage C Assessment – Assessment with Damage Calculation .........................10 9.3.1D General..................................................................................................................10 9.3.5D Calculation of Residual Life ...............................................................................11 11.3.D Treatment of Low Stress Cycles ..........................................................................12 11.6.D Design σr – N relationship for wrought iron.....................................................12

1D SCOPE 1.1D General

Add to the end of the existing BS 5400: Part 10 Clause 1.1 the following: The provisions and procedures in BS 5400: Part 10: 1980 (incorporating amendment no 1 as issue 2, March 1999) should be followed, subject to the modifications given below to clauses within BS 5400: Part 10 and which relate to the clause numbers of that code.




Page 2 of 12

3D. DEFINITIONS AND SYMBOLS

Add the following clauses to BS 5400: Part 10 Clause 3: 3.1.17D Element

A member, part detail or connection of the structure of the bridge which is subject to repeated fluctuations of stress under railway traffic. 3.1.18D Element Identification

A Damage Tolerant Element is an element where there is a reliable alternative load path of adequate strength within that element, other elements or construction including the track and its support. All other elements shall be considered to be Safe Life Elements. 3.2D Symbols

Add the following symbols to BS 5400: Part 10 Clause 3.2:

Ya

age of the element in years at the time of assessment;

YR

residual life in years from the time of assessment which will be the remaining period in which the element may be predicted to perform safely, with an acceptable probability that it will not require repair, or fail due to propagation of fatigue cracks;

γ1

factor on stress range taking account of method of analysis;

γ2

factor on stress range taking account of damage criticality related to inspection access.

4D GENERAL GUIDANCE 4.1D Residual Life

Delete the existing BS 5400: Part 10 Clause 4.1 and substitute the following: The acceptable reliability may be related to the capability of the bridge to carry railway traffic in the event of failure of an element depending on whether the element is a damage tolerant or a safe life element. The required reliability may be deemed to be achieved by applying factors γ1 and γ 2 to the relevant stress ranges. Allowance should be made for fatigue damage that has occurred since construction of the bridge to the element considered and for differences between traffic spectra in the past, present and future whenever this is practicable.




Page 3 of 12

4.4D Methods of Assessment

Delete the existing BS 5400: Part 10 Clause 4.4 and substitute the following: Assessment of steel and wrought iron underbridges for the purposes of determining fatigue endurance should be carried out in stages from (A) through to (D) as relevant for the various elements of the structure which contribute to vertical live load capacity. Where at any of these stages the stated criteria can be satisfied for a particular element, fatigue endurance may be deemed as satisfactory for that element subject to the limitations stated. Subsequent stages representing more rigorous approaches need not be evaluated for that element. Stage D involving the use of measured strains from site investigation should not be undertaken without due consideration of economic factors including the costs of investigation compared with that of remedial works. Stage A Fatigue Assessment - Identify Fatigue Criticality by Inspection and CutOff Stress

(i)

In accordance with Section 3, Clause 3.4.2 carry out inspection of the structure to identify elements which may be particularly susceptible to fatigue. Note the presence such as critical features such as visible cracks, welded repairs and notched railbearer ends. Where visible cracks have been detected measures should be undertaken to identify whether there are fatigue related or not. If so, then further investigations should be carried out even if they occur in elements which have otherwise been determined as non-fatigue critical. As a minimum requirement, the cracks should be monitored whilst detailed assessment is made or repair/strengthening is carried out. Consideration may be given to calculations of crack growth based upon fracture mechanics principles. Such calculations should normally be undertaken or supervised by specialists in this field. It should be noted that calculations as carried out in stages A, B and C may not be valid when significant fatigue cracks exist.

(ii)

Identify Damage Tolerant Elements as defined by 3.1.18D. They may normally include: Main girders and railbearers with effective span of 3 metres or less; Stiffened or unstiffened floor plates spanning less than 3 metres; Cross girders or trimmers supporting a single track; Main girders of deck type bridges interconnected such that in the event of failure of one, the remainder can sustain the required loading at the ultimate limit state with γfL for railway live loading multiplied by 0.85, but with

γfL '1.10;




Page 4 of 12

Multiple flange plates (but excluding any flange angles) which are interconnected by rivets or bolts and not by welding. (iii)

For all elements which contribute to vertical live load capacity, carry out the following procedure: (a) Apply RA railway live loading with the number of units equivalent to that of the route (which may be in excess of the assessed rating of the bridge) to one or two tracks so as to produce the greatest algebraic maximum and minimum values of stress at the critical locations within the element. Include for dynamic increment evaluated in accordance with Section 4 Table 4.5 for fatigue for a speed applicable to the applied live loading (NB: this may be less than the line speed); (b) Determine the maximum and minimum values of principal stress, or vector sum stress for weld throat, σ P max and σ P min , occurring at the critical locations within the element being assessed; (c) Determine the maximum range of stress value of

σ P max

minus

σ P min .

σ R max

equal to the numerical

For non-welded elements the stress range

should be modified in accordance with BS 5400: Part 10 Clause 6.1.3; (d) Multiply the stress range range of stress

σ f max

σ f max =

σ Rmax γ1γ 2

where

σ R max

by factors

γ1 and γ 2

Equation D1

γ1 is obtained from Table D1 and γ 2

Assessment Stage

A C B, D(measuredstrain)

to give the factored

where;

from Table D2.

Method of Analysis Static or Load Distribution Analysis 1.00 1.00 0.80 Table D1 Values of γ1

3D Finite Element NA 0.95




Inspection Access Accessible Inaccessible

Damage Tolerant Element

Page 5 of 12

Safe Life Element

0.80 0.86

0.86 1.00

Table D2 Values of γ 2

Plates which are covered on both sides, for example the areas of webs of girders covered by flange angles and the inner plates of multiple flanges should be considered as inaccessible, unless non-destructive testing or other investigations are undertaken to detect any concealed cracks or other fatigue defects. Single flange plates where are accessible on at least one side can be assumed to be accessible. (e) Compare the value of

σ

f max

with the cut-off stress value

σ

c0

given by

Clause 11.3D in Table D8, or Table D8 as appropriate. Where does not exceed

σ

c0

σ f max

at the critical locations, the element may be

considered to be non-fatigue critical up to the loading assumed. Where σ f max exceeds σ 0 a Stage B fatigue assessment should be undertaken; Stage B Fatigue Assessment - Damage Calculation to Standard Spectrum

This simplified method should be used for all elements which fail Stage A and where a numerical assessment of residual fatigue life YR is required. Stage B assessment should be carried out in accordance with Clause 9.2D. Where YR is evaluated as less than required or the age of the element at the time of assessment, Stage C fatigue assessment should be carried out. Stage C Fatigue Assessment

Damage Calculation to Particular Spectrum

This method should be used for all elements which fail Stage B and where a numerical assessment of residual fatigue life YR is required. Stage C assessment should be carried out in accordance with Clause 9.3D. The application of the method in any particular case should be agreed with the Railtrack Director s Nominee. Where the residual fatigue life is less than required for the age of the element at the time of assessment, Stage D may be undertaken where this can be justified as an alternative to immediate acceptance that the bridge has inadequate fatigue resistance and remedial work should be undertaken in the near future.



The Structural Assessment of Underbridges Appendix D - Fatigue Assessment of Steel and Wrought Iron Stage D Fatigue Assessment

Page 6 of 12

Assessment using Measured Strains

In particular cases where the consequences of accepting that the bridge has inadequate fatigue resistance based on a Stage C assessment, a better assessment may be possible using measured strains. Such assessments require specialist expertise, usually require calibration against theoretical analysis, and can be costly. Any Stage D assessment should be agreed by the Railtrack Director s Nominee. 5.4.D Steel Decks

Add to the existing BS 5400: Part 10 Clause 5.4 the following: Classifications to be used in fatigue assessment of stiffened steel floors and orthotropic steel decks are not included in this Appendix. In these cases the basis and method of assessment should be agreed with the Railtrack Director s Nominee. 5.5.D Classification for Wrought Iron Elements

Add the following additional Clause to BS 5400: Part 10: For wrought iron elements the σr - N relationships given by Clause 11.6D may be used in the absence of fatigue data or testing relating to the material and particular details. Classifications for steel elements should not be used for wrought iron. 6.1.4.D Calculation of Stresses Add to end of the existing BS 5400: Part 10 Clause 6.1.4.1 the following: Due account should be taken of any corrosion losses identified in the calculation of overall stress and local stresses at a section. The stress ranges calculated should be multiplied by factors γ1 and γ 2 taken from Tables D1 and D2. 9.1.3D Load Factors

Add the following additional clause to BS 5400: Part 10: The load factors

γ

fL

and

γ

f3

should both be taken as equal to 1.0.

9.2.D Stage B Assessment - Damage Calculation to Standard Spectrum

Delete the existing 9.2.1D and 9.2.2D. BS 5400: Part 10 Clause 9.2 and substitute the following Clauses 9.2.1D General

This method determines the residual life and should only be used where the following conditions are satisfied: (a)

the detail class is in accordance with BS 5400: Part 10 Table 17 or as specified in this Appendix;




Page 7 of 12

(b)

the assumed load spectrum complies with the standard load spectra for heavy, medium or light traffic given in BS 5400: Part 10 Table 2 based on the typical trains described in BS 5400: Part 10 Appendix E for RU loading;

(c)

on the basis of knowledge of the service history of the bridge, the assumed loading spectrum can be assumed to have been applied throughout the previous life of the bridge.

9.2.2.D Procedure 9.2.2.1.D

(a)

Apply 20 units of RA loading to the first track so as to produce the algebraic maximum and minimum values of stress at the critical locations within the element. Include for dynamic increment evaluated in accordance with Section 4 Table 4.5 for fatigue for a speed applicable at the bridge location to the heaviest live loading normally permitted (NB: this may be less than the line speed).

(b)

Determine the maximum range of stress σ P1 for the first track equal to the numerical value of σ P max minus σ P min . For non-welded elements, the stress range should be modified as given in BS 5400: Part 10 Clause 6.1.3;

(c)

Determine the maximum range of stress range σ P 2 for the second track (where the element is subject to live load stresses for more than one track) as described for the first track in (a) and (b).

(d)

Multiply the maximum range of stress σ P1 for the first track by factor K1 to take account of the second track and by factors γ1 and γ2 to give the factored range of stress σ f max where

σ

f max

=σ

K1γ1γ 2

Equation D1

P1

where:

K1

γ γ

1 2

is obtained from Table D3; is obtained from Table D1; is obtained from Table D2.

Note: A more accurate value for K1 can be obtained using equation D2.

K1 =

  T (1− P )1+   T

2 1

σ  σ

p2 p1

  

m

  σ  + P 1+   σ 

      m

p2 p1

−m

Equation D2




Page 8 of 12

where:

P

is the proportion of cycles of loading on the first track which are coincident with loading of the second track;

T1 T2 m

is the annual tonnage on the first track; is the annual tonnage on the second track; is obtained from BS 5400: Part 10 Table 8 or from Table D8.

σ σ

D, E, F, F2, G and W (m=3)

p2 p1

DETAIL CLASS B, C and wrought iron

S (m=8)

+1.0 +0.9 +0.8 +0.7

1.38 1.31 1.25 1.19

(m=3.5,4) 1.36 1.29 1.24 1.18

+0.6 +0.5 +0.4 +0.3 +0.2 +0.1 0 -0.5 -0.8 -1.0

1.15 1.11 1.07 1.05 1.03 1.01 1.00 1.00 1.11 1.22

1.14 1.10 1.07 1.04 1.03 1.01 1.00 1.00 1.08 1.18

1.51 1.44 1.37 1.30 1.23 1.17 1.11 1.07 1.04 1.01 1.00 1.00 1.01 1.08

TABLE D3 Values of K1 - Second Track Loading

Where

σ

p2

is of opposite sign to

σ

p1

then

p2

σσ

should be taken as of minus

p1

sign. Values of K1 in Table D3 assume P is equal to 0.10 and that T1 is equal to T2 . (e)

Determine the fatigue spectrum using BS 5400: Part 10, Table 2. The load proportions, k wa , should be obtained from Equation D3.




K wa =

K w K 2 K 3 but ≤ 1.0

Page 9 of 12

Equation D3

where:

K wa Kw

is the proportion of 20 units of standard BSU loading which occurs for a total of n R ×10 6 cycles in a period of 120 years; is the proportion of RU loading as defined by BS 5400: Part 10

Clause 7.3.3 for RU loading; K 2 , K 3 are obtained from Table D4;

nR

is as defined by BS 5400: Part 10 Clause 3.2;

Derive K wa , σ fmax for each of the group numbers corresponding to the values of K w shown in BS 5400: Part 10 Table 2. BENDING K3 Heavy Traffic Medium or

length L (m)

2 3 4 5 7 10 15 20 30 m50

Light Traffic Trans-

K2

Longitudinal members

Transverse Members

Longitudinal Members

verse Members

1.01 1.09 1.18 1.21 1.23 1.29 1.34 1.32 1.29 1.22

1.57 1.57 1.52 1.41 1.29 1.20 1.14 1.08 1.03 0.94

1.50 1.39 1.25 1.13 1.02 -

1.56 1.56 1.51 1.40 1.27 1.19 1.12 1.06 1.01 0.93

1.42 1.32 1.19 1.07 0.97 -

SHEAR K3 Heavy Traffic K2

1.12 1.07 1.17 1.20 1.24 1.25 1.23 1.23 1.15 1.13

Medium or Light Traffic Trans-


Transverse Members


verse Members

1.42 1.42 1.42 1.30 1.21 1.14 1.09 1.06 1.02 0.96

1.26 1.19 1.09 1.02 0.94 -

1.40 1.40 1.36 1.29 1.19 1.14 1.08 1.05 1.00 0.95

1.19 1.13 1.04 0.96 0.89 -

TABLE 4D Values of K2 and K3

Note: The values of K2 are a proportion of static RU loading to 20 units of static RA loading. (f)

For each value of K wa , σ f max calculate n N using BS 5400: Part 10 Clause 11.3.



The Structural Assessment of Underbridges Appendix D - Fatigue Assessment of Steel and Wrought Iron Page 10 of 12

(g)

Determine the value of

∑n N

using BS 5400: Part 10 Clause 11.

9.2.2.2.D

Derive the residual life of the element which may be calculated as: 120  27 x10 6  Equation D3   − Ya n ∑  T  N where T is the annual tonnage on each track taken as the greater of T1 or T2

YR =

9.3.D Stage C Assessment

Assessment with Damage Calculation

Delete the existing BS 5400: Part 10 Clauses 9.3.1, 9.3.2 and 9.3.3 and substitute the following: 9.3.1D General

Stage C assessment involves the calculation of Miner s Summation for the stress histories of the various trains which are considered to use the bridge, and may be used for any element for which the σ r − N relationship is known and for any known load or stress spectra. Trains which are to be considered should be representative of trains which have used the bridge or particular element as appropriate since it was built. The traffic spectrum in any particular case should be agreed with the Railtrack Director s Nominee, but should consist of either: (i)

Typical train types assumed as Figure 19 of BS 5400: Part 10. The number of trains per annum to be as given in Table 15 of BS 5400: Part 10 for heavy, medium or light traffic adjusted pro rata to the total annual tonnage advised by the Railtrack Director s Nominee and the annual tonnage of 27 x 10 6 stated. Train speed for each train should be as follows: Train Types 2, 3, 4, 5 & 6 The lesser of the line speed across the bridge or 125 mph. Train Types, 1, 7, 8 & 9 The permitted speed across the bridge for freight trains.

(ii)

Typical “real train” types with numbers of trains per annum of each type and train speeds as advised by the Railtrack Director s Nominee.

9.3.2D Design Spectrum

For all elements which contribute to vertical live load carrying capacity, carry out the following procedure:




(a)

Apply train loading from each train at the appropriate speed to the first track. Include for dynamic increment evaluated in accordance with Section 4 for fatigue for a speed applicable to each train. Each train should be traversed across the relevant point load influence lines for the critical locations within the elements being assessed.

σ

Determine the stress history for each train to derive values of is defined in BS 5400: Part 10 Clause 3.2.

(c)

For elements which are subject to live load stresses from more than one track apply train loading to the second track as described for the first track in (a) and (b). Account should be taken of the possibility of stress fluctuations arising from the passage of trains on not more than two tracks, both separately and in combination. When considering stresses in combination from two tracks it may be assumed that up to 10% (i.e. P=0.1 as assumed in Table D3) of cycles of loading on the first track are coincident with loading of the second track, or unless otherwise advised by the Railtrack Director s Nominee. As an approximation the effects of two track loading may be obtained by multiplying the stress histories by factor K1 as defined in Clause 9.2.2.1D.

(d)

Multiply the values of σ R by factors γ1 and γ 2 to give the factored stress history giving values of σ f max , where γ1 is obtained from Table D1 and γ 2

R

, where

σ

(b)

R

from Table D2. (e)

Analyse the factored stress history for each train by the rainflow method (see BS 5400: Part 10 Appendix F, example 4) to derive the respective stress spectra. These should then be combined with the appropriate total occurrences per annum to compile the overall spectrum. For non-welded elements the stress range should be modified in accordance with BS 5400: Part 10 Clause 6.1.3.

9.3.5D Calculation of Residual Life

Delete the existing BS 5400: Part 10 Clause 9.3.5 and substitute the following: Using the design spectrum obtained in Clause 9.3.1D the residual life of the element should be calculated using Clause 11 of BS 5400: Part 10 but taking n1 , n 2 .......... ..n n as the numbers of repetitions per annum of the various factored stresses and taking account of Clauses 11.3D and 11.6D. The residual life should then be calculated as:




YR

  1 =  n n n    + + ..... N N N 1

2

n

1

2

n

   −Y   

Equation D4

a

11.3.D Treatment of Low Stress Cycles Add to the existing BS 5400: Part 10 Clause 11.3 the following: Values of each stress range σ f max under Clause 4.4D, Clause 9.2D or Clause 9.3D which are calculated as not exceeding the cut-off stress σ c 0 corresponding to N=10 8 may be ignored. Table D8 shows values of σ c 0 for steel and Table D9 shows values of σ c 0 for wrought iron.

DETAIL CLASS

σ

c0

W G F2 F E D C B S

(N/mm²) 16 18 22 25 30 33 51 68 65

TABLE D8 Values of σco for steel - cut-off stress 11.6.D Design σ r N relationship for wrought iron Add the following additional Clause to BS 5400: Part 10.

The σr – N relationship given in Table D9 should be used for wrought iron elements. Detail Class Wrought Iron plain section

m 4.0

K2 17.70x10

Wrought Iron

4.0

3.73x10

σ 13

13

0

(N/mm²)

σ (N/mm²) c0

65

44

44

30

at locations of rivets Table D9 Additional Data for Wrought Iron


The Structural Assessment of Underbridges Appendix E – Model Bridge Assessment Report


CONTENTS

APPENDIX E 1E. MODEL BRIDGE ASSESSMENT REPORT ...................................................................... 1 2E. TYPEFACE AND BINDING............................................................................................... 1 3E.3.1E REPORT SEQUENCE AND FORMAT............................................................................ 1 Cover................................................................................................................................2 3.2E Distribution ..................................................................................................................... 2 3.3E Summary .......................................................................................................................... 2 3.4E Contents .......................................................................................................................... 2 3.5E Location Maps ................................................................................................................. 2 3.6E General Arrangement Drawing .................................................................................. 2 3.7E Introduction .................................................................................................................... 2 3.8E Condition Survey Summary ......................................................................................... 3 3.9E Results Obtained From Calculations ......................................................................... 5 3.10E Conclusions...................................................................................................................5 3.11E Recommendations ....................................................................................................... 5 3.12E Figures ............................................................................................................................ 5 3.13E Appendices....................................................................................................................5 4E. CALCULATIONS.................................................................................................................6 5E. STATUTORY UNDERTAKERS’ SERVICES....................................................................6 6E. TEST RESULTS ...................................................................................................................... 6 APPENDIX E 1E. MODEL BRIDGE ASSESSMENT REPORT

This Appendix provides a recommended model for the contents and format of the Bridge Assessment report. Use of this model by the assessing organisation should ensure that all relevant information is recorded and summarised. A Bridge Assessment report which uses this model will summarise the main Bridge attributes and assessment findings in the first few pages. More detailed (and bulky) information, including the calculations should be recorded as appendices. 2E. TYPEFACE AND BINDING Whenever possible Gill Sans 12 point should be used throughout the report.

Reports should normally be bound to prevent the loss of individual sheets. 3E. REPORT SEQUENCE AND FORMAT

The following report format and contents are recommended:




3.1E Cover

The cover should be one full page, and include, the title, a general photograph, the date, unique structure number, consultants file reference and telephone number and a confidentiality clause. An example cover layout is shown in Figure E2. Paper bearing the name and/or logo of the assessing organisation may be used as the cover for the report. 3.2E Distribution

A distribution list, on one full page, of recipients of the report and their addresses should be included. 3.3E Summary

A short summary on one full page should be given by the assessing organisation describing the assessment methods used and giving the main outcome of the Bridge Assessment in terms of the safe load capacity for the Bridge. The Structures Manager having reviewed the report may add an additional summary sheet giving recommendations. 3.4E Contents

Headings and page numbers of the various sections and appendices of the report should be listed. Each appendix should be provided with a detailed list of contents. 3.5E Location Maps

Two location plans should be included showing:

• •

the position of the Bridge in relation to the local junctions and lines of the rail network, the structure, to an ordnance survey scale, indicating a north point and giving the National Grid reference for the structure. (For multi-span structures this should be the centre of the structure overall).

3.6E General Arrangement Drawing

An A4 or folded A3 sized general arrangement drawing. Existing record drawings may be used if suitable. 3.7E Introduction

Stating the reason for the Bridge Assessment, the location of structure, a brief constructional description, the normal live loading conditions (number of tracks/carriageways and frequency of traffic) and relevant historical information.




3.8E Condition Survey Summary

A description of the main relevant features of the condition of the Bridge including information obtained during the inspection for the assessment. A limited number of photographs (colour, 150 mm by 100 mm in size) may be included in the summary in order to highlight features of particular importance. Close-up photographs should be supported by general views to illustrate the location of the detail shown in close up. The completed condition survey report, giving full details of the condition of the Bridge obtained during the review should be included as an Appendix.




Page 4 of 16

(Consultants Logo with address) (Consultants Telephone number) (If required)

Status of Report (Interim, Draft, Final)

Bridge Name(s)/Road Number(if appropriate) ELR and Structure Number Stations between O.S. location reference

General View Photograph

Date: Assessors Reference:

Confidentiality Clause: e.g. This report was commissioned by Railtrack PLC and is confidential. It is not to be passed to a third party without the permission of the commissioning Railtrack Director (Insert Zone)or his delegated representative. Figure E2 An Example of a Bridge Assessment Report Cover Page




Page 5 of 16

3.9E Results Obtained From Calculations

A statement should be made of the overall Bridge capacity (according to the applicable assessment codes) and identifying the limiting elements. A table should be included at the front of the report summarising the element safe load capacities and the relevant speeds as applicable, including for abnormal road loads whether in traffic or alone and position if alone. Where appropriate, comments should be made on the results obtained, particularly where the calculations are sensitive to the parameters considered. In addition to the foregoing, the results of the assessment should be recorded on the applicable detailed Summary Sheet shown in Tables E1 to E7, which should be included in the Bridge Assessment report. 3.10E Conclusions

A summary (with appropriate commentary) of the main findings of the assessment. 3.11E Recommendations

Recommendations shall be commented on and/or endorsed by the signature of Structures Manager. 3.12E Figures

Sketches and diagrams should be included as necessary. 3.13E Appendices

Typically, the following Appendices should be attached: 1.

Technical Approval in Principle and Check Certificates

Copies of signed completed forms for the assessment and check shall be included. 2.

Record Data

A list of drawings, main dimensions, including span and skew, levels, clearances and any other relevant information on which any aspects of the Bridge Assessment are based. 3.

Inspection for A ssessment I nformation

Sufficient drawings and where applicable, annotated photographs, should be included for each Bridge to record clearly and completely all information obtained from the inspection for assessment. The drawings should show the following: (a)

The minimum clear dimension between the soffit and the highest road level over the road carriageways or rail level as applicable, and the location of such dimension. The position, shape and value of any low Bridge warning plates should be indicated.



(b)

(c)


Relative soffit, carriageway and rail levels. Levels of the road surface or rail levels (and corresponding soffit positions) at positions beneath the Bridge faces and also where the soffit is low should be included. Levels shall, where a ordnance bench mark (OBM) is convenient, be related to Ordnance (Newlyn) Datum. Otherwise, levels shall be related to a well established and permanently marked temporary bench mark (TBM). Details of any components or details potentially susceptible to fatigue and where available the changes from the previous report or examination.

(d)

Details of deterioration including corrosion, cracking, water seepage, spalling of concrete, exposed reinforcement, etc. on the super-structure.

(e)

Details of any stress raising details considered fatigue susceptible, and where available, the change from the previous report or examination.

(f)

Details of any visible distress, cracks, water seepage, lack of verticality, signs of settlement etc of the sub-structure.

(g)

The location, type and size of services and ducts built into the structure or buried in the overburden, whether such services or ducts affect the Bridge Assessment or not.

(h)

The types, sizes and condition of bearings Location and typespositions, of waterproofing membranes and joints shouldand alsojoints. be indicated.

4E. CALCULATIONS Calculations should be clear, objective, legible with a narrative approach and provided with a detailed index. Where applicable, calculations should be annotated to show references to clauses of relevant assessment/design standards. Diagrams and sketches to support the calculations should be provided as necessary.

Material strengths assumed for assessment purposes should be clearly stated, together with their derivation. Computer programmes, where used, should be described with their titles, version numbers and validation status. 5E. STATUTORY UNDERTAKERS SERVICES A list of all Statutory Undertakers and Service Owners including Railtrack contacted

regardingtogether the presence of theirofservices oncorrespondence, or within the structure shoulddrawings be included with copies received and relevant or sketches, from these bodies. 6E. TEST RESULTS

If either load testing or materials testing has been used to obtain information used in the assessment calculations, this appendix should contain the report of the work completed. The format of the report of the tests should be compatible to the main assessment report.




The following pages contain the Assessment Summary Sheets for steel, wrought iron, concrete and masonry arch structures. They may be copied and should form part of the Assessment Report.




Page 8 of 16

Assessment Information Summary Sheet Steel and Wrought Iron Structures D ES CR IP T IONOFB R IDG E

L OCA LN AM E

LINEO RB R A N CH

MI L E A G E

SPANNINGACROSS

EL R

O SGR IDR EF

LINESPEED

Z ON E

SECTIONAL APPENDIX RATING OF LINE mph RA CURRENT FATIGUE RATING

REAL TRAINS SPECIFIED Yes No (if Yes attach details separately)

............... ........ X 106

HEAVY/MEDIUM/LIGHT

Bridge Configuration

Format:

HalfThrough

ThroughDeck

Waybeam

Railbearers:

SpanData:

BeamandDeckingType

Yes

No

Cross Girders:

Yes

No

TrimmerGirders:

Yes

No

NumberofSpans: Supportcondition:

TrackData:

SimplySupported

Continuous

Span Length(s)

metres

Skew angle(s)

degrees

Numberof: Track Support:

Longitudinal timbers

SleeperType:

Transverse sleepers

Timber

BallastedTrack:

Concrete

Yes

Type of Rail:

BH

Electrified:

3rd

Direct fixing

No 113A

UIC60

4th

OHLE

MinimumBallastDepthfromUndersideofSleepers

mm

Radius of Track Curvature

metres

Rail Joints on Bridge:

Yes

No

NearestRailJointsoffBridgefromC/LBearings: FloorType:

Plate

Yes

None

No

Flat

metres BucklePlates:

Up Dn /

Troughing

Yes

No

Transverse

Longitudinal

Timber

Yes

No

Transverse

Longitudinal

Jack Arch Open

Yes Yes

No No

Transverse

Longitudinal

Transverse

Table E1 Steel and Wrought Iron Structure Summary Table

Longitudinal




Page 9 of 16

Assessment Information Summary Sheet Steel and Wrought Iron Structures cntd. MainGirder:

Type

PlateGirder

Depth

BoxGirder

Constant

ReliantonU-Frameaction Bearings:

Type:

Truss

Variable Yes

Spreaderplates

No Linearrocker

Rollers

None

Other: Articulation:

Known

Unknown

Date of Manufacture

Materials

Steel

Yield stress (N/mm²)

Pre 1905 After 1906 After 1948 to BS

Wrought Iron RolledSection

Component Format

Riveted

Welded

Main Girders Cross Girders Rail Bearers Floor Plate Assessed by:

Date:

Table E1 Steel and Wrought Iron Structure Summary Table, cntd.




Page 10 of 16

ASSESSMENT RESULTS SUMMARY FOR STEEL AND WROUGHT IRON STRUCTURES

BENDING CAPACITY KNm REF

1+ϕ

20 BSU x γfl (1+ϕ)

MD CAPACITY

LIVE CAPACITY

2

3

(2x3)

4

(4-1)

DEAD x

20 BSU x

1+ϕ

γfl

γfl

20 BSU x γfl (1+ϕ)

LIVE CAPACITY

1

2

3

(2x3)

CAPACITY RIVETS/ WELDS 4

DEAD x

20 BSU x

γfl

γfl

1

BSU

static

RA

+ impact

static

+ impact

MAIN GIRDERS CROSS GIRDERS TRIMMER RAIL BEARERS FLOOR PLATE SHEAR CAPACITY KN REF MAIN GIRDERS CROSS GIRDERS TRIMMER RAIL BEARERS OTHER HORIZONTAL SHEAR KN/m REF

(4-1)

BSU

static

RA

+ impact

MAIN GIRDERS CROSS GIRDERS TRIMMER RAIL BEARERS OTHER CONNECTIONS - CROSS GIRDER/MAIN GIRDER SHEAR kN BENDING kN SHEAR/ BENDING RATIO RAIL BEARER/ CROSS GIRDER kN BEARING STIFFENERS REACTION M TRANS MLONGIT COMBINATION CURTAILMENT OF FLANGE PLATES

Table E2 Steel and Wrought Iron Structure Assessment Results Summary Table

static

+ impact




Page 11 of 16

Assessment Summary Sheet Masonry Arch Structures D E S CR IP T IONO FBR ID GE

LO CALN AM E

L IN EORBR AN CH

M IL E A G E

SPANNINGACROSS

E LR

OSGR IDR E F

LINESPEED

Z ONE

SECTIONAL APPENDIX RATING OF LINE mph

RA

Bridge Configuration

SpanData:

NumberofSpans: Skew span

metres

Square span

metres

Skew angle

degrees

PierData:

Height

metres

TrackData:

Numberof:

Thickness

metres

Type of Rail:

BH

113A 3rd

Electrified:

UIC60

4th

RadiusofTrackCurvature:

OHLE

None

metres

Minimum Ballast Depth from Underside of Sleepers at crown:

mm

Arch Data: Masonry Type: ArchProfile:

Semicircular

Segmental

Parabolic

Elliptical

Pointed

Number Rings: of

No.

O/A Ring Thickness:

mm

RingThicknessusedinassessment

mm

MEXE Assessment

Provisional Axle Capacity (QP) Modifying Factors: KP = . KS = Permissible Axle Capacity (CF) =

.. KM =

. .. KC = .

.. KD =

.. KV =

..

Assessment by Other Method

Effective width considered Critical Spans/Piers Ultimate Capacity

m . (multi-span structures only)

A SS E SS EDR OUT EA V AIL ABILIT Y:

RA

Assessed by:

Date:

Table E3 Masonry Arch Summary Table




Page 12 of 16

Assessment Information Summary Sheet Concrete Structures D ES CR IP T IONOFB R ID G E

L OCA LNA M E

LINEO RB R A NC H

M IL E A G E

SPANNINGACROSS

E LR

OSG R IDR E F

LINESPEED

Z ONE

SECTIONAL APPENDIX RATING OF LINE mph

RA

REAL TRAINS SPECIFIED Yes No (if Yes attach details separately) Bridge Configuration

ConcreteFormat:

Plain

SpanData:

NumberofSpans:

Reinforced

Supportcondition:

TrackData:

Post-tensioned

SimplySupported

Pre-tensioned

Continuous

Span Length(s)

metres

Skew angle(s)

degrees

Numberof: Track Support:

Longitudinal timbers

Sleeper Type:

Transverse sleepers

Timber

BallastedTrack:

Yes

Type of Rail:

BH

Electrified:

3rd

Direct fixing Concrete

No 113A 4th

UIC60 OHLE

None

MinimumBallastDepthfromUndersideofSleepers

mm

RadiusofTrackCurvature

metres

Rail Joints on Bridge:

Yes

NearestRail JointsoffBridgefrom C/L Bearings: Element Types:

Slab

Type Depth

No

metres

insitu

precast

Beams

Yes

No

insitu

precast

Portal

Yes

No

insitu

precast

Culvert

Yes

No

insitu

precast

Arch

Yes

No

insitu

precast

No

insitu

Other MainBeams:

Yes

No

Yes InvertedT Constant

BoxGirder

precast U

Variable

Table E4 Concrete Structure Summary Table

Rectangular




Page 13 of 16

Assessment Information Summary Sheet Concrete Structures cntd. Type:

Bearings:

Spreaderplates Elastomeric

Articulation:

Rollers

None

Known Assumption

Materials

Linearrocker Pot

f y or f pu (N/mm²)

(*1) Concrete

Other: Unknown Jacking or Pre-stress Force (kN)

Insitu Pre-cast

Steel

Reinforcement Main tendons Other Tendons

Assessed by:

*1

Date:

Abbreviations: SS = Strengthspecified M = Strengthmeasured ASG A

= =

Strength Assumed from specified grade Assumed Table E4 Concrete Structure Summary Table, cntd.




Page 14 of 16

ASSESSMENT RESULTS SUMMARY FOR CONCRETE STRUCTURES

BENDING CAPACITY (kNm)

DEAD x

20 BSU x

γγ

γγ

1

2

fL f3

REF/ SECTION

1+

ϕ

20 BSU x

γγ

fL f3

ϕ

(1+ )

MU CAPACITY

LIVE CAPACITY

4

(4-1)

BSU

3

(2x3)

static

MAIN BEAMS

SLAB

RA

fL f3

Longitudinal Transverse

OTHER

SHEAR CAPACITY (kN) REF/ SECTION MAIN BEAMS vertical

SLAB OTHER

longt./ interface

Table E5 Concrete Structure Assessment Results Summary Table

+ impact

static

+ impact




Page 15 of 16

Assessment Information Summary Sheet Cast Iron Li n eorB ra n ch

Mile ag e

E LR M

Atorbe t we en

B ri dg e

Ch

Z on e

Con s t ru ct i ond at e

Spa n n i n g

GR rie df

Capacity EUDL kN

Member Loaded Length

RAofLine

StaticCapacity kN BSU Units

RA

DynamicCapacity kN BSU RA Units

PermanentSpeedRestrictionon:-

DynamicSummary Tens Comp kN kN

Line Bridge

Stresses N/mm²

Member

Permissible Stress T

CS

Fatigue Member Material Live load stress Allowable Stress Range

Assessed by:

TotalStress forAssessment Loading TCS

Type Speed TCS

TotalStressunderRealVehicles Type Type Speed Speed TCS TCS

Based on Calculation

Based on Tests

Date

Table E6 Cast Iron Structure Summary Table

Shear kN




Page 16 of 16

Assessment Information Summary Sheet Timber Structures Li n eorB ra n ch

Mile ag e

E LR M

Atorbe t we en

B ri dg e

Ch

Z on e

Con s t ru ct i ond at e

Spa n n i n g

GR rie df

Capacity EUDL kN

Member Loaded Length

RAofLine

StaticCapacity kN BSU Units

RA

DynamicCapacity kN BSU RA Units

PermanentSpeedRestrictionon:-

DynamicSummary Tens Comp kN kN

Line Bridge

Stresses N/mm²

Member

Permissible Stress T

Assessed by:

CS

TotalStress forAssessment Loading TCS

Type Speed TCS

TotalStressunderRealVehicles Type Type Speed Speed TCS TCS

Date

Table E7 Timber Structure Summary Table

Shear kN


The Structural Assessment of Underbridges Appendix F - Informative Annex


Appendix F gives additional information about, and guidance on the use of, Issue 1 of RT/CE/C/025. Information is given in Section order for ease of reference. Wherever possible Clause numbers are the same as those used in the main text but with a suffix letter F added.

WARNING: F isallnot intended to structure give comprehensive It should not be assumedAppendix to indicate aspects of the that shouldguidance. be checked in the assessment of an underbridge.

CONTENTS

INFORMATIVE ANNEX Section 1 - Introduction ............................................................................................................. 1 Section 2 - Assessment Philosophy.......................................................................................... 2 Section 3 - Inspection for Assessment .................................................................................... 4 Section 4 - Loading ...................................................................................................................... 6 Section 5 - Steel and Wrought Iron and Appendix A ........................................................ 12 Section 6 - Masonry Arches..................................................................................................... 52 Section 7 - Concrete Structures and Appendix B .............................................................. 65 Section 8 - Composite Structures and Appendix C........................................................... 69 Section 9 - Cast Iron Structures............................................................................................. 73 Section 10 - Timber................................................................................................................... 77 Section 11 - Substructures....................................................................................................... 81 Section 12 - Bearings................................................................................................................. 82 Appendix D - Fatigue Assessment of Steel and Wrought Iron ........................................ 83 SECTION 1 - INTRODUCTION

No further commentary required.




SECTION 2 - ASSESSMENT PHILOSOPHY 2.3F Assessment Situations

In considering a particular structure, the assessing engineer should take account of all loading scenarios that the structure is likely to be subjected to, and using his judgement determine those which will beeffects criticalsuch to the capacity of the bridge. and Theknowledge scenarios should include environmental as due to wind or temperature where relevant. Whilst accidental situations, such as collision and derailment, or unusual loading conditions which may occur during maintenance operations, such as the stock piling of ballast on adjacent tracks, will not govern the load carrying capacity of the bridge it may be necessary to consider these. When required, appropriate guidance will be provided by the Railtrack Director s Nominee. 2.4F Limit States

The governing condition for the assessment of most structures is the Ultimate Limit State (ULS). For individual structural elements it may also be necessary to check that Serviceability Limit State (SLS) criteria are also met, see Clauses 4.2.2A, 4.1.1B and 4.3.2C. In checking for the ULS, it is necessary to make realistic assessment of the interaction which can occur within the structure at ultimate load. In some cases load-induced breakdown of interaction within the structure may reduce its ULS capacity. A prime example is the case where composite behaviour between main girders and a transverse deck structure is required to justify sufficient ULS longitudinal capacity of the main girders. It may not be possible to determine by inspection that the composite behaviour exists, but an SLS check on the interaction may indicate that cracking/separation/loss of bond between main girders and transverse structure is unlikely. This result would give some support to an assumption of composite action in checking the ULS for the main girders. It would not however establish that full composite action is present as main girders reach their ULS. In such cases, the assumptions to be used should be defined by the Railtrack Director s Nominee. Some situations where Operational Safety Limit States (OSLS) may require to be checked are given below:

•

Twist of the deck to minimise the risk of train derailment, especially on skew bridges (refer to Clause 4.5.1);

•

Vertical and longitudinal deflection of the deck throughout each span to ensure acceptable track geometry is maintained;




•

Longitudinal, vertical and rotational displacements at deck ends which may cause disturbance to ballast and adjacent track formation;

•

Run-on details for longitudinal timbers.

Where applicable criteria should be agreed in accordance with Railtrack s Technical Approval Procedures. 2.6F Load Combinations

Table 2.1 gives the various partial factors,

γ

fL

, to be applied to the different elements

of loading which when combined in the manner shown, constitute the assessment loading for a structure. It may be noted that the combinations presented are similar to those given in BD37/88: Loads for Highway Bridges. For the vast majority of structures Load Combination 1 will govern. In certain situations, however, such as for a deep plate girder where the consideration of wind effects may be important, or for a continuous multi-span deck where temperature causing restraint at bearings may have a significant influence, Combinations 2 and 3 should also be considered. The most onerous effect arising from the three combinations should be adopted for the element being assessed. To determine wind and temperature loads, reference should be made to BD37/88. Partial factors shown in Table 2.1 are similar to those given in BD37/88. The figure of γfL = 1.75 for ballast is considered to cover the situations where the ballast is waterlogged and dirty, resulting in a unit weight greater than the nominal value of 1800 kg/m³, and where ballast depths may increase beyond previously measured levels as a result of track lifting or maintenance operations. When control measures are in place to maintain the ballast at a specific level, or the form of construction dictates the main ballast depth, a reduced value of γfL may be adopted.




Page 4 of 87

SECTION 3 - INSPECTION FOR ASSESSMENT 3.4.1F General

Following or during the inspection, identification, sampling or testing of materials may be required. A general description of procedures and test techniques relating to the main structural materials may be found in: Appraisal of Existing Structures. Institution of Structural Engineers, 1996, Sections 5 and 6 and Appendices 7 and 8.

For concrete structures the following may be found useful: BS 6089: Guide to the Assessment of Concrete Strength in Existing Structures; BS 1881: Testing Concrete. Removal of samples should be undertaken in locations of low stress but in the vicinity of the location at which material properties are required to be determined. Sampling should not be carried out in locations where the removal of material would affect the strength of the Bridge, for example at the inner support of a continuous bridge where shear forces and hogging bending moments are at a maximum. The quantity of material removed should be the minimum required to enable satisfactory testing to be carried out. The location and extent of testing should be agreed with Railtrack Director s Nominee before any testing activities commence on site. Some uses of testing are to:

•

confirm assumptions that have been made or need to be made in carrying out the assessment of the Bridge;

•

determine the frequency and distribution of locally identified defects such as reinforcement corrosion;

•

identify possible reasons for the deterioration and possible susceptibility to future deterioration.

The location and number of tests/ samples to be undertaken should take account of factors such as:

•

the likely variation in material properties within and between parts of the Bridge;

• •

the probable number of critical locations; the possible errors that may occur during testing.




3.4.5F Timber Bridges

Inspection for assessment will generally be as for other types of structure. Peculiar to timber bridges is the necessity to identify the species and grading of the timber. This determines density of material for assessment of dead loads, as well as the strength of the timber. If drawings or other records do not confirm timber species and grade, then if the initial assessment calculations, based on the weakest likely timber, show that a lower strength timber is inadequate, or inspection reveals decay that could affect strength, a sample should be obtained at time of inspection for testing by a laboratory. Field grading of the visible parts of the timber may also be required. It is suggested that in the absence of other information the timber is initially taken as Douglas Fir (Canada and USA), SS Grade.




SECTION 4 - LOADING GENERAL

The code gives information on dead loads, superimposed dead loads and live loads including vertical loading with dynamic effects, nosing, centrifugal and longitudinal loads. No soil specific information is given other loadsincluding such as wind, and horizontal pressures. Where load on combinations these temperature effects need to be considered, BD37/88 should be used. In practice load combinations, including wind, temperature or horizontal earth pressures are unlikely to govern assessment. All loads are given as nominal (unfactored) values. For steel, wrought iron and concrete and steel/concrete composite structures loads should be factored as given in Section 2. 4.1F Dead Loads

Table 4.2 gives typical densities of materials. For riveted structures, it is important to make due allowance for stiffeners and rivet heads. In the absence of specific allowance the gross cross sectional area of the beam or girder may be multiplied by 1.12 to allow for stiffeners and rivet heads. Where dead load may be highly critical to the assessment, for example for a long span bridge, the dead load should be calculated more accurately. 4.2F Superimposed Dead Loads

Ballast loading should be based on actual measurement of depth and width using a density of 1800 kg/m³. The γfL factors in Section 2 allow for the possibility of waterlogged ballast and/or whether control measures are present to prevent overballasting. The weights of permanent way in Table 4.1 are an expansion of those taken from the previous code, RT/CE/C/015. Loads from services and ducts should be allowed for and may be significant, for example for parapet girders. 4.3F Live Load 4.3.1F Vertical Loading

This clause is based on the RA classification as used in the permissible stress assessment code, RT/CE/C/015 Issue 1. 20 BSU s should be applied (i.e. RA10) to the member or structure and its capacity factor, C, determined as a proportion of 20 BSU s with the RA number obtained from Table 4.3. EUDL and end shears for simply supported spans are given by Table 4.4 and are the same as those used in RT/CE/C/015. For continuous multi-span bridges, although Table 4.4 will give an approximation of the loading within the “span” taken as the summated length of the spans which are considered as loaded, axle loads should be applied as Figure 4.1.




It should be noted that the values in Figure 4.1 and Table 4.4 have been derived by conversion from the Imperial equivalents given in BS 153 and RT/CE/C/015 (Issue 1) receptively. If the load train in Figure 4.1 is used to determine EUDL s directly, minor discrepancies between the calculated value and that shown in Table 4.4 may result. Note that reduction to 75% of loading on second and subsequent tracks is only allowed when specifically approved. 4.3.2F Dynamic Effects

The dynamic factor is as derived in UIC document 776-1(1) ,with the exception of cross girders dynamic factor I4. The dynamic factor depends on: Train speed; Natural frequency of the member; Quality of track. For members other than cross girders, values of the dynamic increment ϕ may be derived using Clause 4.3.2.2 or conveniently from Figures 4.2 to 4.14, which give values for different train speeds. The natural frequency n0 is calculated assuming the member is simply supported with a span equal to the effective span L . Where n0 as calculated to Clause 4.3.2.2 is within the range between 0.5 x low frequency and 2.0 x high frequency, defined in Clause 4.3.2.3, ϕ should be calculated from the formulae in Clause 4.3.2.2. Where no as calculated is outside the range between 0.5 x low frequency and 2.0 x high frequency impact may need special consideration including the monitoring or witnessing of behaviour under moving trains. During the development of this code, a detailed investigation into the dynamic behaviour of cross girders was undertaken(4). It showed that the track irregularity component (ϕ11 ) of the impact factor was the dominating effect for cross girders. It was also found that, if the calculation procedure developed by the UIC for longitudinally spanning members was also applied to cross girders, the dynamic allowance would be significantly higher than in RT/CE/C/015 Issue 1. Until the applicability of ϕ11 to transverse spanning members can be confirmed, the impact factor for cross girders should be determined from Figure 4.15, which is identical to Factor I4 presented in Issue 1 of RT/CE/C/015. In general “normal” track quality should be assumed unless otherwise authorised. Reduced values of ϕ should be used for any fatigue calculations. The vertical live loading is multiplied by (1+ϕ) to give the live loading including dynamic effects.




The dynamic factor does not allow for the effects of rail joints or points and crossings which if they occur within the bridge itself should be taken into account. 4.3.2.1F Wh eel F lats

Flats on wheels cause short-duration impact loads on rails of up to twice the static wheel loads. This loadingbyhas beenofevident analysis undertaken by AEA of measurements obtained means wheel from impact load detectors. Only parts having a very high natural frequency and low inertia, such as rails, respond to such impact. It will be seen from Figures 4.2 to 4.14 that dynamic increments for short spans are of the order of unity for speeds of 100 mph. It can be assumed that the dynamic factor flats.

ϕ

takes account of the effects of wheel

Longer spans do not respond to wheel flat impulses, since their natural periods considerably exceed the durations of the dynamic forces and their inertias are relatively high. Furthermore, when the governing load effects are due to multiple axles on a bogie or several bogies, the axle load impacts are unlikely to be correlated in time or magnitude. For these reasons wheel flats (like rail irregularities) contribute little to the dynamic increment when the natural frequency of the bridge is less than about 5 Hz. 4.3.3F Dispersal Of Live Loading

Clause 4.3.3 gives rules for dispersal of EUDL and axle loads through the track onto the structure, taken from RT/CE/C/015, but does not take into account any distribution achieved within the structure itself. This distribution should be evaluated, where it will benefit an otherwise unrestricted assessment, by a distribution analysis, for example such as a two dimensional grid analysis of a continuous reinforced concrete slab interconnecting cross girders. The dispersal angle of 15° indicated in Figure 4.16 is in accordance with UIC774-2C(5). 4.3.4F Nosing and 4.3.5F Centrifugal Loads

Nosing and centrifugal loads should be considered. Since centrifugal loads tend to suppress the lateral oscillatory motion of vehicles causing nosing, centrifugal and nosing loads do not need to be considered in combination on the same track. The vertical effects of nosing and centrifugal loads should be included in the assessment of rail bearers, reduced to take account of cant, but generally only have a marginal effect on transverse members and main girders. If bridge floors are capable of diaphragm action, such as continuous concrete or metal plate decks, horizontal loads can be disregarded except on bearings. Nosing forces have been derived from the results of




a parametric study for ERRI D181 which considered lateral forces on railway bridges and showed a dependency on speed. 4.3.6F Longitudinal Loads

The allowance for longitudinal loading is based on BS 5400: Part 2 but is reduced pro rata for loading less than 20 BSU s. to Longitudinal loading is required mainly for checking of bearings or other fixity substructures. 4.5.1F Track Twist

The criterion is based on UIC leaflet 776-3R(2). It is related to risk of derailment. It is particularly likely to be relevant to the ends of skew bridges or where the two railbearers beneath a track have different rigidity. 4.6.1F Collision Loads

For bridges over highways, assessment in relation to collisions by highway vehicles should only carried out where required by Railtrack s Assessment Engineer. Reference should be made to BD 60/94(3). For bridges over railways collision loading resulting from a derailment should be considered only where agreed with the Railtrack Assessment Engineer. An example might be where supports are less than 4.5 metres from rail and pointwork or sharp track curvature exists beneath the bridge. FURTHER INFORMATION ON LIVE LOADING Stress under Real” Vehicles and Speeds

Where the assessed RA number is below the RA of the line, the effects under static EUDL s for the “real” (actual) permitted vehicles and combinations, together with dynamic factors for their respective permitted speeds, may be considered acceptable. This is provided the capacity of the structure is within the limits imposed by this Code. It should be noted that the RA effect of vehicles on a specific “span” (loaded length) is often less than the RA classification for the vehicle since this classification has to allow for a full range of bridge spans. Explanatory Note re EUDLs

The capacity of a simply supported beam can be expressed as the Uniformly Distributed Load (UDL) it can safely carry; this may be governed by bending or shear at various cross sections. If the load coming onto the beam is already a UDL, it can be compared directly to the UDL capacity, for example the load on a box culvert with generous cover could be a single axle load applied at the rail and fully dispersed/distributed onto the slab.




In most practical cases, however, the axle loads are only partially spread and come onto a longitudinal beam as a series of short UDLs or “patch” loads (overlapping as appropriate). The calculation of these loads and their effects is laborious and it is normal practice to proceed, conservatively, as if the axle loads were placed directly on the beam. (Empirical structures these are givenreduction in Sectionfactors 4.) may be applied to the effects; for metal The maximum specified moment or shear force, at a location, of a train travelling across a span, is calculated. The UDL required to produce the same effect is called the Equivalent UDL (EUDL), and can be compared with the UDL capacity per track mentioned above. EUDL tables for selected real trains on a range of spans should be used for:1.

Obtaining the maximum bending moment in span, occurring under an axle at or near mid span. The EUDL is frequently taken as (maximum bending moment x 8/span), but should strictly be derived from the parabola enveloping the real bending momentdiagram. Bending moments at otherpoints in the span may be derived using tabulated information in Section 5, or by a grillage or other suitable method of analysis.

2.

Obtaining the maximum end shear, occurring with an axle at the span end. The EUDL is, by definition, twice the end shear force.

EUDLs should not be used for combined bending and shear calculation for continuous beams. The loading on main girders, transferred as point loads from cross girders, is routinely taken as the EUDL for the train. This approximation is normally acceptable but care should be taken in cases where it could be seriously unconservative (e.g. in the extreme, a single cross girder at mid span). For a transverse member, such as a cross girder, loaded by simply supported rail bearers the appropriate loaded length for applying the EUDL is the influence line length equal to 2 x L where L is the spacing of the transverse members. British Standard Unit (BSU) Loading

The standard BSU loading, based on BS 153 and used for the RA classification, is shown in Figure 4.1. One BSU is defined as one unit of Type RA1 loading. Figure 4.1 is shown in terms of 20 BSUs equivalent to RA10 static loading. This loading should be used where it is inappropriate to apply EUDL s, for example continuous spans.




Type RA1 loading was defined by BS 153 in 1925 as the static unit loading for standard (1432 mm) gauge. For main line railways 20 units of Type RA1 loading was recommended with dynamic effect added. Dynamic effects were investigated for the 1928 report of the Bridge Stress Committee as relevant for steam locomotives and added to Type loading in to1973 derive RB loading whichstock. was used the was introduction ofRA1 RU loading totype suit modern traction RUuntil loading included in BS 5400: Part 2 in 1978.

REFERENCES

1. 2. 3.

UIC. UIC776-1R Loads to be considered in the Design of Railway Bridges. International Union of Railways, 1990; UIC. UIC776-3R Deformation of Bridges. International Union of Railways, 1989; THE HIGHWAYS AGENCY. BD 60/94: The Design of Highway Bridges for Vehicle Collision. The Stationery Office, 1994;

4.

CASS HAYWARD & PARTNERS. Investigation into the Dynamic Behaviour of

5.

Cross Girders. Railtrack plc, June 2000; UIC. UIC774-2R Distribution of Axle-Loads on Ballasted Railway Bridges. International Union of Railways, 1994.




SECTION 5 - STEEL AND WROUGHT IRON

Clause numbers in the guidance notes following do not tie up with the Clause numbers in Section 5. The Clauses giving guidance specific to individual Appendix A Clauses have the same number as the Appendix A Clause but with the addition of the suffix letter F. 5.1F General

This part of Appendix F contains general guidance on common structural forms of steel and wrought iron rail underbridges. Information is given on the theoretical approach for features of construction which are not covered by BS 5400: Part 3 such as buckle plates. The steps in carrying out the assessment of different member types are summarised with specific Clause references to BS 5400: Part 3 and supplemented as necessary. The majority of rail underbridges in steel and wrought iron are riveted structures of half-through type with solid web main girders. Hog-backed girders are common. Tracks are generally carried by cross girders with or without rail bearers and floors of troughing, jack arches, metal plate or timber. Longitudinal timbers often support the track by direct fixing without ballast. Floors are often supported directly by abutments beams, especially on skewfloors bridges. Rigid girders U’-frames bearings at without supportstrimmer are often absent. Joints between or cross andor main girders often do not meet BS 5400 requirements, and sometimes cross girders are not coincident with vertical stiffeners. A 3 girder arrangement is often found on two track lines. Some bridges have half-through or through trusses, often with underslung cross girders. Others use trough main girders beneath each rail carrying longitudinal timbers. Some more modern structures are of welded construction such as Type A’ standard bridges or trapezoidal box girders. BS 5400: Part 3(1), as a modern limit state code, could be used to assess many aspects of these structures. However, many Clauses are not directly applicable or the structural details do not comply with BS 5400. Steel and wrought iron bridges should be assessed, therefore, using Section 5 of this code, making use of Appendix A which is written as a supplement to BS 5400: Part 3, and of Appendix D for fatigue assessment. Appendix A refers to Standards BD 56/96(2) and BD 13/90(3) used by the Highways Agency for road bridges. Wherever possible the requirements in BD 56/96 are adopted to assist uniformity of practice within the industry. However, some Clauses relate to forms of construction not usual in railway bridges, for example, BD 56/96 Clause 9.17 for stiffened support diaphragms in box girders and Clauses for longitudinally stiffened plates and a predominance in highway bridges of deck type rather than half-through arrangements.




5.1.1F Applicability

A limit state approach is used based upon BS 5400: Part 3. Appendix A is based on BD 56/96, where appropriate, and currently proposed amendments to BS 5400: Part 3. These amendments to BS 5400 are expected to be the next revision to BS 5400: Part 3, and are particularly relevant for rail bridge assessments because they incorporate a more rational approach for U-frame bridges. 5.3F Material Properties Steels may be categorised as shown below, whereas the previous code differentiates only between steels before 1905 and after.

•

Pre 1905

No specified mechanical properties, values based on BRR Report LR MF 115;

•

BS 15 1906

UTS specified, but not yield stress;

•

BS 15 1948

Yield stress first specified in addition to UTS;

•

BS 4360 1962 Modern standard for weldable steels.

For wrought iron generally no mechanical properties were specified at the time of manufacture. Production of wrought iron slowed after 1880 with the introduction of steel and ceased by about 1900. The values shown in Table F5.1 for yield stress are based upon the BRR Report LR MF 115(4) and are the mean value minus two standard deviations of test data. The values given in BD 21/97(5) were not adopted as their source is unknown. For steel to BS 15 (1906), the BD 21/97 value of yield stress given in Table F5.1 is considered reasonable in the absence of other information. The value is mid-way between that for pre 1905 steels and 1948 steels when a specified yield stress was first introduced. Section 5 states that detailed theoretical assessment of steel and wrought iron is carried out using Appendix A. The basis of Appendix A is BS 5400: Part 3. It is written as a supplement in a similar way to BD 56/96, which (although containing much which is appropriate) was written for highways structures and tends to give emphasis towards continuous longitudinally stiffened beams and to large box girders. BD 56/96 bases material properties on fairly extensive testing or on mill certificates that may be available from the time of construction of the highway bridge, typically the 1960’s and 70’s when many major viaducts were built. A different approach is required for rail underbridges where the majority are small span riveted structures, many being built before 1900. It is not practicable or justifiable generally to carry out extensive testing on the material of such structures. It is necessary and appropriate to rely on the past information on early steels and wrought iron, although test data in any particular case would always be valuable. Hence information is given on available




Page 14 of 87

early information, but with the option to carry out material testing. Such testing may be appropriate for major structures, for example such as Forth, Tay, Runcorn and Grosvenor bridges.

5.4F Partial Factors

Reliance on the previous permissible stress Code has not resulted in major failures in service. An appraisal was therefore made of the implied safety factors in Issue 1 since it is clearly desirable the present code gives compatible results. At the same time the aim in preparing the present code was to achieve more consistent reliability and not condemn structures which are in fact capable of safely carrying higher loads. Material properties are given for early steel and wrought iron materials in Table F5.1. MATERIAL

PERMISSIBLE BENDING STRESS

YIELD STRESS

UTS minimum

in PREVIOUS assessment code

Steel

Pre 1905 BS 15 1906 BS 15 1948 BS 4360 1962

Present BD 21 Code & LUL 205 * 230 *370 230 **230 430

155 186 186

245 ( 19mm) 245 ( 16mm)

Yield Stress Permissible Stress

UTS YIELD

1.32 1.24

1.80 1.87 1.76

230

430

1.32

240

430

1.32(1.70)

=

186 (144 in BS 153 1962)

=

1.76

(grade 43)

Wrought Iron

130

*190

220

*285

1.46

1.50

* BRR Report LR MF 115 - mean minus two standard deviations ** Origin of BD 21/97 yield stress not known Table F5.1 Bending Stresses Compared with Yield Stress - N/mm²

The values of

Yield Stress Permissible Stress

in Table F5.1 representing the factor of safety in the

previous Code may be compared with the product of the partial factors in the present Code (i.e. γfL x γf 3 x γm ). Parity between the present and previous codes would be achieved if the partial load factor

γ

fL

is about 1.25, that is lower than the

value of 1.4 normally used for new design. Typically for beams in bending and shear, the overall safety factor would be γfL x γf 3 x γm = 1.25 x 1.10 x 1.05 = 1.44. This value compares closely with the total factor of safety in the previous Code for wrought iron (1.46), but is somewhat higher than that for steel (1.32). Parity in steel is, however, achieved if γf 3 is taken as 1.0, i.e. 1.25 x 1.00 x 1.05 = 1.31. This value of




γ

f3

Page 15 of 87

is permitted under Clause 4.3.3A of Appendix A for certain structures.

Therefore, use of the partial load factor

γ

fL

higher than 1.25 will often mean that a

lower assessment rating will be determined under the present code for wrought iron bridges, and more especially steel bridges. 5.6F Structural Form Some typical forms of steel and wrought iron bridges are shown in cross section in Figures F5.1 and F5.4. 5.6.1F Half-through Riveted Bridges

Cross girder

Railbearer

Longitudinal timber

Non-structural floor

Transverse troughing

Timber floor

Cross girder

Non-structural floor

Underslung cross girders (sling bolts or gussets)

Longitudinal troughing

Figure F5.1 Forms of Half-through Riveted Bridge

These bridges can be assessed at Level 1 by their components separately. Rail bearers, cross girders, troughing, and main girders may all be assumed simply supported unless continuity clearly exists, for example continuous rail bearers. Top flange stability generally relies on U-frame behaviour. If this is weak as evidenced by inadequate moment connections or significant inward bowing/tilting of the girders stability may have to rely on end bearing stiffeners. Often floor ends are directly supported on abutments (i.e. without trimmers). U-frame rigidity produced unintentionally by concrete or other haunching of cross girder ends may be taken into account. It is important during inspection to record verticality of main girders to supports and bow of top flanges. If any bow is other than a single wave form, its shape should be recorded. U-frame bridges rely for overall stability on in-plane stability of the floor.




Page 16 of 87

If the floor is an open type, i.e. without continuous floor plating, slab, or troughing, inplane stability may not be properly achieved and U-frame behaviour should be ignored unless there is stiff vierendeel behaviour of the members in plan. Composite behaviour of the floor as part of the main girder section should not be assumed unless there is a continuous plate, slab, or longitudinal trough floor continuously connected to the main girders. Clause 9.9.2.3A: Cross girders with sloped bottom flange may be critical for shear capacity at the ends. A contribution to shear from the flange inclination may be considered. For hog-backed main girders, a shear contribution from the sloping flange may be taken into account. For underslung cross girders, assessment of sling bolts or other tension elements that transfer cross girder reaction plus U-frame forces should be included. The flexibility of the connection should be modelled in a level 2 assessment by a separate study to evaluate the f value. 5.6.2F Half-Through Welded Bridges (e.g. E & A Type)

Shear plate connection

'E' Type

'A' Type

ZED Type

Figure F5.2 Forms of Half-through Welded Bridge

These bridges generally have HSFG bolted end plate/shear plate connections to the cross girders which coincide with any vertical web stiffeners so that U-frame behaviour is achieved. The concrete slabs give improved distribution between cross girders. Trimmers are usually present, but sometimes these are pinned and offer no end U-frame rigidity, except that rigidity may be provided via linear rocker bearings or cross girders near to the ends of the span. Where slabs are haunched at main




Page 17 of 87

girders the joint rigidity is enhanced if positively connected to the web via shear studs or welded reinforcement, but this connection may attract unwanted stresses. Level 1 assessment should consider dispersal from track to cross girders, but static distribution of loads otherwise. Level 2 assessment should include a 2-dimensional grid analysis allowing for distribution between cross girders where concrete covers the top of the cross girder. In Type A decks where concrete is infill only, any longitudinal continuity should normally be ignored. Where the top flange is asymmetric with respect to the web as in Zed’ Type bridges then an additional lateral force effect is applied to the U-frames which has to be resisted by the connections. 5.6.3F Half-Through Riveted Box or Twin Plane Truss Bridges

T 2D p

=

p

P CROSS GIRDERS DIAPHRAGM OR AT CROSS GIRDERS BEARINGS TOP CHORD WEB MEMBERS

+ P 2

p

T 2B

P 2

+ T

T

2D 2D SHEAR TORSION DISTORTION PB T= 2 WEB SHEAR DIAPHRAGMS ddddd INTERMEDIATE CROSS GIRDERS

BTM CHORD UNDERSLUNG CROSS GIRDERS

P SECONDARY BENDING FROM 2 AT INTERMEDIATE CROSS GIRDERS LONGITUDINAL BENDING

Figure F5.3 Forms of Half-through Riveted Box or Twin plane Truss Bridges

Depending on the span length, proportions, and type of bearings, the main girders may or may not rely on U-frame behaviour. If stability can be satisfied without Uframes, cross girder/main girder connections can be assumed as pinned only. Provided adequate intermediate diaphragms or cross bracings exist, the full cross section can be assumed to carry the floor and live loads. Stresses from restraint of




Page 18 of 87

distortion or warping, and transverse distortional bending stresses should be checked if diaphragms are at greater spacings than cross girders. Provided that intermediate diaphragms are capable of transmitting 50% of the cross girder reactions to the outer web, shear may be assessed to be equally resisted by both webs, plus torsional shear flow. Overturning stability of the box at bearings should be checked taking account of fixity reliably provided by end cross girders, bearings or surrounding brickwork. If stability is not achieved at bearings, a Level 2 assessment should consider 3dimensional behaviour, for example using an upstand grillage with representation of the joint rigidities at cross girder ends. 5.6.4F Half-Through Beam and Decking Type Bridges

DISPERSAL FROM SLEEPER

CROSS GIRDER

e.g. BUCKLE

OR TROUGHING PLATES CONTINUOUS FLOOR DISCONTINUOUS FLOOR

GRID DISTRIBUTION

STATIC DISTRIBUTION

Figure F5.4 Half-through and Decking Type Bridges

Some bridges have deck-type girders together with edge or parapet girders which contribute to live load capacity. For a Level 1 assessment, if the floor provides transverse continuity then a distribution analysis, such as a 2-dimensional grid analysis, should be carried out to determine the share of loading between the girders. If the floor is discontinuous, such as buckle plates only or separate spans of troughing, static distribution should be assumed. With ballasted track, the dispersal from sleepers through ballast should be taken into account, see Section 4, Clause 4.3.3.3. When checking the stability of edge girders, an assumption of U-frames may be necessary. In this case the distance B (Appendix A Clause 9.6.5A) may be taken as from the edge girder to the first internal girder.




5.6.5F Deck Type Bridges non-structural parapet

ti mber , troughing or cross girders

TWIN PLATE GIRDER

MULTIPLE PLATE GIRDER

SINGLE BOX

MU LTIPL E BOX

Figure F5.5 Forms of Deck Type Bridges

Where supported by only two plate girders or a single box, static distribution of loads may be applied. With more than two plate girders or a single box, a distribution analysis such as a 2-dimensional grillage should be carried out assuming the floor has transverse continuity and/or effective bracing exists between girders. 5.6.6F Trusses

BOWSTRING HALF THROUGH MULTIPLE INTERSECTION WARREN THROUGH

DECK TYPE

MODIFIED WARREN (1) MODIFIED WARREN (2) PRATT or "N"

Figure F5.6 Forms of Truss Bridges Most trusses are half-through or through type and are simply supported. Cross girders are often underslung. Composite behaviour of the floor as part of the chord is usually not present and should not be assumed unless there is continuously




connected floor plating. Static distribution of floor and cross girder loads to trusses should be assumed. For Level 1 assessment, a 2-dimensional analysis should normally be carried out which may be by a simple manual calculation for trusses such as Warren, modified Warren or N types. If cross girders do not occur at nodes, it is essential to consider local bending of the chord due to the load transmitted by the cross girder. A rigid jointed frame analysis is generally required. However, if there is no off-joint’ loading (other than self weight of members and minor items such as walkways), bending effects within the plane of the trusses can be ignored where the truss is fully triangulated, the centroids at joints all intersect, and the joints have untensioned bolts or rivets. In these cases, the analysis may assume either pinned or rigid joints and the moments can be ignored, see Clause 12.1A. Half-through trusses generally rely on U-frame behaviour for stability. They can be assessed similarly to half-through plate girders with diagonal members included as applicable, see Clause 12.5.1A. The values of I1, I3, and I4 should take account of whether paired members (as in a box form of truss) are effectively connected so as to form a compound section or not. Often tension diagonals in a truss are not connected between intersections, whereas compression members are battened or laced together. Members should only be the U-frame calculations they are adequately connected to included the crossinmembers either directly orwhere via stiffening. Judgement is required in some cases.




Page 21 of 87

5.6.7F Effective Span

The effective span of beams should be taken as given in Table F5.2. TypeofSupport

EffectiveSpan

Onto Outer Abutments and Piers Bearings of all roller, rocker, elastomeric, pot or other type with defined point of rotation

Centre to centre of bearings

Flat plate, mortar, lead or other bedding or directly supported on masonry concrete or brickwork

Centroid of pressure diagram, taken as maximum at front edge giving effective bearing area to zero at back of bearing area. Length of bearing area should be taken as not greater than depth of beam or troughing supported.

Onto Other Beams Cross girders or troughing connected to beam web

Centres of supporting webs of main beams

Cross girders, rail bearers or troughs bearing

Centres of effective stiff bearing areas which

onto flange or seatingweb cleats without connection to beam other than nominal top cleat

are capable of imposed load and eccentricity tocarrying the mainthe beams.

Longitudinal timbers or rail bearers supported on top of cross girders

Centre to centre of cross girders

Onto Metal Columns

Centre to centre of effective support points to beam on columns that are capable of carrying the eccentric imposed load to the columns

Table F5.2 Effective Span of Beams 5.7F Assessment Process 5.7.1F General The following is a description of the assessment steps in calculating the RA number for rail bearers, cross girders and main girders. References are given to Clauses in BS 5400: Part 3 as modified by Appendix A. It is expected that the calculations will be carried out either manually or using spreadsheet computer processes.




Page 22 of 87

5.7.2F Railbearers

Railbearers are discrete members parallel to the track which support one rail only and are carried by cross girders or at the ends of the bridge span by the abutments or trimmers (Figure F5.7). They are usually discontinuous and supported by a web and/or seating cleats onto the cross girders. In some cases they may be integral with floor plates or slab. 1.50m

SHARE OF RESISTANCE DEPENDS ON FIXING

CENTRIFUGAL

NOSING h c 7TRACKS RAILBEARER

A

S

LOADS TO

RAILBEARER

S

Figure F5.7 Railbearers and load Application

For assessment railbearers may be assumed to be simply supported with effective span from centre to centre of the cross girders if web cleated. For Level 2 assessment continuity can be assumed if integral floor plates are also interconnected longitudinally over the cross girders but, if the connections fail assessment simple supports may be assumed. Where top and bottom flange continuity is achieved by the connections or by integral welded construction, continuity may be assumed. Assume static distribution of dead and live loads unless the bearers are interconnected by a slab with transverse continuity. Note γf 3 = 1.0 if rail bearers are assumed simply supported. Assessment Steps - Simply supported I-section rail bearer: Step

1

Determineeffectivespan

2

Calculatedeadloads-staticdistribution

3

Calculate dead loads - moment (WL/8) and end shear (W/2) apply γfL

Clause of Clauses in BS 5400: Code Part 3 or Appendix A TableF5.2 Table4.2 Table 2.1




Page 23 of 87

Assessment Steps - Simply supported I-section rail bearer: Step

Clause of BS 5400: Part 3 or Appendix A

4

Determine live EUDL and end shear for 20 BSU on effective span from one track

5

Determinereductionfactorfordispersal

6

Determine static live loads moments and end shear for 20 BSU - static distribution of track load to railbearers

7

Calculate reductions of cross girder properties

7a

Determine K c factor if any integral plating to

Clauses in Code

Table 4.4

Table4.7

9.4.2.4 9.4.2.4

compression flange 7b

Check compression flange outstand limit if no integral

9.3.1A

plating and adjust σ if required (only likely to apply occasionally to railbearers) ys

8

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span (no

9.7.1A

hole deductions) 9

To derive n0 calculate unfactored deflection under dead WL  δ =  5  384  EI  3

load plus 20 kN EUDL then n0 = 10

0

17.75

δ

0

Using train speed and n0 determine dynamic increment

Figs 4.2 to 4.14

ϕ f or bending (for shear ϕ× 2 3 ). 11

Allow vertical effects of nosing/centrifugal to calculate dynamic vertical live loads

4.3.4/4.3.5





Clause of BS 5400: Part 3 or Appendix A

either

On straight track allow for lateral nosing 11a Calculate vertical load on railbearer:  EUDL(1+ ϕbend ) +2 × nosing load× h  as a UDL   2 s   end shear (1+ ϕ shear ) + nosing load× h  as a shear force   2 s  or On curved track calculate vertical effect of centrifugal 11b 18 + h  Fc  force Vn =   s  h (but not less than 2 x nosing load × ) s Calculate vertical load on rail bearer:  EUDL(1+ ϕ bend ) + V −1.2EUDL(1+ ϕ )× c  as a UDL   n bend 2 s end shear(1+ ϕ shear ) + Vn −1.2end shear(1+ ϕ )× c  as a   shear 2 2 s  shear force 12

Calculate dynamic live loads moment and end shear for 20 BSU

13

If compression flange restrained by plating λ LT = 0, otherwise calculate λ LT Typically le = 0.85 x effective span, η = 1, i = 0.5 (symmetrical)

Clauses in Code

4.3.4 Fig F5.7

4.3.5 Fig F5.7

9.7.2 A




Page 25 of 87


14*

Assume non-compact initially. Calculate Mult (elastic) Calculate β using Equation A20 Determine MR Mult assuming curve for k=0

Clause of BS 5400: Part 3 or Appendix A 9.8A

Clauses in Code

Fig. A10 or A11

9.3.7.2.1, If RA number for bending is restricted try use of compact 9.3.7.2.3A, section. 9.2.7.3.1

β=

If compact

λ

σ LT

yc

355

∴

MR = Figure A10 ratio (or

Figure A11) x Mpe 15

For compression flange MR = Figure A10 ratio (or Figure

9.8A

A11) x Z xc × σ ys 16

For tension flange allow any rivet/bolt holes to tension flange (note: K 2 = 1.15 for wrought iron) Ae nett Z xt = × Z xt , ∴ MR = nett Z xt × σ yt A

17

Mid-Span Bending ( MR − MDead × γ fL ) Static BSU = × 20 Static Live Moment ( MR − MDead × γ fL ) Dynamic BSU = × 20 Dynamic Live Moment

18

Static RA number = Static BSU - 10 = RA.... Dynamic RA number = Dynamic BSU - 10 = RA ....

Table 4.3

19

Fatigue Consider Stage A assessment. Calculate live load stress range under required RA in bottom flange at mid-span. Use ϕ for fatigue. Ignore nosing. Where σ f max

Appendix D

exceeds

σ

0

proceed to Stage B assessment.

11.3.2A & 11.3.3




Page 26 of 87


20

Shear Capacity Calculate shear capacity of web VD =

21

 t d σ   γ γ  3 w

w

m

f3

yw

Clause of BS 5400: Part 3 or Appendix A 9.9.2.2A 4.3.3A

(VD − dead shear × γfL ) × 20 Static Live Load Shear (VD − dead shear × γfL ) Dynamic BSU = × 20 Dynamic Live Load Shear Static BSU =

22

Static RA number = static BSU - 10 = RA.... Dynamic RA number = dynamic BSU - 10 = RA....

23

End connection vertical shear

14.5.3.4A &

Determine shear capacity of rivet/bolt

14.5.3.6A

24

Calculate shear capacity of connection and determine BSU and RA number as steps 20,21

25

End connection - horizontal shear on rivets VAy Calculate horizontal shear on rivet = x spacing I and determine RA number as steps 21,22

26

If nosing/centrifugal resisted by rail bearers and not restrained by plating etc, determine lateral force and shear as steps 11a or 11b

Table 4.3

4.3.5

Decide if resisted by one rail bearer or is shared. 27

Calculate moment My and end shear due to nosing/centrifugal

28

Clauses in Code

Calculate

M Dy

9.9.1.2A




Page 27 of 87


29

Clause of BS 5400: Part 3 or Appendix A 9.9.4.2

Using requirements for combined Mx and My

   20 / BSU vertical BSU combined =  M  20 / BSU vertical + M 

y

Dy

Clauses in Code

   BSU vertical   

∴ RA = BSU combined - 10 5.7.3F Cross Girders

Cross girders are discrete members transverse to the track which carry the bridge floor and span onto main girders. They are normally above flange level, but some cross girders are underslung with hanger bolts. Others span on top of main girders forming deck type bridges. At skew ends they may span from main girder to abutment pier or trimmers. Some form of moment rigidity usually exists at end connections. This rigidity is a necessity where main girders rely on U-frames for stability. Cross girders may be integral with floor plates or slab.

SHEAR B.M.

6 FOOT GIRDER JOINT WITH MOMENT CAPACITY

MAX.

MAX.

BOX

B.M.

MAX.

SHEAR

MAX.

1 TRACK LOAD 2 TRACK LOAD

Figure F5.8 Typical Cross Girders

MAX.




Page 28 of 87

Cross girders can be assumed as simply supported with effective span between centre to centre of main girders and centre to centre of inner webs if the supporting members are box girders. For bridges with centre girders, continuity of interconnecting cross girders can only be assumed if the connections are rigid and able to resist the hogging moments when both tracks are loaded. Static distribution between cross girders should be assumed unless interconnected by effectively continuous rail bearers or slab floor. End connections should be checked for FR + FC restraint forces in U-frame bridges. These effects are also carried at mid-span of the cross girder, but FC will, at the heaviest loaded cross girder, produce a hogging moment of opposite sign to the live load moment. For adjacent lighter loaded cross girders, FC will produce a sag moment which is additive. Effects of centrifugal and nosing forces are generally small on cross girders.

γ

f3

= 1.0 if cross girders are

assumed simply supported. Assessment Steps: - I-Section Cross Girder: Step

Clause of BS 5400:

Clauses in Code

Part 3 Appendix A TableF5.2

1

Determineeffectivespan

2

Calculate dead loads. Assumption of UDL is usually acceptable

Table 4.2

3

Calculate dead loads - moment (WL/8) and end shear (W/2) apply γfL

Table 2.1

4

Determine live loads from (EUDL for 2 x cross girder spacing) ÷ 2, per track. Apportion track load equally between the two rails.

Table 4.4

5

Determine reduction factor FfA

4.3.3.2

6

Determine static live load moment and end shear for 20 BSU

7

Calculate reduction of gross section properties.

7a

Determine K C factor for any integral plating to top flange 9.4.2.4

ordispersal

9.4.2.4




Page 29 of 87

Assessment Steps: - I-Section Cross Girder: Step

7b

Clause of BS 5400: Part 3 Appendix A 9.3.1A

Check compression flange outstand limit if no integral plating and adjust σys if required (likely to apply

Clauses in Code

occasionally to cross girders) 7c

Use effective width of slab if composite slab

8

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span (no

9.15.2.1 9.7.1A

hole deductions) 9

Determine impact factor,

I4.

4.15 Fig.

10

Allow vertical effects of nosing/centrifugal forces to give maximum bending/end shear and add to dynamic live loads. The vertical effects are generally small and can be omitted for Level 1 assessment

11

Calculate dynamic live load moment and end shear for 20 BSU. For U-frame bridges only, add sag moment due to FR buckling force (may disregard for Level 1 assessment)

9.12.2.2A

12

If compression flange fully restrained by plating or slab λ LT = 0, otherwise calculate λ LT .

9.7.2A

13

Assume non-compact initially. Calculate Mult (elastic)

9.8A

4.3.4 & 4.3.5 4.3.6

Calculate β using Equation A20 Determine MR Mult assuming curve for k=0

14

For compression flange MR = Figure A10 ratio (or Figure A11) x Z xc × σ ys

Fig. A11A10 or 9.8A





Clause of BS 5400: Part 3 Appendix A 11.3.2 & 11.3.3

Clauses in Code

15


16

Mid-Span Bending ( MR − MDead × γ fL ) Static BSU = × 20 Static Live Moment ( MR − MDead × γ fL ) Dynamic BSU = × 20 Dynamic Live Moment

17

Static RA number = static BSU-10 = RA.... Dynamic RA number = dynamic BSU-10 = RA.

Table 4.3

18

Fatigue Consider Stage A assessment. Calculate live load stress range under required RA in bottom flange at mid-span. Use ϕ for fatigue. Ignore nosing. Where σ f max

Appendix D

exceeds

σ

0

proceed to Stage B assessment.

19

Calculate shear capacity of web at ends  t w d w  σ yw   VD =  γm γ f 3  3

20

Shear Static BSU =

(VD − dead shear × γfL ) × 20 Static Live Load Shear

Dynamic BSU = 21

(VD − dead shear × γfL ) × 20 Dynamic Live Load Shear

Static RA number = static BSU - 10 = RA.... Dynamic RA number = dynamic BSU - 10 = RA....

9.9.2.2A 4.3.3A




Page 31 of 87


22

Clause of BS 5400: Part 3 Appendix A 9.12.2.2A & 9.12.2.3A

End Connection to Main Girder

Clauses in Code

Calculate FR buckling and Fc (live load) U-frame effects on connections Determine coexistent shear and bending moment on connection 23

Calculate shear capacity of connection using same ratio of shear and bending moment Static BSU =

(Shear capacity - dead loads shear) x 20 static live shear

Dynamic BSU =

(Shear capcity - dead load shear )

x 20

dynamic live shear 24

Static RA number = Static BSU - 10 = RA.... Dynamic RA number = dynamic BSU - 10 = RA...

25

End of Cross Girder – Horizontal Shear Calculate horizontal shear on rivet =

I then determine RA number as steps 21,22

x spacing

Main girders are primary longitudinal members normally parallel to the track. They may be through-truss, half-through or deck-type, plate girder, box girder or trusses. Most older examples of spans up to about 30 metres incorporate no specific bearings and bedstones sometimes with flat bearing plate or bedding medium.




Page 32 of 87

e U-FRAME WEB ONLY

RIVETED OUTER

WELDED

A-TYPE

INNER

e

e STIFF.

º

e e STIFF. U-FRAME

INNER

LOCAL DISTORTIONAL BENDING WARPING

OUTER

LONGITUDINAL DISTORTION

Figure F5.9 Typical Main Girders

Static distribution of loads to main girders can normally be assumed unless the bridge is a deck type with multiple girders and there is transverse continuity. In this case a distribution analysis (say 2-dimensional grillage) is appropriate. Composite behaviour with deck type flooring should be assumed if proper interconnection exists. Halfthrough main girders usually rely on U-frame behaviour. Joint rigidity and the presence of stiffeners in line with cross girders are important parameters in deciding whether proper U-frame rigidity exists. The presence or otherwise of end U-frames and/or of bearings which give torsional restraint is important in determining the effective length of the top flange. Figure F5.9 shows options for the value of δ e used in Clause 9.6.5.2A of Appendix A. Box girder stability requires consideration of cross cross girder connections, where stability is achieved by the end linear rocker bearings centred towards the inner web.

γf3

≤

25o and the girder is simply

supported, it is continuous with splices welded or HSFG bolted/riveted with cover platesortoifboth flanges.




Page 33 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge: Step

Clause in BS 5400: Part 3 Determineeffectivespan

Clause in this Code

TableF5.2

Calculate dead loads – assumption as UDL is usually acceptable 3

Calculate dead loads - moment (WL/8) and end shear

Table 2.1

fL

Determine live loads on all tracks. Apply static distribution for loading to main girders 5

Determine static live load moment and end shear for BSU. Determine moments at curtailment points.

6a

C

factor for any integral plating to top flange

6b

Check compression outstand limits if no integral plating and adjust σys if required.

6c

Check effective web thickness if slender.

7

To derive n0 calculate unfactored deflection under dead

9.3.1A

9.4.2.5 4.3.2.2

5WL  δ =  384  EI  3

load plus 20 kN EUDL then no = 8

o

17.75

δ

0

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span and

9.7.1A

curtailment points 9

Using train speed and n0 determine dynamic increment

ϕ f or bending (for shear ϕ× 2 3 ).

Figure 4.02 to 4.14




Page 34 of 87


Clause in BS 5400: Part 3 Appendix A

10

Allow vertical effects of nosing/centrifugal forces to give maximum bending/end shear and add to dynamic live loads. These effects are generally small and could be omitted for Level 1 assessment.

11

Calculate dynamic live load moments and end shear for 20 BSU

12

For intermediate U-frames calculate δ R . For end U-frame or stiffener calculate δ e . ( δ e should be

4.3.4 & 4.3.5

9.6.5.2A

taken as infinity, i.e. k5 = 3.6 if there is no end frame or effective stiffener within le 3 or L 5 of support). Calculate

le

13

Calculate

λ

14

Assume non-compact initially. Calculate Mult (elastic) Calculate β using Equation A20

9.7.2A

LT

9.8A

Determine MR Mult assuming curve for k = 0 (∆F-0.001x)

y ry 2

= 0 or as measured bow.

Main girders rarely satisfy compact criteria. 15

For compression flange MR = Figure A10 ratio (or Figure A11) x Z xc × σ ys x 0.95 The 5% reduction allows for transverse bending, see Appendix A.

16


Clause in this Code

11.3.2 11.3.3




Page 35 of 87


17


Clause in this Code

Mid-Span Bending

( MR − MDead × γ fL )

× 20 Static Live Moment ( MR − MDead × γ fL ) Dynamic BSU = × 20 Dynamic Live Moment Static BSU =

18

Static RA number = Static BSU - 10 = RA.... Dynamic RA number = Dynamic BSU - 10 = RA.

19

Fatigue Consider stage A assessment. Calculate live load stress range under required RA in bottom flange at point of maximum bending and curtailment points. Use ϕ for fatigue. Ignore nosing. Where σ f max exceeds σ 0

Table 4.3

Appendix D

proceed to stage B assessment. 20

Calculate shear capacity at ends ( VD ).Use Figs. 11 etc for web buckling. For variable depth girders Clause strictly 9.11 should be applied, but 9.9.2.2 is acceptable provided the maximum bending is not greater than 0.5 MR at positions of critical shear, i.e. 9.9.2.2 is applicable to simply supported beams.

21

Shear (VD − dead shear × γfL ) × 20 Static Live Load Shear Dynamic BSU = (VD − dead shear × γfL ) × 20 Dynamic Live Load Shear Static BSU =

22

Static RA number = static BSU-10 = RA.... Dynamic RA number = dynamic BSU-10 = RA...

9.9.2.2




Page 36 of 87


23

Clause in BS 5400: Part 3 Appendix A End of Main Girder Horizontal Shear Calculate horizontal shear on rivet =

VAy

I then determine RA number as steps 21,22

24

x spacing

Intermediate Stiffeners at U-Frames

9.12.2.2A 9.12.2.3A

Check bending capacity. Compare with moments due to FR + FC and determine RA capacity. 25

Intermediate Stiffeners not located at U-Frames

9.13

Check capacity and determine RA number 26

Restraint at Supports Calculate: Fs = Fs1 + Fs 2 + Fs 3 + Fs 4

9.12.4.1A

Fs1 - buckling effect depending on flange bow Fs 2 - buckling effect depending on web verticality Fs 3 - out of verticality effect Fs 4 - skew effect (skew bridges only)

9.12.4.2A

Calculate FL – for U-frames Total force = Fs + FL Calculate moment due to Fs + FL on supports, i.e. bearing stiffener or end U-frame (including, if required, all Uframes within le 3 or L 5 of support). If capacity insufficient for RA number derived from bending capacity of girder, recalculate MR assuming δe = ∞ 27

Assess bearing stiffeners based on combined effects of restraint at supports and vertical bearing load.

9.12.4.1A 9.12.4.2A 9.14.4

Clause in this Code






28

Curtailment points Check bending as steps 17 and 18 at flange curtailment points.

29

Splices Check splice capacities in bending and shear.

Clause in this Code

14.4.1.1A

5.7.4.1F Skew Bridges

Skew tends to reduce the bending effects in main girders which span longitudinally. If static distribution is assumed, Figure F5.10 may be used to derive bending moments in main girders. Alternatively, a grid analysis may be used.




Page 38 of 87 Line through points of "effective" bearing

L

Also applicable when abutments are not parallel.

= Lead on Girders c 7TRACK Live load span m.

= E.U.D.L. fo

r B.M. on m

Live load line w/ft. run

span m

A LI V E L OAD B. M. ON GIRD 0.10

0

0 .1

ER AB = 0 .2

L²

0.3

B

L x SKEW FACTOR FROM 0 .4

0 .5

0 .6

BEL OW

0.7

0 .8

0 .9

1 .0 0.10

0.09

0.09

0.08

0.08

0.07

0.07

0.06

0.06

0.05

0.05

0.04

0.04

0.03

0.03

0.02

0.02

0.01

0.01

0

0

0 .1

0 .2

0.3

DEA D LO AD B .M. F ROM F L OOR =

S 2

Line through po ints of "effective" bearing

0 .4

L²

0 .6

0.7

0 .8

x SKEW FACTOR FROM

Dead Load on AB

S 2

W hen AB is an Inne r Girder B.M. from both fl oors to be added.

0 .5

Girder

0 .9

0 1 .0

BEL OW

L = Lead on Girders

w/ft. run

A

B

L

0.125

0.125

0.100

0.100

0.075

0.075

0.050

0

0 .1

0 .2

0.3

0 .4

0 .5

0 .6

0.7

0 .8

0 .9

0.050 1 .0

Figure F5.10 Bending Moments in Main Girders of Skew Bridges




5.7.5F Bridge Floors 5.7.5.1F General

Floors of pre-1948 riveted bridges are rarely composite with main girders whether they comprise flat plates, buckle plates, troughing, old rails or jack arches. Most modern with maindeck-type girders. bridges use floors of reinforced concrete or steel plate composite 5.7.5.2F F lat Plate F loors

Flat plate floors act structurally when supporting ballasted track. Where tracks are carried by longitudinal timbers, the floor is generally non-structural except for any composite action with rail bearers or cross girders if continuously connected. Where the track is ballasted, floors are subjected to dispersed wheel loads from sleepers. This can be reduced where rail bearers act as rigid supports (Section 4, Clause 4.3.3.3). Support conditions should be considered, i.e. whether 2 or 4 sided, and whether continuity is achieved at the supports. Where plates are in separate panels connected by a single line of rivets to supporting rail bearers/cross girders, continuity should not be assumed. Continuity may be assumed where plates are continuous or connected by more than one line of rivets. The plastic modulus may be assumed in all cases at ULS. The elastic modulus should be used for fatigue checks. Plastic hinge or yield line global analysis may be used for steel plates. Yield line analysis may use standard published expressions or be derived from first principles. Elastic global analysis should be used for wrought iron. Elastic global analysis may use published coefficients (e.g. Steel Designer’s Manual). 5.7.5.3F Bu ckle Plate F loors

Buckle plates consisting of vertically curved steel plates supporting ballast or nonstructural filling and spanning between supporting members may be assessed by Clauses 5.5.3.1 or 5.5.3.2 as appropriate. 5.7.5.3.1F Spans of 1.2 metres or l ess

Where the clear span measured between edges of supporting members is 1.2 metres or less and complies with both of the following: (a) (b)

rise between 1/23 and 1/18 of the clear span, and plate thickness is 6 mm or more

The strength should be assessed assuming arch or catenary action and the horizontal wL2 thrust is taken as per unit width. 8r where:




Page 40 of 87

w

is the pressure on the plate surface due to dead loads and dispersed live loads. Dispersal may be assumed at 1:1. The pressure calculated should be assumed to occupy the full area of the plate;

L

is the span of the buckle plate between edges of supporting members;

r

is the rise of the buckle plate.

5.7.5.3.2F Spans Ex ceedi ng 1.2 metres

Domed (i.e. concave upward) buckle plates may be checked as straight compression members in accordance with Clause 10.6 of BS 5400: Part 3 taking

le

σ

y

as r 355 and an applicable effective length of not less

(λ −15) when calculating the value of σ c than 0.25L where L is the curved length between supports. The strength of suspended (i.e. concave downwards) buckle plates may be checked as tension members under axial load in accordance with BS 5400: Part 3 Clause 5.5.3.1. The fixings and surrounding construction should be capable of resisting the horizontal thrust. Concentrated wheel loads over the plate may be dispersed at 1:1. Alternatively, the buckle plate may be considered as an encastré flat plate in which case the effects of horizontal thrust may be ignored. 5.7.5.4F Tr oughing F loors

Live loading to troughing should be dispersed as defined in Section 4. Troughing generally satisfies compact section criteria (BS 5400: Part 3 Clause 9.3.7.2.3) allowing use of the plastic modulus. The elastic modulus should be used for U-frame effective length calculations where transverse troughing forms cross girders (Appendix A Clause 9.6.5A or BS 5400: Part 3, Clause 9.6.6). Composite action with concrete infill may be assumed in strength calculations and for U-frame rigidity, where relevant, in accordance with Appendix A Clause 8.8.3A, where: (i) (ii)

troughs are filled up to at least 75 mm above crests with concrete known to be dense and without significant evidence of slip or separation, and rivet heads or other positive shear connections occur along at least every alternate crest.

The strength or rigidity determined should not exceed by more than 30% the calculated strength or rigidity for the troughs unless demonstrated by appropriate testing. Plastic global analysis may be used for continuous steel troughing where the section is compact.




Page 41 of 87

5.7.5.5F Old Rail Fl oors

Floors made up of old rails should be assessed assuming that the material has a yield stress of 230 N/mm². Composite action with any infill should be ignored in calculating bending resistance of the rails. Rails may be assumed to be compact sections. In the absence of site measurements, the depth of rail should be assumed to be 5 mm lessmay than srcinalto depth onreinforcement the basis of the railswithin beingconcrete worn andprovided secondhand. Rails bethe assumed act as bars the load stresses permitted by Appendix B are not exceeded.

5.7.5.6F Tr ansver se Sti ff ened F loor - Sta ndar d Box Gir der s

A longitudinal dispersal length of 3.6 metres for calculation of EUDL for Level 1 assessments may be assumed for transverse tee rib stiffened steel floors as used in the Railtrack 1989 standard’ box girders, or for similar floors of welded construction with minimum plate thickness of 20 mm. For Level 2, if required, a local grid analysis should be carried out which takes into account the flexibility of the track and ballast as used for the design of the Railtrack 1989 standard’ box girders. For fatigue assessment using Appendix D, the floor plate fatigue classification should be assumed as E’. 5.7.5.7F L ongitudi nall y Stif fened F loors - Par t of Top Fl ange

Where floors are longitudinally stiffened and are assumed to form part of the top flange of main girders, local and global stresses should be combined under a SLS check (Appendix A Clause 4.2.2A ) since this is not required at ULS. The EUDL for global bending is normally the full span length. RA axle loading should be considered for the calculation of local stresses.

Load Case Centre to centre cross frames EUDL

Load One Bay Between Centre to Centre Cross Frames Up to 3 metres EUDL Fx EUDL (3 metres)

> 3 metres EUDL for centre to centre

Load Two Adjacent Bays Up to 1.5 metres EUDL x FEUDL (3 metres)

> 1.5 metres EUDL for centre to centre

Sketch

Table F5.3 Load Requirements for Longitudinally Stiffened Floor Plates




GUIDANCE NOTES TO APPENDIX A 4.2.3AF Fatigue Appendix D, as required by Clause 4.2.3A, includes a method for fatigue assessment which is subject to further development and trial. It is written as a supplement to BS 5400: Part 10(6). The concept of safe life’ and damage tolerant’ elementsis adopted. The former are primary

elements where ability to redistribute live load exists inthe event of failure. The latter are generally secondary elements, either with capacity for redistribution or the permanent way provides a measure of spanning capability in theevent of their failure. Fatigue endurance is determined in stages from (A) through to (D). Where the stated criteria can be satisfied at any stage, then subsequent stages representing more rigorous approaches need not be evaluated. Stage A compares thecalculated live load stressrange with stated nonpropagating stress values similarly to the previous Code. 4.3.3AF Values of Partial Safety factors

The proposal for using γf 3

=

1.0 aims to give equivalent overall safety of the new code

compared with the existing code as far as possible. 6.1AF Performance The strengths for rivets have been based on tests on full size steel riveted girders recovered from the reconstruction of underbridge LEC1/248, London to Rugby Line, tested in 1998 and reported by Cass Hayward and Partners in a Report to Railtrack dated August 1998. Test results on wrought iron rivets from two other bridges were also studied. Yield and ultimate tensile strengths in Table A2 are based ontests of the rivet material. The use of 0.9σ ult

instead of σ y for the value of

σ

q

in Clause 14.5.3.4A for rivets subjected to shear is based

on tests of riveted web to flange joints taken from underbridge LEC/248. The value of γm for rivet shear in Table A1 for ULS is taken as 1.33, instead of 1.10 as in BS 5400: Part 3, to allow for the effects ofrivet slip as measured during the tests onthe girders. This is so that under SLS loading rivets will be loaded to about 0.5 of their shear capacity, corresponding with a slip of 0.5 mm so as to limit possible overstress in the connected elements through loss of interlock between them. No SLS check is then required. In riveted girders there will be a tendency for a redistribution of bending stress from the flanges to the web causing overstress of the web through slip ofthe rivets in longitudinal shear. The load in slip behaviour of rivets in shear was deemed to be similar to that of headed shear studs used in composite construction. The effect upon assessmentis that the capacity of rivets inshear will be some 50% greater than in RT/CE/C/015 Issue 1. For checking the bearing capacity of rivets in shear then the value ofσ y is taken as the lesser of that of rivet or connected material from Table A2 with

γ

m

taken as 1.05 in a similar way to BS 5400: Part 3. Generally

the capacity will be governed by rivet shear rather than bearing. 6.4AF Ductility

Potentially wrought iron would appear to have insufficient ductility for plastic analysis. However, it is likely to have sufficient ultimate to yield strength for plastic stress analysis. Note draft EC3: Part 2 requires 15% elongation for global plastic analysis. 6.5AF Notch Toughness

The recommendations of BS 5400: Part 3 do not solve the potential difficulty of assessing the notch ductility of steel in existing bridges where there is no test data on the notch ductility of




the steel. Known failures are rare. Most structures willhave shed their secondary stressesin service. Ideally an insitu test method is needed for the future 6.6AF Properties of Steel and Wrought Iron

Properties for wrought iron are from BRR Report LR MF 115 (4). 7.1AF Global Analysis for Load Effects - General Plastic global analysis is permitted by Clause 7.1A, except for wrought iron structures due to the limited elongation ofwrought iron. Plastic stress analysis is,however, considered appropriate for wrought iron because the ultimate stress to yield stress ratio is considered adequate. Clause 9.3.7.5A of Appendix A clarifies that the plastic modulus is permitted for flat plates such as in floor plates. 8.5.1AF Imperfections Allowed For Clause 8.5.1A caters for elements that have deviations or tolerances which vary from those given for new construction to BS 5400: Part 6 and which are implicit within the strength rules of BS 5400: Part 3. In particular, measured bow and verticality of girders should be taken into account as necessary in the assessment where appropriate, for example for halfthrough bridges. The requirement to measure web panel imperfections is probably unnecessary for most small underbridges because out-of-flatness does not affect strength significantly unless the distortion is noticeable. 9.3.1AF Shape Limitations - General

Under Clause 9.3.1A, a “fictitious yield” approach, as in BD 56/96, is used for section outstands. They often exceed the limits of BS 5400: Part 3. 9.6.1AF General This provision is to cater for bridges where the support restraint is not over the support. Its resistance could be added to the gravity cantilever resistance of a bearing stiffener if this is limited by strength or by overturning stability on the bearing. The limits on le are based on Table 8. 9.6.2AF Beams with intermediate lateral restraints Clauses 9.6A and 9.12A provide for assessment of U-frame bridges, other than those with rigid support restraint, as in the proposed amendments to BS 5400: Part 3. Account can be taken of end restraints of any rigidity together with effects of skew. There is a penalty in that the effective length has to be increased in compensation. It is somewhat unfortunate that the proposed amendments are not already published. If they were the bulk of Appendix A would be less. 9.6.3AF Beams (other than cantilevers) without Intermediate Lateral Restraints

This clause is similar toClause 9.6.3 of BS 5400: Part 3 but the expressions forle enable the flexibility of the support restraints to be taken into account. 9.6.5AF Beams with U-frame restraints

U-frames are covered by the following Clauses: 9.6.5A

Effective length –

le




Page 44 of 87

9.12.2.2A 9.12.2.3A

Intermediate U-frames – Force FR Intermediate U-frames - live load on cross girders – Force FC

9.12.4.1A

Support restraints – Force FS

9.12.4.2A

Support restraint – Force FL

The Effective length for U-frames only take account the intermediate U-frame where the lateral stiffeness ofthe deckingcan system meets Clauseof9.12.2.2A. If not the U-frames are to be ignored The selection of δ R (intermediate U-frames) and F5.11.

δ

e

(end supports) is illustrated in Figure

UNIT FORCE

e STIFF.

e (END FRAME) µe AT SUPPORTS OR WITHIN 3

R INTERMEDIATE

SKEW

SKEW

<25°

<25° e (END FRAME)

e (STIFF.) µe

<25°

<25°

3

=º

<

µe

e= R

3

<25°

>25° e (STIFF.)

µe

3

<

µe

e= R

3

>25°

>25° e=º

e = e (STIFF.) Figure F5.11 U-Frame Stiffness

Values of f’ are obtained from Figure 42A. Support restraint force (Fs + FL ) should be applied to the end bearing stiffener, and U-frames if within

le

3

, but (

L . If the RA number 5




Page 45 of 87

is found to be restricted below that of the main girder in bending,

le

should be recalculated

with an appropriate increase in δ e in order to seek a more favourable RA number.

δ δ

e e

f irstAssessment

Amend

δ

(stiff)

∞ or δ e

(end U-frame) if trimmer

e

to:

present

δ

e

(U-frame)

δ

e

(stiff) +

δ

e

∞ (U-frame)

δ

e

(U-frame)

Table F5.4 Magnification of

δ

e

9.7.1AF General It is considered rational to use the short term modular ratio only because the majority of stresses in the composite condition will be from live loading in rail bridges. 9.7.2AF Uniform I , Channel, Tee or Angle Sections For U-frame bridges the transverse section properties can take account of the lateral

stiffnesses of does the decking system even this does not of meet the requirements of Clause 9.12.2.2A, or not form part of the iftension flanges beams. BS 5400: Part 3 uses the long term modulus but this is inconsistent with Clause 9.7.1. See also Clause 9.7.1AF. 9.7.3.1AF Un if orm Rectangular or Tr apezoidal Box s ections

See Clause 9.7.2AF. 9.9.2.1AF General

The only likely cases for rail underbridges where webs need to be checked to the interaction criteria of BS 5400: Part 3 Clause 9.11 arebeams with flanges curved in elevation which are not simply supported. 9.9.2.3AF Shear Resi stance of Sim ply Supported Hog back be ams or sloped Bott om fl ange

Clause 9.9.2.3A includes a method for reducing the shear on webs of hog-back beams. The limit imposed on web slenderness,λ , is to ensure that the compression diagonals of the analogous truss do not buckle. 9.12.2.3AF U- F rames wi th Cross member s subj ected to Ver ti cal Loadi ng

In calculating θ , the differences in rotation between consecutive U-frames, then point loads corresponding to the RA loads shown by Figure 4.1 should be used. The loading however should be compatible with that assumed to derive force FR in Clause 9.12.2.2A. The proposal for 5% reduction in MR to compensate for the effect of lateral flexure is subject to further study.




Page 46 of 87

9.12.4.1AF Restr aini ng F orces

∆ is D/300 in BS 5400: Part 6. This value has been enhanced by 1.2 in accordance with Clause 8.5.1A. The minimum value of ∆ is 3 mm x 1.2. 9.12.4.2AF Additi onal U-f r ame Restr aints

See 9.12.2.3AF. The loading used to derive θ should be compatible with that used to derive force Fs under Clause 9.12.4.1A. 11.3.2AF Ef fective Ar ea

k2 =

0.8σ ULT

σ

which gives 1.20 for wrought iron (i.e., 0.8 ×

Y

285 ). However, a 190

conservative value should be appliedto wrought iron in the absence of testing, i.e.,

1.0 + 0.5

285   −1.2 = 1.15 190  

For beams, the deduction for rivet holes in tension areas may ignore any rivet holes in the portions of web not enclosed by flange angles with the reduced value of section modulus taken as:

nett Z xt =

Ae A

×Z

xt

where:

Ae =

K 2 At but ( A;

At =

A − ∑ Ah + ∑

s2t

4g

.

In some cases the web plate may need to be ignored in bending calculations altogether (see Clause 5.6.3.4F).

GROSS AREA OF FLANGE =A

g2

3 g1 g 2

7

c R.S.A. LEG

Figure F5.12




Page 47 of 87

Rivet Holes in Tension 12.1AF Trusses - General

For black-bolted or riveted triangulated trusses with concentric joint intersections, secondary moments can beignored under, Clause 12.1A. Although fixed-joint analysis is not arduous, the excessive effort in interpreting and checking the moments to all the in-coming members is time-consuming and isirrelevant if the joints Other would require secondary moments to be considered forhave SLS,flexibility. ULSand fatigue astrusses codified. A 3 dimensional analysis can be reserved for second stage assessment. 14.4AF Splices 14.4.1.1AF General

(a)

Girder Splices

In girder splices, bending moment may be assumed to be carried entirely by the flanges (plus flange angles and web plate between) and theshear resisted by the webonly. This assumption is made because web splices often contain only a single row of rivets/bolts and would otherwise lead to a reduced capacity due to overload of web rivets in beam bending plus shear. (b)

Web Joints

Web joints may be concealed at tee or similar vertical stiffeners having two rows of rivets. Bending capacityshows of girders should be assessed therefore ignoring web unless drawings or other evidence that web joints are not located at the pointthe considered. (c)

Flange Joints

Joints may occur in multiple flanges. They should be detected during inspection and from drawings. Unless reinforced by extra cover plates orthe flanges are in compression onlyand the joint is tight fitting, the jointed flange should be ignored in the calculation of bending capacity, as shown in Figure F5.13.

COVER PLATE

JOINT IGNORE FLANGE SPLICED

JOINT

JOINT

Figure F5.13 Flange Joints




Page 48 of 87

Where flange plate joints are located close together, the capacity of the whole flange should be assessed at each joint in the flange and the anchorage of each flange considered with respect to the numbers of rivets connected. (d)

Flange Plate Curtailment

In flange plate curtailment calculations, the point of theoretical curtailment may, unless calculations are carried out to determine the anchorage length, be taken as at the third line of rivets as shown in Figure F5.14.

ACTUAL CURTAILMENT

ASSUMED POINT OF CURTAILMENT Figure F5.14 Flange Curtailment 14.5.3.4AF F astener s Subject to Shear Onl y

The value of γm for rivets is1.33 The value has been increased compared to the previous Code to ensure, in the absence of SLS checks, excessive slip causing redistribution of stresses at working loads is avoided.. For other fasteners γm is 1.0 (see Table A1). The diameter of rivets are taken as 1/16” (1.6 mm) greater than the nominal diameter, i.e. the diameter of the hole. For example a 7/8” in diameter rivet is taken as 15/16” (23.8 mm) in diameter for assessmentcalculations. Rivets are commonly 3/4” or 7/8” nominal diameter. More than one size of rivet often occurs within a bridge. Horizontal shear capacity of rivets should be checked atall span ends. Checks at intermediate locations may be unnecessary where an appropriately uniform rivet spacing is present, typically 4” (102 mm). B3.4.2AF STRENGTH Other amendments to BS 5400: Part 3 B.3.4.2 relating to braced diaphragms have been omitted. APPENDIX D Clause 9.9.6 of BS 5400: Part 3 now refers to a modified Appendix D which corrects errors in the existing in BS 5400: Part 3 clauses for patch loading. Patch loading is relevant for non-




compliant bearing stiffener areas. This Appendix does not cover the common arrangement without bearings and with a stiffener at the bedstone edge and an end plate. APPENDIX E

Further consideration was needed as to how Appendix E of BS 5400: Part 3 should be applied or modified forassessment of transverse bending onU-frame bridges. Appendix E is very andgirder is based onhaving theoretical work assumed a simply supported frameconservative, bridge with the ends infinitely rigidwhich restraint against twist. For U-frameUbridges it is proposed that Appendix E is not used and that a blanket allowance of 5% be added to vertical bending effects in compensation.




REFERENCES - SECTION 5 AND APPENDIX A

1.

BSI, BS 5400: Part 3: 1982 - Steel, Concrete and Composite Bridges. Code of Practice for Design of Steel Bridges(incorporating amendments No 1 & No 2), British Standards Institution;

2.

The Highways Agency. BD 56/96 The Assessment of Steel Highway Bridges and Structures (DMRB 3.4.12). The Stationery Office, 1997;

3.

The Highways Agency. BD 13/90 Design of Steel Bridges. Use of BS 5400: Part 3:1982 (DMRB 1.3). The Stationery Office, 1990;

4.

BEAGLES, M. Static and Fatigue Properties of Wrought Iron and Early Steels.British Rail Research Report LR MF 115, November 1993;

5.

The Highways Agency. BD 21/97 The Assessment of Highway Bridges and Structures (DMRB 3.4.3). The Stationery Office, 1997;

6.

BSI, BS 5400: Pa rt 10: 1980 - Steel, Concrete and Composite Bridges. Code of Practice for Fatigue. British Standards Institution;

Other Sources of reference used in the development of Section 5 and Appendix A included: 7.

BS 7668 Specification for Weldable StructuralSteels. Hot Finished Hollow Sections in Weather Resistant Steels. British Standards Institution;

8.

BSEN10025 Hot Rolled Products of Non-alloy Structural Steels. Technical Delivery Conditions. British Standards Institution;

9.

BSEN10029 Specification for Tolerances on Dimensions, Shape and Mass for Hot Rolled Steel Plates 3 mm Thick and above. British Standards Institution;

10.

BS EN 10034 Structural Steel I and H Sections - Tolerances on Shape and Dimensions. British Standards Institution;

11.

BS EN 10056 Structural Steel Equal and Unequal Leg Angles - Part 2 Tolerances on Shape and Dimensions. British Standards Institution;

12.

BS EN 10113 Hot Rolled Products in Weldable Fine Grain Structural Steels.British Standards Institution;

13.

BS5135 Specification for Arc Welding of Carbon and Carbon Manganese Steels..British Standards Institution;

14.

BS EN 10137 Plates and Wide Flats Made of High Yield Strength Structural Steels in the Quenched and Tempered or Precipitation Hardened Conditions.British Standards

15.

Institution; BS EN 10155 Structural Steels with Improved Atmospheric Corrosion Resistance. Technical Delivery Conditions. British Standards Institution;

16.

BS EN 10210 Hot Finished Structural Hollow Sections of Non-alloy and Fine Grain Structural Steels. British Standards Institution;

17.

DD ENV 1993-1-1 Eurocode3: Design of Steel Structures. Part 1.1 General Rules and Rules for Buildings. British Standards Institution;




18.

DD ENV 1994-1-1 Eurocode 4: Design of Composite Steel and Concrete Structures. Part 1.1 General Rules and Rules for Buildings. British Standards Institution;

19.

ENV 1991-3 Eu rocode 1: Basis of Design and Actions on Structures. Part 3 - Traffic Loads on Bridges (CEN 1994). British Standards Institution;

20.

BSI 96/103556 National Application Document for use in the UK.British Standards Institution;

21.

ENV 1993:3:1994 (EC1: 3) Traffic Loads on Bridges. British Standards Institution;

22.

BSI. Proposed Amendments to BS5400: Part 3 dated 09/02/00. British Standards Institution;

23.

EC3: Part 2 “ Eurocode for Steel Bridges” (Draft). British Standards Institution;

24.

National Application Documentfor EC3: Part 2 (Draft). British Standards Institution;

25.

UIC. UIC776-1R Loads to be Considered in the Design of Railway Bridges. International Union of Railways, 1990;

26

LUL. Standard for Assessment of Railway Underbridges : CED-ST-3114-A1. London Underground Ltd.;

27.

Railtrack. RT/CE/C/015 Railtrack Code of Practice - The Assessment of Underbridge Capacity” Issue 1. Director Civil engineering;

28.

The Highways Agency. BD 37/88 Composite Version of BS5400: Part 2 for the Specification of Loads Used for the Design of Department of Transport Highway Bridges and Associated Structures. The Stationery Office, 1988.




Page 52 of 87

SECTION 6 - MASONRY ARCHES 6.1.1F Applicability

Clause 6.1.1 identifies the various elements of arch structures which are covered by this Section of the code. It should be noted that it is not applicable to structures which have been modified by the replacement of sections of the arch barrel with steel or concrete elements. 6.1.2F Levels of Analysis

Refers back to Clause 1.6 where different levels of assessment are defined. It is intended that techniques presented in Section 6 are related to Clause 1.6 as follows: Single span arches and multi span arches with stocky piers: Level 1 Level 2 Level 3

MEXE analysis 2-D analysis using elastic or mechanism method 3-D finite element

Multi-span arches with slender piers: Level 1

2-D analysis using elastic or mechanism method

6.1.3F Assumptions

All the features from which an arch derives its strength are not directly measurable during the site survey and inspection. Nor may all the information be readily available from existing record drawings. Inevitably certain assumptions have, therefore, to be made during the course of the assessment. The assumptions could include ring thickness and backing levels, as well as the level of passive pressure to be permitted during the analysis. Since some of these assumptions are likely to be judgmental, it is important that the assessor explores the sensitivity of his analysis to changes in assumed parameters before reaching a conclusion on the capacity of the arch. 6.1.4F Loading and Load Distribution

A longitudinal dispersal angle of 1 horizontally to 2 vertically from the underside of sleepers is recommended. In applying this assumed dispersal to a single axle, account may be taken of the fact that the load may also be shared by adjacent sleepers as indicated in Clause 4.3.3.3. If thisbeneath dispersal applied due should taken of the load concentration themodel middleis sleeper. It isaccount recognised thatbe certain proprietary analysis packages incorporate other load dispersal models (for example, ARCHIE uses a sinusoidal dispersal and MAFEA a Boussinesq model (1)) such dispersal methods are acceptable. Transversely, load is assumed to be dispersed at an angle of 45 o. The extent of transverse distribution defines the effective width to be considered for assessment. The point where the dispersal lines of loads from adjacent tracks intersect defines the




limit of the effective width for the situation where adjacent tracks are loaded. The effective width should be limited in circumstances where the arch barrel is cracked, or contains discontinuities, for example, if it has been extended and it is believed prudent to assume that the fill is also cracked and unable to distribute the load. It is recognised that both longitudinal and transverse angles of dispersal are greater than the 15° spread specified in Figure 4.16. This is in recognition of the fact that a significant proportion of the material between the underside of the sleeper and the arch ring will be well compacted fill, rather than ballast. The increased dispersal in the transverse direction also makes some allowance for load distribution by the arch barrel. Where the track over the structure is curved, centrifugal effects may be taken into account by assessing the effective width associated with the more heavily loaded rail. Where the dispersal lines overlap, again the limit of the effective width should be taken at the point of intersection. 6.1.6F Skewed Arches

A 35o limit has been set for permitting skewed arches to be assessed as two dimensional structures based on the skew length of their spans. This limit has been based on judgement, and may be subject to review when further research on this subject has been undertaken. Torsion in the piers of skewed multi-span structures due to non-uniform thrust at the support positions is essentially a three-dimensional phenomenon which cannot be readily dealt with by the two-dimensional methods of analysis most commonly used. Since three-dimensional techniques for the assessment of multi-span structures are unlikely to be commonly available for some time, torsional effects on piers may remain a matter for qualitative assessment (i.e. do the piers exhibit any unusual defects such as horizontal or inclined cracks which may be attributable to torsion). 6.1.7F Permissible Capacity

The permissible capacity for arch structures is dependent on single or multiple axle loading which may be specified for particular trains in the brief. The overall factor of safety on the Ultimate Capacity is based on the value of 3.4 given in BD 21/97. In BD 21, this factor is derived by combining the live load partial factor, γfL, with an impact factor of 1.8, whilst γf3 is assumed to be 1.0. Within this code, the components of the overall factor have been separated and γfL set at 1.4 to maintain consistency with other sections of the code. The maximum value of impact of 1.8 proposed in BD 21/97 has also been considered appropriate for railway structures, and so to achieve the overall level of safety required γf3 has been increased to 1.35. There is scope within this clause to adopt a lower value of impact factor which may be appropriate in certain circumstances, such as when there is a large amount of fill above the arch, or when trains cross the structure at low speeds.




Page 54 of 87

Permanent loads may produce a beneficial effect on the structure in some situations. Two sets of analysis are therefore required, one with the partial factors on permanent loads set to their maximum value and one using the minimum values of the same parameters. 6.2F SINGLE SPAN STRUCTURES 6.2.1F The MEXE Method of Assessment 6.2.2.1F General

The MEXE method contained in this code has been adopted from the previous version with some minor variations. The curves for determining Provisional Axle Capacity are now presented in a metric format. The basis for these is given below. In certain circumstances it may be more convenient to use the formulae presented to calculate values of Q p directly rather by interpolation of Figures 6.7 to 6.12. It is important to note that the MEXE method for railway structures has been developed differently to that for highway structures as defined in BD 21/97. Consequently, if the two methods were used to assess the same structure, different answers would be obtained. To maintain the distinction between the two methods, the nomenclature has not been harmonised with BD 21/97. 6.2.2.2F Provi sion al Ax le Capacit y (F igu r es 6.7 to 6.12) General

The curves were prepared generally by reference to the methods proposed in “A study of the MEXE approach to Masonry Arch Assessment” by A Kennedy and K A Jenkins (2). Unfortunately the text of that report contains a number of errors, although the curves produced in the report appear to be correct. Reference has also been made to the 1976 Underbridge Assessment Code (3). The new curves have been prepared in metric units resulting in several of the equations being amended accordingly. The steps and mathematical expressions used to prepare the curves are described below. Avail able Li ve L oad Str ess

The correct equation for available live load stress in imperial units is: Pa =

13.0 −

L2

672 d

−

L(h + d )

32d

+

L3

3584 d 2

(ton/ft²)

where: Pa

is the available live load stress (ton/ft²);

EquationF6.1


The Structural Assessment of Underbridges Appendix F - Informative Annex L d h


is the span (ft); is the thickness of arch ring at crown (ft); is the depth of fill beneath sleeper soffit and arch ring at crown (ft).

This equation is derived from the srcinal Pippard formulae (1 and 2). The equation’s derivation is correctly described in Appendix G6 of the 1976 assessment code. This equation converts into the following metric expression: Pa =

1400 −

11L2 11L(h + d ) 11L3 − + d 21d 112 d 2

(kN/m²)

EquationF6.2

where: Pa L ,h , d

is in kN; are in metres.

Dead L oad Stress

The above expression for available live load stress can produce values far in excess of 1400 kN/m2 when the span is large compared to the thickness at the crown. Therefore, a check needs to be carried out for dead load overstress. The dead load stress is calculated by extracting the relevant terms for dead load stress from the equation for available live load stress: 11L  L 11L3 + h + d  +  d  21  112d 2 --------axial-------- -bendingPd =

Equation F6.3

When the dead load stress exceeds 1400 kN/m² the curves are terminated. L ongitudi nal L oaded Lengt h

The longitudinal loaded length is the equivalent loaded length on the arch due to the spread of the load through the fill. The imperial expressions previously used are: ls =

6 h + 2 .5 ( 3

ft)

Equation F6.4

lb =

6h + 25 ( 3

ft)

Equation F6.5

where: ls lb

is the loaded length due to axle load (ft); is the loaded length due to bogie load (ft).




Page 56 of 87

These equations convert into the following metric expressions: ls =

2h + 0.(25

lb =

2h + 2( .5

m)

Equation F6.6

m)

Equation F6.7

Ratio of L oaded L ength an d Span

The following ratios are defined to simplify the expressions relating to the influence lines for bending moment and horizontal thrust. xs =

xb =

L − ls

2L L − lb

2L

Equation F6.8

Equation F6.9

where: xs ,xb

are the distances from the abutment to the edge of the loaded area expressed as a fraction of the span.

These expressions are non-dimensional and therefore do not need converting for metric units. I nfl uence L ine for B ending M oment

An influence line curve for (bending moment/span) at the crown due to a unit point load at a distance xL from the abutment is shown in the 1989(2) report. A similar curve is shown in the 1976 assessment code. A very close fit to this curve was obtained from the following expression for values of x between 0 and 0.5 (illustrated in Figure F6.1): F=

0.7 x 3 + 0.14 x 2 −(kN-m/kN-m) 0.135 x

Equation F6.10

Integrating this equation to obtain the area under the curve over the loaded length produces the following expression after minor rationalisation: A=

0.135 x 2 − 0.094 x 3 − 0.(kN-m*m/kN-m) 35 x 4

Equation F6.10

This expression is calculated for the axle and bogie loads as As and Ab .




Page 57 of 87

0.055

F

0.0

0.5

1.0

-0.02 x

Figure F6.1 Bending Moment Influence Line I nfl uence L ine for H oriz onta l Th ru st

The 1989 report and 1976 assessment code show a curve for thrust at the arch crown due to a unit point load at a distance xL from the abutment. An approximation to the curve has previously been shown in those documents to be a parabola with the following equation (illustrated in Figure F6.2): T=

(kN/kN) 0.78( x − x 2 )

Equation F6.11

This expression is non-dimensional since it relates to the thrust at the crown produced by a unit point load on the arch. It is a rearrangement of that shown in the 1989 report so that the thrust is always positive. This expression gives 0.25 times the actual thrust for an arch with a span/rise ratio of 4. The equation has been left in this format so that it is compatible with the previous reports and the Pippard equations. The value of T is multiplied by 4 in the expressions for PUH in the later paragraph entitled “Unit Live Load Horizontal Thrust Stress”.




Page 58 of 87

0.195

T

0 0

0.5

1

x

Figure F6.2 Horizontal Thrust Influence Line Section Properti es

Section properties for area and section modulus are shown in the 1989 report based on a transverse load spread of 4 h + 4 (ft) for values of h ≤1.5 (ft) and 2h + 7 (ft) for values of 1.5 ≤ h ≤ 3.0 (ft). It is assumed that the depth of fill ( h) does not exceed 3ft. h ≤1.5 (ft)

1 . 5 < h ≤ 3 .0 (ft)

B=

( 4 h + 4 )d( d2

ft²)

M=

( 4h + 4 )

B=

( 2h + 7 )d(

M=

d2 ( 2h + 7 ) ( 6

6

(

EquationF6.12

ft³)

EquationF6.13

ft²)

EquationF6.14

ft³)

EquationF6.15

where: B M

is the cross sectional area of loaded arch ring (ft²) is the Section modulus of loaded arch ring (ft³)

These equations convert into the following metric expressions: h ≤ 0.45 (m)

B=

( 4 h(m²) +1.2 )d

Equation F6.16

M=

d2 ( 4 h +1(m³) .2 ) 6

Equation F6.17




0.45 < h ≤ 0.9 (m)

Page 59 of 87

B=

( 2h(m²) + 2.1)d

Equation F6.18

M=

d2 ( 2h + (m³) 2.1) 6

Equation F6.19

It is assumed that the depth of fill ( h) does not exceed 0.9 metres. Un it L ive Load Be ndin g Stress

The unit live load bending stress is calculated from the area under the bending moment influence line as follows: PUB =

AL2

(kN/m2/kN)

lM

Equation F6.20

These expressions are calculated for axle load and bogie load as PUBs and PUBb. Un it L ive L oad H ori zontal Thr ust Stress

The unit live load horizontal thrust stress due to the effect of the load on the loaded area of the influence line is calculated as follows. It relates to the properties of the area under a parabola: PUH =

4( 0.39 + T ) (kN/m2/kN) 3B

Equation F6.21

This expression is calculated for the axle and bogie loads as PUHs and PUHb. Provis ional Axl e Capacity

The provisional axle capacity is obtained by dividing the available live load stress by the sum of the unit live load bending and horizontal thrust stresses: Cs =

P (kN) a PUB s + PUH s

Cb =

(kN)

0.5Pa

PUB

b

+ PUH

Equation F6.22

Equation F6.23 b

The provisional axle capacity Qp is taken as the minimum of Cs or Cb and is shown on the curves expressed in tonnes. Profi le F actor (F igur e 6.13)

A curve for the profile factor Kp is shown in the 1989 report and the 1986 assessment code. A close fit to these curves was found to be obtained from the following expression which has been used for drawing the new curve Figure 6.13.



Kp =

L  R 


− 0 .7

2.64

Equation F6.24

c

6.2.2.3F M odifyin g Facto rs for Pr ovisional Axl e Capacity

Three changes have been made to the modifying factors used in the MEXE method. The first of these is the inclusion of a Material Factor (KM) for mass concrete. The value of 1.2 adopted is consistent with that given in UIC 778-3R (4). Secondly, further definition has been provided on the Crack Factor (KC) for lateral and diagonal cracks. These definitions have been obtained from a report prepared by the Working Party : Assessment of Underbridges (Strength of Masonry Arch Bridges (5) ) dated 1971. This document contains a version of the MEXE method which was rationalised prior to it being included in the 1976 issue of British Rail’s Assessment Code. The final modification is the removal of the span factor, which previously allowed MEXE to be used for the assessment of multi-span arches on slender piers. Hughes (6)and Melbourne (7) have both demonstrated that this approach could be nonconservative. It should be noted that MEXE may still be used for multi-span structures where the piers can be shown to be stocky’ and the criteria in Clause 6.2.1 are satisfied. 6.2.3F Other Methods of Analysis 6.2.3.1 G ener al

i.

Mechanism analysis, although possible by hand, is generally carried out using a proprietary software package. Packages in most common use at present are ARCHIE, ARCH and ASSARC. In carrying out a mechanism analysis there should be no restriction on the position of the line of thrust, provided it lies within the arch ring (that is, the Geometric Factor of Safety should be set at 1.0).

ii.

Elastic analysis may be carried out by two methods: (a)

Elastic analysis based on Castigliano’s Theorems, using computer programme, CTAP.

(b)

Finite element method. The most appropriate computer program using this method for arch assessment is MAFEA-ID.

These methods can be particularly useful since they give additional information on the behaviour of the arch as it is loaded (including stresses and deflections).




Page 61 of 87

6.2.3.2F Limitations

MAFEA-ID can take account of geometric non-linearity and hence is suitable for the analysis of arches which may be susceptible to snap through’ failure. 6.2.4F Advanced Analysis Methods

i.

The MAFEA program also has a 2-D plane strain version which is suitable for investigating the effects of ring separation. This method has not been approved by Railtrack and requires permission of the Professional Head of Structures Engineering before its use. Other finite element packages which have not been developed exclusively for arch assessment may also be suitable.

ii.

It is envisaged that a full 3-D modelling is rarely required. When it is required it will be necessary first to establish firm guidelines for the analytical process with Railtrack’s Assessment Engineer.

6.3F MULTI SPAN STRUCTURES 6.3.1F Modes of Failure

The limiting pier thickness of h/2 has been adopted based on the work carried out hitherto for the Bridgeguard 3 assessment programme. An alternative expression for limiting pier thickness based on research carried out by Hughes is as follows: 220d 1.792 h 0.632 (f +)d ( r 0.185 s 5.937

0.735

t lim =

)w + s

3.963

Equation F6.25

where: d h f s r w

is the arch ring thickness; is the height of the pier; is the depth of fill from underside of sleeper to arch crown; is the clear span length between the faces of successive supports; is the arch rise at mid span; is the loaded length at the underside of sleeper level.

The above expression has been developed assuming constant values of all parameters throughout adjacent spans. Where these vary, the largest values of d , h , f , w and r and the smallest value of s should be adopted. Use of the above expression may be considered but requires permission of the Professional Head of Structures Engineering.




w

Page 62 of 87

Level at underside of sleepers

p

f

r

d t h

s

Figure F6.3 Definition of Parameters for Calculations of tlim 6.3.3F Assessment for Failure Mode 2

Sub paragraph (i) describes computer analysis program MULTI, which is part of the ARCHIE suite of programs. This analysis involves an interactive, iterative process whereby the position of the thrust line within each of the unloaded spans is adjusted in an effort to ensure that it does not fall outside a pre-determined zone within the arch ring and piers. Details of this method are outlined in the paper by Harvey and Smith (8). A mechanism analysis of the type described in sub paragraph (ii) can be carried out by spreadsheet. The calculation process is described by Hughes(6). 6.3.5F Advanced Assessment Methods

Multi-span structures cannot be modelled satisfactorily three-dimensionally at present due to limitations of current computing power. At present, the only viable option is to use two-dimensional plane strain finite element modelling. 6.4F SPANDREL WALLS

Although there has been some recent research on the assessment of spandrel walls for lateral loading, as well as on their interaction with the arch barrel, appropriate assessment techniques have yet to be established. Spandrel walls should, therefore, continue to be assessed on a qualitative basis.




REFERENCES

1.

DUBOST A. Masonry Structures project: Pressure Distribution in the Arch Fill Material. British Rail research Report IM CES 055, 1990;

2.

of the MEXE Approach KENNEDY, A andBritish JENKINS, KA. A Study Arch Assessment.. Rail research Report TM/CES/118, 1989;to Masonry

3.

BRB. Assessment of Live Load Carrying capcity of Underbridges. British Railway Board, 1976;

4.

INTERNATIONAL UNION of RAILWAYS. Recommendations for the Assessment of the Live Load Carrying Capacity of existing Masonry and Mass Concrete Arches. UIC 778-3R (1st Edition), 1995;

5.

BRITISH RAIL LONDON MIDLAND REGION. Working Party: Assessment of Underbridges (Strength of Masonry Arch underbridges), Final Report on Suggested Method of Analysis. 1971;

6.

HUGHES, TG. Analysis of Twin Masonry Arch Bridges. Proceedings of The Institution of Civil Engineers, Structures and Buildings, November 1995;

7.

MELBOURNE, C, GILBERT, M and WAGSTAFF, M. The Collapse Behaviour of Multispan Brickwork Arch Bridges. The Structural Engineer, 2 September 1997;

8.

HARVEY, WJ and SMITH, FW. The Behaviour and Assessment of Multispan Arches. The Structural Engineer, 17 December 1991.

Other documents which have been referred to in the development of Section 6 Masonry Arches are: 9.

HEYMAN, J. The Masonry Arch. Ellis Horwood Ltd., 1982, ISBN 0853125015;

10.

PAGE, J. Masonry Arch Bridges - State of the Art Review. TRL, 1993, ISBN 0115511903;

11.

HARVEY, WJ. Application of the Mechanism Analysis to Masonry Arches. The Structural Engineer, 1 March 1988;

12.

SMITH, FW, HARVEY, WJ and VARDY, AE. Three hinge Analysis of Masonry Arches. The Structural Engineer, 5 June 1990;

13.

GILBERT, M and MELBOURNE, C. Rigid-block Analysis of Masonry Structures. The Structural Engineer, 1 November 1994;

14.

MELBOURNE, C and GILBERT, M. The Behaviour of Multiring brickwork Arch Bridges. The Structural Engineer, 7 February 1995;

15.

HARVEY, WJ. The Origin and treatment of Longitudinal Cracks in Masonry Arches. The Structural Engineer, 5 December 1995;




Page 64 of 87

16.

HUGHES, TG and BLACKLER, MJ. A Review of UK Masonry Arch Assessment Methods. Proceedings of The Institution of Civil Engineers, Structures and Buildings, August 1997;

17.

NG, KH, FAIRFIELD, CA and SIBBALD, A. Finite Element Analysis of Masonry arch Bridges. Proceedings of The Institution of Civil Engineers, Structures and

18.

Buildings, May 1999; HENDRY, AW. Masonry Properties for Assessing Arch Bridges. TRRL Contractor Report 244, 1990;

19.

BROWNHEAD, SF. Review of Arch Assessment Methods. British Rail Research Report RR-CES-007, 1991;

20.

CLARK, GW. Serviceavility of Brick Masonry. British Rail Research Report LRCES-151, 1994;

21.

THOMPSON, DR. Spandrel Wall Failures. British Rail Research Report RR-ICE-009, 1995;

22.

DEAN, PA. The Deflection of Arches.British Rail Research Report ICE-095, 1997.

RR-




SECTION 7 AND APPENDIX B - CONCRETE STRUCTURES 7.2.1F Introduction

The concrete section of this underbridge Code is based largely on BD 44/95: The Assessment of Concrete Highway Bridges and Structures. BD 44/95 is currently being revised. Somebeen of the changessince to BD 44/95 arewas anticipated thisare Code but other changes have proposed this Code preparedinand therefore not included. It was considered desirable to incorporate the known changes included in this Code to reduce unnecessary differences between the documents. In addition the changes have technical merit, to and may help to avoid unnecessary strengthening work and/or reduce requirements for Departures from Standard. Some of the background and justification for the more significant changes is given here. For further general guidance refer to BA 44/96. 7.2.2.3F M ater ial Parti al F actor for Ste el

BS 5400: Part 4, CP 110: 1972 and BS 8110:1985 all used a value for partial factor γm for reinforcement and prestressing steel of 1.15. This value compares with the factor of 1.05 used in BS 5400: Part 3 and BS 5950 for structural steel. This difference is difficult to justify. Following extensive statistical investigation by Beeby(1), it was concluded that the lower factor could be justified for reinforcement and prestressing steel as well. The change has therefore been incorporated into the amendment AMD9882 of BS 8110. During the review of BD 44/95 it was decided that some account of this change should be made. However a rigorous statistical investigation of all the steel sources that could be found in assessment of existing highway structures could not be undertaken. The slightly more cautious factor of 1.1, which is consistent with Eurocode 2 (EC2)(2), has therefore been adopted. Theoretically, since modern practice is to specify characteristic” yield strength whereas past practice used guaranteed minimum” values, the factor could justifiably be smaller than in design. GUIDANCE ON APPENDIX B 4.3.3BF Values of γm

Refer to Clause 7.2.2.3F above. 5.3.3.3BF Shor t Anchorage L engths

2.5 diameters is the limit of test data. The rule may be safe with even shorter anchorages but it has not been tested. A version of this rule was srcinally proposed by Gifford and Partners based on work done on ASR affected specimens. The present version is a modification following more recent work by Clark et al (3) and Cullington et al (4).




5.3.3.5BF Varyi ng Angl e Tr uss Approach

Current codes use the addition principle to design links or to assess the shear strength of members with links. The contribution of the links to shear strength is calculated using a 45° truss analogy and it is added to the calculated strength of the section without links. Historically, this approach was used because the 45° truss analogy had been found to give conservative results. It had no theoretical justification. Indeed, the mechanism of transmitting shear in sections with significant links is fundamentally different from that in sections without. One consequence of the differences is that the addition approach over-estimates the strength of sections with small areas of links. Such links actually have very little effect. Codes for design, however, can overcome this problem by specifying minimum link areas below which links are assumed ineffective. An alternative is to assume all the shear is taken on the truss mechanism but allow the angle of the truss to be optimised for maximum strength. This approach has the great advantage that, unlike other methods, it does have a theoretical justification which is explained by Nielsen (5). However, it is based on plastic theory. It is debatable whether the assumptions of this approach (principally ductility) are justified. Calibration against tests has been done. The results showed a need to apply an effectiveness factor” and to limit the angle of the truss. With these limitations the approach was specified in the 1990 CEB/FIP code (6) and is given as an option in EC2. It can give significantly greater shear strengths than the addition approach particularly for sections with large areas of links. The approach included in this Code is based on EC2. The approach does has two implications. Firstly, the use of flatter angles to increase the shear strength for a given link area implies a greater force in the main flexural reinforcement. This limits the value of the approach in members with curtailments. The second implication is that, rather than having an essentially empirical web crushing limit”, it should theoretically be possible to check the crushing strength of the inclined concrete struts in the same way as the compressive strength of any other concrete compression member is checked. However, tests show that this approach over-estimates strength, hence the need for the effectiveness factor”. Statistical analysis suggests that the varying angle truss approach gives much better correlation with test results than other methods of predicting shear strength. 5.4.1B Moments and Shear Forces in Slabs

References for the particular elastic analysis and yield line methods are listed below: PUCHER A., Influence Surfaces of Elastic Plates. Springer Verlag, Wien and New York 1964, ISBN 3211811389;




WESTERGAARD, HM. Computation of Stresses in Bridge Deck Slabs due to Wheel Loads. Public Roads, Volume 2, Number 1, March 1930; JOHANSEN, KW. Yield Line Theory. CACA London. 1967. 6.3.4.2BF Equation B53

This equation is based on calculating the maximum principal tensile stress at the centroid in a rectangular section using normal elastic beam theory. The maximum shear stress for a rectangular section is 1.5 times the average shear stress (V/bh), hence the 0.67 in the equation. For flanged sections this code takes b” to be the web width but still includes the 0.67 factor. With very heavily flanged sections, the theoretical maximum shear stress can be as little as 1.05B/bh. Hence the equation is conservative. However, if the section is not under a uniform longitudinal stress (that is, if it is subjected to bending) the theoretical maximum principal tensile stress does not necessarily arise at the neutral axis and can be greater than predicted by the equation. In this situation, the only justification for the equation is calibration against tests. The implementation therefore used either the actual calculated maximum principal tensile stress (justified by theory) or that calculated by Equation 28 (justified by tests). There is evidence that the approach is still conservative for flanged sections. Using the maximum principal tensile stress at the neutral axis may be safe for flanged sections too but this has not been proved. 6.3.4.5BF M aximu m Shear For ce

BD 44/95 relates the web crushing limit to the square root of concrete compressive strength. This appears illogical. The limit is based on a crushing failure. It might therefore be expected to be related directly to crushing strength. This relationship is implied by the work on varying truss analogy mentioned above. Some research (7) has indicated that the effectiveness factor reduces with increasing concrete strength but the overall sensitivity to remaining concrete strength is invariably greater than the square root relationship. Batchelor, George and Campbell (7) undertook a major statistical study of all the available crushing on slender and found that concrete strength was not aweb factor in the research effectiveness factor. webs, The crushing strength is therefore directly proportional to concrete strength. For 50N concrete and typical web thickness to depth ratios, their work suggests that the crushing strength could be as much as 70% greater than calculation to BS 5400: Part 4. 6.7.2BF Loss of Prestress other than Friction Loses

Specialist literature that can be referred to is contained in references 2 and 8.




Page 68 of 87

REFERENCES

1.

BEEBY, AW. Partial Safety Factors for Reinforcement. The Structural Engineer, Volume 72 No. 20.18, October 1994;

2.

ENV 1992 -1 -1 Eurocode 2: Design of Concrete Structures;

3.

CLARKE, LA, BALDWIN, MI and GUI, M. Assessment of Concrete Bridges with Inadequately Anchored Reinforcement. Bridge Management 3. London, E&FN Spon. 1996. pp 667-674. pp. 225-232;

4.

CULLINGTON, DW, DALY, AF and HILL, ME. Assessment of Reinforced Concrete Bridges: Collapse Tests on Thurloxton Underpass. Bridge Management 3. London, E&FN Spon. 1996. pp. 667-674;

5.

NIELSEN, MP. Limit Analysis and Concrete Plasticity , Prentice Hall, 1984, ISBN 0849391261;

6.

CEB/FIP: Model Code for Concrete Strucutres 1993, ref MC90;

7.

BATCHELOR, BdeV, GEORGE, HK and CAMPBELL, TE. Effectiveness Factor for Shear in Concrete Beams. Journal of the Structural Engineering Division,

8.

ASCE Vol. 112, No 6. June 1986. pp. 1464-1477; NEVILLE, AM. Creep of Concrete Plain Reinforced and Prestressed. Amsterdam North Holland Publishing Company, 1970.




SECTION 8 - COMPOSITE STRUCTURES 8.1F Introduction

The number of modern composite underbridges owned by Railtrack are small compared with other metal bridges. The first was bridge no 59, Bushey Arches West Coast in 1963.floor. Most After are simply-supported deck-type plate or box girdersMain withLine insitubuilt or precast 1967 these bridges are likely to have been designed to CP 117: Part 2, and should be amenable to assessment using a BS 5400: Part 5 approach since that code followed CP 117. Modern half-through bridges built since the early 1950’s, especially E’ type, have filler beam cross girders, which were usually not designed for composite behaviour. However, their construction often included reinforcement passing through web holes thus providing some composite resistance. A significant number of underbridges are of cased or filler beam construction. This is catered for in BS 5400: Part 5. 8.2F Limit state approach

Appendix C is based on BS 5400: Part 5, and provides a limit state approach to assessment. It is written as a supplementary code to BS 5400: Part 5 in a similar way to Section 5 for steel and wrought iron. For the steel (or wrought iron) and concrete elements, Sections 5 and 7 respectively are referred to in a similar way to BS 5400. Appendix C adopts some amendments due to BD 16/82: Design of Composite Highway Bridges. Use of BS 5400: Part 5. Reference has also been made to BD 61/96: The Assessment of Composite Highway Bridges. BD 61/96 is however considered to be too unwieldy and over-mathematical for assessment of most rail underbridges. It uses the serviceability limit state extensively, whereas it is considered that for assessment of most rail underbridges, only the ultimate limit state (ULS) should be checked. The serviceability limit state (SLS) should be checked only where it is clearly necessary, for example for track twist and for fatigue. Some information relating to different forms of shear connection has been adopted from BD 61/96. The SLS requirements in BS 5400: Part 5 for checking concrete crack widths, deflections and strength of the shear connection are not included in Appendix C. Only in unusual cases, for example when the spacing of shear connectors is excessive, would it be necessary to check serviceability. To compensate for the removal of the SLS check on the shear connection, ULS checks have been introduced which have the same basis as for SLS in BS 5400: Part 5, i.e. elastic distribution of horizontal shear, uncracked modulus for deriving horizontal shear and similar global analysis. Composite behaviour may be assumed in cased and filler beams provided the specified bond stress is not exceeded. The resistance to horizontal shear can be enhanced where transverse bars are continuous through holes in the web as found in




Page 70 of 87

many E’ type bridges. The SLS bending stress given in BS 5400: Part 5 is increased to a value which is applicable to the ULS. Some of the additions and amendments in Appendix C to BS 5400: Part 5 are based on BD 16/82 and BD 61/96. Acknowledgement is made to the Highways Agency for allowing these standards to be referred to so as to form a common basis between the assessment of rail underbridges and highway. GUIDANCE NOTES TO APPENDIX C 4.3.2CF Serviceability Limit State

Checks for serviceability are compatible with Sections 5 and 7. It is desirable to eliminate them as far as possible. The recommendations of Clause 8.2F virtually eliminate their need in practice. 5.2.6CF Control of Cracking in Concrete

Crack control checks should not be necessary for assessment where inspection is carried out followed by maintenance as necessary. 6.1.2CF Deck Slabs Forming the Flanges of Composite Beams

This accords with BS 5400: Part 4 and Eurocode 4(1). The 75% criterion is based on judgement. 6.3.3.3CF I nteraction bet ween L ongitudi nal Shear & Tr ansverse Bending

This Clause was misprinted in BS 5400: Part 5. 6.3.4CF Shear Connectors

The static strength values are as BS 5400: Part 5. In practice assuming γ FL = 1.4 at ULS and γ fL = 1.1 at SLS for live load, the overall safety factor compares thus: At ULS

 γ =   0 .8 P 

(Q × γ fL × γ f 3 )

m

u

At SLS

 γ

(Q × γ fL × γ f 3 )

m

 0.55P

u

=  

(1.4 ×1.1)

1.1  =  0.8 

(1.1×1.0 ) 

=   0.55  1.0

1.925

2..00

This is a conservative approach because no advantage has been taken of the redistribution between connectors, that occurs at ULS, resulting in a tendency towards a uniform rather than triangular distribution of horizontal shear between the point of zero shear and the end of the beam.




It appears inappropriate to consider composite behaviour for type A’ decks, (see Clause 5.2.2F), due to the discontinuity of concrete which tends to be subject to global tension from the main girder bottom flange. These decks generally have 200 mm deep beams. For beams deeper than say 300 mm, it is probably reasonable to assume some composite behaviour. 8.5.2CF Cased Beams

The values of local bond stress for SLS given in BS 5400: Part 5 have been increased by approximately: 0.8 Pu 1.1 = 0.55Pu

1.32

The increase is proportional to the relative ULS/SLS capacities of shear connectors. 8.5.3CF Filler beams

For filler beams the shear capacity includes the transverse bars through the beam web in addition to bond as in Expressions C9 and C10. Expression C9 allows for shear strength of the reinforcement taken as 1.4 Aw f ry . This takes the double shear capacity of the reinforcement 0.7 Ae f ry as in BS 5400: Part 5 Clause 6.3.3.2 and which f ry approximates to which is similar to a calculation for the shear strength of a 2 ×1.05 bolt to BS 5400: Part 3. Expression C10 can be compared with the bearing capacity of a bolt to BS 5400: Part 3 Clause 14.5.3.6, which is 0.85 × 2.5×1.2 ×1.0σ y

γ =1.05

= 2.43σy

m

but 2.0σy is used conservatively.




REFERENCES

1.

ENV 1994 -1 -1 Eurocode 4: Part 2 Design of Composite Steel and Concrete Structures;




SECTION 9 - CAST IRON STRUCTURES 9.1F General

The previous version of this code gave little guidance on the assessment of cast iron structures, merely providing stress limits for tension and compression in bending and shear. Section 9 of this code is based on the Department of Transport Standard BD 21/97 which gives more specific guidance. In addition Section 9 contains provisions for the assessment of members subject to combined axial load and bending, such as arch ribs. There are known to be a small number of such members in service within the existing Railtrack bridge stock. Since the failure of cast iron members in tension is always brittle and sudden, Section 9 gives provisions for assessment based on working loads and permissible stresses. In determining the dynamic factor for the rib of an arch structure in accordance with Clause 4.3.2.2, it should be noted that the fundamental mode of vibration generally involves anti-symmetric bending of the arch rib and any associated stiffening members within the spandrels. The simplified expression for natural frequency given in Clause 4.3.2.2 is based on whole span bending which represents a higher mode of vibration for an arch structure, and will result in a higher value of dynamic factor. For such arch structures, therefore, a more rigorous approach will be required to determine the lowest natural frequency. 9.2F Material Properties

9.2.1F Elastic Modulus Cast iron is a highly variable material whose properties are primarily influenced by the mix proportions and the rate of cooling during manufacture. In addition, it does not behave in a linear elastic manner under load. The elastic modulus, therefore, typically lies anywhere within the range 90,000 N/mm 2 to 138,000 N/mm2. Provided that stresses are kept within the limits outlined in Clauses 9.3.1, the degree of nonlinearity is not significant, and a mean value of E may be adopted for global analysis. 9.3F Strength

9.3.2F Beams Continuously Restrained bySurrounding Fill This clause takes account of the fact that composite action may occur between a girder and surrounding fill in which it is firmly embedded. It is considered unlikely that this clause will be required for railway bridges. It is understood that there are no longer any cast iron girder bridges remaining within the Railtrack bridge stock.




9.3.3F Beams with Intermediate Lateral Restraints Clause 9.3.3 relates to members which are susceptible to lateral torsional buckling and has been included primarily to cater for cast iron arch ribs which are subject to combined bending and axial load. Since no recognised method is available for determining the limiting compressive strength, an appropriate method should be established during the Technical Approval process. One possible method, which is considered to be conservative, may involve the adoption of the curve presented in Figure 9.1F. This is based on the Gordon-Rankine equation, which is generally used for the assessment of cast iron struts, with a maximum value of 154N/mm2 at λ LT = 30 . The value of λ LT was chosen to be compatible with Figure A10 for steel and wrought iron elements. It should be noted that Figure 9.1F has not been validated by tests.

If the procedure outlined in the preceding paragraph is adopted, a less conservative value of the slenderness parameter, λ LT , may be obtained by determining the buckling parameter (k4) using Appendix B2.5 of BS 5950: Part 1, where it is defined as u. This is likely to be beneficial as cast iron members tend to have relatively stocky proportions, with the web and flanges often being of similar thicknesses. 9.3.4F Compression Members The Gordon-Rankine equation, which is considered to be the strut formula most applicable to cast iron, is presented. It is an empirical formula which was developed in the second half of the 19th century. It was also applicable to wrought iron and steel, but was found to be most appropriate for cast iron. The current formula includes a factor of safety of 5, which recent research has indicated may be conservative. 9.3.5F Members Subject to Bending and Axial Compression A rule for combining axial and bending stresses in compression is given together with a re-arranged version of the Rankine-Gordon equation for calculating permissible stresses in axial compression. 9.3.6F Restraints to Elements in Compression Refers to Section 5 to determine forces to be used in assessing the adequacy of bracing members. 9.4F Fatigue The equations placing additional limits on the permissible stresses given in Clause 9.4 are identical to those given in BD 21/97. Although their exact derivation is unknown, sufficient evidence exists to suggest that these equations ensure the live load stresses are always below the non-propagating stress. In determining the live load effects appropriate to this Clause, the dynamic factor for fatigue, as defined in Table 4.5, should be adopted.




Page 75 of 87

160

150

140

130

120

110

100

90

pbc 2

(N/mm )

80

70

60

50

40

30

20

10

0 0

50

100

150

200

250

λLT Figure 9.1F Permissible Compressive Stress due to Bending of Beams with Intermediate Lateral Restraints

300




REFERENCES

1.

THE HIGHWAYS AGENCY. BD 21/97: The Assessment of Highway Bridges and Structures. The Stationery Office, 1997, ISBN 0115519076;

2.

The Strength Iron CHETTOE, CJ,Journal DAVEY, andInstitution MITCHELL, GR. Engineers Girder Bridges. of N The of Civil No.of8,Cast October 1944;

3.

BUSSELL, M. Appraisal of Existing Iron and Steel Structures. Steel Construction Institute Publication 138, ISBN 1859420095;

4.

AITKEN, WK. East Coast Main Line, Bridge No. 184, River Nene Cast Iron Viaduct, Peterborough. British Railways Board Research and Development Division, Track Group, Technical Memorandum TM TS 55;

5.

AITKEN, WK. Cast Iron Railway Bridges - Properties of Cast Iron. British Railways Board Research and Development Division, Track and Structures Section, Technical Memorandum TM TS 19, December 1972;

6.

AITKEN, WK. Cast Iron Railway Bridges - Properties of Cast Iron. British Railways Board Research and Development Division, Track and Structures Section, Technical Memorandum TS 20, September 1973;

7.

SWAILES, T. 19th Cast Iron Beams: their Design, Maintenance and Reliability. The Proceedings of The Institution of Civil Engineers, Civil Engineering, February 1996;

8.

SWAILES, T and MARSH ,J. Structural Appraisal of Iron Framed Textile Mills. ICE Design and Practice Guides - the Institution of Civil Engineers, 1998, ISBN 0727727133.




SECTION 10 - TIMBER 10.1F Introduction

The majority of timber rail underbridges to be assessed take the form of longitudinal main beams supporting transverse timber decking with track on longitudinal sleepers. Timber deckingincluding is also used to support ballasted track onalso timber bridges. Substructures abutments for these bridges are likelyand to metal be of timber comprising a timber crosshead carried on a number of braced pile bents. 10.2F Permissible Stress Approach

British Standard BS 5268: Part 2: 1996 Structural Use of Timber, Part 2, Code of Practice for Permissible Stress Design, Materials and Workmanship”(1) is written primarily for timber applications within buildings and associated structures; in the absence of bridge specific codes BS 5268 should be used for guidance. The permissible stress principles for the design of timber structures is the approach familiar to Engineers. North American and Canadian practice is similar. 10.4.5F Impact and Load Duration Factors

The allowable load for timber varies with the length of time the load is applied. The shorter the duration of load, the higher the allowable load that can be sustained by the structure. The timber grade stresses and joint strengths given in BS 5268: Part 2: 1996 are applicable to long-term loading and are increased for shorter-term loading by use of modification factors. However, it is the cumulative effect of the individual loading occurrences that determines the allowable load; this is not stated in BS 5268. It is necessary, therefore, to arrive at the most appropriate duration modification factor. North American and Canadian practice is similar to that in the UK. In AASHTO Standard Specification for Highway Bridges”(2) stresses are based on a normal load duration which contemplates that the member is stressed to the maximum stress level, either continuously or cumulatively, for a period of approximately 10 years, and/or stressed to 90% of the maximum design level continuously. Modification factors are provided for either cumulative or continuous periods other than 10 years. These factors are reproduced in Table 10F.1




LoadDuration

Page 78 of 87

C

Permanent years 10 2 months (vehicle live load) days 7 day1 5 minutes (railings only)

D

0.9 1.0 1.15 1.25 1.33 1.65

Table 10F.1 Load Duration Factor, DD

American railway engineering practice(3) adopts a different approach which is related to consideration of impact due to live load: The dynamic increment of load due to the effects of speed, roll and track irregularities is not well established for timber structures. Its total effect is estimated to be less than the increased strength of timber for the short cumulative duration of loading to which railroad bridges are subject in service and is taken into consideration in the derivation of allowable working stresses for design”. Stresses given in the AREMA manual equate to 90% the value of stress quoted in AASHTO for highway bridge design. The railway value of stress includes a 0.9 form factor. In AASHTO impact effects are not added to the static loads for timber structures. It can be argued from the American approach that increases in working stress due to short duration loading is at least equivalent to the magnitude of impact that could have been applied. However, it is advantageous in assessment to include impact in the loading effects so that variabilities due to speed can be included. There may be a case for placing a limit on values for impact, especially for high speed lines where the value of assessment impact can be higher than UIC impact(4) for a new bridge. Similarly for decking, where timbers are likely to be short spans and continuous over a number of bays, the American approach could be adopted. Values of impact in accordance with different practices are as follows:



The Structural Assessment of Underbridges Appendix F - Informative Annex U.K.

Page 79 of 87 U.S.A.

SPEED (MPH)

UIC CODE FOR NEW BRIDGES

776-1R FOR ASSESSMENT

DIESEL LOCOS

STEAM LOCOS

10

1.69

1.06

1.28

1.32

20

1.69

1.13

1.39

1.49

30

1.69

1.22

1.48

1.62

40

1.69

1.30

1.54

1.71

50

1.69

1.38

1.58

1.77

60

1.69

1.45

1.59

1.79

Table 10F.1 Values of Imapct for Various Practices

A speed-related impact allows live capacity to be maintained or improved at reduced speed if necessary. A duration factor of 1.5 is proposed.




REFERENCES

1.

2.

BSI, BS 5268: Part 2 Structural Use of Timber, Part 2, Code of Practice for Permissible Stress Design, Materials and Workmanship”, British Standards Institution, 1996; AASHTO. Standard Specification for Highway Bridges. American Association of State Highway and Transportation Officials, 16th Edition, 1996, ISBN 1560510404;

3.

AREMA. AREMA Manual for Railway Engineering. American Railway Engineering and Maintenance of Way Association, CD Rom, 2000;

4.

UIC. UIC776-1R Loads to be considered in the Design of Railway Bridges. International Union of Railways, 1990;




SECTION 11 - SUBSTRUCTURES 11.2F Assessment

To allow the calculation of lateral pressures on earth retaining structures due to varying levels of live loading, Table 11.1 containing live load surcharge loading has been included. The values in this table areload based on(four the total loads from theacting locomotive component of the Type RA1 train axles,axle each of 200 kN) over an area defined by it s length (7.85 metres) and the effective width of a sleeper (2.6 metres).


The Structural Assessment of Underbridges Appendix F - Informative Annex SECTION 12 - BEARINGS

No additional commentary required.





Page 83 of 87

APPENDIX D - FATIGUE ASSESSMENT OF STEEL AND WROUGHT IRON 1.1DF. GENERAL

General philosophy for fatigue assessment follows the Highways Agency draft (1)

standard steel bridge fatigue assessment , which has a steel similar to the (2). AASHTOfor guide specifications forlife fatigue evaluation of existing bridges (3) Appendix F comprises a series of amendments to BS 5400: Part 10 as being in a form with which designers will be familiar in a similar way that Section 5 is an adaptation of BS 5400: Part 3. 4.1DF Definitions

The concept of damage tolerant’ and safe-life’ elements is adopted. Damage tolerant’ elements are those which have capacity for redistribution or the permanent way is capable of in effect spanning the gap left by a failure of the element, whereas safe life’ elements have no ability to redistribute load in the event of failure. For rail underbridges longitudinal members spanning less than 3 metres or cross girders carrying only a single track can normally be taken as damage-tolerant’. This will effectively enhance the fatigue capacity of short span members such as railbearers and single track cross girders through reduction of the calculated fatigue stresses by application of the factors γ1 and γ 2 as used by the Highways Agency draft. Factor γ1 allows a reduction in applied fatigue stresses (i.e. γ1 < 1.0) where an advanced analysis is used such as 3D finite element analysis or where strains are measured directly such that secondary effects are taken into account. Factor γ 2 takes account of whether the element is damage tolerant’ or safe life’ and whether access is feasible for inspection against fatigue defects. In effect γ 2 modifies the σ - N (stress against numbers of cycles) capacity by adjusting the probability factor from the minus 2 standard deviations below mean of fatigue results used in BS 5400: Part 10. The value of γ 2 is approximately equivalent to failure probability as below using the relationships given by BS 5400: Part 10, Appendix A. Approximate value of γ 2

Probability of failure

0.75 0.80 0.86 1.0 1.25

50% 31% 16% 2.3% 0.14%

Number of deviations below the mean, d 0

Table D1F Failure Probability

0.5 1.0 2.0 3.0




Page 84 of 87

For safe life’ elements which are accessible the BS 5400: Part 10, BS 7608: 1993 (4) and ECCS recommendations (5) on new design is used, i.e. the minus 2 standard deviations corresponding to a 2.3% probability of failure, i.e.

γ

2

= 1.0

4.4DF Method of Assessment

Fatigue assessment is carried out in stages varying from elementary calculations comparing the live load stress range with a cut off stress, through simplified calculations to a full design spectrum and consideration of measured stresses. If at any of these stages fatigue life is found to be satisfactory then further fatigue consideration is not required. Stages are: A - identify fatigue criticality by inspection and cut-off stress - The live load stress range multiplied by γ1 γ2 is compared with σco the cut-off stress, below which no fatigue damage should occur such that no further calculation is necessary. At this stage any particular fatigue defects such as cracks must be noted and acted upon. It is important to appreciate that any calculations using Appendix D may be invalid in the presence of significant fatigue cracking; B - damage calculation to particular spectrum - Stage B involves use of the standard load spectra of BS 5400: Part 10, Table 2 which is based on the heavy, medium or light traffic types with standard trains as in BS 5400: Part 10, Appendix E (BS 5400: Part 10, Clause 9.2 is not applicable because it does not cater for applied stresses below σco ). It should be noted that Table 2 is strictly only applicable to simply supported spans. In critical cases then a Stage C assessment should be carried out in verification. Use of Table 2 involves a modification to the load proportions Kw in that RA unit not RU loading is applied, and that Section 4 applies a minimum length of 4 metres for the calculation of the dynamic factor (1+ϕ), whereas the values of Kw in Table 2 imply higher impact factors for lengths less than 4 metres; C - damage calculation to particular spectrum (similar to BS 5400: 10 Clause 9.3) Stage C may be used to evaluate the residual fatigue life for typical trains such as figure 19 of BS 5400: Part 10 with numbers of trains as in Table 15. It represents a more accurate assessment by use of the rainflow method compared with Stage B. Stage C may also be used to evaluate fatigue under real trains representing past, present and future traffic. If 3D finite element analysis is carried out which properly represents the stresses itemised in BS 5400: Part 10, Clause 6.1.5 (but




excluding residual stress and stress concentrations as itemised in BS 5400: Part 10, Clause 6.1.6) then γ1 may be assumed as 0.95; D - assessment using measured strains - Strain measurements in the vicinities of potentially fatigue prone details may be used to verify theoretical calculations for an assessment carried out under Stage C. The strains should be recorded during the slow passages or under stationary vehicles of known geometries and axle loads and compared with those calculated by elastic methods. Adjustments should be made to account for any discrepancies detected. Where the stresses adjacent to the relevant detail are recorded at a distance from the detail such as to avoid any local stress concentration, any magnification due to local discontinuities should be subsequently allowed for in accordance with BS 5400: Part 10. In appropriate cases the strains may be recorded in the locations of peak strains and the corresponding stresses used directly in the assessment. Strains at locations in which uniaxial stresses occur may be recorded by portable extensometers when records are to be taken at increments of movement of a test vehicle. These have the advantage of being mountable without removal of protective coatings, but are not suitable for recording spectra under moving traffic. However, mechanical strain range counting instruments are available which may be used for deriving spectra. Where principal stresses may not be derived from uniaxial strains electrical resistance strain gauge rosettes may be used to derive principal stress ranges. Measurement of local strains in the vicinities of stress concentrations requires the use of gauges of very short gauge lengths and electrical resistance gauges are most appropriate for such purposes. These require removal of any protective coatings and careful preparation of surfaces before bonding and subsequent weather protection of the gauges and their connections to the electrical circuits. They have the advantage that once installed recording and processing of measurements may be undertaken remotely from the gauge positions. Any gauges and their associated equipment should be calibrated before use. 11.3.DF σ-N Relationship

σ-N relationships are assumed as in BS 5400: Part 10 for steelwork, but modified by introduction of the cut-off stress σco as noted above. For wrought iron, the results reviewed by Beagles (6) have been used to determine a σ-N relationship for plain and riveted wrought iron. The proposed σ-N curve for riveted wrought iron is: Log10

σ

R

= 3.393 – ¼ Log 10 N

The value of σo is retained at 1 x 107 cycles as in BS 5400: Part 10 but with a slope m=4 to fit the Beagles data with a cut-off stress σco corresponding to 10 8 cycles. The




σ-N relationship for riveted wrought iron is close to the ECCS-71 curve proposed by Netherlands Railways (7) for low stress ranges where most fatigue damage will be apparent. However, there are no test results at low stress levels reported by Beagles so this is an area where further research is needed. The σ-N relationships for plain wrought iron is based upon a cut-off stress of 65N/mm² equivalent to 0.29 x UTS as (8)

put forward by Beagles for , but is otherwise parallel that forwith riveted m=4 to m=3 flatter σ-N relationships wrought iron, i.e. compare the curve. slope ofThe for steel. There is support for a flatter slope in wrought iron in that Beagles determined (8) that for fatigue crack growth wrought iron showed slower growth at lower ∆k values whereas at high ∆k values fatigue cracks grow marginally slower in steel.




Page 87 of 87

REFERENCES

1.

HIGHWAYS AGENCY. Draft of Standard for Steel Bridge Life Assessment. Draft dated 1 December 1998;

2.

AASHTO. Guide Specifications for Fatigue Evaluation of Existing Steel Bridges. American Association of State Highway and Transportation Officials, 1990;

3.

BS 5400: P art 10 Code of Practice for Fatigue, incorporating amendment no 1, 1999;

4.

BS7608: Fatigue Design and Assessment of Steel Structures, 1993;

5.

ECCS Recommendations for the Fatigue Design of Steel Structures, No 43, ECCS, 1985;

6.

ARMITAGE, JD and BEAGLES, M. Fatigue Strength of Riveted Connections. British Rail Research, September 1993;

7.

MAARSCHALKERWAART, H. Evaluation of Existing Structures;

8.

BEAGLES, M. Static and Fatigue Properties of Wrought Iron and Early Steels. British Rail Research Report LR MF 115, November 1993.

RAILTRACK BRIEFINGNOTE

RT/CE/C/025(Issue1)

RAILTRACK LINE CODE OF PRACTICE: The Structural Assessment of Underbridges

This Code of Practice gives recommendations for the appropriate standards and analytical methods which should be used to determine the load carrying capacity of existing Railtrack underbridges, in terms of British Standard Units of Type RA1 loading. The load carrying capacity is determined in the context of the performance requirements of an underbridge of meeting safety and serviceability criteria whilst regularly carrying rail traffic up to a level of traffic load and speed in accordance with operational system requirements. This Code of Practice may be used for the assessment of all Railtrack owned underbridges and is applicable for line speeds up to a maximum of 125 mph. Recommendations are provided for the assessment of underbridges constructed from steel, wrought iron, cast iron, concrete, timber, or composite steel/concrete construction, and for masonry arches, substructures and bearings. The document is divided into 12 Sections with 6 accompanying appendices as follows: Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8 Section 9 Section 10 Section 11 Section 12 Appendix A Appendix B

Introduction Assessment Philosophy Inspection For Assessment Loading For Assessment Steel And Wrought Iron Structures Masonry Arches Concrete Structures Composite Structures Cast Iron Structures Timber Structures Substructures Bearings Assessment of Steel and Wrought Iron Assessment Version of BS 5400: Part 4

Appendix C Appendix D Appendix E Appendix F

Amendments To BS 5400: Part 5 Fatigue Assessment of and Wrought Iron Model Bridge Assessment Report Informative Annex

Limit state principles are used in Sections 5, 7 and 8 and the corresponding appendices A, B and C for the assessment of underbridges of steel, wrought iron, concrete and composite construction respectively. Permissible stress or capacity principles are used in Sections 6, 9, 10, 11 and 12 for assessment of underbridges of other materials and forms of construction.

RT/CE/C/025

Where appropriate, guidance on the use of simple and more rigorous methods of analysis is given. Unusual forms of construction such as cable stayed, moveable or combined road/rail bridges are not specifically covered, but the principles outlined may be applied in checking the elements of such structures. Requirements for the assessment of superstructures and supports under accidental loading conditions are excluded from RT/CE/C/025. In comparison with RT/CE/C/015 (which has been retained for use for existing assessments and record purposes) , the principle changes are:

• • • • • • •

clarification of the principles of assessment and description of the three levels of assessment; use of limit state principles based on the relevant section of BS 5400 for the sections on steel and wrought iron; inclusion of sections for concrete, composite construction, cast iron, timber, substructures and bearings; inclusion of revised summary report forms for steel and wrought iron bridges and new summary report forms for other materials and forms of construction; extensive revision of recommendations for inspection, and inclusion of detailed recommendations for inspection of fatigue; revised limits for capacities of rivets based on research carried out during the development of the document; inclusion of criteria for assessment of U frames; revised criteria for determination of effective length of transverse members.

• RT/CE/C/025 has been extensively trialed in comparison with the permissible code. Overall the capacity of underbridges assessed using the limit state code should be not less than that obtained using RT/CE/C/015. It is expected that significant benefits in terms of assessed capacity will accrue in respect of riveted bridges and masonry arches assessed using the MEXE method. It is possible that the assessed capacity of some bridges will be lower. A series of appreciation courses are being arranged to give Railtrack staff and Railtrack s assessment consultants an understanding of the document content and details of the significant changes. Such training will be provided in April and May 2001. It is also proposed that technical support will be provided for 24 months in order that clarification on interpretation of recommendations and advice on validity of any apparent errors may be given by the drafters of the document. In conjunction with the implementation of RT/CE/C/025, RT/CE/P/016: The Assessment of Bridge Capacity is currently being revised.

Keith Ross Senior Asset Manager Structures Asset Management Group

November 2000

CEC025 - Rail Track Code

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