95765932-Design-Hybrid-Concrete-Buildings.pdf

A

cement and concrete

Design of Hybrid Concrete Buildings A guide to the design of buildings combining in-situ and precast concrete

R. Whittle MA (Cantab) CEng MICE H. Taylor FREng, BSc, PhD, CEng, FICE, FIStructE

industry publication

Acknowledgements The authors would particularly like to thank the following people for their support in the development of this design guide: Tony Jones Ian Feltham

Arup Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged: John Stehle Graham Hardwick Peter Kelly Alex Davie David Appleton Kevin Laney Norman Brown

Laing O’Rourke John Doyle Construction Ltd Bison Concrete Products Ltd Consultant Hanson Concrete Products Strongforce Engineering Plc British Precast Concrete Federation Ltd

Published by The Concrete Centre

Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 +44 (0)1276 606800 Fax: +44 (0)1276 606801 www.concretecentre.com

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0 Price Group P © The Concrete Centre Cement and Concrete Industry Publications (CCIP) are produced through an industry initiative to publish technical guidance in support of concrete design and construction. CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All advice or information from The Concrete Centre is only intended for use in t he UK by those who will evaluate t he significance and limitations of its contents and take responsibility for its use and application. No liability(including that for negligence) for any loss resulting from such advice or i nformation is accepted by The Concrete Centre or their subcontractors, suppliers or advisors. Readers should note that the pub lications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in p ossession of the latest version. Cover photo: Courtesy of Outinord International Ltd. Printed by Information Press Ltd, Eynsham, UK

Acknowledgements The authors would particularly like to thank the following people for their support in the development of this design guide: Tony Jones Ian Feltham

Arup Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged: John Stehle Graham Hardwick Peter Kelly Alex Davie David Appleton Kevin Laney Norman Brown

Laing O’Rourke John Doyle Construction Ltd Bison Concrete Products Ltd Consultant Hanson Concrete Products Strongforce Engineering Plc British Precast Concrete Federation Ltd

Published by The Concrete Centre

Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 +44 (0)1276 606800 Fax: +44 (0)1276 606801 www.concretecentre.com

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0 Price Group P © The Concrete Centre Cement and Concrete Industry Publications (CCIP) are produced through an industry initiative to publish technical guidance in support of concrete design and construction. CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All advice or information from The Concrete Centre is only intended for use in t he UK by those who will evaluate t he significance and limitations of its contents and take responsibility for its use and application. No liability(including that for negligence) for any loss resulting from such advice or i nformation is accepted by The Concrete Centre or their subcontractors, suppliers or advisors. Readers should note that the pub lications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in p ossession of the latest version. Cover photo: Courtesy of Outinord International Ltd. Printed by Information Press Ltd, Eynsham, UK

Design of Hybrid Concrete Buildings

Contents 1.

2.

3.

Introduction Introductio n

5

1.1 Single point of responsibility 1.1 1.2 Design considerations 1.3 Best practice procurement guidance

5 6 6

Overview of hybrid solutions

7

2.1 2.2 2.3 2.4 2.5

7 9 10 12

Type 1: Precast twin wall and lattice girder slab with in-situ concrete Type Typee 2: Precast column with in-situ floor slab Typ Typee 3: Precast column and floor units with cast in-situ beams Typ Typee 4: In-situ columns or walls and beams with precast floor units Typ Typee 5: In-situ column and structural topping with precast beams and Typ floor units 2.6 Typ Typee 6: In-situ columns with lattice girder slabs with optional spherical void formers

14

Overall structural design

15

3.1 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3. 10 3.11 11

15 18 18 19 22 25 26 29 29 29 33

Robustne ss Robustness Stability Diaphragm action Shear at interface of concrete cast at different times Interface shear Shear and torsion design Long-line prestressing system Secondary effects of prestressing and the equivalent load method Temperature Temper ature effects Differential Different ial shrinkag shrinkagee Designing for construction

13

4.

5.

6.

7.

Bearings and movement joints

34

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

34 35 36 36 37 37 38 42

Horizontal forces at bearings Restrained bearings Movement joints Actions and restraints Design considerations Allowance for anchorage of reinforcement at supports Bearings that allow limited movement Connections between precast floors and in-situ concrete beams

Structural elements and connections

43

5.1 5.2 5.3 5.4 5.5 5.6 5.7

43 52 55 58 68 74 82

Twin wall construction (type 1) Precast columns, edge beams and in-situ slabs (type 2) Biaxial voided slabs Prestressed hollowcore units Double tee beams Stairs Corbels, nibs and half joints

Construction issues

87

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9

87 89 89 89 91 91 91 91 92

Method of construction Composite action between precast units and in-situ structural topping Specially shaped standard units Long and short units adjacent to each other Differences of camber in double tees Method of de-tensioning double tee units Checking strand or wire pull-in for hollowcore units Placing hollowcore units into the correct position Production tolerances

Special structures - case studies

93

7.1 Lloyd’s of London 7.2 Bracken House

93 100

References

104

List of worked examples Worked example 1 Worked example 2 Worked example 3 Worked example 4 Worked example 5 Worked example 6 Worked example 7 Worked example 8 Worked example 9

Hollowcore floor acting as a diaphragm Interface shear between hollowcore slab and edge beam Upwards camber on slab due to temperature gradient Differential shrinkage Bearing of a hollowcore unit Vertical tie Anchorage length of longitudinal tie bar Dowel bar for connection of precast stairs Corbel design

20 23 30 31 41 56 65 80 84

Introduction 1

1. Introduction Hybrid construction allows the most appropriate use of different materials and methods of construction to produce a pleasing and effective form of structure. The search for greater economy, in terms of material costs and reduced construction time, has resulted in innovative approaches that seek to combine construction materials and methods to optimum effect. Hybrid concrete construction (HCC) is one such development that combines in-situ and precast concrete to maximise the benefits of both forms of concrete construction. Further guidance on the benefits of HCC is given in Section 2.1. This design guide is aimed at the designer and considers a range of hybrid concepts and the overall structural aspects. It provides design and detailing information for some of the common systems used and structural elements involved. Where applicable the information is in accordance with BS EN 1992-1-1 1, together with the UK National Annex (Eurocode 2 is used to refer to BS EN 1992-1-1 throughout this guide unless noted otherwise). This incorporates a section on the design of members by strut and tie methods, which is particularly useful when considering ‘hybrid’ design details. This guide also considers and refers to the following European Concrete Product Standards for precast concrete elements: 2 Common Rules for Precast Concrete Products  BS EN 13369 3  BS EN 1168 Precast Concrete Products – Hollowcore Slabs 4 Precast Concrete Products – Floor Plates for Floor Systems  BS EN 13747 5 Precast Concrete Products – Ribbed Floor Elements  BS EN 13224 6 Precast Concrete Products – Linear Structural Elements  BS EN 13225 7 Precast Concrete Products – Wall Elements  BS EN 14992 8 Precast Concrete Products – Stairs  BS EN 14843

1.1 Single point of responsibility

The use of precast and in-situ concrete may well lead to the design of the individual elements by designers working for different companies. Therefore, it is essential that there should be a single named designer or engineer who retains overall responsibility for the stability of the structure and the compatibility of the design and details of the parts and components, even where some or all of the design, including details, of those parts and components are not carried out by this engineer. This is particularly important for the design of hybrid structures where misunderstandings as to who is responsible have occurred. It is the responsibility of the designer, before incorporating any proprietary system as part of the structure, to ensure that the assumptions made in the design and construction of such are compatible with the design of the whole structure. This should include:  an adequate specification for that part.  ensuring that any standard product designed and detailed by the precast manufacturer, is suitable for that particular structure.  the design of any such part is reviewed by the designer to ensure that it satisfies the design intent and is compatible with the rest of the structure.

5

1 Introduction

1.2 Design considerations

The design of each component should include consideration of:  its performance in the permanent condition  the construction method and loading  any temporary supports required during construction. The design should be carried out following the requirement of Eurocode 2, Cl. 1.3, which assumes:  Structures are designed by appropriately qualified and experienced personnel.  Adequate supervision and quality control is provided in factories, in plants and on site.  Construction is carried out by personnel having the appropriate skill and experience.  The construction materials and products are used as specified in Eurocode 2 or in the relevant material or product specifications.  The structure will be adequately maintained.  The structure will be used in accordance with the design brief. 9  The requirements for execution and workmanship given in EN 13670 are complied with. The design assumptions should generally include the following construction related information:  sequence of construction  exposure requirements  pour sizes assumed (if appropriate)  concrete strength at time of striking formwork and back-propping requirements  breakdown of loading including allowance for construction loads  loading history assumed. It should be noted that some of the advice given in this design guide is a result of failures that have occurred on completed structures.

1.3 Best practice procurement guidance

Best Practice Guidance for Hybrid Concrete Construction 10 looks at the procurement

process from concept stages through to design and construction, suggesting processes that allow the capture of best practice. It is supported by a number of case studies. The guidance explains the benefits that result from:  early involvement of specialist contractors  using a lead frame contractor  using best value philosophy  holding planned workshops  measuring performance  trust  close cooperation – with an emphasis on partnering. It is recommended that this guidance document is used to maximise the advantages of using HCC.

6

Overview of hybrid solutions 2

2. Overview of hybrid solutions This section considers a range of possible hybrid concrete construction (HCC). The ideal combination of precast and in-situ is influenced by the project requirements. There is a wide range of possible options, a selection of which is presented here as representative of current UK practice. This is not intended to be exhaustive, but to reflect the spectrum of possibilities. The planning and detailed design of hybrid structural systems will almost always require the involvement of specialist precast concrete manufacturers. These manufacturers are willing and able to assist early in the design process to produce an efficient design. There are advantages to using both precast and in-situ concrete summarised in Table 2.1; more detailed discussion on the benefits of concrete can be found in other publications11, 12, 13. The key to maximising the benefits of HCC is to use the most appropriate technique for each element to produce an economic structure. Table 2.1 Benefits of concrete.

Precast concrete

Precast or in-situ concrete

In-situ concrete

Economic for repetitive elements

Inherent fire resistance

Economic for bespoke areas

Long clear spans

Durability

Continuity

Speed of erection

Sustainability

Inherent robustness

Buildability

Acoustic performance

Flexibility

High-quality finishes Consistent colour

Thermal mass that can be utilised for fabric energy storage Prestressing

Services coordination later in programme Locally sourced materials

Accuracy

Mouldability

Short lead-in times

Reduced propping on site

Low vibration characteristics

Reduced skilled labour on site

Six of the most regularly used HCC options are shown in Figure 2.1 and are described in more detail in the remainder of this chapter. They will be referred to by type number throughout this guide where the detailed design of the various elements is discussed. Suggested span limits are given for each type of construction. Further guidance for initial sizing can be found in Economic Concrete Frame Elements 14.

2.1 Type 1: Precast twin wall and lattice girder slab with in-situ concrete

Hybrid concrete wall panels are increasingly being proposed on projects throughout the UK and are often known as ‘twin wall’. They comprise two skins of precast concrete connected by steel lattices, which are filled with concrete on site, see Figure 2.2. The external skins of the twin wall system are factory made, typically using steel moulds. This results in a higher quality finish than a typical in-situ wall. The panel surfac e quality is suitable to receive a plaster finish or wallpaper. The panel surface is not normally ‘architectural’ concrete and the colour may not be consistent or easy to specify. Joints are cast using in-situ concrete and either have to be expressed as a feature or concealed. This option offers potential advantages to the contractor in terms of speed of construction, as well as reducing the number of skilled site staff required to construct the walls.

7

2 Overview of hybrid solutions

Type 1 Precast twin wall and lattice girder slab with in-situ concrete

Type 2 Precast column and edge beam with in-situ floor slab

Type 3 Precast column and floor units with cast in-situ beams

Type 4 In-situ columns or walls and beams with precast floor units

Type 5 In-situ column and structural topping with precast beams and floor units

Type 6 In-situ columns with lattice girder slabs with optional spherical void formers

Figure 2.1 Typical hybrid concrete options. Please note this diagram is repeated on the inside back cover for ease of reference.

Figure 2.2 Type 1 construction, twin wall erection. Photo: John Doyle Construction Ltd

8


Often the twin wall system is combined with the use of lattice girder precast soffit slabs, with or without spherical void formers. These provide permanent shuttering for an in-situ slab that can be relatively easily fitted to the wall system. Spans up to 8 m are common and spans up to 14 m are possible. (The manufacturer should be consulted early on to ensure the longer spans are viable.) Potential structural uses of the twin wall system include:  cellular type structures for residential use  walls carrying vertical loads only  shear and core walls; this has significant implications for the design, as discussed in Section 5.1  retaining walls; this has significant implications for the design, as discussed in Section 5.1  ‘single sided’ formwork situations, where there is no access to one side of the wall to erect formwork, for example wall construction on a party wall line against neighbouring buildings. The major advantage is that it is an ‘in-situ structure’, fully continuous and tied together, but without the need for shuttering on site. Twin wall can also be cast with fully trimmed openings and with ducts for cables and other services. Advantages:  Quality finish for walls and soffits.  No formwork for vertical structure and horizontal structure when lattice girder slabs are used.  Structural connection between wall and slabs is by standard reinforced concrete detail and inherently robust.  Reduced propping. Disadvantages:  Propping of precast required prior to sufficient strength gain of in-situ concrete.  The smaller dimension of the precast units is typically a maximum of 3.6 m, so joints in walls and soffits must be dealt with: expressed or concealed.  Reduced flexibility of layout as this option requires walls rather than columns.

2.2 Type 2: Precast column with in-situ floor slab

The combination of an in-situ slab, e.g. post-tensioned flat slab, with precast columns can provide an economic and fast construction system. Precast concrete edge beams may also be used to avoid edge shutters on site and to allow perimeter reinforcement, cladding fixings or prestressing anchorages to be cast in. This reduces the time required for reinforcement fixing and erecting the formwork. The maximum span for this form of construction depends largely on whether the in-situ slab is post-tensioned. For flat slabs with spans greater than 10 m punching shear is likely to be a critical design issue.

9


Where long-span thin slabs are used vibration limits should be checked, see A Design Guide for Footfall Induced Vibration of Structures15. This form of construction relies on the structure being braced. This is achieved by the lift core(s) or separate shear walls. Advantages:  Columns can be erected quickly.  Quality finish for columns.  Precast edge beam contains post-tensioning anchorages (if required), slab edge reinforcement and cladding fixings, and avoids need for slab edge shuttering.  Can be used with a variety of in-situ slabs, selected to suit individual project requirements.  More flexible for late changes. Disadvantages:  In-situ slab requires falsework, formwork and curing time.

2.3 Type 3: Precast column and floor units with cast in-situ beams

This form of construction allows a high proportion of the structure to be manufactured in quality controlled factory conditions off site leading to fast construction on site. A variety of precast floor products could be used with this type of construction, including hollowcore units, double tees or lattice girder slabs (with or without spherical void formers) or bespoke cofferred floor units, see Figures 2.3a and 2.3b. The latter have successfully been used in high quality buildings designed for energy efficiency, where the light fittings, architectural features and cooling systems have all been incorporated into the unit. Advantages:  Vertical structure can be erected quickly; no formwork required.  Precast floor structure can be erected quickly; no formwork required.  Quality finish for columns and soffits (although this is not always possible with hollowcore units).  Structural connection between precast elements is via standard reinforced or posttensioned concrete. Disadvantages:  Precast flooring must be temporarily propped.  Sealing between precast units is required.

10


Figure 2.3a Example of type 3 projects. Paternoster Square and office building. Photo: John Doyle Construction Ltd

Figure 2.3b Example of type 3 projects. Homer Road, Solihull. Photo: Foggo Associates

11


2.4 Type 4: In-situ columns or walls and beams with precast floor units

This is a similar form to type 3 discussed above, the key difference being that the columns are cast in-situ rather than being precast, see Figure 2.4. The advantage of this form of construction over a fully in-situ concrete structure is the ability to use long spans (up to 16 m) precast floor units, e.g. hollowcore slabs, double tees. These obviate the need for slab formwork and provide a relatively lightweight floor. This construction system does not require the involvement of a specialist subcontractor beyond the manufacture and supply of the standard precast units.

Figure 2.4 Example of type 4 project, car park, West Quay, Southampton Photo: Hanson Concrete Products

Advantages:  Precast floor structure can be erected quickly.  Quality finish for soffits (although this is not always possible with hollowcore units).  Short lead time for standard precast products. Disadvantages:  Precast flooring must be temporarily propped.  Sealing between precast units is required.

12


2.5 Type 5: In-situ column and structural topping with precast beams and floor units

In this form of construction the floor consists entirely of precast elements, which are tied together with an in-situ structural topping, see Figure 2.5. (A structural topping is now defined as wearing screed in BS 820416.) The column formwork can be designed as a temporary support for the precast beams and slabs to reduce the requirement for propping of the precast floor. The joint between the beam and columns and any structural screed is concreted with the columns to form a monolithic, robust structure. This system requires particular attention to the connection details between the precast beam and floor units. It should be ensured that adequate structural ties are provided to achieve a robust structure. Advantages:  Precast floor structure can be erected quickly.  Precast beams support precast floor units, minimising floor propping.  Precast quality finish for soffits.  Formwork for in-situ columns can be used to prop precast beams.  Structural connection between precast elements is via standard reinforced concrete.  In-situ structural topping to beam permits beams to be continuous over columns. Disadvantages:  Downstand beams need to be coordinated with the services distribution.

Figure 2.5 Example type 5 project, Home Office Headquarters, London. Photo: Pell Frischmann Consulting Engineers Ltd and Bouygues (UK) Ltd

13


2.6 Type 6: In-situ columns with lattice girder slabs with optional spherical void formers

The main feature of this system is the use of the lattice girder panels to act as permanent formwork for a flat slab. A variation is to include spherical void formers, which reduce the self-weight of the slab, for only a small reduction in flexural strength and stiffness. Lattice girders and void former cages are cast into (usually class C40/50) concrete panels containing reinforcement in two directions, providing a precast panel that acts as the permanent formwork, see Figure 2.6. The slab may be designed as a flat slab. If the spherical void formers are used, they are removed in areas of high shear where a solid section provides greater shear resistance. The slab may be designed as a flat slab, although propping of the panels will be required, to reduce the overall floor zone of the building and to simplify installation of services. The quality of the factory produced soffits provides the opportunity to take advantage of the thermal mass properties of the concrete slab by exposing them. Advantages:  Precast floor structure can be erected quickly; no formwork required.  Structural connection between precast elements is via standard reinforced concrete.  Quality finish for soffits.  More flexible for late changes. Disadvantages:  Precast flooring must be temporarily propped.

Figure 2.6 Type 6: Lattice girder soffit panels used as permanent formwork. Photo: John Doyle Construction Ltd

14

Overall structural design 3

3. Overall structural design This section gives specific guidance on the aspects of structural design that will apply to most forms of hybrid concrete construction (HCC). HCC requires special design care because the connections of elements within the structure are unlikely to use standard in-situ reinforcement details; more detailed guidance is given in Sections 4 and 5 on bearings, movement joints, various elements and their connections. The designer must be confident that the details will work satisfactory for all situations that the structure is likely to experience. The introduction to this design guide emphasizes the importance of a single named engineer responsible for the design of a hybrid concrete structure. This is particularly important in the design of the connection details.

3.1 Robustness

The design and detailing advice provided in this guide assumes that the structure falls into Approved Document A17, class 2B (risk group 2B in Scotland) or above. It is essential to create a robust structure and this may require special details to be developed to allow the precast elements to be properly integrated. The UK Building Regulations18 through Approved Document A refers to BS EN 1991-7, Actions on Structures – Accidental Actions19 and Eurocode 2. The full requirements are given in Eurocode 2, Cl. 9.10, its UK National Annex20 and PD 6687, Background Paper to the UK National Annexes to BS EN 1992-1 21. The design of ties should take account of the minimum reinforcement requirements (related to the tensile strength of concrete) and the anchorage capacity of the bars.

Continuity of ties A tie may be considered effectively continuous if the rules for anchoring and lapping bars given in Eurocode 2, Cl. 8.4 and 8.7 are followed and the minimum dimension of any in-situ concrete section in which tie bars are provided is not less than the sum of the bar size (or twice the bar size at laps), twice the maximum aggregate size and 10 mm. The tie should also satisfy one of the following conditions:  A bar or tendon in a precast member lapped with a bar in connecting in-situ concrete, bounded on two opposite sides, by rough faces of the same precast member, see Figure 3.1.  A bar or tendon in a precast concrete member lapped with a bar in in-situ structural topping or connecting concrete anchored to the precast member by enclosing links. The combined ultimate tensile resistance of the links should be not less than the ultimate tension in the tie, see Figure 3.2.  Bars projecting from the ends of precast members joined by any method conforming with Eurocode 2, Cl. 8.7.  Bars lapped within in-situ structural topping or connecting concrete to form a continuous reinforcement with projecting links from the support of the precast floor or roof members to anchor such support to the topping or connecting concrete, see Figure 3.3.

15

3 Overall structural design

Figure 3.1 Continuity of ties: Bars in precast member lapped with bar in in-situ concrete.

Figure 3.2 Continuity of ties: Anchorage by enclosing links.

Tie

Tie

Figure 3.3

Tie

Tie

Continuity of ties: Bars lapped within in-situ concrete.

Peripheral ties The peripheral tie should be capable of resisting a design tensile force: F tie,per = (20 + 4n0) ≤ 60 kN

where n0

= number of storeys

Internal ties The internal tie should be capable of resisting a design tensile force: F tie,int = [(qk + gk)/7.5](l r /5)(F t) ≥ F t kN/m

where (qk + gk) = sum of the average permanent and variable floor loads (in kN/m2) = greater of the distances (in metres) between the centres of the columns, l r frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration, and = (20 + 4n0) ≤ 60 kN F t Maximum spacing of internal ties = 1.5l r

16


Horizontal ties to columns and/or walls Edge columns and walls should be tied horizontally to the structure at each floor and roof level. The tie should be capable of resisting a design tensile force: F tie, fac = F tie, col = Maximum (Minimum (2F t; l sF t/2.5); 0.03 N Ed)

where = in kN/m run of wall = in kN/column = as defined in above = floor to ceiling height (in metres) = total design ultimate vertical load in wall or column at the level considered Tying of external walls is only required if the peripheral tie is not located in the wall. F tie,fac F tie,col F t l s N Ed

Vertical ties For class 2B and 3 buildings Approved Document A (and similarly the Technical Handbooks for Scotland for risk group 2B and 3 buildings) has the following requirements: a) Each column and each wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of carrying a tensile force equal to the design load carried by the column or wall from any one storey under accidental design situation (that is loading calculated using BS EN 1990, Eurocode: Basis of Structural Design22, Expression (6.11b)). b) Where ties described in a) are not provided a check should be carried out to show that upon notional removal of each supporting column and wall, and each beam supporting columns or walls (one at a time in each storey of the building) that the building remains stable and that the area of floor at any storey at risk of collapse does not exceed 15 per cent of the floor area of that storey or 70 m2, whichever is the smaller, and does not extend further than the immediate adjacent storeys. c) Where the notional removal of such elements would result in damage or is in excess of the limit above then these elements should be designed as ‘key elements’. A key element should be capable of withstanding a design load of 34 kN/m2 at ultimate limit state applied from any direction to the projected area of the member together with the reaction from the attached components, which should also be assumed to be subject to 34 kN/m2. The latter may be reduced to the maximum reaction that can be transmitted by the attached component and its connections.

Anchorage of precast floor and roof units and stair members PD 6687, Background Paper to the UK National Annexes to BS EN 1992-1-1 and BS EN 1992-1-221, Cl. 2.20.2 Anchorage of precast floor and roof units and stair members states that: a) In buildings that fall into class 2B and 3 as defined in Section 5 of Approved Document A all precast floor, roof and stair members should be effectively anchored whether or not such members are used to provide other ties required in Eurocode 2, Cl. 9.10.2. (Similar requirements apply in Scotland.) b) The anchorage described in a) should be capable of carrying the dead weight of the member to that part of the structure that contains the ties.

17


3.2 Stability

HCC frames may be designed as either braced or unbraced. The design of unbraced frames requires extra care to ensure that the joint details can resist the applied moments without excessive rotation.

3.3 Diaphragm action

Where floor diaphragm action is used in the design, type 3 and 4 structures have the precast elements carrying horizontal shears for diaphragm action to take place. Types 2 and 6 structures have the in-situ floor acting as a diaphragm, and type 1 and 5 structures can have the diaphragm action shared by the precast units and the in-situ structural topping. Multi Storey Precast Concrete Framed Structures 23 describes the design approaches for

floor diaphragm action formed from different types of precast units supported by tests. One approach is the use of precast units, either alone or with a structural topping, having sufficient horizontal shear capacity between them, such that together they can be considered as horizontal beams with longitudinal steel at each gable and tie steel across the unit-tounit joints, see Figure 3.4a. An alternative method, appropriate to hollowcore floors with no structural topping considers the hollowcore unit as a member in a virendeel girder and with reinforcement in the embedment zone in the edge beams acting as the stiffening component in the virendeel joints, see Figure 3.4b. Figure 3.4 Typical diaphragm action from precast floor systems.

a) Floor carrying horizontal forces from wind by beam action

18

b) Floor carrying horizontal forces from wind by virendal action


BS EN 1168, Precast Concrete Products – Hollowcore Slabs 3 has an informative annex that gives some advice on the design of horizontal diaphragms to carry lateral loads, usually wind loading. This, in turn, refers to Eurocode 2, Cl. 10.9.3 where the maximum longitudinal shear stress for grouted connections v Rdi is limited to 0.15 MPa for smooth and rough surfaces, as found at the edges of hollowcore, and 0.1 MPa for very smooth surfaces as found in the ex-mould finish of bounding edge beams, see Figure 3.2. A considerable amount of test work has also been carried out on hollowcore diaphragms and is discussed by Elliott23.

3.4 Shear at interface of concrete cast at different times

Eurocode 2, Cl. 6.2.5 also covers the design approach for shear at the interface between concrete cast at different times. A design example (worked example 1) is included here to illustrate the process, as it is required in many areas of hybrid design where precast and in-situ concretes are combined to produce composite sections. The example using hollowcore without structural topping is a useful one as it is more critical than diaphragms with any topping. A further consideration is the shear connection between the hollowcore units and also between the end unit and the bounding beam. In this case, the connection to the main support beams and the longitudinal steel in the support beams is usually sufficient to ensure that the hollowcore units cannot move apart and so the structural model used in worked example 1 remains valid.

19

3 Overall structural design Project details

Calculated by

RW

Job No.

Worked example 1 Hollowcore floor acting as a diaphragm

Checked by

OB

Sheet No.

WE 1/1

Client

TCC

Date

April 08

Check the design of the hollowcore diaphragm, without structural topping, carrying wind load to walls at each end, as shown below.

Edge beam

vs

vs

s Hollowcore unit

vs

vs

A

s

A

KEY

vs - Very smooth surface s - Smooth surface

Section A - A

Plan:

20

15 m x 9 m with 250 mm thick hollowcore unit

CCIP-030

Overall structural design 3 Project details

Calculated by

RW

Job No.

CCIP-030


Checked by

OB

Sheet No.

WE 1/2

Client

TCC

Date

April 08

Wind load: 2 kN/m2

(A high wind load)

Assume a 3 m high storey, calculate maximum moment, MEd, from the diaphragm edge wind load/m run. w d = 1.5 x 3 x 2 = 9 kN/m γQ is taken as 1.5 MEd = 9 x 152/8 = 253 kNm Calculate shear reaction at the diaphragm edges, V Ed. V Ed = 9 x 15/2= 67.5 kN Assume 2 No. hairpins (U bars), 12 mm diameter, in each 1.2 m wide hollowcore unit.

Check shear at interface: v Edi < v Rdi gives: v Edi = β V Ed/( z bi) where β = 1 V Ed = 67.5 kN at end of diaphragm d = 0.83 h and z = 0.67 h (assuming elastic stress distribution) Hence: z = 0.67 x 9 = 6 m bi = 250 – 50 (say) = 200 mm

∴

v Edi =

Eurocode 2, Cl. 6.2.5 Eurocode 2, Exp.(6.24)

Eurocode 2, Figure 6.8

67.5 x 1000/(6000 x 200 ) = 0.056 MPa

r Rdi is limited to 0.10 MPa (> 0.056 MPa → OK)

Eurocode 2, Cl.10.9.3(12)

Check v Rdi (which is unlikely to control); for this example the first and second terms are small and may be ignored as a first estimate. v Rdi = ρf yd (µ sin α + cos α) ≤ 0.5 υ f cd

Eurocode 2, Exp.(6.25)

where ρ µ f yd As Ai α υ

= = = = = = =

As/ Ai

0.5 (very smooth surface) the design yield strength of reinforcement the area of reinforcement crossing the interface the area of the joint 90 for reinforcement perpendicular to the joint 0.6 (1 – f ck/250)

Eurocode 2, Cl.6.2.5 (2)



21


Calculated by

RW

Job No.

CCIP-030


Checked by

OB

Sheet No.

WE 1/3

Client

TCC

Date

April 08

For this example: As Ai

= =

Hence: ρ = and: v Rdi = =

2 x 2 x 113 = 452 mm2 1200 x 200 = 240 000 mm2

452/240 000 = 0.00188 0.00188 x 500 x (0.5 x 1 + 0)/1.15 ≤ 0.5 x 0.6(1 - 25/250) x 1 x 25/1.5 0.41 ≤ 4.5 MPa

Use 2 No. hairpins (U bars) - 12 mm diameter This check demonstrates that Exp. (6.25) is not usually a limiting control. The design would now normally continue to calculate the tensile steel required in the edge beam to carry the diaphragm tensile boom force, taking into account that this calculation must also consider the other actions for the appropriate combination of actions.

3.5 Interface shear

For many beams in HCC there is an interface between concrete cast at different times. The interface may be between precast and in-situ, two precast elements or in-situ concrete with a construction joint. All interfaces and critical sections in the composite section must be considered in accordance with Eurocode 2, Cl. 6.2.4 and 6.2.5 (see example in Section 3.4). Typical interfaces are shown in the Figure 3.5, and typical calculations are presented in worked example 2.

Figure 3.5 Typical interfaces between precast and in-situ joints.

Interface 3

Interface 2

Interface 4

Interface 1

22

Overall structural design 3 Project details

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Job No.

Worked example 2 Interface shear between hollowcore slab and edge beam

Checked by

OB

Sheet No.

WE 2/1

Client

TCC

Date

April 08

CCIP-030

Consider Example 13.7 in the Precast Eurocode 2: Design Manual 24. Interface shear check is between the edge beam and in-situ concrete provided in the joint (see figure). In this example the contribution of the horizontal surface is ignored. The shear resistance of the interface between the upstand of the precast unit and the main body below should also be checked.

Shear interface 600

175

110

200

In-situ concrete

The flange over each hollowcore is cut out and therefore the units should be temporarily propped. 1 No. H16 U-bar is placed in each void t o interlock with projecting reinforcement in the edge beam as shown. Assume that the compression flange of the edge beam is 600 + 175 + 110 = 885 mm wide. Check shear at interface according to Eurocode 2, Cl. 6.2.5. f ck f y

= 35 MPa = 500 MPa Maximum sagging moment, MEd = 267 kNm V Ed = 223 kN Maximum design shear, bi = 200 mm d = 540 mm MEd/bd 2f ck = 267 x 1000000/(885 x 5402 x 35) = 0.0296

23


Calculated by

RW

Job No.

Worked example 2 Interface shear between hollowcore slab and edge beam

Checked by

OB

Sheet No.

WE 2/2

Client

TCC

Date

April 08

CCIP-030

From Figure B1 of the Precast Eurocode 2: Design Manual 24 find value of z (alternatively find z by calculation or with any suitable design aid): z = 0.97 v Edi =

Eurocode 2, Exp (6.24)

where β

bi

βV Ed / z bi

Hence: v Edi = v Rdi

= ratio of the longitudinal force in the new concrete and the total longitudinal force = width of new concrete/total flange width = 775/885 = 0.88 = 200 mm

0.88 x 223 x 1000/(0.97 x 540 x 200) = 1.87 MPa

= c f ctd + µ σ n + ρf yd (µ sinα + cosα) ≤ 0.5 υf cd

Eurocode 2, Exp (6.25)

c σ n α f ctd υ

Eurocode 2, Exp (6.6N)

where = = = = =

0.35 and µ = 0.6 for a smooth surface 0 90º 1 x 2.2/1.5 = 1.47 MPa 0.6(1 – 35/250) = 0.52

v Rdi = 0.35 x 1.47 + 0 + ρ x 0.6 x 500/1.15 ≤ 0.5 x 0.52 x 1 x 35/1.5 (= 6.07 MPa) v Edi ≤ v Rdi ≤ 0.515 + 260.9 ρ

Hence: ρ

≥ (1.87 – 0.515)/260.9 = 0.005

Now: ρ

∴

= As / Ai As,req = ρ Ai = 0.005 x 1200 x 200 = 1200 mm 2

Using 3 No. voids each containing 1 No. H16 U bar. As,prov = 3 x 2 x 16 2 π /4 = 1210 mm2

24

OK


3.6 Shear and torsion design

Shear and torsion are predominately critical at the ultimate limit state and the composite sections can be considered to be monolithic if the interface shear calculations have been carried out appropriately, as discussed in Section 3.4 (see Eurocode 2, Cl. 6.2.4 and 6.2.5). The variable strut inclination method used in Eurocode 2 is based on the shear load being applied at the top of the beam element. When it is applied near to the bottom, the load must be ‘carried up’ to the top with vertical reinforcement additional to the vertical reinforcement required by the shear calculation. This is sometimes called ‘hang up steel’, as its effect is to hang up the applied load to the top compression chord of the beam (Eurocode 2, Cl. 6.2.1(9)), see Figure 3.6.

Figure 3.6 ‘Hang up steel’ requirement. Beam shear strut

Slab shear strut

“Hang up steel” additional to reinforcement required to carry shear Eurocode 2, Cl 6.2.1 (9)

Slab shear strut

Types 2, 3 and 4 apply the floor permanent actions to the spine beams at the bottom of the section and this element of the load must be carried by hang up steel. Whether the subsequent variable actions should also be covered in this way depends on the form of the composite connection. In any event, the load only needs to be carried up once to the top of the truss and the extra link requirement is not onerous. Where type 5 is used a further check is required for edge beams or where there is out-ofbalance loading on an internal beam. The edge beam and internal spine beam with unequal loading in this form of construction must be designed to resist the torsion set up by the eccentric loading. Both the transient situation during construction and the ultimate limit state must be considered. The joint between the beam and its support must also be designed to take this torsion, see Figure 3.7.

25


Figure 3.7 Design for torsional restraint.

Centre of resistance of column

Shear centre of beam

V

h1

h2

For the torsional design of the edge beam, the design torque is equal to the load multiplied by the distance from its line of action to the shear centre of the edge element Vh1. For the design of the temporary support system to give equilibrium, the overturning torque is equal to the torsional force multiplied by the distance from the line of action of the force to that of the restraining system Vh2.

3.7 Long-line prestressing system

Many prestressed precast elements are produced by the long-line pre-tensioning system on prestressing beds of up to 200 m in length with built-in jack heads at each end, see Figure 3.8. The normal construction procedure is as follows:  The moulds are placed in a continuous line along the bed (the number depending on the length of each unit) and end plates are fitted to the required dimensions of the units to be cast.  The tendons are laid out and stressed from fixed external jack heads. They pass through each unit as straight horizontal tendons.  The secondary reinforcement is then fixed within each mould.  The concrete is poured into each mould.  When the concrete reaches the required transfer strength (confirmed by test cubes), the stress is gradually released from the jack heads and is transferred into the concrete by anchorage bond. A typical detail of the placing of moulds on the long-line system is shown in Figure 3.9.

26


Figure 3.8 The long-line pre-tensioning system.

Gradual detensioning mechanism Stressed strands

Unit moulds or continuously extruded units

Jack blocks and embedded cantilever upright in concrete strong floor

Figure 3.9 Typical detail of placing of moulds on the long-line system.

Unit in mould

Strand

Mould end plate

Detail of gap between moulds

Debonding tendons The position of the strands in the section is normally determined by the length of the unit and the design loading at mid-span. Stress limits are set for the serviceability limit state (for further information see Precast Eurocode 2: Design Manual 24 and Post-tensioned Concrete Floors Design Handbook 25). Since the tendons are straight the prestress is the same at the end of the units as it is at mid-span (apart from within the transmission zone), but there is little balance from the stresses due to permanent actions at the ends. This creates high-tension stresses at the top of the section that will be a maximum immediately after transfer of prestress. In order to reduce these stresses locally some of the tendons are debonded by placing tubing over them at the end of the unit for the required length, see Figure 3.10. It should be noted that the bottom strand should not be debonded, as it ensures that the concrete near the end of the unit has less chance of being damaged. It is advisable to provide two links just beyond the debonding point in the beam span to restrain anchorage stresses. Two 10 mm diameter links, the first at 100 mm from the debonding point and the second 40 mm beyond that, are typically sufficient. The proximity of the links to the bonding position ensures sufficient restraint to bursting even if the transmission zone is less than that assumed in design in accordance with Eurocode 2.

27


Figure 3.10 Typical detail showing the debonding of a strand.

Typically 7 - 8 protruding links Extra links at debonding point

Fully bonded stressed strand

Debonded strand

Debonding is used in double tee design because it is such a simple and cost-effective option. An alternative to debonding some of the tendons is to deflect them at the ends of the unit. This method is very seldom adopted, as it requires special features to be built into the long-line system to take account of the vertical forces involved. The difference between the effects of straight bonded and debonded tendons is shown in Figure 3.11. Figure 3.11 Comparison between straight bonded, debonded and deflected tendons. Unit with straight bonded tendons

Moments from quasi-permanent loading

Moments from prestress

Balance of moments

Resulting camber

28

Unit with straight debonded tendons


Prestressed units camber because of the hogging moment provided by the prestress. A pretensioned prestressed beam with no camber, unless it has a very short span or is debonded, should be viewed with caution. Camber is equivalent to the deflection of a reinforced concrete beam; in fact for a permanent and variable action balanced by prestress, the upwards camber would be less than the downward deflection of the reinforced section. This is because the prestressed section would be uncracked and stiffer than the cracked reinforced beam. Thus, camber should not be a problem but should be allowed for when setting floor levels. An estimate of camber should be obtained from the manufacturer of the prestressed unit. It will be affected by the strength of concrete at the time of transfer. Debonding has the advantage of reducing camber, as the debonded prestressed moment diagram is closer to the permanent load diagram than the fully bonded one. The typical camber of a fully bonded 16 m double tee beam carrying car park loading is 35 to 45 mm and this can be reduced by debonding to the range of 10 to 25 mm. Debonding, however, reduces the net prestress at the support and this reduces the design shear strength, but for double tees this reduction is seldom a critical design issue.

3.8 Secondary effects of prestressing and the equivalent load method

The occasions where secondary effects (sometimes referred to as parasitic effects) need to be considered relate to indeterminate frames and continuous beams/slabs. The most likely example for HCC is where post-tensioned slabs are used. Section 5.6 of the Post-tensioned Concrete Floors Design Handbook 25 describes the phenomena and the use of the equivalent load method.

3.9 Temperature effects

The deflection of a floor in response to a temperature gradient can be large and this can result in rotational movements at supports, which can produce unwanted local damage such as cracking and spalling. This problem is particularly acute in un insulated roofs, often found in car parks. The following simple calculation, worked example 3, gives an idea of the magnitude of the displacements. Further guidance can be found in Movement, Restraint and Cracking in Concrete Structures 26.

3.10 Differential shrinkage

When an in-situ screed is added onto a first stage cast floor of either reinforced or prestressed construction, the shrinkage of the screed after its initial hydration will develop a compressive strain in the top of the first stage cast and will induce a downwards deflection in the span of the composite unit and, if the floor is of continuous construction, a hogging moment at the supports. Note that these effects are of importance at the serviceability limit state only, as at the ultimate limit state these imposed strains will have little effect. Figure 3.12 shows how the strains are built up through the height of the composite section for a given free differential shrinkage strain, ε fds. The final curvature, φ , is constant across the section. Design equations can be developed as follows:

29


Figure 3.12

ε fds

The effect of differential shrinkage across a section.

ε cp εi

y i,b

εp

In-situ

y p,t

ε ci

Precast φ

Force equilibrium: εi E i Ai = εp E p Ap

(1)

εp = εi E i Ai /E p Ap

Section equilibrium (φEI = M): φ (E i I i + E p I p) = εi E i Ai ( y i,b + y p,t)

(2)

Strain equilibrium: εfds = εi + εci + εcp + εp = εi + φ y i,b + φ y p,t + εp φ = (εfds - (εi + εp))/( y i,b + y p,t) φ = (εfds - (εi + εi E i Ai /E p Ap))/( y i,b + y p,t)

(3)

Project details

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Job No.

Worked example 3 Upwards camber on slab due to temperature gradient

Checked by

OB

Sheet No.

WE 3/1

Client

TCC

Date

April 08

Calculate the upwards deflection of a 16 m span 300 mm deep simply supported floor resulting from a temperature gradient of 20ºC with the upper surface being the hotter. Assume that the gradient is linear and steady state, and that the temperature coefficient for concrete, α, is 10 x 10 -6. The curvature, φ, from this temperature gradient is = 20 x α/300 = 20 x 10 x 10-6/300 = 0.67 x 10-6 The curvature is constant along the length of the unit. From the second moment area theorem, the mid-span deflection: δ = φ x l2/8 = 0.67 x 8000 x 4000/1000000 = 21.4 mm

30

CCIP-030


Combining (2) and (3): φ { y i,b + y p,t + (εfds - (E i I i + E p I p)) (1/E i Ai + 1/E p Ap)/( y i,b + y p,t)} = εfds φ = εfds /{ y i,b + y p,t + (E i I i + E p I p) (1/Ei Ai + 1/E p Ap)/( y i,b + y p,t)}

(4)

εi = εfds /{1 + E i Ai /E p Ap + ( y i,b + y p,t)2 E i Ai /(E i I i + E p I p)}

(5)

εp = εfds /{1 + E p Ap /E i Ai + ( y i,b + y p,t)2 E i Ai /(E i I i + E p I p)}

(6)

From equations (4) to (6) all the strains, stresses and forces can be determined. Worked example 4 describes the method for determining the effect of differential shrinkage where in-situ concrete is placed on a precast concrete T section.

Project details

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Job No.

Worked example 4 Differential shrinkage

Checked by

OB

Sheet No.

WE 4/1

Client

TCC

Date

April 08

CCIP-030

Calculate the effect of differential shrinkage in a beam constructed in two stages as shown below. The element is simply supported and 20 m span. The free differential shrinkage strain is 0.0002.

B785 mesh 1000

100 50

B283 mesh In-situ concrete

300 2 x 2 No 7.9 super strand

Precast concrete

150

B785 fabric in in-situ concrete B283 fabric in precast concrete flange 2 x 2 No. 7.9 mm super strand in precast rib

31


Calculated by

RW

Job No.

CCIP-030

Worked example 4 Differential shrinkage

Checked by

OB

Sheet No.

WE 4/2

Client

TCC

Date

April 08

In-situ concrete f ck,in = E c,in,long =

= =

25 MPa, f cm,in = 33 MPa, creep coefficient, ϕ = 1.5 22 [f cm,in/10]0.3/(1 + ϕ ) 22 x (33/10)0.3/(1 + 1.5) 12.59 GPa

Eurocode 2, Table 3.1 and Cl.3.1.4

Section properties, including the reinforcement, are as follows: Ain = 112 x 103 mm2 I in = bd 3/12 = 1000 x 1003/12 = 87.5 x 106 mm4 y inbar,b = 52.1 mm z in,b = 1680 x 103 mm3

Precast concrete f ck,p = E c,p,long =

=

50 MPa, f cm,p = 58 MPa, Creep coeficient, ϕ = 1 22 x (58/10)0.3/(1 + 1) 18.64 GPa

Eurocode 2, Table 3.1 and Cl.3.1.4

Section properties, including the tendons and reinforcement, are as follows: Ap = 101.5 x 103 mm2 I p = 1220 x 106 mm4 y pbar,b = 237.4 mm y pbar,t = 112.6 mm z p,t = 10900 x 103 mm3

Curvature Using expression (4) above: Curvature:

φ =

1000 x 0.0002

(

0.00058/m

Deflection Deflection from differential shrinkage δ = φ l 2/8 = 0.00058 x 202/8 = 29 mm

32

)

52.1 + 112.6 + (12.59 x 87.5 x 10 + 18.64 x 1.22 x 109) x (1/(12.6 x 112 x 103) + 1/(18.6 x 101.5 x 103))

=

6

50 + 112.6


3.11 Designing for construction

Designers should take into account the stability of the structure during construction:  Precast elements are heavy. Bearings must be adequate and be robust enough to withstand normal unit fixing operations including landing and ‘barring’ (see Section 6.7).  Beams must be securely fixed and have adequate safe bearing at each end to avoid overturning, excessive deflection or collapse when the precast elements are placed.  Consideration must be given to the unequal loading when precast elements are being placed.  Where precast elements are tilted or twisted to allow them to be placed in their final position consideration should be given to ensuring there is sufficient clearance to place the unit and achieving the minimum end bearing required in the final position.  Special requirements, such as special fixing techniques, temporary measures or sequencing, should be clearly conveyed.

33

4 Bearings and movement joints

4. Bearings and movement joints The design of bearings and joints for hybrid concrete construction (HCC) is critical to the serviceability and lasting integrity of the structure. Careful design can avoid problems which lead to deterioration of joints, which ultimately compromise the whole safety of the structure. Where a bearing is introduced between precast elements or between precast and in-situ elements great care is required to take account of all the forces and movements that may be imposed on the elements connected to the bearing. In addition consideration must be given to:  how the robustness of the structure is attained  effects of composite action  practical tolerances  temperature changes  shrinkage  differential settlement  effects of repeated changes in imposed deformations  ensuring construction meets the assumption made in design. The decision to design a full continuity joint or one that allows some movement is critical. The design must then follow the decision to reach a practical and lasting solution. The joint detail must be robust and must not deteriorate with time due to the effects of movement. Joints that are designed to be monolithic are considered in Chapter 5.

4.1 Horizontal forces at bearings

Horizontal forces at a bearing can reduce the load carrying capacity of the supporting member considerably by causing premature splitting or shearing. The forces may be due to creep, shrinkage and temperature effects or may result from misalignment, lack of plumb or other causes. Allowance should be made for these forces in designing and detailing by the provision of: a) bearings that allow limited movement or b) suitable lateral reinforcement in both the supporting and supported members or c) sufficient continuity reinforcement through the joint to resist the lateral forces. Where type a) bearings are used then conservatively the horizontal design force should be taken as 20 per cent of the vertical force. A more detailed assessmen t may show this force can be reduced. For type b) and c) bearings the design horizontal force should be not less than half of the design vertical force on the bearing. Unless top and bottom continuity reinforcement is provided precast floor slabs, e.g. hollowcore slabs, spanning more than 8 m should be supported on elastomeric bearings, e.g. neoprene.

34

Bearings and movement joints 4

These can normally be attached to the support surface. They allow:  the forces resulting from variation of bearing surfaces to be absorbed  any small horizontal movements to be absorbed without causing cracking and  limited rotation (as a result of cyclic upward and downward deflection) of the precast slab. Where top and bottom continuity reinforcement is provided, to make a homogenous joint it may be acceptable not to provide elastomeric bearings. In this case great care must be taken in construction to ensure that the precast element is not damaged during placing and that it can absorb the movements that take place during and after construction without damage.

4.2 Restrained bearings

For bearings that offer significant restraint to sliding or rotation, e.g. dry bearing on concrete or mortar bedding, actions due to creep, shrinkage, temperature, misalignment, lack of plumb and other things must be taken into account in the design of adjacent members. Further guidance on creep, shrinkage and temperature effects can be found in Movement, Restraint and Cracking in Concrete Structures 26. The effect of such actions may require transverse reinforcement in supporting and supported members, and/or continuity reinforcement for tying elements together. They may also influence the design of the main reinforcement in such members. Such joints are not considered suitable for external situations or for spans greater than 8 m for internal situations. It should be noted that it is unlikely that a dry connection without bedding material will have a uniform contact surface and that concentrated loading will result that may cause local cracking. For joints with bedding material, e.g. mortar, concrete, polymers, relative movement between the connected surfaces should be prevented during hardening of the material. The bearing width should not be greater than 600 mm unless specific measures are taken to obtain a uniform distribution of the bearing pressure. In the absence of other specifications, the bearing strength, f Rd, of a dry connection should not exceed 0.4 f cd and the average bearing stress between plane surfaces should not exceed 0.3 f cd. The bearing strength for joints with bedding material should not exceed the design strength of the bedding material, f bed ≤ 0.85 f cd where f cd is the lower of the design strengths for supported and supporting members.

35


4.3 Movement joints

It is possible to deal with movement at bearings using movement joints, and care should be given to the design and construction, as for bridge decks, to minimise the risk of failures. In general it is recommended to seek solutions that do not require movement joints. Figure 4.1 describes potential failure mechanisms that can occur even with a structural topping.

Figure 4.1

Expanding material to plug gap

Examples of potential failures at movement joints.

Movement

If the bearing material creates large friction forces (use neoprene or similar to avoid this), this can lead to large tension stresses in both the support and the precast slab or beam.

Friction can cause cracking

If no plug, hard material can prevent rotation

Rotation

Rotation

If the space between the precast slab or beam and the face of the supporting member is not adequate for the required movement or if in time it it fills up with hard material, then cracking can occur.

If the effects of movement and/rotation cause the line of action to move too close to the edge of the support, local spalling can occur.

Rotation can cause spalling

4.4 Actions and restraints

4.4.1 Action effects

36

In addition to the effects of direct loading (imposed variable and permanent actions) the following action effects on the elements supported by the bearing must be considered:  shrinkage (both long term and early thermal)  temperature changes (both seasonal and short term)  creep.


4.4.2 Restraints

In addition to the above action effects the following restraints must be considered:  internal, e.g. from reinforcement, differential shrinkage  edge restraints  end restraints. For detailed consideration of these effects and restraints refer to Movement, Restraint and Cracking in Concrete Structures 26.

4.5 Design considerations

When designing bearings the following details should be checked:  calculation of the bearing area  bearing layout  the detail of the reinforcement in the end of the supported member  the detail of the reinforcement in the supporting member  tolerances  construction issues – especially any additional forces imposed on the bearing through ‘barring’ the units into final position, see Section 6.8.

4.6 Allowance for anchorage of reinforcement at supports

The design and detailing of the reinforcement at supports is critical. The supported member has to be designed to bear safely onto the support without spalling of the end cover and also to sustain any forces that may come from shrinkage of the floor, through shortening of the floor, if prestressed, and from thermal, live and fu rther dead load movements, see also Section 4.1. Prestressed members used for flooring are commonly pre-tensioned and the main prestressed steel continues to the end of the member. Reinforcement in supporting and supported members should be detailed to ensure effective anchorage, allowing for deviations, see Figure 4.2. d i d i c i ∆ai r i

= = = = =

c i + ∆ai with horizontal loop bars c i + ∆ai + r i with vertically bent bars

nominal concrete cover a deviation (see Section 4.8) radius of bend (see Table 4.1)

37


Figure 4.2 > a1 +  a3

d 2

Effect of reinforcement on bearing dimensions.

c3

r 3

r 2

c2

Table 4.1 Minimum bend radii for reinforcement to avoid damage to reinforcement.

38

> a1 +  a2

d 3

Bar diameter

Minimum radius of bend

φ ≤ 16 mm

2φ

φ >

3.5 φ

16 mm

4.7 Bearings that allow limited movement

Bearings that allow limited movement, e.g. neoprene pads, not only distribute the bearing forces over uneven supports but also allow limited rotational and longitudinal movement of the supported member to take place. The bearing pad also defines the area of load transfer and thus has a direct effect on the detailed design of the ends of the supporting and supported members.

4.7.1 Design of the bearing area

In the absence of other specifications, the bearing strength, f Rd = f bed ≤ 0.85 f cd where f bed is the design strength of the bearing material may be used.

4.7.2 Bearing layout

The layout of a bearing is critical to its successful execution. The concrete surfaces must be separated in areas where load transfer is not intended and must be bedded appropriately where load transfer is required. To ensure that spalling does not take place in the contact area at the end of the supported and supporting concrete, the provision of sufficient bearing length must be provided. This should allow for constructional tolerances and ensure the overlap of reinforcement between the supporting and supported concrete. The required allowances are shown in the Figure 4.3 and are described in Eurocode 2, Cl. 10.9.5.2. These will lead to the design of minimum bearing shelf and nib sizes.


Figure 4.3 Critical dimensions for bearings.

> a2 + a3

b1

a3 +  a3

a1

a1

a2 +  a2

a

The nominal length, a, of a simple bearing may be calculated as: a = a1 + a2 + a3 + √(∆a22 + ∆a32)

where a1 F Ed b1 f Rd a2 a3

∆a2 ∆a3 l n

Table 4.2 Minimum value of a1 (mm).

= net bearing length with regard to bearing stress = F Ed /(b1f Rd) but not less than the values in Table 4.2 = design value of the support reaction = net bearing width = design value of the bearing strength = 0.85f cd = distance assumed ineffective beyond outer end of supporting member (see Table 4.3) = distance assumed ineffective beyond outer end of supporting member (see Table 4.4) = allowance for distance between supporting members (see Table 4.5) = allowance for deviation of the length of the supported member = l n /2500 = length of member in mm

Relative bearing stress, σ Eda/ f cd

≤ 0.15

0.15 to 0.4

> 0.4

Line supports (floors and roofs)

25

30

40

Ribbed floors and purlins

55

70

80

Concentrated supports (beams)

90

110

140

Key: a σ Ed is the design bearing stress

39


Table 4.3 Distance a2 (mm) assumed ineffective from outer end of supporting member.

Relative bearing stress, σ Eda/ f cd

≤ 0.15

0.15 to 0.4

> 0.4

5 10

10 15

15 25

10 20

15 25

25 35

Reinforced concrete ≥ C30/37 Line Concentrated Reinforced concrete < C30/37 Line Concentrated Key a σ Ed is the design bearing stress

Table 4.4 Distance a3 (mm) assumed ineffective from outer end of supported member.

Table 4.5 Allowance for deviations for the clear distance between the face of the supports.

Detailing of reinforcement

Type of support Line

Concentrated

Continuous bars over support (restrained or not)

0

0

Straight bars, horizontal loops, close to end of member

5

15, but not less than end cover

Tendons or straight bars exposed at end of member

5

15

Vertical loop reinforcement

15

End cover + inner radius of bend

Support material

∆ a2

Precast concrete

10 ≤ l /1200 ≤ 30 mm 15 ≤ l /1200 + 5 ≤ 40 mm

Cast in-situ concrete Note: l is clear distance between supports in mm

An example calculation is shown in worked example 5.

40

Bearings and movement joints 4 Project details

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Job No.

CCIP-030

Worked example 5 Bearing of a hollowcore unit

Checked by

OB

Sheet No.

WE 5/1

Client

TCC

Date

April 08

A 1.2 m wide hollowcore slab seated on an in-situ concrete nib, treated as a non-isolated member. The length of hollowcore unit is 9 m. The in-situ concrete beam is class C35/45 concrete.

Actions Self weight Variable load Partitions Finishes

= = = =

3.33 kN/m2 4 kN/m2 1 kN/m2 0.7 kN/m2

Bearing stress F Ed

= Assume a 30 mm wide = σ Ed σ /f = Ed cd

9 x 1.2 x {1.35 (3.33 + 0.7) + 1.5(4 + 1)}/2 = 69.9 kN neoprene bearing. 69.9 x 1000/(30 x 1200) = 1.94 MPa 1.94/(0.85 x 35/1.5) = 0.098

Geometry Minimum value of a1 from Table 4.1 for a line support is 25 mm. Hence: a1 = 30 mm OK a2 = 5 mm a3 = 5 mm ∆a2 = 15 mm ∆a3 = 9000/2500 = 4 mm say 5 mm The reinforcement in the in-situ concrete nib is assumed to be 20 mm vertically bent with a nominal cover of 20 mm. d 2 = c 2 + ∆a2 + r 2 = 20 + 15 + 3.5 x 20 = 105 mm a2 + ∆a2 ≥ d 2 ∴ a2 + ∆a2 = 105 mm

Table 4.2 Table 4.3 Table 4.4

Allowance for clearance at end of unit ∆a2 + ∆a3 ≥ 15 + 4 mm = 19 mm say 20 mm The bearing stress should also be checked for the hollowcore unit. 20

H20 bar

10 30

105 20

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Figure 4.4 Typical methods to avoid spalling of bearing corners.

Chamfer option

Lowered support area option

Spalling of the support is avoided if a large chamfer is provided on the outer corner or alternatively a local part of the bearing shelf is lowered, see Figure 4.4. This and the compressed thickness of any bedding material in the bearing must be sufficient to avoid contact, taking into account any long-term movements, deflection, hogging and if the floor is laid to a fall for any reason, the difference in angle of the floor soffits at its end and that of the bearer beam. Neoprene is recommended as a suitable material for bearings but other materials may be used (see also PCI Design Handbook27). In an HCC situation, the bearing may be in a different state when it carries construction actions and when it is fully constructed and carries superimposed permanent actions and variable actions. These interactions should be considered and very soft bearing materials may be inappropriate if the final objective is to have a fully continuous connection.

4.8 Connections between precast floors and in-situ concrete beams

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Type 3 and 4 systems that use precast floors with in-situ beams do not always have a direct bearing since the in-situ concrete is often cast against the precast unit. The floor is propped and the formwork for the edge beam is fixed. The steel protruding from the floor units is incorporated into the reinforcement of the edge beam that is then cast. The continuity steel must be fully anchored in both the in-situ and precast concrete. Consideration should be given to the possibility of tension occurring in the bottom steel at the support. This can be caused by temperature and shrinkage effects. The design of the interface for shear requires the provision of ‘hang-up steel’ as the shear load in the floor is concentrated near to the bottom of the section. This is described in Eurocode 2, Cl. 6.2.1 (9) and is also shown in Figure 3.6.

Structural elements and connections 5

5. Structural elements and connections 5.1 Twin wall construction (type 1)

Twin wall panels comprise two skins of precast concrete, connected by steel trusses, which hold the precast skins apart at a constant spacing to produce a wall of a particular thickness. Figure 5.1 shows a typical view of a twin wall panel system.

Figure 5.1 Typical example of a twin wall panel. Photo: John Doyle Construction Ltd

The panels are supplied to site, erected and then filled with in-situ concrete to form a solid concrete wall. The trusses, therefore, also act t o hold the skins together against the pressure exerted by the in-situ concrete before this has cured. A typical layout is shown in Figure 5.2. The precast skins function as permanent formwork. The precast skins contain the main horizontal and vertical reinforcement for the wall, in the form of a cross-sectional area of fabric or bars, which can be specified by the designer. However, starter bars and continuity reinforcement must be provided within the in-situ concrete. Figure 5.2 Simple layout of a twin wall system.

The precast skins are connected and spaced by steel lattice

Main horizontal and vertical reinforcement for the wall is fitted within the precast skins

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5 Structural elements and connections

The twin wall system is often combined with a precast concrete permanent shuttering system, e.g. lattice girder slabs. This allows the minimum use of temporary formwork on site. The wall system is ideally combined with a precast lattice or composite slab floor, as the in-situ element of both the wall and floor can be combined to produce a monolithic structure.

5.1.1 Manufacturing process

The precast skin on one side of the panel is cast horizontally on a steel mould, with the trusses projecting. After curing, the assembly is rotated so that the trusses face down and can be cast into the pour for the precast skin on the other side, see Figure 5.3.

Figure 5.3 Precasting sequence for twin wall manufacture.

a) One side of panel cast with outer face down with trusses projecting upwards

b) Assembly then turned over and the second side of panel cast with outer face down

Fabric reinforcement, which can be specified by the designer, is cast into each precast skin, see Figure 5.4. A 60 mm thick precast skin could accommodate, for example:  25 mm cover to external face (or as appropriate to meet durability bond and fire requirements)  16 mm vertical bar  8 mm horizontal bar  10 mm cover to internal face (whilst not required for durability in the permanent condition, some cover here is advisable). Clearly, walls that require larger bar sizes to achieve required levels of reinforcement, or walls in exposed conditions, will in turn need thicker precast skins to achieve required covers.

Overall panel thickness The final wall thickness can range typically from 200 to 350 mm in total width, although thicker walls are possible. A typical 250 mm panel thickness may comprise:  60 mm precast skin  130 mm gap for in-situ concrete, starter bars, continuity reinforcement  60 mm precast skin.

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Figure 5.4 Twin wall connection with a lattice g irder slab. Precast concrete Lattice reinforcement In-situ concrete

Vertical reinforcement

Slab reinforcement

In-situ concrete

Lap length

Vertical reinforcement

Tie reinforcement

With two layers of reinforcement an overall wall thickness less than 250 mm is difficult to achieve. This is because the precast skin thickness is typically 50-70 mm each side (plus tolerance), and the thickness of the in-situ concrete in between must accommodate starter and continuity reinforcement with sufficient space for the concrete to flow around the bars, see Figure 5.5. With one layer of reinforcement it is possible to reduce the overall section thickness to 200 mm. It is worth noting that, due to the manufacturing process, tolerances on the inside faces of the precast skin are easily controlled and can reduce the space available for in-situ concrete or starter bars by 10-15 mm each side. Tolerance for the hybrid panel to be erected over the starters is a related issue and it is advisable to use a single row of starters, rather than one row each side as for a traditional in-situ wall.

45


Figure 5.5 The available space for vertical continuity reinforcement is restricted. Photo: Hanson Concrete Products

Overall panel sizes Typically the maximum panel dimensions are 10 m x 3.5 m as shown in Figure 5.6. These dimensions are often limited by the capacity of the lifting equipment, transportation or size of moulds. The minimum dimension of a panel is typically 1.20 m. Figure 5.6 Typical twin wall maximum panel dimensions.

10 m max. 3.5 m max.

3.5 m max.

46

10 m max.


5.1.2 Planning implications

A key impact of introducing twin wall panels, as with many prefabricated forms of construction, is to increase the amount of coordination required early in the programme. To assist in this planning it is advised that the points below are considered.

Which walls are twin walls? Agreement on the extent of twin walls is likely to involve architect, client, contractor, and structural and services engineers. Until fully defined, this decision may have an impact on the design programme – an issue that should be communicated to the proposer of the system (often the contractor).

Manufacturers’ requirements and their impact on co-ordination Often the twin wall manufacturer will require the following to be fully defined before commencing manufacture:  dimensioned CAD wall elevations showing all walls to be manufactured  locations of all cast-ins (e.g. junction boxes, conduit)  locations and sizes of all holes and cut-outs (e.g. for services, drainage, builders work, windows, downstand beams)  reinforcement to be cast into the precast skins  locations and details of any bend-out bars required  information showing which side of the panel is to be propped (to permit the prop attachments to be cast in). To produce CAD elevations showing this level of detail, the des ign of the services must be well progressed (and any builders’ work holes assumptions agreed and recorded); the architect must have frozen the wall layout; all suspended and ground slab levels, soffit levels and upstands/downstands must be fully defined and frozen; and the contractor must have defined a pour sequence so that the side to be propped can be identified. The designer should allow for the additional time required to coordinate the work.

Detailing continuity rebar at joints The catalogues of the twin wall manufacturer often show a number of typical joint details where fabric or loose bars are used, within the in-situ concrete, to provide reinforcement continuity. It is important to realise that the designer is responsible for detailing and scheduling such bars despite what may be implied in the catalogues.

Checking of fabrication drawings It is important that the designer checks the key panel layout drawings. The twin wall manufacturer produces shop drawings for each panel. They are likely to be presented to the designer just as the project begins on site. A plan for checking these should be set up in advance to avoid the confluence of site queries with panel drawing checking creating possible resourcing difficulties at a key project stage.

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5.1.3 Site erection and filling

The panels are typically propped on one side only, using typically two ‘push–pull’ props to achieve verticality. This requires a cast base slab on at least one side of the wall. If panels are being erected before the whole slab is cast, coordination with the contractor’s pour sequence will be required. Where no slab is adjacent, e.g. walls inside lift shafts, there should be a clear method statement on how these panels will be safely erected. The methods of fixing the continuity reinforcement, particularly if the walls are acting as shear walls, should be clearly stated. The contractor should provide a method statement for the following:  At panel base level, how the panel is fitted over the projecting reinforcement in the lower slab taking account of the accepted tolerances. Figure 5.7 indicates other points that should be considered.  At top of panel, how the vertical continuity reinforcement is fixed. One method may be to tie horizontal fixing bars onto the trusses (say two each side) and tie the vertical projecting bars onto those. The alternative proposal of pushing them into the wet in-situ concrete is not recommended. A template for the vertical bars should be considered to ensure that the next lift of wall panel will fit over them correctly.  For the fabric reinforcement at joints between adjacent panels at the same level, how this is held in position within the pour (see Figure 5.8).

Figure 5.7 Typical issues to consider in the layout design. Decide which side props should be positioned

Decide from which level the wall should spring

Consider tolerances for starter bars

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Figure 5.8 Plan view showing horizontal continuity reinforcement.

Use of U-bars or links recommended to ensure reinforcement remains in correct position during concreting

Distance to 1st truss is typically 340 mm

Check sufficient lap length with reinforcement in skin

As the precast skins take up a fair proportion of the overall width of the wall, the gap between them is often very narrow in comparison to their height. This may make it difficult to remove all the air when concreting (‘blowing out’) and the contractor should provide specific proposals for this. Due to the very low volume of in-situ concrete required to fill the walls on site, the contractor’s preference may be to erect a large number of wall panels at one level, before arranging a concrete pour to fill them. As the precast skins are functioning as permanent formwork, resisting the pressure of the wet in-situ concrete, the wall manufacturer’s catalogue may have rate of rise limits – typically less than 1 m/hr. Coupled with the low fill volume, this leads to a relatively slow filling process on site, and one that the operatives may be tempted to speed up! The operatives should be made aware of and respect the wall manufacturer’s rate of rise limits. The panels are typically erected on chocks to leave a gap at the base of around 30 mm. This is the principal means of checking that the in-situ concrete reaches the base of the pour. Timbers acting as grout checks are placed along each side at the base of the panel.

Precast lattice girder slab units Figure 5.9 shows a typical section of a composite floor using precast lattice girder units. The lattice girder is cast into (usually class C40/50) concrete reinforced with high-yield reinforcement. The width of the precast slab is typically 2.4 m with a depth of 50 mm or 75 mm. They are used for spans of up to 10 m (larger spans are possible with careful planning). Figure 5.9 Section showing typical lattice girder floor.

In-situ concrete

Lattice

Precast concrete slab Main steel Distribution steel

49


Figure 5.10 Typical details of a lattice girder.

Top bar 8 mm to 14 mm dia.

Diagonal bar 4.5 mm to 7 mm dia. Bottom bar typically 5 mm dia.

The design of the lattice girder is dependant on the thickness of the composite floor, final loading and propping system. Typical details of a lattice girder are shown in Figure 5.10. Figure 5.10 shows a typical layout of a 2.4 m wide unit containing four sets of lattice girders. Propping to support the self-weight and in-situ concrete can be reduced or eliminated by increasing the stiffness of the slabs through increasing the diameter of the reinforcement to the top of the lattices and/or reducing the spacing of the lattices. Unpropped spans of up to 5 m can be achieved depending upon the design loads and the overall depth of the slab. Temporary propping is required where the end bearing is small. An example of this is at end supports where the slab unit is seated on just one leaf of the wall. Normally the minimum cover to the reinforcement will be 20 mm; however, the cover to the reinforcement can be adjusted to meet the specific bond, durability and fire resistance requirements for individual contracts.

5.1.4 Design of panels

Design moments Design moments about the minor axis of a wall should be considered even where central bars are placed in the joint, as these do not represent a hinge.

Flexural, shear and axial design When checking the strength of a section of a wall more than a full lap length from a joint the full width of section may be included. Otherwise just the in-situ part should be considered. If the whole section is in compression, it is reasonable to assume that the full section can provide axial resistance.

Lap lengths At the top and bottom of the wall there will be a lap between the main vertical reinforcement and the vertical continuity reinforcement, see Figure 5.4. When the distance between these bars is greater than 4φ or 50 mm the lap length should be increased by a length equal to the space between the bars (Eurocode 2, Cl. 8.7.2(3)).

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Minor axis bending If the decision to use a single row of starters has been adopted, minor axis bending on such walls should be checked. If significant, the decision should be revisited – with potential impacts on wall thickness, as noted above.

Horizontal joint between panels stacked one above the other (no slab adjacent) Horizontal joints commonly occur, for example, in lift shaft walls, or in walls adjacent to risers, stairs, or double-height spaces. At the panel joint level, effective design to resist minor axis buckling moments would tend towards the use of two rows of vertical continuity reinforcement (one layer on each face) within the in-situ portion of the wall. Due to the position of the continuity bar within the in-situ portion, and the possible tolerance and positional control issues, a realistic effective depth should be used in assessing the moment capacity of the wall at this point, see Figure 5.11. Figure 5.11

C

Detail at an unrestrained horizontal panel joint in compression.

The tendency to buckle under compression at an unrestrained horizontal joint, is resisted by the vertical continuity reinforcement, acting at a reduced lever arm.

d

5.1.5 Concrete and finishes

Concrete mix The nature of the in-situ concrete mix used to fill the panels on site should be considered. As the gap between the precast skins may be as little as 100 mm for a 250 mm wall, and starters and continuity reinforcement may protrude into this gap, using a vibrator poker may be difficult or impossible. The use of self-compacting concrete should be considered. A smaller aggregate size, for example 10 mm, may also be appropriate.

Surface finish Typically, the use of steel moulds gives the external faces of the panels a smooth finish. The finish quality is suitable to receive a plaster finish or, on request, wallpaper. It should be noted, however, that the finish is not ‘architectural’ concrete as colour is not consistent or easily specified.

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5.1.6 Detailing

Vertical joints between adjacent panels at same level At junctions between adjacent panels and at corner junctions, horizontal continuity reinforcement is recommended within the in-situ portion. Detailing this reinforcement in the form of fabric or prefabricated cages is likely to be the easiest way of fixing it within the pour. As noted above, the designer should be responsible for detailing this reinforcement. It should be noted that the presence of the trusses at a typical distance of 340 mm from the ends of each panel effectively constrains the volume in which continuity reinforcement can be provided. If the forces applied to the wall are such that they cause significant shear or tensile forces to develop at the vertical panel joints, the suitability of twin wall panels as a design solution may need to be revisited.

Interface with reinforced concrete ground slab. It is important to obtain the contractor’s pour sequence for the ground slab – at locations where the ground slab steps (changes level) this will often define the panel base level. Agree with the contractor whether the panel will sit on the higher-level slab, or on the lower-level slab with the higher-level slab poured up against the wall. Also agree details at the edge of slabs or at lift pits. Agree from which side the panels are to be propped. It is likely that the twin wall panels will need to be installed over projecting starter bars cast into the foundations. As well as the use of a template and the consideration for using a single row of starters, as noted above, the starters will need to coordinate with the horizontal continuity reinforcement provided at locations of vertical joints between panels. This means five or six layers of reinforcement locally overlapping within the gap between the panels – a potential congestion issue, see Figure 5.12. Figure 5.12 Horizontal continuity reinforcement to fit with twin wall reinforcement.

Horizontal continuity reinforcement lowered into position after placing twin walls. This must be detailed to miss the wall trusses

5.2 Precast columns, edge beams and in-situ slabs (type 2)

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The type 2 system uses precast columns and edge beams, often with a prestressed in-situ floor slab. The complex fixing of steel and anchorages in the edge strips is more safely and accurately carried out in the precast concrete factory. The use of precast concrete columns speeds up the time between the casting of the floor plates. The precast edge strip is supported on the same shutter system that is used for the floor.


5.2.1 Column to floor joint

The column to floor joint in this form of construction is assumed to be semi-monolithic, i.e. the in-situ concrete is cast up to the surface of the column or a fully grouted connection is made. It may be desirable that levelling devices, for example nuts and wedges, having no load bearing function in the completed structure should be slackened, released or removed as necessary. Where this is necessary, the details should be such that inspection (to ensure that this has been done) can be carried out without undue difficulty. The design of the vertical continuity or tying reinforcement requires careful consideration. Three examples are shown in Figure 5.13. Where a central dowel bar, as shown in Figure 5.13, is also acting as a vertical tie, the load on the grouted connection between the slab and the dowel bar can be significant. The designer should ensure that the detail can carry this load either by design or through testing.

Bearing under the precast column In the absence of more accurate information (derived from a comprehensive programme of suitable tests), the area of concrete that should be considered in calculating the strength of the joint should not be greater than 90 per cent of the area of column assumed to be in contact with the joint, unless specific means are taken to ensure that no voids exist in the grout. The strength of the concrete in the precast column may be taken as f cd (= 0.85f ck/1.5). The area of any bar passing through the joint should be deducted from the bearing area. The design force of such a bar may be deducted from the applied force on the bearing when calculating the capacity of the concrete provided that the bar has sufficient anchorage beyond the joint.

Grouting The contractor should provide a method statement for the grouting work. This should ensure that no pockets of air are trapped in the ducts and that the interface between the base of the column and support is fully grouted. Trials may be necessary to demonstrate the method.

Maximum compression through floor For axial load with moment transfer Eurocode 2, Cl. 6.7 limits the compression within the slab. Exp. (6.63) is modified to: F Rdu = Ac0,eff f cd √( Ac1/ Ac0,eff ) ≤ 3.0 f cd Ac0,eff

where Ac0,eff Ac0 f cd Ac1 h b1 d 1

= = = = = = =

0.9 x Ac0 area of precast column design strength of the slab (h/2 + b1) (h/2 + d 1) depth of slab breadth of the precast column depth of the precast column

53


a) Column shoe

b) Column bar coupler

Hole grouted before placing column

Grouting ring

Bars welded to dowel table and column reinforcement

c) Central dowel bar

Figure 5.13 Typical column floor connections.

54


Where moment is transmitted through the joint Ac0,eff should be reduced to 0.9 x the area of the stress block shown in Figure 5.14, where Ac0,eff = 0.9 b1 x 2(d 1/2 - e). Figure 5. 5.14 14 CL of column

Stress block in slab where moment is transmitted from column.

d 1 e = M/N

CL of compression block

f cd

2 x (d1/2 - e )

5.2.2 Vertical tie

For class 2B and 3 buildings (risk group 2B and 3 in Scotland) the vertical tie must be designed to take the full floor load in tension under ‘accidental’ loading conditions. The partial factors for the accidental combination of actions are equal to 1 (see BS EN 1990, and UK National Annex, Table Table NA.A1.3), see worked example e xample 6. If a central dowel bar system is considered for such a floor, i.e. span > 7 m, it should be effectively continuous throughout the height of the building. Full tension mechanical couplers should be used where joints are required.

5.3 Biaxial 5.3 Biaxial voided slabs

Figure 5.15 15 shows a typical section of a composite floor using precast lattice girder units with spherical void fomers (biaxial voided slabs). The lattice girder and the void former cages are cast into a (usually class C40/50) concrete panel containing reinforcement in two directions. The width of the precast slab is typically 2.4 m with a depth of 50 mm or 70 mm. Normally the minimum cover to the reinforcement will be 20 mm; however, howe ver, the cover to the reinforcement can be adjusted to meet the specific bond durability and fire resistance requirements for individual projects.

Figure 5. 5.15 15 Typical layout of biaxial voided slab. Photo: Cobiax Technologies Ltd

55

5 Structural elements and connections Project details

Calculated by

RW

Job No.

Worked example 6 Vertical tie

Checked by

OB

Sheet No.

WE 6/1

Client

TCC

Date

April 08

CCIP-030

Consider a 9 m x 9 m flat slab floor 300 mm thick with imposed variable load of 3 kN/m2 and finishes of 1 kN/m2.

Actions The total design force in vertical tie = Gk + Ad F Ed = 9 x 9 x {(25 x 0.3 + 1.0) + (0.5 x 4) = 851 kN.

Eurocode, Table NA.A1.3

Resistance Using a column shoe system: 4 No 25 mm bars will provide a resistance of = γs f yk As F Rd = 1.0 x 500 x 252 x 4 x π/4/1000 = 981 kN F Rd > F Ed → OK Use 4 No 25 mm bars

5.3.1 Slab geometry

Initial sizing can be determined from manufacturers literature. The manufacturer literature will also advise the size of the spheres available, the spacing requirements and the general configuration of the slab.

5.3.2 Flexural design

The benefit of the reduced self-weight should be taken into account in the design. The design may assume a flat slab model, which has been demonstrated as appropriate through testing of the slabs. A check should be carried out to ensure that the concrete compression zone remains outside of the depth of spherical void formers. Where this is not the c ase, as in heavily loaded slabs, the manufacturers will be able to offer appropriate guidance on determining the permissible compression zone that can be used in the calculation of the flexural strength.

5.3.3 Shear design

Testing has been carried out to determine the shear strength of this type of slab, alongside a theoretical assessment of the reduction in the shear plane due to the inclusion of the voids. The manufacturers recommend that shear strength of a solid slab of the same depth should be reduced by a factor of between 0.55 and 0.6 to obtain the design shear resistance for the voided slab, see s ee Figure 5.16. For punching shear it is recommended that the void formers are left out o ut where the design shear stress exceeds the reduced shear resistance of a voided slab, see Figure 5.16. Punching shear checks may then be carried out on the solid slab areas around the columns.

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Figure 5. 5.16 16 Typicall layout with final reinforcement in Typica place. Photo: Cobiax Technologies Ltd

5.3.4 Deflection control

Manufacturers have carried out testing to determine the reduced stiffness of the slabs due to the voids. Conservatively, the stiffness of the voided slab may be taken as 0.87 times the stiffness of a solid slab, although in some configurations the factor may be increased to 0.96. The manufacturers have data available to take advantage of t hese situations. When using a finite element analysis, the stiffness of the slab (by adjusting the modulus elasticity) can be reduced accordingly. The use of the span-to-effective depth rules of Eurocode 2 is not valid for this form of construction since it is not clear how the slab stiffness is incorporated in the manufacturers design expressions.

5.3.5 General considerations

Splice bars are used across the panel joints so that the slab may be designed as a continuous member. Figure 5.16 16 shows a typical layout including the final reinforcement.

Buoyancy of voids Whilst the concrete is being place and vibrated, the buoyancy force can reach the displaced weight. The void formers are held in place by:  firm tying of the void former to the lower and upper reinforcement  casting of concrete in several stages (normally two, but three may be required where the voids are larger than 360 mm).

Slab edges Voids are not normally provided near slab edges to ensure a robust and continuous edge detail.

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5.4 Prestressed hollowcore units

Prestressed hollowcore units are produced by an extrusion or slipform process with a typical width of 1200 mm, in lengths of up to 200 m. Each length is prestressed before casting. After curing, the units are sawn to the required length. Figure 5.17 shows a typical production layout. It should be noted that where the only reinforcement in the units is the prestressing strands, as is common, it makes the support zone particularly vulnerable since this is where the maximum stresses due to bearing, shear and anchorage occur. The design should be in accordance with Eurocode 2.

Figure 5.17 Typical hollowcore unit production.

Hollowcore units have lateral edges provided with a longitudinal profile in order to make a shear key for transfer of vertical shear through joints between contiguous elements. For diaphragm action these joints are designed to resist horizontal shear. Hollowcore units are often specified from manufacturers’ tables rather than designed from first principles. These tables are based on assumed loading, support and reinforcement details, and where the actual situation varies from that assumed in the tables, e.g. the existence of concentrated loads or different fire rating, detailed calculations should be made to verify such units are appropriate. BS EN 11683 describes the requirements and the basic performance criteria and specifies minimum values where appropriate. It covers terminology, performance criteria, tolerances, relevant physical properties, special test methods and special aspects of transport and erection. Reference should also be made to Precast Prestressed Hollowcore Floors28. An example of the design of a hollowcore unit is given in Precast Eurocode 2: Worked Examples29.

5.4.1 Anchorage of prestressing tendons

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Resistance at the end of the hollowcore unit relies on the interaction of shear and bond, therefore it is very important to understand the end prestressing conditions of hollowcore units. Figure 5.18 shows how the stress in the prestressing wires or strands and the moment of resistance, builds up from the end of a unit and further guidance is given in Eurocode 2, Cl. 8.10.2.2.


5.4.2 Transmission length

The transmission length, l pt, for the prestressing wires or strands is that length required to transmit the full prestress, σ p. l pt is defined in Eurocode 2, Cl. 8.10.2.3 where σ pt1 and l pt1 are the values at ‘transfer’ and σ pt2 and l pt2 are the values after all losses (as shown in Figure 5.18). The ultimate design strength of the tendon requires further anchorage length. The slope of the line between σ pt2 and σ pd is less than that for the transmission length, l pt2, because the tendon reduces in size as it is stressed. The reverse is true within the transmission length over which there is a wedging effect. One reason for assuming a linear build-up of stress is because any flexural stress in this region will tend to reduce the section size and nullify the wedge effects.

Figure 5.18 Build-up of stress in prestressing wires or strands from end of unit.

Tendon stress σ pd σ pt1 σ pt2

Distance from end of unit I pt1 I pt2 I bpd

5.4.3 Cracking length

The cracking length, l cr, is the distance from the end of the unit to the point where the bottom fibre stress resulting from all actions (bending, prestress and horizontal forces at the bearings) equals f ctd. Figure 5.18 shows the components of actions and the net effect on the bottom fibre stress. Note that if l cr is less than l pt2, the prestress is reduced. Figure 5.19 indicates the results from the example given in the Precast Eurocode 2: Worked Examples29. The following points are of particular note:  Consider all action effects to determine where the unit is likely to crack.  Where dry or mortar bearings are used large horizontal forces may arise from temperature and shrinkage effects.  In this example the horizontal force at the bearing may cause cracking close to the end of the unit, before l cr is reached, see Figure 5.19(d). If cracking does occur close to the support, the shear resistance is likely to be exceeded.

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Figure 5.19 Build-up of bottom fibre stress in concrete from end of unit.

C L of unit

C L of unit

Support Bottom fibre stress

Bottom fibre stress f b,m = Mx/Z b

Support

a) Stress due to flexure

f b,P = P / Ac + P e/Z b

b) Stress due to prestress

C L of unit

Support Bottom fibre stress f b,H = H / Ac + Hy b/Z b

c) Stress due to horizontal force at support

Bottom fibre stress

Possible overstress near end of unit Compression

CL of unit

0 f ctd

Tension l cr

f b,Net = f b, M + f b,P + f b,H d) Net bottom fibre stress showing cracking length, l cr

5.4.4 Total anchorage length

The total anchorage length, l bpd, is the distance from the end of the unit to the point beyond which the full design resistance of the wires or strands can be obtained, as shown in Figure 5.18.

5.4.5 Tendon slip at ends of units

When the prestress is transferred from the anchor blocks to the hollowcore units, there is anchorage bond along the full length of the strand, apart from the transmission length at each end of the prestressing line. The concrete is then cut into the required lengths and at each end a further transmission length is introduced. Although expressions have been developed to determine the relationship between the end slip of the strands and the transmission length, it has been shown27 that, for hollowcore units that have been sawn, there is no simple relationship between transmission length and initial slip at these positions. This is discussed further in Section 6.6.

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Shear tension crack

Anchorage slip

Horizontal splitting cracks

Large crack close to support

b) Shear tensio tension n failu failure re

a) Ancho Anchorage rage bond failure failure

c) Horizontal splitting cracks

Figure 5.20 Types of end failure.

5.4.6 Types of end failure

5.4.7 Anchorage bond failure

Figure 5.20 shows the three typical types of end e nd failure that may occur. It should be noted that types a) and b) can interact, one reducing the resistance of the other.

Anchorage bond failure, see Figure 5.20a, may occur due to cracking close to the support which does not allow the full anchorage resistance to develop and strands start to slip. This causes the crack to grow until the unit fails. The most common cause of anchorage failure is when the end of the unit is subject su bject to movement relative to its bearing. This may be the result of the effects effec ts of one or more of the following:  shrinkage  temperature changes  humidity changes  vertical loading. It is important that the designer considers each of these possible effects. This is especially important for units with spans greater than 8 m. Reference should be made to Movement, Restraint and Cracking in Concrete Structures 26.

5.4.8 Shear resistance

Cracked sections The cracked shear resistance should be checked at positions likely to be cracked at the ultimate limit state. The position at which this check should be carried out is at a distance l cr from the end of the unit, see Section 5.4.3. The shear tension resistance is calculated in accordance with Eurocode 2, Exp (6.2a and b) together with UK National Annex: (100ρl f ck)1/3 + 0.15σ cp]bwd V Rd,c = [0.12k (100 with a minimum of V Rd,c = (0.035k 3/2 f ck1/2 + 0.15σ cp)bwd

61


where k ρl σ cp

= 1 + (200/d )0.5 ≤ 2.0 = Asl /bwd ≤ ≤ 0.02 (normally = 0 since the distance to the end of the unit < l bpd + d ) = N Ed/ Ac < 0.2f cd (N Ed should be taken as γp times the prestress force)

Uncracked sections Shear tension failure, see Figure 5.20, occurs occ urs when the tension in the webs of the slab becomes too high causing a sudden failure. For a circular core section the critical section for a shear tension failure is likely to be at h/2 from the inner face of the support, see Figure 5.21. For oval core shapes the critical section is likely to be closer to the bottom of the section. Figure 5.21 Critical section for shear tension failure. Critical position

For circular core shapes = h /2 Forr ova Fo ovall co core re sh shap apes es say h / 3

s

The shear tension resistance is calculated in accordance with Eurocode 2, Exp (6.4): V Rd,c = I bw/S {(f ctd)2 + αlσ cp f ctd }0.5

where I bw S αl l x l pt2 σ cp

= = = = =

second moment of area width of the cross-section at the centroidal axis first moment of area above and about the centroidal axis l x/l pt2 ≤ 1.0 distance of the section considered from the starting point of the transmission length = upper bound value of the transmission length of the prestressing element according to Exp (8.18) of Eurocode 2 = concrete compressive stress at the centroidal axis due to prestress (this should include γp = 0.9)

For cross-sections where the width varies over the height, the maximum principal stress may occur on an axis other than the centroidal axis. In such cases the minimum value of the shear resistance should be found by calculating V Rd,c at various axes in the cross-section. (Note: At the time of writing a revision to this expression was being considered by the Eurocode 2 Committee in discussion discuss ion with the Committee for BS EN 1168.)

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5.4.9 Further design checks

BS EN 1168, Precast concrete products – Hollowcore slabs 3 sets out further design checks that are required: 1)  Prevention of horizontal splitting cracks (Cl. 4.3.3.2.1)  Combined shear and torsion (Cl. 4.3.3.2.2) 4.3.3.2.2)  Shear capacity of longitudinal joints (Cl. 4.3.3.2.3)  Punching shear capacity (Cl. 4.3.3.2.4)  Transverse bending caused by concentrated loads (Cl. 4.3.3.2.5) 4.3.3.2.6)  Additional torsion where one long edge cannot deflect (Cl. 4.3.3.2.6) It is also critical that the requirments for bearings, see Section 4, are fully satisfied, otherwise there is a danger of deterioration de terioration of the supporting nibs nibs and ends of the hollowcoree units that could lead to a shear and anchorage failure of the hollowcore units. hollowcor

5.4.10 Lateral distribution of vertical loads

5.4.11 Multi-span without structural topping

Floors are not always uniformly loaded; they often are required to carry point loads and line loads from partitions to supporting beams. BS EN 1168, Appendix C, Transverse Transverse Load Distribution, charts factors that can be used to determine the loads on units adjacent to the loaded unit. These charts are for use with units in floors with no or one free edge. They apply to units without structural topping and are therefore conservative for units with structural topping. BS EN 1168, Cl. 4.3.3.2 provides a method of assessing transverse tensile stresses in the hollowcore units that are un-reinforced in the transverse direction.

Longitudinal tie bars Hollowcore units should be connected to the supports or to the adjacent floor bay by means of longitudinal tie bars. Tie arrangements should realise the structural integrity and meet the requirements with regard to:  diaphragm action  transverse distribution of vertical loads  differential settlements  restrained deformation  robustness (in accordance with Section 3.2). The longitudinal tie bars should be equally distributed and their spacing should not normally exceed 0.6 m at edge supports and 1.2 m at intermediate supports. The ties pass through grouted longitudinal joints between units, see Figure 5.22, provided that they are anchored into the members supporting those units (see also section 3.1), or in the concreted cores of the units, see Figure 5.23; in either case it important that the bars are fixed in the correct position, as shown. If the latter method is used, note that it is essential that, after removing the top flange, the open core is thoroughly cleaned to allow good bonding of the new and old concrete.

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Figure 5.22 Placing tie bars between hollowcore units. h/2 Limits to the placing of tie bar

≤ 2φ, ≤ 25 mm

≤ 2φ, ≤ 25 mm

Limits to the placing of tie bar

40 mm

a) With grouting key at top

b) With grouting key at bottom

Figure 5.23 Placing tie bars in hollowcore unit. h/2

Normal limits to placing of tie bar φ

The yield load of the tie bars anchored in any core of a unit or between units should not exceed 80 kN and the total yield load per unit should not exceed 160 kN. If the yield load for a tie bar between units is greater than 30 kN, hooked bars should be used. In such cases the anchorage length should not be less than 75φ , as shown in Figure 5.24. Otherwise straight bars may be used with a minimum anchorage length of 100φ . Figure 5.24 Minimum length of tie bar between units.

h/2 ≤ 75 φ ≥ lcr

The anchorage length of a tie bar should not be less than l cr (see Section 5.4.3). The anchorage length should normally be sufficient to anchor the yield load of the tie bar (see also Precast Prestressed Hollowcore Floors28). In order to prevent progressive collapse the anchorage length should be increased by ∆l b in accordance with Table 5.1 (see worked example 7). Table 5.1 Additional anchorage length, ∆ lb, for ribbed tie bars with regard to design against progressive collapse.

Concrete grade Grout Concrete

64

C20/25

C30/37

13φ 11φ

10φ 9φ

Structural elements and connections 5 Project details

Calculated by

RW

Job No.

Worked example 7 Anchorage length of longitudinal tie bar

Checked by

OB

Sheet No.

WE 7/1

Client

TCC

Date

April 08

Consider the use of 20 mm size straight bars with C30/37 grout. l cr = 1080 + 10 x 20 = 1280 mm

CCIP-030

For example 10 of Precast Eurocode 2: Worked Examples 29

Minimum length of 10φ = 10 x 20 = 200 mm. Use anchorage length = 1280 mm

Where further strengthening of the support zone is required the tie bars should be anchored to transfer their yield load at any cracked section within the critical support zone. In such situations the tie bar should be placed above the mid-height of the hollowcore unit to provide moment capacity and should be anchored with a hook. An additional anchorage length, l add, should be provided to ensure the shear transfer between the in-situ concrete or grout and the hollowcore unit. l add = F st/f ctu

where F st f ct u

= tensile capacity of the tie arrangement in one core or joint = tensile strength of the in-situ concrete or grout = perimeter is of the core or 2h for anchorage in joints (h is height of the hollowcore unit)

Alternatively, straight bars may be used. In this case the anchorage length should be increased to l cr + l bd (see Eurocode 2, Cl. 8.4.4) (+∆l b) for anchorage in concreted cores and to l cr + 100φ for anchorage in grouted joints.

Connections to walls If the wall supports more than three floors, it is advisable to provide hollowcore units with slanted ends and for the ties to be anchored in the concrete cores (not between units) as shown in Figure 5.25. It is important that the reinforcement is detailed to interlock as shown. If the wall supports less than three floors, it will normally be satisfactory for the units to have a square cut, but the reinforcement details should be as shown in Figure 5.25. Details that do not provide a mechanical link should not be used.

65


a) Edge support

b) Intermediate support

Figure 5.25 Connections to walls. In-situ concrete

In-situ concrete

a) Edge support

b) Internal support

Figure 5.26 Connections to beams.

Connections to beams Typical connections to beams are shown in Figure 5.26.

Connections to ledge beams The continuity tie reinforcement should interlock with the reinforcement of the supporting beam. A typical detail is shown in Figure 5.27. The flange width of the supporting ledge beam should be limited to the continuous solid section at the ends of the hollowcore units or confined to the depth of their top flanges.

66


Figure 5.27

Transverse tie bars

Typical detail for connections to ledge beam.

/b (+/b) /cr

Figure 5.28 Typical detail showing the tie reinforcement within the structural screed. Tie reinf orcement within structural screed

Tie reinf orcement within hollowcores

Tension lap length

a) For Class 2A structures

b) For Class 2B and over structures

Figure 5.29

Tying reinforcement within hollowcores

Typical detail showing connection of tying reinforcement to an edge beam.

Tension lap length

a) For Class 2A structures

5.4.12 Multi-span with structural screed

b) For Class 2B structures

Figure 5.28 shows a typical detail where the tie/flexural reinforcement is placed within the structural screed. Where the structural screed is used to provide the tie and flexural continuity reinforcement it should be adequately tied to the perimeter ties. Figure 5.29 shows a typical detail for this.

5.4.13 Dimensions

The permitted deviations are specified in BS EN 1168, which are complementary to those given in Eurocode 2. BS EN 1168 provides further restrictions with respect to cover based on the geometry of the hollowcore units. The following are extracts from that standard.

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5.4.14 Tolerances for construction purposes

The maximum deviations, unless declared by the manufacturer, shall satisfy the following:  slab length ± 25 mm ± 5 mm  slab width  slab width for longitudinally sawn slabs ± 25 mm

5.4.15 Minimum concrete cover and axis distances of prestressing steel

The minimum cover c min to the nearest concrete surface and to the nearest edge of the core, as stated in BS EN 1168 shall be:  for the exposed face, the one determined in accordance with Eurocode 2, Cl. 4.4.1.2;  for preventing longitudinal cracking due to bursting or splitting and in the absence of specific calculations and/or tests as follows: when the nominal centre to centre distance of the strands ≥ 3φ : c min = 1.5φ ;   when the nominal centre to centre distance of the strands < 2.5φ : c min = 2.5φ ; c min may be derived by linear interpolation between the values of above  where φ is the strand diameter (mm). In the case of different diameters of strand, the average value shall be used for φ .

5.5 Double tee beams

a) Without structural screed

b) With structural screed

Figure 5.30 Typical double tee units.

68

Double tee beams are ribbed units, usually with two ribs in each 2.4 m wide unit. Other widths can be provided. It is also possible to obtain an inverted trough unit with the ribs at each unit edge. The double tee is the lightest precast unit for spans in the 9 to 20 m range thus requiring a lighter support structure than hollowcore, for example, see Figure 5.30. Alternatives to double tees exist in the form of multi-rib units, usually with three ribs.


The shape of the of the double tee unit is particularly suitable and economical for prestressing because of the high position of the neutral axis, which maximises the lever arm, and because the ratio of the top and bottom fibre modulus is similar to the concrete to steel modular ratio. Double tee units can be procured in a variety of depths, from 300 to 800 mm and even beyond, but the most common unit is 600 mm deep as this conveniently carries office loading to 12 m and car park loading to 16 m. The most common application of double tee units is in car park structures. The top flange is usually 50 or 60 mm deep and the ribs taper from a minimum of 140 mm at the base, widening towards the underside of the top table, the taper of 1 in 20 each side allowing for easy lifting out of a fixed mould. There are variations to dimensions as some manufacturers have fixed moulds set for the full depth, e.g. 800 mm, and fit pallets inside to make units of less depth; thus the shallower the unit is, the wider the bottom of the web. It is advisable to check what dimensions are available from the manufacturers at the time of design, although these variations are not usually critical. In order to achieve maximum economy, grids should be at 2.4 m modules, 7.2 m being the most common. Specially shaped units, to cover irregular grid areas, narrow or tapering units, units with splayed ends and notched units to fit round columns and others, can be supplied. Double tee units are normally designed by the precast manufacturer and a typical example of this is given in the Precast Eurocode 2: Worked Examples 29. BS EN 13224, Precast Concrete Products – Ribbed Floor Elements 5 provides the specification for materials, production, properties, requirements and methods of testing for ribbed floor elements. This includes a section on permitted deviations and minimum dimensions.

5.5.1 Self-stressing moulds

5.5.2 Welded joints

A less common system for manufacturing double tees is by using self-stressing moulds. These can incorporate deflected strands (see Taylor30).

Welding is commonly used in double tee construction as tests and experience show that the welded connection between flanges is the only method of connection that is positive, taking account of differential camber, and that gives excellent long-term performance with respect to controlling cracking at the flange joints in car park construction from the rolling loads (see Figure 5.31). In car parks it is common for the weld plates and the welded cross-bar to be in stainless steel with the anchor bars beneath the concrete surface in mild steel. Manufacturers have procedures for ensuring the stainless to mild steel welds are made correctly, and account for the higher temperatures required with stainless steel. This can result in more expansion of surface mounted plates and spalling on site.

69


Figure 5.31 Typical double tee connection details.

100 x 40 shear connector with 25 φ bar in welded connection 100 x 100 steel plate anchored into double tee unit (The plate should be welded to the anchorage bar)

Where welding is permitted it should be the responsibility of an erection subcontractor and carried out before the structure, including any areas immediately below, is released for access by other trades. Thus safety issues with respect to personnel (arc eye) and fire in debris beneath are controlled. It is essential to ensure that the erection subcontractor is experienced in welding work, that modern gas shielded weld procedures are used by trained and tested welders and that site procedures take account of welding hazards with respect to shielding from arcs and in the removal of any flammable material from the workplace. Weld inspection procedures should be agreed with the welder. End connectors are critical and should all be de-slagged and inspected. The number of flange connectors usually allows inspection to be on an agreed statistical basis.

5.5.3 Structural topping

70

Double tee secondary reinforcement usually consists of end cages, which commonly protrude from the top surface of the unit to bond into the structural topping, and a special light fabric in the top table (flange), sometimes held in place by five or seven stressed wires. To assist shear flow from the rib to flange at the ends, it is also usual to provide some transverse steel in the flange at the end, see Figure 5.32.


Steel end plate with internal anchor for tie to support

May protrude into structural topping

Lifter position decided by supplier

Transverse bars for shear flow with flange

U-bars to close links at end of web.

Upper strand layer may be debonded

Link cage nominally 10 mm at 50, 100 & 200 ctrs in end 2d of unit to aid anchorage of strands

Lower strand layer should never be debonded

End U-bar or anchored angle to restrain spall potential at end of rib

Figure 5.32 Typical double tee end detail.

Where the designer has designed the double tee floor as a slab in accordance with Eurocode 2, Cl. 6.2.1 (4) minimum shear reinforcement is not required when VEd ≤ VRd,c. Apart from the main stressed strands, double tee beams often only have reinforcement in the form of a light fabric in the top flange to control shrinkage and transportation stresses and a light end cage in the web to control transfer transmission zone stresses. Structural concrete screed with fabric reinforcement is often provided for the final structure. This is also used to augment the welded connections between units. The lateral shear connectors, which should be welded, provide lateral continuity between the double tee units and can spread concentrated loads from one unit to another. The fabric size is defined by the need for transverse ties (they augment the welded shear connection tie capacity) and in some cases for load distribution of point loads on the floor to adjacent units.

5.5.4 Transverse distribution of concentrated loads

Floors are often required to carry point loads and line loads from partitions to supporting beams. Eurocode 2, Cl. 10.9.3 (5) states that transverse distribution of loads should be based upon analysis or tests. The designer should check any test report carefully to ensure that it covers the specific design situation. It is not recommended that differences between the deflection of units are removed by jacking and then welding the shear keys. Any shear forces resulting from such an operation or any other load variation should be considered in the design of the connections.

71


It is recommended that the width of slab assumed to contribute to the support of concentrated loads (including partitions in the direction of the span) should not exceed the width of three precast units and joints, plus the width of the loaded area, or extend more than a quarter of the span on either side of the loaded area. In some forms of construction, for example long span wide units, these limits may be inappropriate and more detailed considerations should be made. Where there is a reinforced structural topping the width of four precast units and joints may be allowed to contribute. Elliott23 gives further information. Double tee floors can be designed either to carry line partition loads by providing extra strength in the unit beneath or by a 2D elastic analysis. The double tee deck can be taken as being comprised of a two-way beam grillage with the beam stiffness in one direction and the flange stiffness from the full flange depth in the other, even where the flanges between adjacent double tees meet. Double tee beams can be provided with additional reinforcement, for example links and additional longitudinal steel for more than the normal one hour of fire resistance, shear reinforcement for exceptionally heavily loaded cases and top steel for cantilever ends.

5.5.5 Tying requirements

Typical end and side connections are shown in Figure 5.33; these connections can be part of the tying strategy of the complete design. Free standing double tee beams with end and side shear connectors should always be put on elastomeric bearings. A mortar bed may only be used if sufficient reinforcement is provided through the joint to ensure that it behaves monolithically as shown in Figure 5.33, see also Section 4.1. The welded connection in Figure 5.33a is formed from two surface plates with anchoring reinforcement welded to it cast into and anchoring around the beam longitudinal steel, and in the double tee rib anchoring to the end cage. A surface plate is then placed on top and welded down with fillet welds. This anchorage can be used as part of the transverse tying of the structure.

Figure 5.33 Typical double tee connection detail. End of web End connector with welded tie bar

End of flange

Structural topping

Double tee

a) Standard double tee support with welded connection

72

b) Support of double tee with full continuity. Note: Temporary support of beam may be required


Double tees cast into in-situ edge beams should have protruding steel as a tie and this steel should be taken far enough into the double tee to ensure that it is fully lapped with the stressed reinforcement.

5.5.6 Half joints

To create half joints the ends of double tee units may be scarfed, as shown in Figure 5.34. Ends should not be scarfed to more than two-thirds of their depth, for example a 600 mm deep unit may be scarfed to 400 mm. Scarfing allows the edge beams to still support the double tee in the temporary situation and be no deeper than the double tee itself. The scarf may also be extended so as to provide a convenient path for services between double tee ribs. Figure 5.34 shows typical reinforcement in a scarfed end. Debonding should never be applied to the bottom strands in the rib or to the strands immediately above a scarf.

Figure 5.34 Double tee with scarfed end.

Strut and tie (1)

Strut and tie (2)

Reinforcement and anchorage provided for struts and tie layouts 1 and 2

Strand must be present and must not be debonded

Chamfer allows inclined tie to be in optimum position

Strand must not be debonded Strand must not be debonded

Only additional reinforcement for the mechanism of strut and tie is shown. This figure is to be read with Figure 5.32

5.5.7 Billet support of double tee units

A variant of the half joint support is to use a billet protruding from the rib at the end of the double tee at a high level, as shown conceptually in Figure 5.35. This has the advantage that a nibbed bearer beam is not required and that the bearer beam does not need ‘hang up steel’. A disadvantage is that the bearer beam has no restraint to rotation from the bearing force of the double tee at its soffit. This lack of restraint should be considered in the temporary condition, when there may be out-of-balance moments on the bearer beam and in the permanent condition for edge beams, or beams supporting

73


Figure 5.35 Support of double unit using billet connection.

Tie

Strut Double tee

Tie

The tension in the vertical tie will be about double the value of the compression forces

floor spans of varying length on each side. Where there is a permanent torsion applied to the beam, the connection to the supporting column should be capable of providing torsional fixity. This should not be a problem if the bearer beam and column are in-situ concrete, but this would be an important design consideration if the bearer beams are precast. The billet assembly can be purchased as a proprietary item. The designer should ensure that the fitting is adequate, meets the specification and is suitable for use in the UK. Galvanised fittings can have corrosion problems in chloride bearing environments so expert advice should be sought before the fittings are used in swimming pool roofs, car parks and exposed coastal locations. Finally, the fitting has to be incorporated into the double tee in such a way that it interacts with the other reinforcement in the unit to develop the strut and tie action, conceptually illustrated in Figure 5.35. Internal tie forces required for robustness may also have to be carried by the fitting. These may not have been considered in the development of the fitting, particularly if it was manufactured overseas where the traditions of tying structures may not be the same as in the UK.

5.5.8 Transportation of long double tee beams

5.6 Stairs

74

Two beams are usually supplied in a load and should be secured in such a way that holdingdown straps do not bear on the top flange edges. The site access must be firm without irregularity. Careless handling and the loading of the top flanges with site construction material can crack the top flange of a unit, typically at the ends at the interface between the flange and web. Such cracking is unsightly rather than hazardous in the long term and the manufacturer can be consulted to suggest repair procedures that should be carried out before the structural topping is cast.

Precast concrete stairs are produced to be incorporated within many forms of construction. This section considers their use within in-situ and precast concrete frames. Their use has become common, especially within ‘design and build’ contracts, where the speed of construction is a benefit.


It should be noted that stairflights are the primary means of escape if a building is subject to fire or explosion and thus the robustness of the structure is vital. BS EN 14843, Precast Concrete Products – Stairs 8 provides the specification for materials, production, properties, requirements and methods of testing for precast stairs. This includes a section on production tolerances and minimum dimensions. It also describes terms and definitions that are used. With regard to detailing it requires the technical documentation to include the construction data, such as the dimensions, the tolerances, the layout of reinforcement, the concrete cover, the expected transient and final support conditions, and lifting conditions. In particular, the technical documentation must include the maximum acceptable gap between components when erected to ensure the design overlap of the reinforcement is achieved, see Eurocode 2, Cl. 10.9.4.7. When considering the use of any proprietary system it is essential to consider:  how the stairflight is adequately tied to the adjacent parts of the structure  sequence of construction  temporary works involved  chain of responsibility in achieving the final structure (often the temporary actions, say due to props, are the critical design condition). The following procedure and points should be followed.  The working drawings should include complete propping instructions related to the cube strength of the in-situ concrete (in any event a minimum of four floors should be propped).  The sequence of construction and grouting-up instructions (if required) should be stated on the drawings. The method of levelling should be determined and agreed with the contractor and the method stated on the drawing.  The waist dimension should not be less than 100 mm.  For a precast stair flight on an in-situ landing nib section the precast flight should be positioned first before the in-situ landing is cast up against it.

5.6.1 Single stair flights

Figure 5.36 shows the main features of a typical single stair flight.

Figure 5.36 Typical single stair flight.

Tread

Going

Riser

Waist

75


Production tolerances The tolerances are given in BS EN 13369, Common Rules for Precast Concrete Products 2 and BS EN 148438, table 1, see Table 5.2. Unless stricter tolerances are given in the project specification these should apply. Table 5.2 Tolerances for stairs.

Target dimension of the cross-section in the direction ∆La to be checked (mm) L ≤ 150 mm

+10

L ≥ 400 mm

-5 ±15

∆cb (mm) ±5 +15 -10

Key: a The difference between two consecutive risers must not exceed 6 mm. b The minimum concrete cover defined in BS EN 14843, Cl. 4.3.7 must take into account the depth of any concrete removed by a finishing process. The positioning of reinforcement shall ensure that the minimum cover defined in BS EN 14843, Cl. 4.3.7 is achieved.

Minimum dimensions The minimum dimensions given in Table 5.3 should apply. Table 5.3 Minimum dimensions for stairs.

Dimension

Minimum dimension (mm)

Thickness of a step or landing

45a

Thickness of a wall

80

Thickness of a parapet

60

Wall thickness of a hollow element

45

Plan dimension of a column

120

Key: a Special care should be taken to ensure the correct position of the reinforcement

5.6.2 Top and bottom supports with in-situ connections

Where precast stair flights are used supported on in-situ landings, the landings should be cast against the precast flight. This avoids the problems of tolerances where precast flights are placed on in-situ landings previously cast. Temporary propping will also be required for the precast stairs, see Figure 5.37. Figure 5.38 shows alternative preferred arrangements of the reinforcement at the joints.

Figure 5.37 Temporary support of precast stairs.

In-situ concrete

In-situ concrete

Precast stair flight Precast stair flight Temporary 2 way braced props

Temporary 2-way braced props

Note: It is important that the temporary braced props are supported by a permanent structure.

76


Screed H12 bar

Screed

H12 bar

a) Connection with dowel bar only H12 bar

Screed

b) Connection with hanging and dowel bar

Figure 5.38 Preferred arrangements of reinforcement for connection with in-situ concrete.

5.6.3 Top and bottom supports with precast concrete

Layout Figure 5.39 shows the preferred dimensions for the detailing of the top joint between a precast stair flight and a precast support. The design of the bearings shall be in accordance with Eurocode 2, Cl. 10.9.5 and due allowance shall be made for erection tolerances. For the application of this rule, two classes of stair nibs are defined: Class A: The stair nib is manufactured with the design end cover in accordance with BS EN 14843, Cl. 4.3.1.1. Class B: The stair nib is similar to Class A but with reduced end cover. In this case the full concrete cover is achieved on site with a non shrink mortar. The result shall be in accordance with Eurocode 2, Section 4.

Recommended bearing type The recommended bearing type for precast stairs to precast concrete supports is a 10 mm thick mortar bedding.

77


90

15

10 90

15

100 min

100 min 15

90

a) Landing with sloping interface

10 100 min 90

15

15

90

90 min

10

100 min

15 b) Landing with square interface

90 c) Wall with square interface

Figure 5.39 Preferred dimensions for top joint between stairflight and precast support.

Design and supervision considerations The following should be considered during the design and construction process:  an allowance for a very generous impact factor on self-weight (say 2 or 3) of the precast flight  checking the consequence if the support is assumed to be at the edge of the in-situ nib (or designing seating layer to even out the loading)  failure mode in shear and hanging tension behind the nib  the construction procedure and temporary propping loads are properly understood  ensuring that the concrete reaches the required strength  no shims are included  the reinforcement is checked prior to concreting.

Lapped horizontal connection Figure 5.40 shows a preferred layout of reinforcement. This may not be the easiest way to construct an acceptable cage but ensures that the dimensions and the positioning of the loop and link reinforcement is correct.

78


Tie reinforcement in structural topping 15

375

Tie reinforcement in structural topping

15

375

Figure 5.40 Preferred layout of reinforcement for precast joints.

Figure 5.41 Dimensions to allow for dowel hole.

120 Screed

35 40

70 120

Figure 5.42 Reinforcement arrangement for dowel connection.

375

15

15

Screed

375

Dowel connection To provide sufficient room for a dowel hole the dimensions of the nib need to be as shown in Figure 5.41. Figure 5.42 shows the preferred layout of reinforcement for dowel connections, and worked example 8 shows a typical calculation.

79


Calculated by

RW

Job No.

CCIP-030

Worked example 8 Dowel bar for connection of precast stairs

Checked by

OB

Sheet No.

WE 8/1

Client

TCC

Date

April 08

Consider a 1.5 m wide stair flight spanning 4 m, with a vertical spacing between precast units of 10 mm and using a 20 mm diameter bar for the dowel. The tying force, F Ed required should be at least the permanent action of the stair flight.

Actions Assume average vertical thickness of stair flight is (150 + 100)√2 = 350 mm Self weight of stair flight F Ed = 25 x 0.35 x 1.5 x 4 = 52.5 kN

Resistance It can be shown that the maximum dowel force, F Rd, is F Rd = φb2.√(f cd.f yd).{√(1 + ε2) - ε } ≤ Asf yd/√3 (shear resistance of the dowel) where ε = 3(e/φb) x √(f cd/f yd) e = equal to half the vertical spacing between the units Hence: e = 10/2 = 5 mm ε = 3 x (5/20) x √{(0.85 x 40/1.5)/(500/1.15)} = 0.171 and F Rd = 202 x √(0.85 x 40/1.5 x 500/1.15) x { √(1 + 0.1712) – 0.171}/1000 = 33.5 kN ≤ (π x 202/4) x (500/1.15)/(√3 x 1000) = 78 kN ∴ F Rd = 33.5 kN No req’d = 52.5/33.5 = 1.57 Use 2 No. 20 mm dia. dowel bars

5.6.4 Top and bottom supports using steel angles Figure 5.43 Support using steel angles.

80

Steel angles are used to allow the stair flight to rest directly onto walls or floor units, see Figure 5.43.


Full strength weld to reinforcement to angle. Lap with structural reinforcement.

Structural topping

Structural screed Structural reinforcement

Structural reinforcement

In-situ structure

Full strength weld of reinforcement to angle. Lap with structural reinforcement.

Figure 5.44 Bottom and top support details using steel angles.

The angle provides the bearing onto the supporting structure but often does not have any joint continuity reinforcement. The designer should ensure that the design of the precast unit incorporating such a steel angle is adequate for the particular situation and provides an adequate tie to the structure. One method of achieving this is to weld reinforcement to the steel angle and anchor it to the structure through the screed. Typical top and bottom details are shown in Figure 5.44. The tension forces transmitted from the angle to the reinforcement within the precast unit in the top joint requires links welded to the bottom of the top angle. These should be designed to resist the forces from the angle with the force of the support at the worst possible position, i.e. when the joint between units is the widest permitted by the tolerances. The stability of the staircase before the screed has been cast is not normally considered by the manufacturer. It is essential to ensure that any temporary supports are provided and clearly identified in the construction sequence. One example is to provide a positive tie between the flight and the landing by reinforcement welded to the bearing angle (at the precast factory) to lap with the fabric in the structural topping.

5.6.5 Stairs with integral landings

Stair flights can be provided with an upper or lower integral landing as shown in Figure 5.45. It is important that an insert (typically 50 mm) is provided on the top surface of the landing. This allows the top finish to be laid un iformly over the whole of the landing surface, avoiding any steps, due to construction and installation tolerances. In order to establish an adequate tie to the supporting structure the reinforcement projecting from the precast unit should interlock with that of the support.

81


Figure 5.45 Stairs with integral landings.

Bar inserted to lap with wall reinforcement

50 mm recess for finish surface

Temporary 2 way braced props

Two hor izontal bars inserted within ‘U’ bar to lap with wall reinforcement In-situ wall

a) Stair flight with integral lower landing

50 mm recess for finish surface

Bar inserted to lap with wall reinforcement Two horizontal bars inserted within ‘U’ bar to lap with wall reinforcement

In-situ wall

Temporary 2 way braced props

b) Stair flight with integral upper landing

5.7 Corbels, nibs and half joints

Corbels, nibs and half joints are common to many forms of hybrid concrete construction. The correct position of and cover to the reinforcement is critical to the performance of this type of element. The design should carefully specify the requirements through the layout and reinforcement detail drawings. Corbels should be designed using strut and tie models when 0.4hc ≤ ac ≤ hc or as cantilevers when ac > hc, see Figure 5.46 for definitions of hc and ac. Unless special provision is made to limit the horizontal forces on the support, a minimum horizontal force of H Ed should be combined with the vertical force F Ed. Reference should be made to Section 4.1 concerning the value of H Ed.

5.7.1 Design by strut and tie model

82

Corbels, nibs and half joints are examples where non-linear strain distribution exists. For such situations design using strut and tie models is appropriate. Eurocode 2, Cl. 6.5 provides advice and stress limitations for the struts and nodes.


Figure 5.46 Layout of strut and tie for a typical corbel. ac

F Ed

H Ed

aH

F td

ac’



z 0

d

hc

x

Strut and tie model for a corbel Figure 5.46 shows the layout of the strut and tie layout for a corbel. The following procedure may be adopted to check the strength of the corbel:  The stress σ in the strut of width x should be limited to σ Rd,max = 0.34f ck(1-(f ck /250)), see Eurocode 2, Exp (6.56). The value of x effects the angle of the strut and hence the force in the strut.  The position of the top of the strut should be determined by the resolution of F and Ed H Ed, and the depth to F td (aH), as shown in Figure 5.46.  The angle and width of strut may be found by iteration or by use of the charts given in Figure 5.36 of the Manual for the Design of Concrete Building Structures to Eurocode 2 31.  It is recommended that z should not exceed 0.75d . 0  The bearing stress under the load should not exceed 0.48 f (1- (f /250)), see Eurock ck code 2, Exp (6.61).  Check the tie force, F = F ’ + H where F ’ is the horizontal component of the strut td td Ed td force caused by F Ed.  The total area of secondary links should be at least 0.5 area required to resist F , see td worked example 9.

83


Calculated by

RW

Job No.

CCIP-030

Worked example 9 Corbel design

Checked by

OB

Sheet No.

WE 9/1

Client

TCC

Date

April 08

Design ultimate load F Ed = 300 kN, f ck = 40 MPa, distance to the centre of the tension reinforcement is assumed to be 45 mm. The width of the corbel is 300 mm, other details are as shown below.

300 kN 175

bc =

300

160 60 kN

F td

45







605

Z 0

605

o

650

67.64

Z

400

x/2

300 x =

y

120

a) Chosen solution

b) Geometry of solution

Actions H Ed

=

0.2 F Ed = 0.2 x 300 = 60 kN

y z α β γ z 0/d

= = = = = =

175 + 60/300 x 45 = 184 mm √(1842 + 6052) = 632.4 mm sin -1 (120/(2 x 632.4)) = 5.44º tan -1 (184/605) = 16.92º 90 – 5.44 – 16.92 = 67.6º (184 tan 67.6)/605 = 0.73 < 0.75 ➝ OK

Geometry

Strut design Maximum stress in the strut is: σ Rd,max = 0.34 f ck(1-(f ck 250)) = 0.34 x 40 x (1 – 40/250) = 11.4 MPa For an angle of strut to the horizontal of 67.6º and strut force is: F Ed = 300/sin 67.6º = 325 kN Hence the stress: σ Ed = 326 x 1000/(120 x 300) = 9.1 MPa σ Rd,max > σ Ed ➝ OK Note: Further iteration could be carried out to maximise the strut efficiency.

84

Eurocode 2, Exp.(6.56)

Structural elements and connections 5 Project details

Calculated by

RW

Job No.

Worked example 9 Corbel design

Checked by

OB

Sheet No.

WE 9/2

Client

TCC

Date

April 08

CCIP-030

Reinforcement The tension force in the reinforcement: F td = F td’ + H Ed = 300 cot 67.6º + 60 = 183 kN Area of reinforcement required: As,req = 183 x 1000/(500/1.15) = 421 mm2 Try H20 bars: No. req’d = 421/(π x 202/4) = 1.34 Use 2 H20 bars Area of secondary links required = 421/2 = 211 mm 2 Try H8 links: No. req’d = 211/(82 x π/4) = 5.2 Use 5 H8 links See the figure below for layout of reinforcement in accordance with The Standard Method of Detailing Structural Concrete 32. 2 H20 bars H32 bar

5 No H8 links

Strut and tie model for nibs Where a nib is connected to the bottom of a beam, Figure 5.47 shows the arrangement of strut and ties for a given arrangement of reinforcement. The angle of the strut should be determined by the position of the centre of the bottom corner bar of the beam, up to the point of intersection of the resultant of the applied forces and the centre of the tension bar in the nib. It should be noted that the reaction, F t2d in the link bar is F Ed ( z b + ac)/ z b. The value of z b may be taken as 0.8 d b. Note this force is in addition to any shear force in the beam link.

85


Figure 5.47 Nib connected to bottom of beam. db

Zb

ac

F Ed

F t2d HEd

F t1d Zn

Strut and tie model for half joints

Figure 5.48 Layout of strut and ties for a typical half joint.

Figure 5.48 shows the arrangement of struts and ties for a typical half joint. The addition of a diagonal bar is not considered essential but does provide a more direct route for the forces and better crack control (see also PD 668721 and The Standard Method of Detailing Structural Concrete32). Full depth links to resist total reaction equally spaced

Tension anchorage

hh

Distance between edge of bearing and inside of bar to be a minimum of the bar diameter or 0.75 x cover, whichever is greater

Cranked bars improve crack control a) Section

Tension lap Horizontal ‘U’ bar with standard mandrel size

b) Plan

86

Nominal links at 150

Construction issues 6

6. Construction issues 6.1 Method of construction

The performance of an HCC structure may be affected significantly by the construction method. In order to achieve consistency between design and construction of structures it is important for the designer to include a method statement as part of the project specification indicating the assumptions regarding construction. This will bring clarity to the project and set a benchmark for pricing. The contractor is, of course, free to submit an alternative price based on different assumptions, if any, from the original design. In this proces s, the performance criteria agreed with the client should not be compromised. Although precast elements generally require less propping than in-situ elements, it is important to note that the forces in the props are also generally higher and therefore more care is required when considering the temporary works.

Static equilibrium during construction BS EN 1991-1-6 Eurocode 1: Actions on Structures – Part 1-6: General Actions – Actions during Execution33 and BS 5975, Code of Practice for Formwork 34 provide information on the design of temporary works. The designer should also consider transient situations, for example the effect of temporary overturning forces during construction. BS EN 1990, Eurocode: Basis of Structural Design 22, Table A1.2(A) describes the load factors that should be used. Figure 6.1 shows the single arrangement that includes both equilibrium (EQU) and structural resistance (STR). Figure 6.1 Temporary loading during construction. Overturning 1.35* Gk,f + 1.5 Qk, c

Resisting 1.15* Gk,b

Qk,c Construction

Resistance beam Gk,f floor

Gk,b

*

Check that using a factor of 1.0 for both favourable and unfavourable does not give a more unfavourable effect

87

6 Construction issues

For type 5 form of construction (see Figure 2.1) the in-situ concrete is used to knit the precast floor and beams together. The support of the precast floor should be designed as a bearing in the temporary case and, even though the bearing will eventually be part of an integral system, it will benefit from neoprene pads beneath the floor elements. The outer edge of the supporting beam should include a chamfer to eliminate spalling when the floor is loaded onto the precast beam and the full load comes onto the combined system. The chamfer also gives a visually clean edge to the joint between the precast and in-situ concrete if the precast unit is ribbed – a double tee for example. The support and restraint of the beam onto the column should also be considered in the temporary situation, as this connection may not be fully made before the in-situ composite concrete is placed. For construction types 3 and 4, see Figure 2.1, the precast floor is supported on some form of propped system before the in-situ edge beam is poured. The props should be designed for the construction loading and a means to gradually release the supported load onto the composite floor should be devised with back propping if necessary to support the floors above. The deflection of the shuttering of the in-situ edge beam during casting should be c onsidered. If the floor and edge beam shuttering are supported from separate propping systems during the pouring of the in-situ concrete the support struts of the in-situ area will take up load and may shorten slightly. The floor is on a different set of props and will not shorten as no extra load is applied to it. This can result in cracking of the top of the floor near to the support as the moment from the wet concrete is applied. To avoid this risk entirely, the same support system should be used for the floor and edge beam shutter, see Figure 6.2. A neat lower end to an embedded floor unit can be achieved by forming a small groove in the in-situ concrete. This allows the edge of the in-situ to fill properly, avoids the likelihood of spalling and masks any slight difference in the soffit level, see Figure 6.3. Figure 6.2 Support for connection.

a) Separate support systems can cause cracking in precast unit.

88

b) Common support of precast floor and insitu beam.


Figure 6.3 In-situ/precast joint showing groove detail.

Construction joint

Groove detail

6.2 Composite action between precast units and in-situ structural topping

The preparation of the base is of paramount importance. The surface of the precast units should be left rough during production and contained shot blasting equipment (which will avoid damaging the unit) used to prepare the surface, unless it can be shown that there will be adequate bond. All loose debris should be removed. Where required, the joints between the units should be grouted at least one day before the screed is placed.

6.3 Specially shaped standard units

Hollowcore units are manufactured to a 1200 mm module and double tees are normally to a 2400 mm module. It is possible to introduce narrow units into a layout or units tapered in plan if the building layout requires it. In the case of hollowcore, these are cut after manufacture, but double tees are cast to the required dimensions. In such cases, the manufacturer will be able to advise on how to detail the special units so that they are sufficiently robust to be delivered and incorporated into the building successfully and to ensure that exposed soffits look acceptable. In the case of long span units, for example, it may be preferable to take up a required taper in the last two units rather than have the last unit tapering excessively. Double tees can also be cast as single tees allowing a greater taper in plan than can be provided in a double tee unit, see Figure 6.4.

6.4 Long and short units adjacent to each other

In situations where long and short units are side by side, for example where lift and stair cores shorten spans, differential cambers can produce difficulties. This is particularly the case with long span double tees, for example in car parks. A clear span double tee car park unit may have a camber of 30 mm whereas the unit next to it, spanning from a common bearing position at one end to a ramp or stair core, may be 12 m long and have a camber of 10 mm. This difference in level is usually accommodated in practice by bearing the non-common end of the shorter span at a higher level than the long span unit, as shown in Figure 6.5. The designer should consult with the manufacturer to obtain an estimate of these cambers and mark the drawings accordingly.

89


Figure 6.4 Specially shaped standard units.

Key Web beneath

Figure 6.5 Long and short unit adjacent to each other. 30 mm camber

15 mm camber

Outer supports at same level

Inner supports set approx. 15 mm higher to reduce camber step between long and shorter unit

90


6.5 Differences of camber in double tees

In some countries, it is considered good practice to jack double tee flanges at mid-span to even out camber differences. This is sometimes carried out by casting loops of reinforcement that protrude from the flanges, vertically at mid-span, which are then used as purchase points for a crowbar or jack. While the built-in stresses from this process do n ot affect the ultimate strength of the structural system because of plasticity at the ultimate limit state, it is not recommended, as it can induce local cracking in the flanges, see also Section 5.5.4.

6.6 Method of de-tensioning double tee units

Double tee units should always be de-tensioned using release jacks that release all of the tendons simultaneously and gradually. This is standard practice in the UK, but not throughout the world. Engineers should be aware of the different practices and ensure that gradual release is specified and carried out. Otherwise bond checks should be carried out.

6.7 Checking strand or wire pull-in for hollowcore units

Hollowcore units are almost unique in that they are manufactured in a continuous length and are sawn to the required length only after the concrete has reached the appropriate strength. The de-tensioning process only de-tensions the strands at each end gradually whereas at the saw cuts a gradual release of tendon stress is not possible. The integrity of the anchorage bond of the tendons can be checked by examining the ‘pull-in’ of the strands at the ends of the unit. Assuming that the anchorage length is in the order of 1000 mm and that the build-up of strain is linear in that length, as stated in Eurocode 2, a pull-in design value of 2 mm can be calculated. However, this does not allow for the thickness of the saw-cut and in practice the measured pull-in is normally less than 1 mm. Manufacturers should check pull-in on units routinely and reject any with excessive pull-ins.

6.8 Placing hollowcore units into the correct position

On site, hollowcore units are often lifted into their final position using clamp lifting devices that clamp onto the sides of the unit near to each end. The clamp arms are of such a width that a unit cannot be placed exactly next to an already erected adjoining unit; thus, when the lifting device is removed, the unit has to be moved laterally to close up the gap. This is often accomplished by moving the unit, or ‘barring’ it with a crowbar. While this may not cause damage to a short span light unit, there is a risk of breaking a corner of a long span unit. Manufacturers recognise that barring of long span and heavy units is not good practice and provide other means of lifting hollowcore units for this situation, e.g. ‘L’ shaped lifting arms or lifting loops cast into the hollowcore units. Lifting loops should be used for the last unit that has to fit into an exact space. If lifting clamps are used, the unit would have to be placed at an angle, resting on the edge of the previously placed unit, while the clamps are removed and then barred until it drops into place. Guidance on the safe practice of barring is given in Code of Practice: For the Safe Erection of Precast Concrete Flooring and Associated Components 35.

91


6.9 Production tolerances

Table 6.1 Permitted deviations of cross-section.

Production tolerances are specified in BS EN 13369, Common Rules for Precast Concrete Products2, Cl. 4.3.1.1. For cross–sectional dimensions L, the permitted deviation is ∆L, and for position of reinforcing steel, prestressing steel and for the design cover c the permitted deviation is ∆c . The permitted deviations of cross-sections for structural elements are reproduced in Table 6.1. Target dimension of the cross-section in the direction to be checked

∆L (mm)

∆c (mm)

L ≤ 150 mm

+ 10 -5

±5

L = 400 mm

± 15

+ 15 - 10

L ≥ 2500 mm

± 30

+ 30 - 10

Notes: 1. Linear interpolation may be used for intermediate values. 2. ∆L and the positive values of ∆ c (upper permitted deviation) are given to ensure that deviations in cross-sectional dimensions and in position of the reinforcement do not exceed values covered by the relevant safety factors in the Eurocodes. 3. The negative values of ∆ c (lower permitted deviation) are given for durability purposes. 4. In particular, functional specificities of the products may require tighter tolerances. 5. The given values may be modified by product standards.

The upper permitted deviation for the location of the reinforcement may be determined as the mean value of the bars or strands in a cross-section over 1 m in width, e.g. slabs and walls. The design cover c of the reinforcement shall be at least the minimum cover, c min, plus the permitted deviation , ∆c dev, or the producer’s guaranteed deviation, whichever is lower. For principal dimensions other than cross-sectional dimensions: ∆L = ± (10 + L/1000) ≤ ± 40 mm where L is the target size of the linear measure expressed in millimetres Other types of tolerances may be given by product standards together with the values of the related permitted deviations, e.g. camber of beams. These values will not include the deformation effects of any applied load or of prestressing. In the verification of the measured deviations, such deviations shall be taken into account by computing their value for the test situation, including all the relevant time-related effects.

92

Special structures - case studies 7

7. Special structures - case studies This chapter describes two projects that relied upon hybrid concrete construction to realise an architectural requirement: Lloyd’s of London36, 1986, and Bracken House37, 1992. Both these buildings were constructed within a traditional contract procedure led by the architect. The design engineers and contractors found solutions to ensure that the architect’s intent was achieved with the most suitable use of materials. This required very close cooperation between engineer and architect, with a particular contribution from the specialist precaster. The input of the contractor to the design solution was small on these projects. One of the most important themes common to both projects related to the design of the structural joints. These were designed either to be:  made of in-situ concrete that connected precast elements to in-situ elements or other precast elements, allowing for reasonably large construction tolerances or  made with close tolerance templates that ensured that great care had to be taken to construct them correctly.

7.1 Lloyd’s of London

In 1977 the Committee of Lloyd’s decided to redevelop their site located either side of Lime Street, London. Architects Richard Rogers & Partners, with Ove Arup & Partners as structural and service engineers, won a competition by defining a design strategy rather than a building. The key points were that it:  allowed for maximum flexibility of use  gave continuity of trading and preserved the Lloyd’s tradition  did not rely exclusively on providing a new ‘Room’ as quickly as possible but gave Lloyd’s a means of maintaining expansion of business in the short term. The Room is the heart of Lloyd’s and is where the underwriters work. Two important architectural features included in the design brief were:  to show the columns cleanly throughout their height both on the external face and within the atrium as shown in Figure 7.1.  to show an exposed soffit of diagrid beams at 1.8 m centres. The resulting design produced a rectangular ring floor with a central atrium. The span of the floor was 16.2 m (9 x 1.8 m) with a floor-to-floor height of 4.5 m. The floor depth was 1500 mm of which 1150 mm was structural. Prestressed in-situ beams span between external columns and those at the atrium as shown in Figure 7.2. Further prestressed beams were required in the corner areas of the building and precast concrete was used for the column brackets, bearing yokes and stub columns.

7.1.1 Achieving a clean column appearance

The design included in-situ columns with precast brackets to support the floors, see Figure 7.3a and 7.3b.

93

7 Special structures - case studies

Figure 7.1a

Figure 7.1b

Lloyd’s of London redevelopment, external view.

Lloyd’s of London redevelopment, internal view.

Photo: Copyright Arup

Photo: Copyright Arup

94


Precast yoke

Structural topping

Prestressed inverted U beam

Stub column Steel permanent formwork panels

Precast bracket

Figure 7.2 Layout of the floor components.

Tolerance pocket with steel inserts

Precast yoke

Stainless steel flange

Dip groove Precast bracket

Steel plate with shear studs under In-situ node Steel dowel

Figure 7.3a left Precast concrete bracket connection. Schematic layout of brackets. Waterproofing detail

Figure 7.3b above Prefabricated bracing Elastomeric bearings

Precast concrete bracket connection. Precast bracket and yoke. Photo: Copyright Arup

95


7.1.2 Precast brackets

The design of the bearing had to fulfil a number of functions:  to carry the vertical load from the floor while allowing for relative rotation as the floor and bracket deflected, see Figure 7.4a  to transmit the wind and stability forces from the main building into the bracing system via the bracket, see Figure 7.4b  to restrain the bracket from rotating in plan because this provided stability restraint to the column at each level, see Figure 7.4c  to allow construction tolerance.

Figure 7.4 Design of bearings.

a) Bearing allowing rotation between filter and bracket

b) Bearing restrains column

c) Bearing transmits shear from building into bracing

It was decided that all the forces should be carried on the top face of the bracket. The vertical loads were transmitted through elastomeric bearings. The bearing was bonded to a plate that was screwed down on an epoxy levelling bed and so could be replaced if necessary. The horizontal forces were transmitted through four steel dowels. The load on the dowels was too great to transmit directly into the concrete, so steel bearing-plates were cast into the top surface of the bracket with welded shear studs projecting down to transmit horizontal load into the brackets, see Figure 7.3a.

96


Connection of precast bearing to column The way the bracket was connected to the column was one of the key points in its design. It was essential that this provided a straightforward construction operation, and the details had to have a proper architectural quality. The solution chosen was to make the bracket an extension of a ring that would be formwork for the column at that point. The ring would identify the bracket on the column, visually, and express the connection. The main bracket reinforcement passed into the column zone within the ring where it turned up and down, while the ring itself contained nominal reinforcement. The ring gave two possible sequences of construction:  The bracket could be placed on the column formwork and the c olumns and the bracket filled together.  The column could be cast first up to the soffit of the bracket, then the bracket placed and concreted. The second solution was chosen because it was thought that it would be too difficult to hold the five tonne bracket and column form in place with sufficient accuracy, since this took place outside the slab. The details of the bracket and column profile were worked out with the contractor to give grout tight joints while having the necessary visual articulation. The top of the column was slightly tapered to draw the bracket into the correct position on a sealing strip. Because the brackets and some of the columns were heavily reinforced, great care had to be taken in the design and detailing to ensure that there was no clash. The fact that the columns were circular made the problem worse. The steel was detailed and fixed, with templates, to precise dimensions that gave a clearance of a few millimetres. As is often the case with such a sensitive and potentially disruptive detail, so much care was taken that all went well.

7.1.3 Connection of precast bearing to in-situ prestressed inverted U-beams

A precast yoke was designed to transmit the loads from the in-situ prestressed beam to the precast bracket, see Figure 7.3b. The bearing and pockets in the precast bearing were designed to allow the elastic shortening of the prestressed U-beams to take place before grouting the precast yoke. However, the action of prestressing relieved the props of some of the load of the beam grid and transferred it to the bracket. This applied a moment to the column that caused an inward horizontal displacement. It was found to be better to grout the dowels before prestressing, which restrained the column against this displacement. The columns were pre-cambered outwards to allow for the prestress shortening of the U-beam. When a floor was cast it was propped down through two levels to limit the amount of load applied to the bracket and hence rotation.

97


7.1.4 Stability

The main building contains none of the usual lift, stair and riser cores that can be used for stability, as these were provided through the satellite towers. A form of bracing between some of the columns was chosen, see Figure 7.5. Where the bracing was required extra connections were built into the precast brackets.

Floor grid construction The floor-to-floor height is 4.5 m, of which 1.5 m is the floor itself. Both the structure and the services are exposed, with no false ceilings. Air is supplied through the raised floor and extracted at high level through the light fittings. The return air is taken out through duc ts at stub column level. The permanent formwork panels were made of profiled metal sheets welded to pressed channels, see Figure 7.6. The channels were lipped on the underside to support the anchors for service hangers in the zone of the stub columns. A typical section through the floor is shown in Figure 7.7. Figure 7.5 T5

Main building stability system. T3

T4

T6

T2 T1

98


Figure 7.6 Permanent formwork panel with acoustic trays. Photo: Copyright Arup

Figure 7.7 Typical section through floor.

Structural slab

Floor finish

50 300 100 60

Stub columns

440 1500

Permanent formwork & acoustic insulation

550

Diagrid beams 300 1800

The subcontractor developed a formwork system to produce the diagrid beams, see Figure 7.8. Their design was based on folded and welde d steel frames with ply faces. Neoprene gaskets were built into the metal sections that also formed rebates at joint lines. The components were fixed together with bolts and wedges with adjustment for tolerance. The reinforcement cages were supported on purpose-made plastic cradles bolted down to the soffit form. These ensured accurate cover, and the threaded insert could be used later to restrain the top of partitions. This formwork was excellent; it gave a first-class finish and could be put together and taken apart very quickly. It was the key to success of this subc ontract.

99


Figure 7.8 Diagrid beam formwork system. Photo: Copyright Arup

7.1.5 Points of interest







 





7.2 Bracken House

100

This was a bespoke building and although time of construction was of the essence, the budget was generous. The interaction between client, architect and engineer was crucial and favoured the ‘traditional’ form of contract. Much time and effort was spent to provide the most suitable form of construction and materials, but the contractor provided very little input to the development. The subcontractor developed a very efficient formwork system. Precast and in-situ concretes were used appropriately to ensure maximum benefit to the aesthetics, speed of construction and accuracy of construction. Considerable effort and money was spent on setting up mock-ups and prototypes to identify the most appropriate form of construction. Where it was made clear that great accuracy was required in construction it was achieved without fuss.

Bracken house is on Cannon Street close to St Paul’s Cathedral, London. In 1986 Obayashi appointed Michael Hopkins as architect and Ove Arup as structural and service engineers to redevelop a building designed by Sir Albert Richardson. When this was listed it was decided to retain the two wings of the building and rebuild the centre block. From an engineering point of view one of the main features of the design was the integration of the structure and services in the centre block and the way this linked to the construction of the facade. The design was based on the principle of a wheel in which circumferential primary services routes around the outside of the building and inside the atrium connect to radial secondary routes running between radial beams, see Figure 7.9.


Figure 7.9 Combined structure and services concept.

The concept wheel

Structural organisation

Ceiling extract

Supply Floor services supply between structure Extract

The outer circumferential route is supplied from risers located in the cores between the wings and the centre block. The inner circumferential ring connects to air exhaust risers contained within quadrant shaped columns in the corners of the atrium, see Figure 7.10. For speed of construction the beams were precast, whereas the columns were cast in-situ because their construction had no time penalty. Alternating in-situ and precast permitted a very simple connection detail; the beam swelled out at the column position and a pocket was left out at this point: the column reinforcement passed through the pocket (see Figure 7.11), which was concreted up before casting the next lift of column. The beams are 650 mm deep and span 12 m from a column at the atrium to a column that is set back 4.2 m from the facade, and then continue with a reduced depth of 350 mm on to a support at the facade. In each of the quadrant corners, eight radial beams are supported on a continuous corbel that springs from the quadrant columns. There are no circumferential beams. The structural slab is in-situ concrete placed on metal decking permanent formwork between the precast beams.

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Above: Figure 7.10

Radial beams and quadrant-shaped column at atrium corner. Photo: Copyright Arup

Above right: Figure 7.11

Column beam connection. Photo: Copyright Arup

The soffit of the slab is above the soffit of the beam and this zone is used for false ceiling, sprinklers, lighting, and the extract air plenum, see Figure 7.12. The zone above the 150 mm slab is used for the floor-based air supply, electrical power and communications. The raised floor is 300 mm above the beam. Figure 7.13 shows the floor layout during construction. Figure 7.12 Typical section through floor zone.

Floor finish

Air supply, electrical services and communications

300 950 150

650

250

False ceiling Lighting, sprinkers and air extraction

Precast concrete radial beams

In-situ concrete slab on metal decking

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Figure 7.13 Floor layout during construction. Photo: Copyright Arup

7.2.1 Points of interest

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Apart from the plan of the site and the retention of the wings, the most important factor governing the design was the St Paul’s height rule, which restricted the height of the building to that of the wings to avoid obstructing the view of the cathedral. To fit six floors within the superstructure height available, while maintaining the clear heights and raised floor depth required of a modern City office, the depth of the floor zone had to be as small as possible. The result is a 12 m clear span, with a 950 mm overall, which provides a clear zone of 300 mm for telecommunications and small power. By placing the slab towards the middle of the beams the benefit of T-beam action is lost, but it is this, combined with the radial interleaving of structure and services, that leads to the minimum possible depth of the structural and services zone. The financial benefit of the extra floor that this allowed far outweighed the reduction in structural efficiency. Similar to the Lloyd’s contract, the interaction between client, architect and engineer was crucial and favoured the ‘traditional’ form of contract. Precast and in-situ concretes were used appropriately to ensure maximum benefit to the aesthetics, speed of construction and accuracy of construction. Metal decking permanent formwork for the slab was chosen for its simplicity and ease of construction. As the structural slab was in the middle of the floor zone, the metal decking was hidden by the false ceiling. There was a strong belief that the joints between precast concrete units should be in in-situ concrete and that the architecture should reflect this principle.

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References

References 1

BRITISH STANDARDS INSTITUTION, BS EN 1992-1-1, Eurocode 2: Design of concrete structuresPart 1-1: General rules and rules for building s, BSI, 2005.

2

BRITISH STANDARDS INSTITUTION, BS EN 13369, Common rules for precast concrete products, BSI, 2004.

3

BRITISH STANDARDS INSTITUTION, BS EN 1168, Precast concrete products - Hollowcore slabs, BSI, 2005.

4

BRITISH STANDARDS INSTITUTION, BS EN 13747, Precast concrete products - Floor plates for floor systems, BSI, 2005.

104

5

BRITISH STANDARDS INSTITUTION, BS EN 13224, Precast concrete products - Ribbed floor elements, BSI, 2004.

6

BRITISH STANDARDS INSTITUTION, BS EN 13225, Precast concrete products – Linear structural elements, BSI, 2004.

7

BRITISH STANDARDS INSTITUTION, BS EN 14992, Precast concrete products – Wall elements: Production properties and performances, BSI, 2007.

8

BRITISH STANDARDS INSTITUTION, BS EN 14843, Precast concrete products – Stairs, BSI, 2006.

9

BRITISH STANDARDS INSTITUTION, BS EN 13670, Execution of concrete structures, BSI, due 2008.

10

GOODCHILD, C. and Glass, J. Best practice guidance for hybrid concrete construction. The Concrete Centre, 2002, Ref. TCC/03/09.

11

THE CONCRETE CENTRE. Hybrid concrete construction. The Concrete Centre, 2005, Ref. TCC/03/010.

12

THE CONCRETE CENTRE. Precast concrete in buildings. The Concrete Centre, 2007, Ref. TCC/03/031.

13

THE CONCRETE CENTRE. Concrete framed buildings. The Concrete Centre, 2006, Ref. TCC/03/024.

14

GOODCHILD, C.H. Economic concrete frame elements. The Concrete Centre, 2008, Ref. CCIP-025.

15

WILFORD, M. and YOUNG, P. A design guide for footfall induced vibration of structures. The Concrete Centre, 2006, Ref CCIP-016.

16

BRITISH STANDARDS INSTITUTION, BS 8204, Screeds, bases and in-situ floorings, BSI, 2003.

17

DEPARTMENT FOR COMMUNITIES AND LOCAL GOVERNEMENT, Building regulations (England and Wales) Approved document A (2004) . DCLG, revised 2006.

18

THE BUILDING REGULATIONS 2000 (Amended), Statutory Instrument 2000 No 2531 Building and Buildings, The Stationery Office, 2000.

19

BRITISH STANDARDS INSTITUTION, BS EN 1991-1-7, Eurocode 1: Actions on structures – Part 1-7: General actions – Accidental actions, BSI, 2006.

20

BRITISH STANDARDS INSTITUTION, UK National Annex to Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings, BSI, 2005.

21

BRITISH STANDARDS INSTITUTION, PD 6687: Background paper to the UK national annexes to BS EN 1992-1, BSI, 2006.

22

BRITISH STANDARDS INSTITUTION, BS EN 1990, Eurocode: Basis of structural design, BSI, 2002.

23

ELLIOTT, K S. Multi storey precast concrete framed structures. Blackwell Science, 1995.

24

NARAYANAN, R. Precast Eurocode 2: Design manual. British Precast, 2007.

25

CONCRETE SOCIETY. Technical Report 43: Post-tensioned concrete floors design handbook, second edition. CS, 2005.

26

CONCRETE SOCIETY. Technical Report 67: Movement, restraint and cracking in concrete structures. CS, 2008.

27

MARTIN, L. and PERRY, C. PCI design handbook, sixth edition. Precast/Prestressed Concrete Institute, 2004.

References

28

INTERNATIONAL FEDERATION FOR PRESTRESSING. FIP Recommendations: Precast prestressed hollowcore floors. Thomas Telford, 1988.

29

NARAYANAN, R. Precast Eurocode 2: Worked examples. British Precast, 2008.

30

TAYLOR, H. Strand deflection systems in pretensioned, prestressed concrete. The Structural Engineer , Vol. 70, No. 5, March 1992.

31

INSTITUTION OF STRUCTURAL ENGINEERS. Manual for the design of concrete building structures to Eurocode 2. IStructE, 2006.

32

INSTITUTION OF STRUCTURAL ENGINEERS/CONCRETE SOCIETY. The Standard Method of Detailing Structural Concrete, third edition. IStructE, 2006.

33

BRITISH STANDARDS INSTITUTION, BS EN 1991-1-6, Eurocode 1: Actions on structures – Part 1-6: General actions – Actions during execution, BSI, 2005.

34

BRITISH STANDARDS INSTITUTION, BS 5975, Code of practice for formwork, BSI 1996.

35

PRECAST FLOORING FEDERATION. Code of practice: For the safe erection of precast concrete flooring and associated components. PFF, 2007.

36

RICE, P. and THORNTON, J. Lloyd’s redevelopment. The Structural Engineer , Vol. 64, No. 10, October 1986.

37

UNKNOWN. Inside job: Bracken House. Architects’ Journal, 27 May 1992, pp. 26–37. Anon.

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A

cement and concrete

Type 1 Precast twin wall and lattice girder slab with in-situ concrete

Type 2 Precast column and edge beam with in-situ floor slab

Type 4 In-situ columns or walls and beams with precast floor units

Type 5 In-situ column and structural topping with precast beams and floor units

Typical hybrid concrete options.

Please note this diagram is a repeat of Figure 2.1, page 8.

industry publication

Type 3 Precast column and floor units with cast in-situ beams

Type 6 In-situ columns with lattice girder slabs with optional spherical void formers

95765932-Design-Hybrid-Concrete-Buildings.pdf

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