Bridge Engineering Lecture Note.pdf

ETHIOPIAN CIVIL SURVICE UNIVERSITY INSTITUTE OF URBAN DEVELOPMENT STUDY URBAN ENGINEERING DEPARTMENT FUNDAMENTALS OF BRIDGE DESIGN (UE 575) Course: Fundamental of Bridge design (UE575) Prerequisite: UE 452 -Design of RC Structures II, & UE 342 Credit Hours: 3 (2 Lecture hours and 3 Tutorial hours) Academic Year: 2016/17/semester I

COURSE OUTLINE 1. Introduction 2. Investigation for Bridges - Site Selection - Data Collection, Span Determination 3. Types of Bridges and their Selection - Types of Bridges - Selection of Bridges 4. Bridge Loading - Types of Loads - Distribution of Loads According to AASHTO 5. Superstructure - Reinforced Concrete Superstructures - Steel Superstructures - Composite Superstructures - Arches, Cable stayed, Suspension 6. Substructures - Piers - Abutments - Wing Walls - Scour Protection 7. Bearings and Railings 8. Culverts and Low Level Water Crossings

Theory of Structures II Targeted group: Year 5th Instructor name: Yonas A.

References: 1. ERA Bridge Design Manual, 2002 2. AASHTO LRFD Bridge Design Manual 3. Bridge Engineering-Superstructure (Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) 4. Bridge Engineering Handbook, Wai-Fah Chen and Lian Duan. (2000) 5. Structural Engineering Handbook, Gaylord, E.H (1997) 6. Bridge Engineering, Ponnuswamy, S( 1999) 7. Design of Modern Concrete Highway Bridges, Heins and Lawrie (1984)

Evaluation: Attendance and Class Activity……………………10% (90% attendance for all student) Project …………………………………………….....40% [Min./Max.5 &6 Group] Mid Exam…………………………………………….20% Final Exam………………………………………........30%

“Everything should be made as simple as possible. But not simpler!” (Albert Einstein)

1.

Introduction

Bridges are important structures to pass obstacles, such as rivers, gorges, roads and railways. They are not seen or understand in the same way by everyone. For instance: A simple bridge over a small river will be viewed differently by different people because the eyes each one sees it with are unique to that individual. -

Some one travelling over the bridge everyday while going to work may only realize a bridge is there because the road way has posts and railing on either side. Others may remember a time before the bridge was built how far they had to travel to visit friends and to get the children to school. Civic leaders see the bridge as a link between neighborhoods and a way to provide fire and police protection and access to hospital. In business community, the bridge is seen as opening up new markets and expanding commerce. An artist will consider the bridge and its setting as a possible subject for a future painting. A theologian may see the bridge as symbolic of making a connection between God and human beings. While a boater on the river, looking up when passing underneath the bridge, will have a completely different perspective.

Bridges affect people. People use them and engineers design them and later build and maintain them. Bridges must be planned and engineered before they can be constructed. Bridge engineering is one of the fascinating fields in civil engineering calling for expertise in many areas: structural analysis and design, geotechniques, traffic projection, surveying, runoff calculation and methods of construction. Mankind takes lessons from nature to construct bridges • • •

Tree fallen accidentally across a stream was the earliest example of a beam type bridge. Similarly, the natural rock arch formed by erosion of the loose soil below was the earliest forebear of arch bridges. Creeper hanging from tree to tree allowing monkeys to cross from one bank to the other was the forerunners of suspension bridges.

Transportation System and Bridges Transportation system which is implemented on land needs bridges. Basically in Road Transportation System Bridge is mandatory for two reasons. 1. To pass obstacles Fundamental of Bridge Design

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2. To facilitate the transportation system. This is to say in larges cities there is traffic jam when two or more roads meet at a point. For such cases bridges, called interchanges are provided. E.g. Gotera interchange, Addis Ababa A bridge controls the capacity of the transportation system. For instance: If the strength of the bridge is unable to carry heavy trucks, loads limits will be posted and heavy trucks will be rerouted. Bridges are expensive structures. The cost per meter of a bridge is high in comparison to the road. If the bridge fails, the transportation system will not be in a position to give function. Therefore, bridge designer has control over the • • •

2.

capacity, cost and Safety.

Investigation for Bridges

Bridge Site Selection In locating a bridge crossing the following considerations come in to picture. • The reach of the river should be straight. Especially this has to hold on upstream side of the crossing. This is necessary so that the approach flow is not angular and the obstructions caused by piers, etc have minimum disturbance effect on the flow. • The river in the reach should have a regime flow free of excess of currents. If this is present, it will be aggravated by the piers that have to be put up and will result in excessive scour which endanger the foundation. • The channel in the reach should be well defined. • The crossing site should be as narrow as possible. • The crossing site should have firm high banks which are fairly inerodable. In this case the river flow will be defined and confined and any excessive velocity will not cause erosion. • The site on a meandering river should be a nodal point. A nodal point is defined as the location where the river regime does not normally shift and the location serves as a fulcrum about which river channel swing laterally ( both upstream and down stream) • The site should have suitable strata at reasonable and workable depth for founding piers and abutments. • The site should allow for constructing approach road. • The site should be selected where skewness can be avoided.

Fundamental of Bridge Design

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For a river crossing it is important to identify the type of river to be crossed. There are two types of rivers namely alluvial and incised. Alluvial rivers are winding and they erode their banks and scour their beds; they have flood plains on either side of the channel and the flow regularly overtops the channel banks to spread across the flood plain. They are continually active, scouring and depositing materials on the banks and transporting sediments. Incised rivers have a relatively stable banks and arc generally narrower and deeper than alluvial rivers. Bridge crossing over alluvial rivers nearly always require training works to stabilize the channel flow with in tile bridge waterway opening. Data Collection Once the engineer has identified a likely site for the bridge, he/she needs to obtain field information on the catchment area and run off, local terrain conditions and water levels, navigational (like Baro River) and other clearance requirements. Field reviews shall be made by the designer in order to become familiar with the site. The most complete survey data cannot adequately depict all site conditions or substitute for personal inspection by someone experienced in bridge design. •

River Survey

Information required by the designer for analysis and design should include all features that can affect the magnitude and the frequency of the flood flow which will pass the site under study. These are: Climatologically characteristics, land runoff characteristics, stream gauging records, high water marks and size and performance of existing structures in the vicinity. High water marks can be obtained from gauges or from local people. In addition, they can be identified from small debris, such as grass or twigs caught in tree branches, elephant grass or similar matted down, mud lines on stones or bridges, are all high water indicators. The hydrologic characteristics of the basin or watershed of the stream under study are needed for any predictive methods used to forecast flood flows. Although many of these characteristics can be found from office studies, some are better found by a field survey of the basin. The size and configuration of the watershed, the geometry of the stream network, storage volumes of ponds, lakes, reservoirs, and flood plains, and the general geology and soils of the basin can be found from maps.


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Having determined these basin characteristics, runoff times, infiltration values, storage values, and runoff coefficients can be found and used in calculating flood flow values using different methods . Like: Reve’s formula, rational formula, Area-Velocity formula

Soil Investigation Once at the site it is easy and of great value to sample for soil, rock, stone, water, etc. in cooperation with the soil investigators. Soil investigation is required to get soil profile, engineering property of the foundation material and foundation level of the abutments and piers for design of the foundation. This information is obtained by analyzing samples taken from boreholes, test pits or geophysical surveying. Samples of at least 2Kg each should be collected marking station number and river name where the crossing site is fixed.

Field Sketching and Photos It has proved very practical to make a simple sketch of the bridge site with approximate water shores, existing structures, scour holes, main stream location, etc including very rough dimensions with approximate measurements As a minimum, photos shall be taken looking upstream and downstream from the site as well as along the contemplated highway centerline in both directions. Details of the streambed and banks should also be photographed along with any existing structures in the vicinity both upstream and downstream. Close-up photographs complete with a scale or grid shall be taken to facilitate estimates of the stream bed gradation.

Check List of Site Investigation A form or checklist that can be used by the field investigator/designer in identifying and cataloging field information is shown on .A checklist for Inspection of existing bridges is shown below.

FIELD VISIT INVESTIGATION FORM •

PROJECT:…………..................................................………………......………………

Date: …................……….

Inv. by ……….........................…………………………….........................… Site Situated @ STA: .................………. •

WATERWAY: Name: .............................................................................. Direction of flow: ....................................


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High Water Mark:... ...................... Level: +...............Side Slopes: ..................................degrees: ............ Diversions/ Flow concentration / Flood Hazards year, level: ............................................................................................. % Grade of Stream: .........

Channel, Base: ……(m) Height of Banks:…...... (m) Manning's Value n=..................

Crossing angle (estimated): …………degrees Meandering: ……………………………………(show figure below) Bottom/Base material............................................................ Material on channel side: ......................... n=.......... Up or Downstream Restriction (debris/sedimentation/scour/soil mtrl.): ............................................................................ .............................................................................................................................................................................................. •

STRUCTURES : Bridges/ Buildings upstream and downstream: ............................................. @ M up/down:.......

Type: ..........................................................................................Piers: Type: ............................................................ Abutment Types: ........................................Width: ...............(m)

Size of Spans: ...................................................

Clear Height: ……(m) @............; ............m@..............; .............m@ .............; Total water width at HWL:............(m); (Overflow? Year? Level: +...) ............................................................................................................................................. •

MISC. Land Uses upstream and downstream: ..............................................................................................................

Vegetation (Location, Type, Name): ................................................................................................................................... Wildlife (Paths, Traces, Type, Name): ............................................................................................................................... •

Soil Conditions: .......................................................................................................... at Roadway STA: .....................

Sample no: ............. @ STA: .............; Sample no: ............ @ STN: ................; Sample no: ............ @ STA: ................; •

Photos no:............@STN:............... Shows: ................... Photos no:..........@STA:............... Shows: .........................

Photos no:............@STN:............... Shows: ..................... Photos no:............@STN:............... Shows: ......................... •

REMARKS: ........................................................................................................................................................................

............................................................................................................................................................................................... ............................................................................................................................................................................................. ............................................................................................................................................................................................. (please, make simple plan sketch incl. water shores/Road alignment and continue the text on back side of this page, if needed)

Economical Span Span determination is usually dictated by the hydraulic requirement. However, there are conditions where lengthen spans are chosen for the sake of road alignment. Fundamental of Bridge Design

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For a given span the most economical span is the length at which superstructure cost equals to substructure cost. Hydraulic Requirements Bridges are designed to accommodate design discharge at design flood. When a river has a wide flood plain, the economical solution may be using short span bridge with proper scour and erosion protection for the embankment, abutments and piers. Piers should be located in such a manner that they can provide the required lineal waterway and navigational clearance. The alignment of piers and abutments should, if possible, be set parallel to the direction of flow during maximum flood. Free Board The waterway below the superstructure must be designed to pass the design flood and the floating debris carried on it. The free board allows for uncertainty in determining DFL also. The minimum free board above the design water level is given in table below unless refined hydraulic analyses have been made. Table: Free Board

Discharge (m3/s)

Vertical Clearance/Free board (m)

0 to 3.0

0.3

3.0 to 30.0

0.6

30.0 to 300

0.9

> 300

1.2

These clearance measurements should be increased for backwater effects when the flow is restricted by short span bridge or when the river has history of unusual large floating items or in case of navigational requirements. Grade Requirements Often in mountainous areas the road way grade is governed by the capacity of heaviest vehicle to Fundamental of Bridge Design

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climb, vertical curve and sight distance. These requirements may increase the span beyond the hydraulic requirement.

3. 3.1

Types of Bridges and their Selection Types of Bridges

Bridges can be classified in different ways (a) Traffic type/functionality -

Road bridge Railway bridge Pedestrian bridge Aqueduct Viaduct Equipment bridge

(b) Life Span -

Temporary bridges Permanent bridges Semi-permanent bridges

(c) Horizontal Arrangement -

Straight/Normal bridge Skewed bridge Curved bridge

(d) Vertical Arrangement -

Horizontal/ Flat/ Normal Inclined

(e) Span -

L ≤ 6m (Culvert) 7m < L ≤ 15m (Small span bridges) 16 ≤ L ≤ 50m (Medium span Bridges) 50 ≤ L≤ 150m (Large Span Bridges) L≥150m (Extra Large Span Bridges)


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(f) Construction Materials - Timber Bridges - Masonry Bridges - Reinforced Concrete Bridges - Prestressed Concrete Bridges - Steel Bridges - Composite Bridges (g) Span Arrangement -

Simply Supported Continuous Cantilever

(h) Structural Arrangement -

Slab Bridges Girder (Deck girder Bridges) Box Girder Portal Frame Bridges Arch Bridges Truss Bridges Plate Girder Bridges Cable Stayed Bridges Suspension Bridges Box Cell/ Box culvert

(i) Movements -

3.2

Movable Bridges Fixed Bridges

Selection of Bridge Type

In selection of a bridge type, there is no unique answer. For each span length range there is more than one bridge type that will satisfy the design criteria. Generally the following factors should be considered. • Geometric Condition of the Site: The type of bridge selected will often depend on the horizontal and vertical alignment of the highway route and on the clearances above and below the road way. E.g. If the alignment is on a curve, box and slab type bridges are best options.


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• Subsurface Conditions of the Site: The foundation soils at a site will determine whether abutments and piers can be founded on spread footings, driven piles and etc. The potential for seismic activity at a site should be a part of the subsurface investigation because this will change details of the substructure. E.g. an inclined leg rigid frame bridge requires strong foundation • Functional Requirements: Bridge should serve the present and future traffic. E.g. If future widening or replacement of bridge decks is a concern girder type bridge is best option. • Aesthetics: It is necessary to understand what qualities and features of a bridge tend to make that aesthetics statement a good one. This understanding requires training and time. -

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When a bridge is placed across a relatively shallow valley, the most pleasing appearance occurs when there are an odd number of spans with span lengths that decrease going up the side of the valley. Harmony between the whole structure and its surrounding needed to be addressed. Repeating similar spans too many times can become monotonous, just as hearing the same music with a heavy beat that is repeated over and over again can be uncomfortable.

Moreover, contrast and texture, light and shadow are additional aesthetic parameter in bridge design. • Economics and ease of maintenance: In comparison of the economics of different bridge types, the construction cost and maintenance cost should be taken together. A general rule is that the bridge with minimum number of spans, fewest deck joints and widest spacing of girders will be the most economical. By reducing the number of spans, the construction cost of one pier is eliminated. Deck joints are a high maintenance cost item, so minimizing their number will reduce the life cycle cost of the bridge, Generally, concrete structures require less maintenance than steel structures. • Construction and erection considerations: The selection of the bridge type to be built is often governed by construction and erection considerations. In general, the larger the prefabricated or precast member, the shorter the construction time. However, the larger the members, the more difficult they are to transport and lift into place.


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The availability of skilled labor and specified materials will also influence the choice of a particular bridge type. • Legal Considerations: Applicable laws like environmental laws also govern the type of bridge.

4. Bridge Loadings 4.1

Types of loads

The following permanent and transient loads and forces shall be considered for design of bridges where applicable. The load provisions may also be applied to the structural evaluation of existing bridges. •

Permanent Loads DC DD DW EH EL ES EV

•

= = = = = = =

dead load of structural components and nonstructural attachments down drag dead load of wearing surfaces and utilities horizontal earth pressure load accumulated locked-in effects resulting from the construction process earth surcharge load vertical pressure from dead load of earth fill

Transient Loads BR CE CR CT EQ FR IM LL LS PL SE SH TG TU WA WL

= = = = = = = = = = = = = = = =

vehicular braking force vehicular centrifugal force creep vehicular collision force earthquake friction vehicular dynamic load allowance vehicular live load live load surcharge pedestrian live load settlement shrinkage temperature gradient uniform temperature water load and stream pressure wind on live load


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WS = wind load on structure

4.2 Dead Loads Dead load shall include the weight of all components of the structure, appurtenances and utilities attached thereto, earth cover, wearing surface, future overlays, and planned widening. In the absence of more precise information, the densities, specified in table below, shall be used for dead loads. Densities and Force Effects of Different Materials MATERIAL DENSITY (kg/m3) Bituminous Wearing Surfaces 2250 Cast Iron 7200 Cinder (volcanic stone) Filling 960 Compacted Sand, silt, or Clay 1925 Concrete Normal 2400 Loose Sand, Silt, or Gravel 1800 Soft Clay 1700 Rolled Gravel or Ballast 2250 Steel 7850 Stone Masonry 2725 Wood Hard 960 Soft 800 Water Fresh 1000

Force effect (kN/m3) 22.5 72 9.6 19.3 24 18 17 22.5 79 27.3 9.6 8 10

4.3 Live Loads Number of Design Lanes: Generally, the number of design lanes should be determined by taking the integer part of the ratio w/3600, where w is the clear roadway width in mm between curbs and/or barriers. Multiple Presence of Live Load: The provisions of this subchapter shall not be applied to the fatigue limit state for which one design truck is used, regardless of the number of design lanes. Trucks will be present in adjacent lanes on roadways with multiple design lanes but this is unlikely that all adjacent lanes will be loaded simultaneously. This will be considered by the multiple presence factors. Number of Loaded Lanes Multiple Presence Factors “m”


1 1.20

2 1.0

3 0.85

>3 0.65

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When the loading condition includes the pedestrian loads combined with one or more lanes of the vehicular live load, the pedestrian loads shall be taken to be one loaded lane.

4.3.1 Vehicular Live Loads Vehicular live loading on the roadways of bridges structures, designated HL-93, and shall consist of a combination of the: • Design truck or design tandem, and • Design lane load •

Design truck: The weights and spacing of axles and wheels for the design truck shall be as specified in Figure below. Plan of Design Truck Load showing tire contact areas

4.3 m

4.3 –9.0 m

1.8 m

3.000 mm

Fig. Characteristics of the Design Truck •

Design Tandem: The design tandem used for Strategic Bridges shall consist of a pair of 110 kN axles spaced 1.2 m apart. The transverse spacing of wheels shall be taken as 1.8 m. See below. 110 kN 1.2 m 110 kN 1.8 m

Fig. Design Tandem Load


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• Design Lane Load: The design lane load shall consist of a load of 9.3 kN/m, uniformly distributed in the longitudinal direction. Transversely, the design lane load shall be assumed to be uniformly distributed over a 3.0-m width. The force effects from the design lane load shall not be subject to a dynamic load allowance.

4.3.2 Dynamic Load Allowance Ø (IM = Vehicular Dynamic Load Allowance): Dynamic effects due to moving vehicles shall be attributed to two sources: 1-Hammering effect is the dynamic response of the wheel assembly to riding surface discontinuities, such as deck joints, cracks, potholes, and delaminations, and 2-Dynamic response of the bridge as a whole to passing vehicles, which shall be due to long undulations in the roadway pavement, such as those caused by settlement of fill, or to resonant excitation as a result of similar frequencies of vibration between bridge and vehicle. The frequency of vibration of any bridge should not exceed 3 Hz. Dynamic load allowance need not be applied to: • •

Retaining walls not subject to vertical reactions from the superstructure, and Foundation components that are entirely below ground level.

The dynamic load allowance shall not be applied to pedestrian loads or to the design lane load. The factor to be applied to the static load shall be taken as: (1 + IM/100).

Component Deck Joints – All Limit States All Other Components • Fatigue and Fracture Limit State • All Other Limit States

IM 75% 15% 33%

Table Dynamic Load Allowance, IM The dynamic load allowance for culverts and other buried structures, in %, shall be taken as: IM = 33 (1.0 - 4.l*10 -4 DE) > 0% Where: DE = the minimum depth of earth cover above the structure (mm)


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Centrifugal forces (CE= Vehicular Centrifugal Force):

4.3.3

Centrifugal force is due to inertia force of vehicles on curved bridges at speed. Centrifugal forces shall be applied horizontally at a distance 1.8 m above the roadway surface. Centrifugal forces shall be taken as the product of the axle weights of the design truck or tandem and the factor C, taken as: C = 4 v2 3 g*R where: v g R

= highway design speed (m/s) = gravitational acceleration: 9.81 (m/s2) = radius of curvature of traffic lane (m)

Breaking Force (BR= Vehicular Braking Force):

4.3.4

From AASHTO Commentary 3.6.4 Based on energy principles, and assuming uniform deceleration (retardation), the braking force determined as a fraction "b" of vehicle weight is: v2

b =

2ga Where a = the length of uniform deceleration. From AASHTO Article 3.6.4 Braking forces shall be taken as 25 % of the axle weights of the design truck or tandem per lane placed in all design lanes headed in the same direction. These forces shall be assumed to act horizontally at a distance of 1800 mm above the roadway surface in either longitudinal direction to cause extreme force effects. 4.3.5

Vehicular Collision (CT= Vehicular Collision Force):

Unless protections are provided a horizontal force of 1800KN applied at 1.2m above the ground should be considered.

4.3.6 Pedestrian Loads A pedestrian load of 3.6 kPa (kN/m2) shall be applied to all sidewalks wider than 0.6 m and considered simultaneously with the vehicular design live load.


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4.3.7

Water Loads (WA= Water Load and Stream Pressure)

• Static Pressure: Static pressure of water shall be assumed to act perpendicular to the surface that is retaining the water. Pressure shall be calculated as the product of height of water above the point of consideration, the density of water, and "g" (the acceleration of gravity = 9.81 m/s2). p = γ * g * z * 10-9 Where p = static pressure (Mpa) γ = density of water (kg/m3) z = height of water above the point of consideration (mm) g = Gravitational acceleration (m/s2) 4.3.8

Buoyancy: Buoyancy shall be considered an uplift force, taken as the sum of the vertical components of static pressures, acting on all components below design water level.

4.3.9

Stream Pressure

Longitudinal: The longitudinal drag force shall be taken as the product of longitudinal stream pressure and the projected surface exposed thereto. p = 5.14*10-4 CDV2 Where: p = pressure of flowing water (MPa) CD = drag coefficient for piers as specified in Table below V = design velocity in m/s of water for the design flood in strength and service limit states and for the check flood in the extreme event limit state Type Semicircular-nosed pier Square-ended pier Debris lodged against the pier Wedged-nosed pier with nose angle 90o or less

CD 0.7 1.4 1.4 0.8

Table. Drag Coefficient


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Lateral: The lateral, uniformly distributed pressure on substructure due to water flowing at an angle, θ, to the longitudinal axis of the pier (see Figure below) shall be taken as: PL = 5.14 x 10 -4CLV2 Where: PL = lateral pressure (MPa) CL = lateral drag coefficient specified in Table below.

Plan View of Pier Showing Stream Flow Pressure Angle, θ, between direction longitudinal axis of the pier 0o 1o 10o 20o ≥30o

of flow

and C L 0.0 0.5 0.7 0.9 1.0

Table. Lateral Drag Coefficient

The lateral drag force shall be taken as the product of the lateral stream pressure and the surface exposed thereto. 4.3.10

Wind Loads (WL= Wind on Live load; WS= Wind load on Structure)

§ Wind Pressure on Structures, (WS): For small and medium sized concrete bridges below 50m length the wind load on structures shall be neglected. In the absence of more precise data, design wind pressure, PD in kPa, shall be determined as:


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VDZ  PD = PB   VB 

2

Where PB = base wind pressure specified in Table below: VDZ = design velocity of wind at design elevation, Z (Km/hr) VB= Base wind velocity (Km/hr)

STRUCTURAL COMPONENT

WINDWARD LOAD, kPa

Trusses, Columns, and Arches 2.4 Beams 2.4 Large Flat Surfaces 1.9

LEEWARD LOAD, kPa

1.2 Not applicable Not applicable

Table 3-12 Base Pressures, PB Corresponding to VB = 160 km/h (45 m/s) The wind loading shall not be taken less than 4.4 kN/m2 in the plane of a windward chord and 2.2 kN/m2 in the plane of a leeward chord on truss and arch components, and not less than 4.4 kN/m2 on beam or girder components. § Wind Pressure on Vehicles, (WL): When vehicles are present, the design wind pressure shall be applied to both structure and vehicles. Wind pressure on vehicles shall be represented by an interruptible, moving force of 1.46 kN/m acting normal to, and 1.8 m above, the roadway and shall be transmitted to the structure § Aeroelastic Instability: Many bridges, decks, or individual structural components have been shown to be aeroelastically insensitive if their length-to-width or length-to-depth ratios are under about 30.0. Wind tunnel testing of bridges and other civil engineering structures is a highly developed technology, which shall be used to study the wind response characteristics of a structural model or to verify the results of analysis. This is especially applicable to long spans. 4.3.11 Earthquake Effects (EQ= Earthquake) Earthquake loads are given by the product of the elastic seismic response coefficient Csm and the equivalent weight of the superstructure. These are inertia forces due to mass of the bridge when a sudden shaking of the ground occurs. Minimum seat width requirements shall be at least 500 mm at each abutment. For multispan structures, the minimum analysis requirements shall be as specified below:


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Seismic Single-Span Zone Bridges

Multispan Bridges Other Bridges Regular Irregular No Seismic Analysis * * Seismic Analysis SM/UL SM

1-3 4

Essential Bridges Regular Irregular * * SM/UL MM

Critical Bridges Regular Irregular * * MM MM

Minimum Analysis Requirements for Seismic Effects In which: * UL SM MM

= = = =

no seismic analysis required (Zone 1-3) uniform load elastic method single-mode elastic method multimode elastic method

The selection of the method of analysis depends on seismic zone, regularity, and importance of the bridge. Essential bridges are generally those that should, as a minimum, be open to emergency vehicles and for security/defense purposes immediately after the design earthquake, i.e., a 475-year return period event. However, some bridges must remain open to all traffic after the design earthquake and be usable by emergency vehicles and for security/defense purposes immediately after a large earthquake, e.g., a 2500 year return period event. These bridges should be regarded as critical structures. 4.3.12

Earth Pressure

(EH = Horizontal Earth Pressure; ES = Earth Surcharge; LS = Live Load Surcharge; DD = Down drag)

Earth pressure shall be considered as a function of the: • • • •

Type and density of earth, Water content, Soil creep characteristics, Degree of compaction

• • • •

Location of groundwater table, Earth-structure interaction, Amount of surcharge, and Earthquake effects.

Walls that can tolerate little or no movement should be designed for at-rest earth pressure. Walls that can move away from the soil mass should be designed for pressures between active and atrest conditions, depending on the magnitude of the tolerable movements. Movement required to reach the minimum active pressure or the maximum passive pressure is a function of the wall height and the soil type. Some typical values of these mobilizing movements, relative to wall height, are given in Table below:


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Approximate Values of Relative Movements Required to Reach Minimum Active or Maximum Passive Earth Pressure Conditions

Type of Backfill Dense sand Medium-dense sand Loose sand Compacted silt Compacted lean clay Compacted fat clay

Values of ∆/H Active 0.001 0.002 0.004 0.002 0.010 0.010

Passive 0.01 0.02 0.04 0.02 0.05 0.05

Where: ∆ = movement of top of wall required to reach minimum active or maximum passive pressure by tilting or lateral translation (mm) H = height of wall (mm) For walls that are backfilled with cohesive materials, the effects of soil creep should be taken into consideration in estimating the design earth pressures. Where activity by mechanical compaction equipment is anticipated within a distance of one-half the height of the wall, taken as the difference in elevation between the point where finished grade intersects the back of the wall and the base of the wall, the effect of additional earth pressure that shall be induced by compaction shall be taken into account. Wherever possible, the development of hydrostatic water pressure on walls should be eliminated through use of free-draining (rapid-draining) backfill material and/or the use of weep holes and crushed rock, pipe drains, gravel drains, perforated drains, or geofabric drains that provide drainage. Where soils are subject to both saturation and seismic or other cyclic/instantaneous loads, special consideration should be given to addressing the possibility of soil liquefaction. • EH = Horizontal Earth Pressure There are two earth pressure theories used. These are Rankin and Coulomb Earth Pressure Theories.


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Coulomb theory is recommended by AASHTO for masonry and RC abutment since this theory holds better for the actual situation. Basic earth pressure (p, in MPa) shall be assumed to be linearly proportional to the depth of earth and taken as: p = k h*γs*g*z *10-9 Where: kh = coefficient of lateral earth pressure taken as ko, from table below for walls that do not deflect or move,

Soil type Loose sand Medium Sand Dense Sand Silt (ML) Lean Clay (CL) Highly Plastic Clay (CH)

Coefficient of Lateral Earth Pressure, ko OCR = 1 OCR = 2 OCR = 5 0.45 0.65 1.10 0.40 0.60 1.05 0.35 0.55 1.00 0.50 0.70 1.10 0.60 0.80 1.20 0.65 0.80 1.10

OCR = 10 1.60 1.55 1.50 1.60 1.65 1.40

Or k a, specified in Equations below, walls that deflect sin 2 (θ + ϕ/)

ka =

Γ* sin2θ sin (θ - δ) In which: 2

Γ = 1 + sin (ϕ/ + δ) sin (ϕ/ - β) sin (θ - δ) sin (θ + β) Where: δ = friction angle between fill and wall β = angle of fill to the horizontal θ = angle of backfill of wall to the vertical ϕ/ = effective angle of internal friction (°)


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Notations γs = density of soil (kg/m3) z = depth below the surface of earth (mm) g = Gravitational acceleration (m/s2) • ES = Earth Surcharge; LS = Live Load Surcharge Where a uniform surcharge is present, a constant horizontal earth pressure, ∆p (MPa), shall be added to the basic earth pressure. This constant earth pressure shall be taken as: ∆p = ks qs Where: ks = coefficient of earth pressure due to surcharge qs = uniform surcharge applied to the upper surface of the active earth wedge (MPa) live load surcharge shall be applied where vehicular load is expected to act on the surface of the backfill within a distance equal to the wall height behind the back face of the wall. The increase in horizontal pressure due to live load surcharge shall be estimated as: ∆p = k*γs*g*heq *10-9 where: ∆p = constant horizontal earth pressure due to uniform surcharge (MPa) γs = density of soil (kg /m3) k = coefficient of earth pressure heq = equivalent height of soil for the design truck (mm) Equivalent heights of soil, heq, for highway loadings shall be taken from Table below. Linear interpolation shall be used for intermediate wall heights. Fundamental of Bridge Design

21

The “Wall Height” shall be taken as the distance between the surface of the backfill and the bottom of the footing.

Wall Height (mm)

heq (mm)

≤1500 3000 6000 ≥9000

1700 1200 760 610

Equivalent Height of Soil, heq for Different Wall Heights Due to Vehicular Loading N.B If the vehicular loading is transmitted through a structural slab, which is also supported by means other than earth, an appropriate reduction in the surcharge loads shall be permitted. • Down Drag (DD): When soil surrounding piles settle, it applies a downward force. In this case, the force should be considered. 4.3.13 Force Effects Due to Superimposed Deformations: TU, TG, SH, CR, SE -

Uniform temperature, (TU): Bridge materials expend and contract in response to rise and fall of temperature. The difference between the lowest or the highest temperature and the base construction temperature assumed in design shall be used to calculate thermal deformation effects.

-

Temperature Gradient, (TG): Temperature rise can differ on the top and bottom surfaces of abridge because the top surface is subjected to direct solar radiation.

-

Differential Shrinkage, (SH): Where appropriate, differential shrinkage strains between concretes of different age and composition, and between concrete and steel or wood, shall be determined. The designer may specify timing and sequence of construction in order to minimize stresses due to differential shrinkage between components.

-

Creep, (CR): In determining force effects and deformations due to creep, dependence on time and changes in compressive stresses shall be taken into account.

-

Settlement, (SE): This will cause internal forces in continues structures. Force effects due to extreme values of differential settlements among substructures and within individual substructure units shall be considered.


22

4.4 Design Philosophy In engineering design the general principle is that the resistance of a cross section has to exceed the effects come from the applied loads. That is Resistance ≥Effect of Loads When a particular loading condition reaches and just exceeds the resistance capacity of the provided section failure is the result. Such a condition is referred to as a Limit State. A limit state is a condition beyond which a bridge system or bridge component ceases to full fill the function for which it is designed. Preventing a limit state from being reached is the central goal of design of bridges. In addition to this function, appearance and economy must get due attention. Safety is achieved by using reasonable margin of safety factors. These factors are results of collective experience and judgment of qualified group of engineers and officials. In Highway Bridge design AASHTO LRFD provision is used for bridge design. The resistance side of the inequality of Equation above is multiplied by a statistically based resistance factor, whose value is usually less than one, and the load side is multiplied by a statistically based load factor, whose value is usually greater than one. The load effect at a particular limit state involves a combination of a different load types (Qi) that have different degrees of predictability. Due to this reason the load effect side is written in a summation form. The equation is Φ ∗ Rn ≥ ∑ γi ∗ Qi

And this equation involves both load factors and resistance factor due to this the design method is called load and resistance factor design method. In AASHTO LRFD bridge design specification the equation is given by

η ∗ ∑ γi ∗ Qi ≤ Φ ∗ Rn The additional parameter η is known as load modifier which is incorporated to consider ductility, redundancy and operational importance of the bridge. Under the umbrella of the LRFD the strength limit state, extreme event limit state, service limit state and fatigue and fracture limit state exist. Load Factors and Load Combinations: Fundamental of Bridge Design

23

The total factored force effect shall be taken as:

Q = Σηiγi Qi Where: ηi= load modifier Qi = force effects from loads specified herein γi = load factors specified in Tables B and C below Rn= is resistance Components and connections of a bridge shall satisfy the above equation for the applicable combinations of factored extreme force effects as specified at each of the limit states presented in Table A: Table A Limit States STRENGTH I

Basic load combination relating to the normal vehicular use of the bridge without wind.

STRENGTH II

Load combination relating to the use of the bridge by ERA-specified special design or permit vehicles, without wind.

A reduced value of 0.50, applicable to all strength load combinations, specified for uniform temperature (TU), creep (CR), and shrinkage (SH), used when calculating force effects other than displacements at the strength limit state, represents an expected reduction of these force effects in conjunction with the inelastic response of the structure. The calculation of displacements for these loads utilizes a factor greater than 1.0 to avoid undersized joints and bearings.

The permit vehicle should not be assumed to be the only vehicle on the bridge unless so assured by traffic control. Otherwise, the other lanes should be assumed to be occupied by the vehicular live load as specified herein. For bridges longer than the permit vehicle, the presence of the design lane load, preceding and following the permit load in its lane, should be considered.

STRENGTH III

Load combination relating to the bridge exposed to wind velocity exceeding 90 km/h.

STRENGTH IV

Load combination relating to very high dead load to live load force effect ratios.

Vehicles become unstable at higher wind velocities. Therefore, high winds prevent the presence of significant live load on the bridge.

The standard calibration process for the strength limit state consists of trying out various combinations of load and resistance factors on a number of bridges and their components. Combinations that yield a safety index close to the target value of β = 3.5 are retained for potential application. From these are selected constant load factors γ and corresponding resistance factors ϕ for each type of structural component reflecting its use. This calibration process had been carried out for a large number of bridges with spans not exceeding 60 m. For the primary components of large bridges, the ratio of dead and


24

live load force effects is rather high, and could result in a set of resistance factors different from those found acceptable for small- and medium-span bridges. It is believed to be more practical to investigate one additional load case than to require the use of two sets of resistance factors with the load factors provided in Strength Load Combination I, depending on other permanent loads present. For bridges with up to 180 m spans, Load Combination IV will govern where the dead load to live load force effect ratio exceeds 7.0.

STRENGTH V

Load combination relating to normal vehicular use of the bridge with wind of 90 km/h (25 m/s) velocity

EXTREME EVENT I

Load combination including earthquake

SERVICE I

Load combination relating to the normal operational use of the bridge with a 90 km/h (25 m/s) wind and all loads taken at their nominal values. Also related to deflection control in buried metal structures, tunnel liner plate, and thermoplastic pipe and to control crack width in reinforced concrete structures. This load combination should also be used for the investigation of slope stability.

This limit state includes water loads, WA. The probability of a major flood and an earthquake occurring at the same time is very small. Therefore, consideration of basing water loads and scour depths on mean discharges shall be warranted. Live load coincident with an earthquake is discussed elsewhere in this chapter.

Compression in prestressed concrete components is investigated using this load combination. Service III is used to investigate tensile stresses in prestressed concrete components.

SERVICE II

Load combination intended to control yielding of steel structures and slip of slip critical connections due to vehicular live load. This load combination corresponds to the overload provision for steel structures, and it is applicable only to steel structures. From the point of view of load level, this combination is approximately halfway between that used for Service I and Strength I Limit States.

SERVICE III

Load combination relating only to tension in prestressed concrete structures with the objective of crack control. The live load specified in these Specifications reflects, among other things, exclusion weight limits. The statistical significance of the 0.80 factor on live load is that the event is expected to occur about once a year for bridges with two traffic lanes, less often for bridges with more than two traffic lanes, and about once a day for bridges with a single traffic lane.

FATIGUE

Fatigue and fracture load combination relating to repetitive gravitational vehicular live load and dynamic responses under a single design truck having a constant axle spacing of 9.0 m between 145 kN axles. The load factor, applied to a single design truck, reflects a load level found to be representative of the truck population with respect to a large number of return cycles of stresses and to their cumulative effects in steel elements, components, and connections.


25

The load factors for various loads comprising a design load combination shall be taken as specified in Table B. All relevant subsets of the load combinations shall be investigated. For each load combination, every load that is indicated to be taken into account and that is germane to the component being designed, including all significant effects due to distortion, shall be multiplied by the appropriate load factor and multiple presence factor specified in, if applicable.. The factors shall be selected to produce the total extreme factored force effect. For each load combination, both positive and negative extremes shall be investigated. In load combinations where one force effect decreases another effect, the minimum value shall be applied to the load reducing the force effect. For permanent force effects, the load factor that produces the more critical combination shall be selected from Table C. Where the permanent load increases the stability or load-carrying capacity of a component or bridge, the minimum value of the load factor for that permanent load shall also be investigated. The larger of the two values provided for load factors of Uniform Temperature (TU), Creep (CR), and Shrinkage (SH) shall be used for deformations and the smaller values for all other effects. Table B - Load Combinations and Load Factors Load Combination

Limit State STRENGTH 1 (Unless noted) STRENGTH II STRENGTH III STRENGTH IV EH, EV, ES, DW DC ONLY STRENGTH V EXTREME EVENT I SERVICE I SERVICE II SERVICE III FATIGUE LL, IM and CE ONLY

DC DD DW EH EV ES

WA

γp

LL IM CE BR PL LS EL 1.75

1.00

-

-

1.00

0.50/1.20

γTG

γ SE

γp γp

1.35 -

1.00 1.00

1.40

-

1.00 1.00

0.50/1.20 0.50/1.20

γTG γTG -

γ SE γ SE -

γp 1.5 γp γp

-

1.00

-

-

1.00

0.50/1.20

1.35 γ EQ

1.00 1.00

0.50 -

1.0 -

1.00 1.00

0.50/1.20 -

γTG -

γ SE -

1.00 1.00 1.00

1.00 1.30 0.80

1.00 1.00 1.00

0.30 -

1.0 -

1.00 1.00 1.00

1.00/1.20 1.00/1.20 1.00/1.20

γTG γTG

-

0.75

-

-

-

-

-

-


WS

WL

FR

TU CR SH

TG

SE

Use one of these at a time

EQ -

CT -

-

-

-

-

γ SE γ SE

1.0 0 -

-

-

-

-

26

Where (see following text): BR = vehicular braking force CE = vehicular centrifugal force CR = creep CT = vehicular collision force DC = dead load of structural components DD = downdrag DW = dead load of wearing surfaces and utilities EH = horizontal earth pressure load EL = accumulated locked-in effects resulting from the construction process EQ = earthquake load ES = earth surcharge load EV = vertical pressure from dead load of earth fill

FR = friction IM = vehicular dynamic load allowance LL = vehicular live load LS = live load surcharge PL = pedestrian live load SE = settlement SH = shrinkage TG = temperature gradient TU = uniform temperature WA = water load and stream pressure WL = wind on live load WS = wind load on structure

Table C - Load Factors for Permanent Loads, γp Type of Load Load Factor (γp) Maximum DC: Component and Attachments 1.25 DD: Downdrag 1.80 DW: Wearing Surfaces and Utilities 1.50 EH: Horizontal Earth Pressure 1.50 • Active 1.35 • At-Rest EL: Locked-in Erection Stresses 1.0 EV: Vertical Earth Pressure 1.35 • Overall Stability 1.35 • Retaining Structure 1.30 • Rigid Buried Structure 1.35 • Rigid Frames • Flexible Buried Structures other than 1.95 Metal Box Culvert • Flexible Metal Box Culverts ES: Earth Surcharge

Minimum 0.90 0.45 0.65 0.90 0.90 1.0 N/A 1.00 0.90 0.90 0.90

1.50

0.90

1.50

0.75

For example, at Strength I Limit State where the permanent load reaction is positive and live load can cause a negative reaction, the load combination would be: 0.9DC + 0.65DW + 1.75(LL+IM) Fundamental of Bridge Design

27

If both reactions were negative, the load combination would be: 1.25DC + 1.50DW + 1.75(LL+IM). Load Modifiers, ηi = η D ηR ηI : ηD = a factor relating to ductility, as specified below ηR = a factor relating to redundancy as specified below ηI = a factor relating to operational importance as specified below Ductility, redundancy, and operational importance are significant aspects affecting the margin of safety of bridges. • Ductility: The structural system of a bridge shall be proportioned and detailed to ensure the development of significant and visible inelastic deformations at the strength and extreme event limit states prior to failure. For the strength limit state: ηD ≥ 1.05 for non-ductile components and connections ηD = 1.00 for conventional designs and details complying with these Specifications ηD ≥ 0.95 for components and connections for which additional ductility-enhancing measures have been specified beyond those required by these Specifications For all other limit states: ηD = 1.00 • Redundancy: Multiple load-path structures should be used unless there are compelling reasons not to use them. For the strength limit state: ηR ≥1.05 for nonredundant members =1.00 for conventional levels of redundancy ≤0.95 For exceptional levels of redundancy For all other limit states: ηR = 1.00


28

•

Operational importance: This definition shall apply to the strength and extreme event limit states only. Some bridges or structural components and connections shall be declared to be of operational importance. Such classification should be based on social/survival and/or security/defense requirements. For the strength limit state: ηl ≥ = ≤

1.05 for important bridges 1.00 for typical bridges 0.95 For relatively less important bridges

For all other limit states: ηl =

1.00

For Conventional Construction, Resistance Factors Ф shall be taken as For flexure and tension of RC For flexure and tension of PSC For shear and torsion For bearing on concrete For compression in strut-and-tie model For compression in anchorage zones For tension in steel in anchorage zones For resistance during pile driving

= 0.9 = 1.0 = 0.9 = 0.7 = 0.7 = 0.8 = 1.0 = 1.0

5. Superstructure Types An efficient design of bridge superstructure is essential to achieve overall economy in the whole bridge structure in that the superstructure dead weight may form a significant portion of the gravity load the bridge must sustain and transmit to the foundation. A light superstructure is economical not _only material requirements of the superstructure but also requires smaller size for substructure and foundations. A clear understanding of the structural behavior of structural behavior under loads is essential for efficient design. A bridge superstructure is an integrated body of various members of reinforced concrete, prestressed concrete, steel, composite, diaphragms, trusses, arches, etc. Determination of' forces in theses components is essential for design purposes. The following types of bridges are discussed as follows. RC Bridge, Steel Bridge, Arch Bridge, Cable Stayed Bridge and Suspension Bridge Fundamental of Bridge Design

29

Reinforced concrete bridges: Steel and concrete are construction materials. Reinforced concrete bridges possess several advantages over steel bridges. -

adaptability of concrete wide variety of structural shapes

-

Low maintenance cost Long life and better resistance to temporary overloads and dynamic loads than steel bridges. Cast-in-place Reinforced concrete structures are continuous and monolithic, attributes, which translate into easy construction, low cost and good seismic resistance. They can also be given the desired aesthetic appearance.

-

The disadvantage -

large dead weight difficulty to widen longer construction time requires formwork and false work

Bridge live loads occupy partial area of the decks unlike live loads in buildings which is taken uniformly distributed all over the floor area. Live load on bridges can occupy random positions both longitudinally and transversely, and this affects the live load shared by various beams. This aspect of live load distribution is one of the primary concerns in the analysis of bridge decks. Influence lines will be used to determine load position for maximum effect and the magnitude of these effects. The following RC bridges will be discussed. Slab bridge T-girder bridge Box girder bridge Continuous RC bridge RC rigid frame bridges Slab Bridge: Slab Bridge may be in the form


30

Uniform thickness

Voided slab Slab Bridge normally requires more concrete and reinforced steel than Girder Bridge of the same span but the formwork is simpler and less expressive, hence they are economical when these cast factor balance favorably. The structural configuration of bridge is shown below.

Post & raining Curb

Notch Slab Edge beam Typical Cross-section of Slab Bridge Slab bridges are most commonly used to span short spans up to 12 meters. The load carrying mechanism is by plate action, i.e., by bending and twisting due to continuity in all directions. Application of a load on the portion make the slab deflect into a dish shape locally, causing a two-dimensional system of bending and twisting moments, the mechanism through which the load is transferred to the adjacent elements of the deck, which are less severely loaded.


31

Where: Z is the downward deflection of the plate q is intensity of uniform load D is flexural rigidity of tile plate In the absence of closed form solution to the above equation, approximate methods are developed. One method is the method of influence surfaces, which uses design charts. These charts have been prepared by Pucher or slabs of various shapes and support conditions (1964), and by Rusch and Heregnroder (1961) and Dalas and Hanuska (1964) for simply supported skew slabs. Grillage method is also used for analysis and softwares are available for this. Another method that AASHTO recommends is the Strip Method. Load distributions: The equivalent width of longitudinal strips per lane for both shear and moment with one lane, i.e., two lines of wheels, loaded shall be determined as: E = 250 + 0.42 L1W1 The equivalent width, E of longitudinal strips per lane for both shear and moment with more than one lane loaded shall be determined as: W NL equivalent width (mm) modified span length taken ≤ of the actual span or 18,000 (mm) modified edge-to-edge width of bridge taken to be ≤ of the actual width or 18,000 mm for multilane loading, or 9,000 mm for single-lane loading (mm) physical edge-to-edge width of bridge (mm) number of design lanes as specified

E = 2100 + 0.12 L1W1 ≤

Where: E = L1 = W1 = W = NL =

Where decks span primarily in the direction of traffic, the effective width of a strip, with or without an edge beam, shall be taken as the sum of the distance between the edge of the deck and the inside face of the barrier, plus 300 mm, plus one-half of the strip width. The effective width shall not exceed either the full strip width or 1800 mm. T-girder Bridge: T- Girders are used for bridges spanning from about 10meters-25 meters. These usually consist of equal1y spaced beams (generally with spacing of 1.8-3.6m) spanning longitudinally between Fundamental of Bridge Design

32

supports. The slab is structural1y continuous across the top. The slab serves dual purpose of supporting the live load on the bridge and acting as the top flange of the longitudinal beams. Diaphragms are provided transversely between the beams over the supports and depending on the span, at midspan and other intermediate locations. The purpose of providing diaphragms is to ensure lateral distribution of live loads to various adjacent stringers, the magnitude of the share of each stringer depends on the stiffness of the diaphragms relative to the stringers and on the method of connectivity. Design of T- girder bridges consists of deck slab analysis and design, and the T-girder analysis and design. Structural analysis of the deck slab involves taking a continuous strip perpendicular to the girders (AASHTO Art.9.6.1) and analyzing by moment distribution or using design aid given by AASHTO, ILS for critical position of loads. Extreme positive moment at any point will be taken to apply to all positive moment regions (Art 4.6.2.1.1). The width of equivalent interior transverse strip over which the wheel loads can be considered distributed longitudinally in cast in - place concrete decks is given as [Table Art. 4.6.2. I .3-1] • • •

overhang, 11401-0.83lX positive moment, 660+0.55S negative moment, 1220+0.25S

Where X is the distance from the wheel load to centerline of support and S is the spacing of' the T-beams. In the design of overhang deck slab design forces acting on the post and railings or barrier should be considered. Load Distribution Factors for the Girders:

For moment: -

Interior girders: The live load flexural moment for interior beams with concrete decks shall be determined by applying the lane fraction specified in Table below Type of Beams

Concrete Deck on Wood Beams

Applicable Cross-section from Figure 13-2 l


Distribution Factors

One Design Lane Loaded: S/3700 Two or More Design Lanes Loaded: S/3000

Range of Applicability

S ≤ 1800

33

Type of Beams

Concrete Deck, Filled Grid, or Partially Filled Grid on Steel or Concrete Beams; Concrete T-Beams, T-and Double TSections

Applicable Cross-section from Figure 13-2

Distribution Factors

a, e, k and also One Design Lane Loaded: i, j if sufficiently 0.06 + S 0.4 S 0.3 Kg 0.1 connected to 4300 L Lts3 act as a unit Two or More Design Lanes Loaded: 0.075 +

Multicell Concrete Box Beam


S 4300

0.6

S L

0.2

Kg Lts3

0.1

Use lesser of the values obtained from the equation above with Nb = 3 or the lever rule One Design Lane Loaded:

d

1.75 + S 300 0.35 1 0.45 1100 L Nc Two or More Design Lanes Loaded: 13 Nc Steel Grids on Steel Beams

a

0.3

S 430

1 L

1100≤ S≤4900 110 ≤ ts ≤ 300 6000≤L≤73000 Nb ≥ 4

0.25

One Design Lane Loaded: S/2300 If tg < 100 mm S/3050 If tg ≥ 100 mm Two or More Design Lanes Loaded: S/2400 If tg < 100 mm S/3050 If tg ≥ 100 mm

Nb = 3

2100≤S≤4000 18 000 ≤ L ≤ 73000 Nc ≥ 3 If Nc>8 Nc=8

use

S ≤ 1800 mm

S ≤ 3200 mm

Table Distribution of Live Load per Lane for Moment in Interior Beam -

Exterior girders: The live load flexural moment for exterior beams shall be determined by applying the lane fraction, g, specified in Table below Type of Superstructure

Wood Deck on Wood or Steel Beam Concrete Deck on Wood Beams Concrete Deck, filled Grid, or Partially Filled Grid on Steel or Concrete Beams:

Applicable Crosssection from Figure 13-2 a, l

One Design Lane Loaded Lever Rule

Two or More Design Lanes Loaded Lever Rule

L

Lever Rule

Lever Rule

N/A

a, e, k and also i, j if sufficiently connected to act as

Lever Rule

g = e ginterior

-300 ≤ de ≤ 1700


Range of Applicability N/A

e = 0.77 + de 2800

34

Concrete T-Beams. T and Double T Sections

a unit

Use lesser of the values obtained from the equation above with Nb = 3 or the lever rule

Nb = 3

Table. Distribution of Live Loads per Lane for Moment in Exterior Longitudinal Beams

For shear: -

Interior girders: The live load shear for interior beams shall be determined by applying the lane fractions specified in Table Type of Superstructure

Concrete Deck on Wood Beams Concrete Deck, Filled Grid, or Partially Filled Grid on Steel or Concrete Beams: Concrete T-Beams. T and Double T Sections Multi-cell Concrete Box Beams, Box Sections

Applicable Cross-section from Figure 13-2 l a, e, k and also i, j if sufficiently connected to act as a unit

One Design Lane Loaded

Two or More Design Lanes Loaded

Lever Rule

Lever Rule

0.36 +

0.2 + S - S 2.0 3600 10700

S 7600

Lever Rule

Lever Rule

d S 2900

0.6

d L

0.1

S 0.9 d 2200 L

0.1


N/A 1100 ≤ S ≤4900 6000 ≤ L ≤ 73000 110 ≤ ts ≤ 300 4x109 ≤ kg ≤ 3x1012 Nb ≥ 4 Nb = 3 1800 ≤ S ≤ 4900 6000 ≤ L ≤ 73000 890 ≤ d ≤ 2800 Nc ≥ 3

Table -Distribution of Live Load per Lane for Shear in Interior Beams -

Exterior girders: The live load shear for exterior beams shall be determined by applying the lane fractions specified in Table Type of Superstructure



Wood Deck on Wood or steel Beams Concrete Deck, Filled Grid, or Partially Filled Grid on Steel or Concrete Beams; Concrete T-Beams, Tand Double T-Beams Multi-cell Concrete Box Beams, Box Sections

a, l

Lever Rule

N/A

a, e, k and also i, j if sufficiently connected to act as a unit

Lever Rule

g = e ginterior

-300 ≤ de ≤ 1700

θ = 0.6 + de . 3000 Lever Rule

Nb = 3

d

Lever Rule

g = e ginterior

-600 ≤ de ≤ 1500


Two or More Design Lanes Loaded


θ = 0.64 + de . 3800

35

Type of Superstructure


Steel Grid Deck on Steel Beams

a


Two or More Design Lanes Loaded Lever Rule


N/A

Table Distribution of Live Load Per Lane for Shear in Exterior Beams Where: S = spacing between girders (mm) L = Length of Girder (mm) t s = thickness of slab (mm) The lever rule involves summing moments about one support to find the reaction at another support by assuming that the supported component is hinged at interior supports. When using the lever rule on a three-girder bridge, the notional model should be taken as shown in Figure 13-1. Moments should be taken about the assumed, or notional, hinge in the deck over the middle girder to find the reaction on the exterior girder.

Figure 13-1 Notional Model for Applying Lever Rule to Three-Girder Bridges Multiple presence factors shall not be used with the approximate load assignment methods other than statical moment or lever arm methods because these factors are already incorporated in the distribution factors. Box Girder Bridge: Concrete box girder bridges are economical for spans of above 25 to 45m. They can be reinforced concrete or prestressed concrete. Longer span than 45m will have to be prestressed. They are similar to T-beams in configuration except the webs of T-beams are all interconnected by a common flange resulting in a cellular superstructure. The top slab, webs and bottom slab are built monolithically to act as a unit, which means that full shear transfer must be provided between all parts of the section. Reinforced concrete box girders have high torsional resistance due to their closed shape and are particularly suitable for structures with significant curvature. This construction also lends itself to aesthetic treatment. Concrete box girder bridges have several advantages over other types; Fundamental of Bridge Design

36

1. The relatively shallow depth of box girders is all advantage where headroom is limited like in urban overpasses. 2. Monolithic construction of the superstructure and substructure offers structural as well as aesthetic advantage. The pier caps for continuous box girders can be placed with in the box, facilitating rigid connection to the pier. 3. They provide space for utilities such as water and gas lines, power, telephone and cable ducts, storm drains and sewers, which can be placed in the hollow cellular section. Typical cross section is shown in Fig. below. While the interior webs are all vertical the exterior webs may be vertical, inclined or curved. When the exterior webs are inclined their slope should preferably be IH: 2V. Design Consideration: The structural behavior of box girders is similar to T-beams. Box girders are essentially T-beams with transverse bottom flange resulting in a closed, torsionally stiff multi-cell configuration. The interior webs resist shear and often only a small portion of girder moments. Consequently they are usually thinner than the webs of T-beams. This is so because, in the case of continuous T-beams, the webs must resist the negative girder moments as well as all the shear, and contain all the reinforcement for positive moments. The bottom slab (soffit) contains reinforcement for the positive moment and also acts as a compression flange in the negative moment regions of continuous spans. The bottom slab also affords a superstructure considerably thinner than a T- beam bridge of the same span and permits even longer spans to be built. Cross-sections are taken as shown in Fig. below a and b for analysis for exterior and interior girders respectively. The structural analysis is same as for T-beams. Section analysis is also same except the compression bottom flange for continuous spans. The entire slab width is assumed effective for compression.

a. Exterior girder

b. Interior girder

Continuous RC bridge: Advantages Fundamental of Bridge Design

37

-

Less number of bearings than simply supported bridge since one line of bearings is used over the piers Reduced width of pier, thus less flow obstruction and less amount of material Requires less number of expansion joints due to which both the initial cost and maintenance cost become less. The rigidity quality over the bridge is thus ~proved. Lesser depth of girder, hence economical supports. Better architectura1 appearance Lesser vibration and deflection

Disadvantages -

Analysis is laborious and time consuming Not suitable on yielding foundations

RC rigid frame bridges: In rigid frame bridges, the deck is rigidly connected to the bridge and piers. All the advantages of a continuous span bridge are present here. Additional advantages of rigid frame bridges over continuous ones are - More rigidity of the structure - Less moments in deck being partly transferred to the supporting members - No bearings are required - Better aesthetic appearance than the continuous span structure As in continuous span bridges, these structures also require unyielding foundation materials. The analysis is however, more laborious than the former. Steel Bridge: Steel bridge construction consists of rolled steel beams, plate girders or trusses with reinforced concrete deck or steel plate deck-beam bridges. -

Steel has got several advantages.

-

It is a high quality, homogeneous, isotropic material that is perfectly elastic to its yield point. It has high tensile and compressive strengths. Past the yield point it offers considerable ductility to provide a large reserve of strength. Steel bridges can be built faster than reinforced concrete or prestressed concrete bridge.

-


38

-

-

They can be erected with ease and this minimizing construction costs. Steel superstructures are usually lighter than concrete superstructures which translate into reduced substructures costs, which can be significant when soil conditions are poor. Steel superstructures can be designed with shallower depth than RC, which is an important consideration when overhead clearance is required. Steel bridges are easy and faster to repair than RC.

Steel bridges have some major disadvantages that make then much less favorable than RC or PSC bridges -

Corrosion of steel is the major drawback which requires prohibitively high maintenance cost. Corrosion can reduce cross section of structural members and weaken the superstructure also.

Some steel bridge types: -

Rolled steel beam bridge Plate girder and steel box girder bridges Steel truss bridges Plate Deck-Stringer Bridges

Arch Bridge: Arches are generally characterized by the development of inclined rather than vertical reactions under vertical loads. Cross-sections are designed for thrust, moment and shear, with magnitudes depending on the location of the pressure line as shown in Figure below.


39

If the pressure line coincides with the axis of structure, (as in a uniformly loaded parabolic arch), all cross sections will be subjected to compression, with no moment or shear. If the pressure line falls with in the kern, there will not be tension. But if shape of structure and pressure line differs moment may become dominant. Figure below shows parts of arch bridges.

Compared to the girder bridges, arch bridges are economical because the dead load moments in arch bridges are almost absent when the arch is properly design. The loads on the arch are carried by the arch ribs mainly through direct axial thrusts, the bending moment and shear forces being small compared to Girder Bridge which requires larger section. This is due to the hogging moment which balances the sagging moment created by the horizontal force, H, at the support. The main parameter of an arch is the rise to span ratio, r/l (1/6 to 1/10). From economic point of view it is attempted to coincide the center of pressure of a given load with center of line of the arch. Cable Stayed Bridge: Cable stayed bridges are ideal for spanning natural barriers of wide rivers, deep valleys and for vehicular and pedestrian bridges crossing wide interstate highways because they can provide long spans unobstructed by piers. Span arrangement types: -

Two span (symmetrical or asymmetric) Three spans Multi Span


40

The arrangement of the cable stays is one of the fundamental items in the design of cable-stayed bridges. It influences, in fact not only the structural performance of the bridge but also the method of erection and economies. Longitudinal cable arrangement: -

Radiating (converging)

-

Harp

-

Fan /Modified fan/

Suspension Bridge: The twin main cables from the tower of a suspension bridge form a catenary from which the hangers are suspended and fixed to the deck. Sag ratio for cables should be L/9-L/13 Suspension bridges are economical when the span exceeds 300m. Suspension bridges consist of one main span and two side spans. L1/L =0.17100.50 Fundamental of Bridge Design

41

The cables being very flexible do not take any bending moment and arc subjected only to tensile forces. The stiffening truss stiffens the deck and distributes the live load of the deck on to the cables. Otherwise the cables would be subjected to local sag due to action of concentrated live load and thus causes local angle change in the deck system. The stiffening trusses arc hinged at the towers and suspended at node points from suspenders, which are usually high tensile cables. Vertical suspenders have been used in many bridges but diagonal suspenders have the advantage of increasing the aerodynamic stability Simple Suspension Bridge

Suspension Bridge with Stiffening Truss

Suspension Bridge with Braced Chain


42


43

6. SUBSTRUCTURE Piers Piers provide vertical supports for spans at intermediate points and perform two main functions: • •

transferring superstructure vertical loads to the foundations resisting horizontal forces acting on the bridge

Although piers are traditionally designed to resist vertical loads, it is becoming more and more common to design piers to resist high lateral loads caused by seismic events. Generally piers are subjected to: • Dead loads • Live loads and impact from the superstructure • Wind loads on the structure and the live loads • Centrifugal force from the superstructure • Longitudinal force from live loads (vehicular braking force) • Drag forces due to the friction at bearings • Earth pressure • Stream flow pressure • Ice pressure • Earthquake forces • Thermal and shrinkage forces • Ship impact forces • Force due to prestressing of the superstructure • Forces due to settlement of foundations

Pier Types Solid Wall Pier: A solid wall pier (also known as a continuous wall pier) as its name would imply, consists of a solid wall which extends up from a foundation consisting of a footing or piles. The top of the wall is equipped with individual pedestals upon which the superstructure rests. Their slender and streamlined proportions provide a minimal resistance to flood flows. Using solid wall piers in stream or river crossings is advantageous since they are not as prone to accumulating debris as multiple column piers.

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Figure: Solid wall pier Hammerhead Pier: A hammerhead pier utilizes one or more columns with a pier cap in the shape of a hammer. They are predominately found in urban settings because they are both attractive and occupy a minimum of space, thereby providing room for underpass traffic.

Figure: Hammerhead pier Column Bent Pier: A column bent pier, as its name would imply, consists of a cap beam and supporting columns in a frame-type structure. In dense urban interchanges, use of column bent piers can lead to a cluttered image producing a “concrete jungle” effect.


44

Figure: column bent pier Pile Bent Pier: The pile bent pier is a variation on the column bent pier with the supporting columns and footing replaced with individual supporting piles. The end piles are generally equipped with a batter in the transverse direction.

Figure: Pile bent pier Finally selection of pier is made based on the following considerations. •

Aesthetics

•

Economy

•

Flow of water and traffic under the bridge

•

Superstructure

Design Criteria: In general, the design of a highway bridge pier should address: Safety against overturning Safety against sliding Safety against bearing failure of the soil Fundamentals of Bridge Design

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Safety against excessive or differential settlement

Backwall and Wingwall

Figure: Backwall and Wingwall

Wingwall A wingwall is a side wall to the abutment back wall or stem designed to assist in confining earth behind the abutment backwall.

Backwall A backwall, sometimes called the stem, is the primary component of the abutment acting as a retaining structure at each approach.

Abutments Abutment is component of a bridge • provides the vertical support to the bridge superstructure at the bridge ends • connects the bridge with the approach roadway • retains the roadway base materials From the view of the relation between the bridge abutment and roadway or water flow that the bridge overpasses, bridge abutments can be divided into two categories: open-end abutment and closed-end abutment. For the open-end abutment, there are slopes between the bridge abutment face and the edge of the roadway or river canal that the bridge overpasses. Those slopes provide a wide open area for the traffic flows or water flows under the bridge. It imposes much less impact on the environment and the traffic flows under the bridge than a closed-end abutment. Also, future widening of the roadway or water flow canal under the bridge by adjusting the slope ratios is easier. However, the existence of slopes usually requires longer bridge spans and some extra earthwork.


46

Figure: open-end abutment The closed-end abutment is usually constructed close to the edge of the roadways or water canals. Because of the vertical clearance requirements high abutment walls must be constructed. The high abutment walls and larger backfill volume often result in higher abutment construction costs and more settlement of road approaches than for the open-end abutment.

Figure: Closed-end abutment Generally, the open-end abutments are more economical, adaptable, and attractive than the closedend abutments. However, bridges with closed-end abutments have been widely constructed in urban areas and for rail transportation systems because of the right-of-way restriction and the large scale of the live load for trains, which usually results in shorter bridge spans. Fundamentals of Bridge Design

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Based on the connections between the abutment stem and the bridge superstructure, the abutments also can be grouped in two categories: the monolithic abutment and the seat type abutment. The monolithic abutment is monolithically constructed with the bridge superstructure. There is no relative displacement allowed between the bridge superstructure and abutment. The seat-type abutment is constructed separately from the bridge superstructure. The bridge superstructure seats on the abutment stem through bearing.

Figure:

a. Monolithic abutment

b. Seat-type abutment

Abutments can be classified based on the load resisting mechanism as: 1. Gravity abutments: resist the load acting with its dead weight and dead weight of retained soil or backfill on its inclined back face. They can be constructed using stone masonry or concrete masonry. 2. Cantilever abutments: load resistance derived from cantilever action and usually constructed from reinforced concrete. 3. Counterfort abutments: are similar to cantilever ones but ties called counterforts are provided to tie the stem to the footing. 4. Reinforced earth abutments: The essential concept is the use of multiple-layer strips or fibers to reinforce the fill material in the lateral direction so that the integrated fill material will act as a gravity retaining structure. Overturning and sliding are needed to be checked under the assumption that the reinforced soil body acts as a gravity retaining wall.

Figure:

(4)


(1)

(2)

(3) 48

Design of Backwall Abutments Loads: All loads acting on piers act on abutments except forces that result from buoyancy, stream flow and collision of barge. Impact will not also be considered for the design of abutments. Earth pressure is the major force acting on abutments in addition to those forces mentioned. In seismic areas, seismic forces should be considered for the back face of the wall due to inertia of the retained soil mass. Mononobe-Okabe method is used for seismic analysis. In this analysis for free standing abutment The horizontal inertia force FH=Kh*Ws The vertical inertia force FV=Kv*Ws Where Kh=αo and Kv is in between 0.3*Kh and 0.5*Kh αo is bedrock acceleration, in Ethiopia case refer to EBCS 8,1995. If the abutment is restrained from horizontal displacement, i.e. abutment supported on batter piles, Kh=1.5 αo. In case of seismic active pressure force, the load is applied at a larger distance from static active pressure case i.e. at 0.6*H.

Design: Length and shape of abutment wingwalls are usually dictated by terrain, height of bridge and level of HWM. Length of abutment backwall is governed by width of the superstructure. Design of abutments is started by providing preliminary dimension then checked for different design criteria. Fundamentals of Bridge Design

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Masonry abutments shall be checked for the followings. •

The location of the resultant on the base is determined by balancing moments about the toe of the wall. Location of resultant X from the toe is:

B/3≤x≤2B/3

base on soil

B/4≤x≤3B/4

base on rock

•

Check for sliding

Фs*Fru≥∑γi*Phi Where: Фs=performance factor [used to account for uncertainties in structural properties, soil properties, variability in workmanship, and inaccuracies in the design equations to estimate the capacity.] Fru=Nu*tanδb+Ca*Be Nu=factored vertical resultant (force/length) δb=friction angle between base and soils (degrees) Ca= adhesion (force/length2) Be=effective length of base in compression (length) γi= load factor force component Phi=horizontal earth pressure force causing sliding (force/length) •

Check for bearing σmax ≤ σall

Scour Scour is the washing away of streambed material by water channel flow. Typically, scour occurs when the water channel becomes narrowed or constricted. From basic fluid dynamics, we know that a smaller opening leads to a greater velocity (Q = Av). If constrictions in the channel are significant, the velocity of the stream can increase such that the substructure footings and/or piles become undermined. Sharp bends or curves in the water channel also increase velocity which can create cross-currents and turbulence. Scour can either occur locally at specific substructure components (local scour) or over the entire structure crossing the channel (general scour). The removal of material from under a pier’s foundation, often associated with scour, is known as undermining or undercutting. Fundamentals of Bridge Design

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Figure: An exposed pier footing shows evidence of undermining.

Scour generally takes place because one or a combination of the following conditions occurs: Natural conditions in the channel A constriction in the channel The bridge itself being a constriction When scour occurs at a specific localized point in the channel, such as a pier, abutment, drainage structure, or some other obstruction, it is known as local scour. Local scour is evidenced by turbulence around piers which erodes material from under the foundation. If scour takes place over a large area of the channel it is known as general scour. General scour occurs over a long period of time and is initiated by an alteration in channel flow patterns. Scour which results from a reduction in the cross-sectional area of a channel due to the placement of an obstruction such as a highway bridge or drainage structure is known as constriction scour. The total potential scour at a highway crossing usually comprises the following components: •

Aggradation and degradation:

When natural or human activities cause streambed elevation changes over a long period of time, aggradation or degradation occurs. Aggradation involves the deposition of material eroded from the channel or watershed upstream of the bridge, whereas degradation involves the lowering or scouring of the streambed due to a deficit in sediment supply from upstream. • stream contraction scour: Contraction scour usually occurs when the flow area of a stream at flood stage is reduced, either by a natural contraction or contraction of a bridge. The decreased flow area causes an increase in average Fundamentals of Bridge Design

51

velocity in the stream and bed shear stress through the contraction reach. This in turn triggers an increase in erosive forces in the contraction. Hence, more bed material is removed from the contracted reach than is transported into the reach. • local scour: When upstream flow is obstructed by obstruction such as piers, abutments, and embankments, flow vortices are formed at their base as shown in Figure below (known as horseshoe vortex). This vortex action removes bed material from around the base of the obstruction. A scour hole eventually develops around the base.

Figure: Schematic representation of local scour at a cylindrical pier • lateral stream migration: Streams are dynamic. The lateral migration of the main channel within a floodplain may increase pier scour, embankment or approach road erosion, or change the total scour depth by altering the flow angle of attack at piers. Since scour problem is dangerous to a bridge, proper consideration should be made in design of substructure. If the scour depth exceeds the foundation depth there is possibility of bridge failure. Therefore, to avoid bridge failures due to erosion, the depth of the pier should be more than the scour depth. A general rate is to take depth of erosion as four times the height which the water level raises at the obstruction section. So the foundation of the substructure should be located at least the depth of erosion below the stream bed.

Scour Protection: Scour protection is required when some restriction is made to the flow of the flood.

Riprap: It is carpet of loose stones, which protects the loose bed material for bed protection and loose bank or embankment material from erosion by water current. The stone elements must be heavy enough not to be carried away by water.

Gabion and Reno mattresses: The standard gabion is a rectangular basket made of steel wire mesh with sizes of 2m, 3m and 4m long by 1m wide by 0.5m or 1m high. Reno mattress is a mattress - shaped version of the gabion for Fundamentals of Bridge Design

52

use when the gabions height and bulk are not needed as in lining river beds and banks. Their standard size is 6m length, divided in to 6 compartments of 1m width.

Filter Blankets: If the river bed and bank material are non - cohesive and fine, they may be washed away through the spaces between the voids in riprap or gabion lining. This can be prevented by using textile filter or using gravel filter.

Vegetation: Many plant types can be used to protect river banks against erosion. The most successful ones are almost always those found growing naturally along the river being bridged.


53

7. BEARINGS AND RAILINGS BEARINGS Bearings are structural devices positioned between the bridge superstructure and the substructure. Their principal functions are as follows: 1. To transmit loads from the superstructure to the substructure, and 2. To accommodate relative movements between the superstructure and the substructure. Movements should be accommodated properly and these result from a) longitudinal movement due to temperature changes-reversible b) rotation due to deflection of girders- live load deflection is reversible c) vertical movement due to settlement of support- irreversible d) movement due to shrinkage, creep or prestressing-irreversible

Bearings are categorized in to two as a) Fixed bearings – are fixed against translation, but free to rotate b) Expansion bearings-are free to translate and rotate

Selection of the type depends on the type of superstructure, type of substructure supports and the length of the span.

-

A simply supported span requires fixed bearings at one and expansion joints at the other.

-

A continuous girder require fixed bearing at one and expansion bearing on all other supports

-

A two span girder will have fixed bearing at the centre support and expansion bearings at the two abutments

Bearings are important components of bridges and call for care in design, skill in execution, regular attention in maintenance. Faulty design or malfunctioning bearings due

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54

to clogging by debris have often resulted in collapse of bridges due to the high stresses induced due to restrained motion. On certain major bridges bearing cost is as high as 10-15% of the total cost. It is worth designing carefully and inspecting regularly for maintenance. Types of Bearings For slab bridges no special bearings are provided because the translational movement is very small. Edges of supports are beveled to allow for rotation. The following are the different types of bearings for girder bridges -

Sliding plate bearing- is the simplest type of bearing. This utilizes one plate sliding on another. This type is used for bridges up to 30m span. Flat plates were common but due to freezing and high frictional forces the surface of the upper plate is slightly curved as shown in Figure below.

Figure: Sliding plate bearing Fundamentals of Bridges Design

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-

Roller bearing- in this type rotation is accommodated by the pin and translation by the use of a roller or a group of rollers. See Figure below. This is used for spans of moderate length.

Figure: Roller bearing

-

Rocker bearing- this is used for longer spans and is used to accommodate large live load deflections. See Figure below.

Figure: Rocker bearing

In seismic areas suitable guides should be provided to prevent the roller and rocker components from being displaced off abutments and piers during earthquakes.

-

Elastomeric bearing- is made of a polymeric substance called elastomer (either natural or synthetic rubber).


56

This can be reinforced with steel plates, which are usually the case, or may be unreinforced. This is very strong in compression and relatively week in shear, thus allowing horizontal displacement. This has shown good weathering performance and so incurs low maintenance cost. This type is favored for bearings. This is used for medium to large spans. See Figure below.

Figure: Elastomeric bearing -

Lead Rubber bearing-is an elastomeric bearing equipped with lead core and is used in highly seismic areas. See Figure below. Under normal loading conditions, the lead core resists lateral loads. Under seismically induced dynamic loads, however, the lead core yields and assists the elastomeric material in resisting loads while at the same time accommodating lateral flexibility.


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RAILINGS Railings are provided along edges of bridges for protection of traffic and pedestrian. There are three types of railings. 1. Traffic railings 2. Pedestrian railings 3. Combination railings

The following guideline indicate the application of various types of rails

-

Traffic railings is used when a bridge is for the execution use of highway traffic

-

A combination railing in conjunction with raised curb and side walk is used on low-speed highways.

-

On high speed highways the pedestrian path should have both an out board pedestrian railing and an inboard combination railing

Aesthetics and freedom of view from passing vehicles should also be taken in to consideration.

Traffic Railing The primary purpose of traffic railings is to contain and redirect errant vehicles. Specifically they serve the purposes of

-

primarily containing the average vehicle

-

protecting the occupants of a vehicle in collision with the railing

-

protecting other vehicles near the collision

-

protecting people and property on roadway and nearby areas underneath

A traffic railing should provide a smooth and continuous face on the traffic side. Steel and concrete posts should set back from the face to the rail.


58

Pedestrian Railing Pedestrian railing can be made from concrete, metal and timber. A pedestrian walkway should be separated from roadway by a barrier curb, traffic railing or combination railing.

Combination Railing Consists of traffic and pedestrian railings


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8. CULVERTS AND LOW LEVEL WATER CROSSINGS CULVERTS Culverts are cross drainage structures used to drain rain run off collected by side ditches from one side of the highway to the other. In any highway or railway project, the majority of cross drainage works fall under this category. Where the waterway opening is less than about 15m2, and particularly where the road crosses the waterway on a relatively high embankment, a culvert will usually be cheaper than a bridge. In addition to providing culverts at the lowest point, it is good practice to install, culverts for cross drainage at regular intervals down a long grade. As a general rule, there should be at least one culvert every 300m, unless the road follows a ridge. Therefore, cost of culverts account for a significant proportion of the project cost. Culverts are usually constructed from: - Precast concrete jointed pipes - Single or multi-cell reinforced concrete boxes, prefabricated or built in situ - corrugated steel pipes made of prefabricated panels Single precast concrete pipe culverts are commonly used for small openings up to 2m2, while multiple concrete pipes with common headwalls cater for larger areas. Alternatively, reinforced concrete box culverts are used with internal box size up to 4m x 3.5m. Twin or multiple boxes may be required for larger waterway openings. In difficult ground conditions a flexible steel pipe has an advantage over a rigid concrete culvert through its ability to accommodate a certain amount of differential settlement over the length of the culvert without overstressing the material. A culvert made of rigid concrete sections will not be tolerant to differential settlement unless it is specifically designed for such conditions either by increasing its structural strength or by segmenting the culvert along its length to allow it to flex. In theory such a solution may appear to be satisfactory, but the provision of watertight joints to permit flexing to occur can be costly and the satisfactory performance of the joints cannot always be guaranteed. Leakage from culverts, particularly those carrying embankments, can have serious consequences, since weakening of the embankment material may lead to an embankment failure. If properly constructed a reinforced concrete culvert is likely to have a service life in excess of 60 years and will almost certainly be more durable and require less maintenance than a steel pipe. By comparison, a corrugated steel pipe culvert, well protected against corrosion by plating and bitumen coating, can be expected to have a working life in the order of 30 to 40 years in a non-aggressive environment. It is usual to design culverts to last the life of the highway. Despite the best efforts of the engineer, it must be expected that some culverts will become silted or obstructed by debris. For this reason pipes of internal diameter less than 0.6m are not recommended since they are difficult to clean. Like bridges, culverts are designed to be large enough to pass the design flood without Fundamentals of Bridge Design

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damage to the embankment or surrounding land. Highway drainage ditches collect general run-off water which is cross drained using culverts. This concentration of flow is almost certain to cause erosion damage to many types of soil. Before or at the entrance to culverts it is important to control the flow velocity, particularly since an abrupt change in direction of flow may occur at these points making erosion damage more likely. Control may be achieved by reducing the gradient, installing check-drains or by building a drop inlet. It is also important to control the velocity of flow at the outlet, because erosion damage to the road embankment or surrounding farmland occurs more frequently at the discharge end. For this reason, where gradients arc steep, the arrangements shown in Figure below are preferred. A curtain wall is often necessary at the outlet of a culvert carrying more than a minimal flow, in order to prevent scour undermining the structure. A concrete or rip rap apron may be required to dissipate the energy of the discharge, or it may be necessary to contain the flow in a paved channel until the natural gradient becomes small enough not to induce fast flows with consequential deepening of the channel.

Figure: Check dams The proper location of each culvert is important because it affects the drainage pattern and hence the strength of the surrounding soils, the area of waterway, ditch and culvert maintenance, and erosion damage. There are four rules concerning location and alignment of culverts - Where a road crosses a valley, the lowest point requires a vent, whether there is an established stream or not. - Where there is an established stream, the culvert should follow the existing alignment, unless the alignment can be improved. - The gradient of the culvert should be the same as the gradient of the stream. - Measures may be necessary to ensure that the watercourse does not move. This could cause severe damage and the consequent change of location of the culvert would be expensive. The gradient of the culvert is important because it affects future maintenance. If it is too steep, it will encourage erosion at the outlet. If it is less steep than the stream, there is likely to be a sediment problem. A gradient of 2 to 4 per cent is advisable where silts are carried in the flow; a minimum of 0.5 per cent is recommended for clear water. It is also important to set the culvert invert at the same level as the natural stream bed. Culverts are frequently set low to avoid humps in the road above. Where the road embankment is not high enough to provide adequate clearance above the pipe, the ditch has to be lowered on both sides of the road or the embankment raised. This may necessitate widening the side Fundamentals of Bridge Design

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slopes and demonstrates the importance of detailing drainage structures early in the design of a road. Where an established stream is met at an angle to the road alignment, it is usually better to follow the line of the stream with a skewed culvert, even though the construction cost will be increased by the greater length. An abrupt change in the direction of flow at the inlet and outlet of a square crossing may be accommodated without problem for most of the year, but when heavy rains cause maximum flow the change of direction can result in severe erosion or wash-out. It may also be necessary to stabilize a shifting channel or even move it to improve the geometry where stream and road interact. The principal objectives here are: -

to avoid a bend at either end of the culvert if possible if a bend is unavoidable, to place it at the outlet

Any change of stream channel must be constructed so that there is no possibility of the stream regaining its original course. HEADWALLS Headwalls retain and protect the embankment at the ends of a culvert and help to counteract the dislocation of jointed pipes due to the spreading forces at the base of large embankments. They also divert debris from falling into the waterway, reduce seepage of water into the embankment and prevent small animals from burrowing alongside the pipe. Where conditions favor erosion, headwalls should incorporate deep curtain walls and side slope protection in the form of gabions or hand-pitched stone. Since large headwalls are expensive to build, it is advisable initially to make the culvert pipe long enough to reach the toe of the embankment. Headwalls are usually made of masonry or concrete. CONCRETE RIPE CULVERTS On most road constructions, sections of concrete pipe are butted together and connected by mortar, and the embankment material is compacted around them. This is cheap and often serves for several years. A far better quality culvert is obtained by surrounding the pipe with concrete. If the sub grade is likely to settle under the weight of the embankment, the concrete surround should be reinforced to take tensile stresses. All soft clay material and rock should be excavated from below the pipe to a depth of at least 200mm and replaced with good granular material, well compacted. FLEXIBLE STEEL CULVERTS These are made from pre-formed corrugated sections bolted together to form a circular or elliptical tube. They can accept a certain amount of longitudinal or radial deformation without sustaining damage and so are better suited to unstable conditions than rigid concrete structures. They are also easier to transport and faster to construct than the equivalent size of pre-cast pipe culvert, but require specialist manufacture. All soft unstable material should be excavated and back filled to foundation level with Fundamentals of Bridge Design

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gravel mixture, crushed stone or other suitable material. In the case of rock foundations, the rock should be excavated to at least 250mm below the foundation level and wide enough to prevent the pipe resting directly on rock at any point. It should then be back filled to provide a cushion for the pipe. Back filling will be made in layers of approximately 200mm, loading each side and layer evenly and fully compacted before placing the next. The inside shape may need to be protected by strutting as the fill is built up on each side and eventually above the pipe. All the fill material should be of good quality, extending to a distance roughly equal to the pipe diameter each side and above, or up to the road base if this distance is less. The degree of protection against corrosion required to extend the working life of a corrugated steel pipe structure is determined on the basis of: - The importance of the structure and its required working life - The corrosive nature of the environment in which it exists. Durability studies indicate that metal loss associated with corrosion usually originates on the interior surface of the structure and progresses towards the exterior or buried face. The conclusion is that protective treatment to the exposed surfaces should generally be superior to that applied to the buried surfaces. All corrugated steel structures should be hot-dipped galvanized and hot coated with bitumen on the inside. Bituminous paving to the invert of the pipe will also help extend the life of the structure. REINFORCED CONCRETE BOX CULVERTS Reinforced concrete box culverts may be constructed either as a monolithic box or as a U section with a simply supported top slab propping the sidewalls. Twin or multiple cell box culverts are used where the horizontal opening is more than about 4m. These too may be constructed with monolithic top slab and walls or with a separately cast lid. Culverts with separately constructed top slabs are simpler to construct because they require less complicated steel reinforcement and, though containing more material, they generally offer a more economical form of construction. Owing to reversal of bending moment in the walls and bottom slab, reinforcement to both faces of these elements is required but the top slab of a single cell culvert requires only bottom reinforcement. For a twin box culvert with a separately cast top slab, the outer walls are again propped apart and the centre wall may be positively located to the top slab by the provision of dowel bars. The top slab of a twin box culvert is designed as a two span continuous beam spanning over the central wall and requires top reinforcement in the region of the central wall. LOW LEVEL WATER CROSSINGS In favorable conditions, low level water crossings can provide economical and relatively simple alternatives to conventional bridges. These are of two basic types: - fords and bed-level causeways - vented causeways and submersible bridges Both types are appropriate for sites where traffic volumes are low or where a reasonably short detour provides access to an all-weather bridge. For most of the year the maximum depth of water over the carriage way should be less than 150mm. The success or service Fundamentals of Bridge Design

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life of the structure will depend considerably on its hydraulic design. Fords and bed-level causeways, like conventional bridges, will be constructed so that they cause little interference with the design flood. Vented causeways and submersible bridges inevitably disrupt river flow and so are liable to sustain damage or indirectly cause scour to the river bed or banks which in turn may affect the road approaches to the crossing. Since fords and bed-level causeways arc overtopped by all water flowing in the river channel there is no reason to raise the road surface above the stream bed. Vented causeways and submersible bridges usually present a dry carriage way for ordinary flows and are overtopped during the design flood. The best location for a low level crossing is similar to that recommended for a conventional bridge, with the exception that a broad, straight stretch of river will provide slower and shallower water as well as easier road approaches. The maximum gradient for motor traffic should be 10 per cent, and for animals 5 per cent. The change of gradient should be gradual to prevent the underside of vehicles touching the road and to preserve long sight distances. Equal cut and fill construction of the approach roads requires less work than the cut and remove spoil, but the placing of the spoil in the river channel may cause scour problems during a flood. On roads where speeds are naturally slow and where the horizontal curve on the approach side is clearly visible, steep approaches may be relieved by a diagonal descent of the river bank. Even where the road is a single track, it is advisable to make the crossing and its approaches two lanes in width, so as to allow traffic to pass any vehicle, which breaks down or fails to mount the gradient. FORDS Fords are the simplest form of river crossing at places where the stream is wide, shallow and slow, the approaches gentle, and the surface firm. The running surface in the stream can be strengthened and made more even using stones brought in and buried just below the surface. Alternatively, if stones are carried in the flow, these may be trapped by barriers made of boulders, gabions or piles. a. Boulders [fig. a] Large stones placed across the river bed at the downstream side of the crossing are claimed to filter the flow of water and retain gravel and sand, which eventually form a more level and even surface for vehicles. b. Gabions A more expensive but durable improvement may be made by replacing the boulders with gabions to trap river gravel or retain imported material, as shown in Fig b. It is important that gabions protrude only 150-300mm above the natural bed level of the river, depending on the nature of the bed material, otherwise they will act as a weir and cause heavy scour downstream of the crossing. As an alternative to conventional gabions, tube baskets can be made from a roll of fencing mesh filled with stone or shingle. During filling, the edges are raised and then bent over at the top to form a tube and tied; finally a wire rope is attached as shown in Fig c. After installation, sand and gravel transported by the stream is trapped behind the baskets and provides a firmer fairly level surface suitable for vehicles. Fundamentals of Bridge Design

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c. Piles Where gabions are unobtainable, timber piles driven into the river bed with or without stone protection make an acceptable cut-off wall. To be fully effective, timber piles should have a length of about 2m, and should be placed at no more than 0.6m centers. If the river is fast a continuous line of piles may be necessary. A curtain wall may be required on the upstream as well as the downstream side if the roadbed is erodible. The top of the piles should be no more than 0.3m above bed level. N.B: All types of ford may require scour protection on the downstream side, as shown in the Figures.


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BED LEVEL CAUSEWAYS Where the type of traffic or the distance to an alternative crossing justifies the expense, a pavement may be laid on the river bed. A paved ford is also called a bed-level causeway, drift, paved dip or Irish bridge. Three common designs are shown in Figure. To protect the pavement from scour damage, curtain walls or aprons are usually required on both the upstream and the downstream side and these must continue up the approaches to the height of the design flood.

Figure. Bed level causeways Fundamentals of Bridge Design

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If the bed is inerodible the causeway need not be provided with curtain walls, but the bed on both the upstream and downstream sides of the crossing should be trimmed flat to reduce turbulence. Fig (a) Shows a section through a basic bed-level causeway suitable for maximum water flows below2m/sec and for light traffic. The crossing shown in Fig (b) requires good concrete technology and may sustain damage to the apron that is difficult to repair. Fig (c) shows a design employing a practical combination of concrete pavement with flexible protection that is more suited to routine repair than the rigid concrete curtain wall. VENTED CAUSEWAYS Vented causeways are designed to pass an ordinary flood with very little water overtopping the carriageway, but may still be inundated and unusable for a few days each year. These structures present a considerable obstacle to the free flow of both normal flow and the design flood, so they must be built massive enough to withstand water pressure and debris impact.

Figure: Vented causeways They must also be provided with adequate scour protection where the bed is erodible, and marker posts.


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The vents are usually concrete or corrugated steel pipes from 0.6 to 1.0m diameter, set in a block of concrete or masonry. Where prefabricated pipes are not available, vaulted masonry tunnels have proved successful. Concrete or masonry retaining walls and aprons are required to channel the flow and prevent scour at both entrance and exit. In order to prevent blockage of the stream by debris or silting, careful attention is required to setting the pipes level with the stream bed and at the same gradient. No part of the vents should be narrower than the entrances and wedge-shaped deflector ramps may be required on the upstream side to; guide large floating debris above the vents. Alternatively, a grill of posts installed upstream of the causeway will collect tree branches before they reach the structure. The capacity of the vents is sufficient to pass all ordinary floods without damage and with no more than 150mm of water over-topping the structure. SUBMERSIBLE BRIDGES Where the traffic is dense enough to justify a dry crossing of a substantial ordinary flood and the design flood is much greater, a submersible bridge is an alternative to a vented causeway. Submersible bridges are able to pass a larger flow than the vents of a causeway of the same height but are more susceptible to damage by the design flood. The overturning moment at the pier foundations becomes very large unless the piers are kept short, and the horizontal and vertical forces on the deck require solid restraint. Because of these difficulties, submersible bridges are not recommended for any foundation other than rock, and even then a vented causeway or conventional bridge may be a more durable alternative. Construction is usually of reinforced concrete with continuous reinforcement between the sub-structures and the deck.


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Bridge Engineering Lecture Note.pdf

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