Design Hybrid Concrete Buildings

CC

IP-030D

esign of Hybrid C

oncrete Buildings

R. W

hittle MA (Cantab) CEng M

ICE H. Taylor FREng, BSc, PhD

, CEng, FICE, FIStructE

Design of Hybrid Concrete Buildings

This design guide is intended to provide the structural engineer with essential guidance for the design of structures that combine precast and in-situ concrete in a hybrid concrete structure. It introduces the options available for hybrid concrete structures, and goes on to explain the key considerations in the design of this type of structure.

Bearings, interface details, consideration of movement, composite action, robustness and the effects of prestressing are all explained in this guide and design examples are included where appropriate. The importance of overall responsibility and construction aspects are also described.

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0Price Group P

© The Concrete Centre

Riverside House, 4 Meadows Business Park,Station Approach, Blackwater, Camberley, Surrey, GU17 9ABTel: +44 (0)1276 606 800 www.concretecentre.com

CI/Sfb

UDC624.072.33:624.012.3/.4

Robin Whittle has extensive knowledge and experience of designing all types of concrete buildings. He regular contributes to concrete industry publications and is a consultant to Arup. He was a member of the project team which drafted Eurocode 2.

Howard Taylor has extensive knowledge and experience of designing precast concrete elements and buildings, including developing alternative production methods. He is a past president of the Institution of Structural Engineers and is currently chairman of the British Standards Institution Building and civil engineering structures Technical Committee B/525.

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

A cement and concrete industry publication

R. Whittle MA (Cantab) CEng MICE

H. Taylor FREng, BSc, PhD, CEng, FICE, FIStructE

Hybrid cov-.indd 1Hybrid cov-.indd 1 29/01/2009 16:43:3729/01/2009 16:43:37


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

CCIP-030Published January 2009 ISBN 978-1-904482-55-0Price 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 the UK by those who will evaluate the signifi cance 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 information is accepted by The Concrete Centre or their subcontractors, suppliers or advisors. Readers should note that the publications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Cover photo: Courtesy of Outinord International Ltd.Printed by Information Press Ltd, Eynsham, UK

AcknowledgementsThe authors would particularly like to thank the following people for their support in the development of this design guide:

Tony Jones ArupIan Feltham Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged:

John Stehle Laing O’RourkeGraham Hardwick John Doyle Construction LtdPeter Kelly Bison Concrete Products LtdAlex Davie ConsultantDavid Appleton Hanson Concrete ProductsKevin Laney Strongforce Engineering PlcNorman Brown British Precast Concrete Federation Ltd

Type 1Precast twin wall and lattice girder slab with

in-situ concrete

Type 2Precast column and edge beam with in-situ

fl oor slab

Type 3Precast column and fl oor units with cast in-situ

beams

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

fl oor units

Type 5In-situ column and structural topping with precast

beams and fl oor units

Type 6In-situ columns with lattice girder slabs with

optional spherical void formers

Typical hybrid concrete options.Please note this diagram is a repeat of Figure 2.1, page 8.



Contents

1. Introduction 5 1.1 Single point of responsibility 5 1.2 Design considerations 6 1.3 Best practice procurement guidance 6

2. Overview of hybrid solutions 7 2.1 Type 1: Precast twin wall and lattice girder slab with in-situ concrete 7 2.2 Type 2: Precast column with in-situ fl oor slab 9 2.3 Type 3: Precast column and fl oor units with cast in-situ beams 10 2.4 Type 4: In-situ columns or walls and beams with precast fl oor units 12 2.5 Type 5: In-situ column and structural topping with precast beams and fl oor units 13 2.6 Type 6: In-situ columns with lattice girder slabs with optional spherical void formers 14

3. Overall structural design 15 3.1 Robustness 15 3.2 Stability 18 3.3 Diaphragm action 18 3.4 Shear at interface of concrete cast at different times 19 3.5 Interface shear 22 3.6 Shear and torsion design 25 3.7 Long-line prestressing system 26 3.8 Secondary effects of prestressing and the equivalent load method 29 3.9 Temperature effects 29 3.10 Differential shrinkage 29 3.11 Designing for construction 33

Design of Hybrid Concrete Buildi1 1Design of Hybrid Concrete Buildi1 1 29/01/2009 16:47:3229/01/2009 16:47:32

4. Bearings and movement joints 34 4.1 Horizontal forces at bearings 34 4.2 Restrained bearings 35 4.3 Movement joints 36 4.4 Actions and restraints 36 4.5 Design considerations 37 4.6 Allowance for anchorage of reinforcement at supports 37 4.7 Bearings that allow limited movement 38 4.8 Connections between precast fl oors and in-situ concrete beams 42

5. Structural elements and connections 43 5.1 Twin wall construction (type 1) 43 5.2 Precast columns, edge beams and in-situ slabs (type 2) 52 5.3 Biaxial voided slabs 55 5.4 Prestressed hollowcore units 58 5.5 Double tee beams 68 5.6 Stairs 74 5.7 Corbels, nibs and half joints 82

6. Construction issues 87 6.1 Method of construction 87 6.2 Composite action between precast units and in-situ structural topping 89 6.3 Specially shaped standard units 89 6.4 Long and short units adjacent to each other 89 6.5 Differences of camber in double tees 91 6.6 Method of de-tensioning double tee units 91 6.7 Checking strand or wire pull-in for hollowcore units 91 6.8 Placing hollowcore units into the correct position 91 6.9 Production tolerances 92

7. Special structures - case studies 93 7.1 Lloyd’s of London 93 7.2 Bracken House 100

References 104


List of worked examples

Worked example 1 Hollowcore fl oor acting as a diaphragm 20Worked example 2 Interface shear between hollowcore slab and edge beam 23Worked example 3 Upwards camber on slab due to temperature gradient 30Worked example 4 Differential shrinkage 31Worked example 5 Bearing of a hollowcore unit 41Worked example 6 Vertical tie 56Worked example 7 Anchorage length of longitudinal tie bar 65Worked example 8 Dowel bar for connection of precast stairs 80Worked example 9 Corbel design 84


5

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 benefi ts of both forms of concrete construction. Further guidance on the benefi ts 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:

BS EN 133692 Common Rules for Precast Concrete Products

BS EN 11683 Precast Concrete Products – Hollowcore Slabs

BS EN 137474 Precast Concrete Products – Floor Plates for Floor Systems

BS EN 132245 Precast Concrete Products – Ribbed Floor Elements

BS EN 132256 Precast Concrete Products – Linear Structural Elements

BS EN 149927 Precast Concrete Products – Wall Elements

BS EN 148438 Precast Concrete Products – Stairs

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.

1.1 Single point of responsibility

Introduction 1


6

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. The requirements for execution and workmanship given in EN 136709 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.

Best Practice Guidance for Hybrid Concrete Construction10 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 benefi ts 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.

1.2 Design considerations

1.3 Best practice procurement guidance

66

1 Introduction


7

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 infl uenced 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 refl ect 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 effi cient design.

There are advantages to using both precast and in-situ concrete summarised in Table 2.1; more detailed discussion on the benefi ts of concrete can be found in other publications11, 12, 13. The key to maximising the benefi ts of HCC is to use the most appropriate technique for each element to produce an economic structure.

Precast concrete Precast or in-situ concrete In-situ concreteEconomic for repetitive elements Inherent fi re resistance Economic for bespoke areas

Long clear spans Durability Continuity

Speed of erection Sustainability Inherent robustness

Buildability Acoustic performance Flexibility

High-quality fi nishes Thermal mass that can be utilised for fabric energy storage

Services coordination later in programme

Consistent colour Prestressing 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 Elements14.

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 fi lled 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 fi nish than a typical in-situ wall. The panel surface quality is suitable to receive a plaster fi nish 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.

Table 2.1Benefi ts of concrete.

77

2.1 Type 1: Precast twin wall and lattice girder slab with

in-situ concrete

Overview of hybrid solutions 2


8

Figure 2.1Typical hybrid concrete options.

Please note this diagram is repeated on the inside back cover for ease of reference.


in-situ concrete


fl oor slab


beams


fl oor units





Figure 2.2Type 1 construction,

twin wall erection.Photo: John Doyle Construction Ltd

2 Overview of hybrid solutions


9

Often the twin wall system is combined with the use of lattice girder precast soffi t slabs, with or without spherical void formers. These provide permanent shuttering for an in-situ slab that can be relatively easily fi tted 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.

The combination of an in-situ slab, e.g. post-tensioned fl at 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 fi xings or prestressing anchorages to be cast in. This reduces the time required for reinforcement fi xing and erecting the formwork.

The maximum span for this form of construction depends largely on whether the in-situ slab is post-tensioned. For fl at slabs with spans greater than 10 m punching shear is likely to be a critical design issue.


2.2 Type 2: Precast column with in-situ fl oor slab


10


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.

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 fl oor 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 fl oor units, see Figures 2.3a and 2.3b. The latter have successfully been used in high quality buildings designed for energy effi ciency, where the light fi ttings, 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 post-

tensioned concrete.

Disadvantages: Precast flooring must be temporarily propped. Sealing between precast units is required.

2.3 Type 3: Precast column and fl oor units with cast

in-situ beams


11

Figure 2.3aExample of type 3 projects.

Paternoster Square and offi ce building.Photo: John Doyle Construction Ltd


Figure 2.3bExample of type 3 projects.

Homer Road, Solihull. Photo: Foggo Associates


12


Figure 2.4Example of type 4 project, car park, West

Quay, SouthamptonPhoto: Hanson Concrete Products

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 fl oor units, e.g. hollowcore slabs, double tees. These obviate the need for slab formwork and provide a relatively lightweight fl oor. This construction system does not require the involvement of a specialist subcontractor beyond the manufacture and supply of the standard precast units.

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

precast fl oor units

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.


13


In this form of construction the fl oor consists entirely of precast elements, which are tied together with an in-situ structural topping, see Figure 2.5. (A structural topping is now defi ned 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 fl oor. 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 fl oor 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.

2.5 Type 5: In-situ column and structural topping with

precast beams and fl oor units

Figure 2.5Example type 5 project, Home Offi ce

Headquarters, London.Photo: Pell Frischmann Consulting Engineers Ltd and Bouygues (UK) Ltd


14


The main feature of this system is the use of the lattice girder panels to act as permanent formwork for a fl at slab. A variation is to include spherical void formers, which reduce the self-weight of the slab, for only a small reduction in fl exural 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 fl at 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 fl at slab, although propping of the panels will be required, to reduce the overall fl oor zone of the building and to simplify installation of services. The quality of the factory produced soffi ts 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.

2.6 Type 6: In-situ columns with lattice girder slabs

with optional spherical void formers

Figure 2.6Type 6: Lattice girder soffi t panels used as

permanent formwork.Photo: John Doyle Construction Ltd


15

Overall structural design 3

3. Overall structural design

This section gives specifi c 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 confi dent 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.

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 1992-121. 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 tiesA 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.

3.1 Robustness


16

3 Overall structural design

Figure 3.1Continuity of ties: Bars in precast member

lapped with bar in in-situ concrete.

Figure 3.3Continuity of ties: Bars lapped within in-situ

concrete.

Tie

Tie Tie

Figure 3.2Continuity of ties: Anchorage by enclosing

links.

Tie

Peripheral tiesThe peripheral tie should be capable of resisting a design tensile force:

Ftie,per = (20 + 4n0) ≤ 60 kN

where n0 = number of storeys

Internal tiesThe internal tie should be capable of resisting a design tensile force:

Ftie,int = [(qk + gk)/7.5](lr /5)(Ft) ≥ Ft kN/m

where (qk + gk) = sum of the average permanent and variable floor loads (in kN/m2) lr = greater of the distances (in metres) between the centres of the columns,

frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration, and

Ft = (20 + 4n0) ≤ 60 kNMaximum spacing of internal ties = 1.5lr


17


Horizontal ties to columns and/or wallsEdge columns and walls should be tied horizontally to the structure at each fl oor and roof level. The tie should be capable of resisting a design tensile force:

Ftie, fac = Ftie, col = Maximum (Minimum (2Ft; lsFt/2.5); 0.03 NEd)

where Ftie,fac = in kN/m run of wall Ftie,col = in kN/column Ft = as defined in above ls = floor to ceiling height (in metres) NEd = total design ultimate vertical load in wall or column at the level consideredTying of external walls is only required if the peripheral tie is not located in the wall.

Vertical tiesFor 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 fl oor 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 fl oor 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.


18


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.

Where fl oor 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 fl oor 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 Structures23 describes the design approaches for fl oor 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 suffi cient 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-to-unit joints, see Figure 3.4a.

An alternative method, appropriate to hollowcore fl oors 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 3.4Typical diaphragm action from precast fl oor

systems.

3.3 Diaphragm action

b) Floor carrying horizontal forces from wind by virendal actiona) Floor carrying horizontal forces from wind by beam action

3.2 Stability


19


BS EN 1168, Precast Concrete Products – Hollowcore Slabs3 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 vRdi 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 fi nish 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.

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 suffi cient to ensure that the hollowcore units cannot move apart and so the structural model used in worked example 1 remains valid.

3.4 Shear at interface of concrete cast at different

times


20


Project details

Worked example 1Hollowcore floor acting as a diaphragm

Calculated by Job No.

Checked by Sheet No.

Client Date

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

Plan: 15 m x 9 m with 250 mm thick hollowcore unit

Section A - A

vs - Very smooth surfaces - Smooth surface

vs

s

vs

vsvs

s

A

A

Edge beam

Hollowcoreunit

KEY

RW

OB

TCC

CCIP-030

WE 1/1

April 08


21


Project details




Client Date

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. wd = 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, VEd. VEd = 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: vEdi < vRdi

gives: vEdi = β VEd/(z bi) where β = 1 VEd = 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 ∴ vEdi = 67.5 x 1000/(6000 x 200 ) = 0.056 MPa

rRdi is limited to 0.10 MPa (> 0.056 MPa → OK)

Check vRdi (which is unlikely to control); for this example the first and second terms are small and may be ignored as a first estimate.

vRdi = ρfyd (μ sin α + cos α) ≤ 0.5 υ fcd

where ρ = As/Ai μ = 0.5 (very smooth surface) fyd = the design yield strength of reinforcement As = the area of reinforcement crossing the interface Ai = the area of the joint α = 90 for reinforcement perpendicular to the joint υ = 0.6 (1 – fck/250)

Eurocode 2, Cl. 6.2.5

Eurocode 2, Exp.(6.24)

Eurocode 2, Figure 6.8

Eurocode 2, Cl.10.9.3(12)


Eurocode 2, Cl.6.2.5 (2)

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Project details




Client Date

For this example: As = 2 x 2 x 113 = 452 mm2

Ai = 1200 x 200 = 240 000 mm2

Hence: ρ = 452/240 000 = 0.00188 and: vRdi = 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.

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.

3.5 Interface shear

Interface 3 Interface 2

Interface 1

Interface 4

Figure 3.5Typical interfaces between precast and in-situ

joints.

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Project details

Worked example 2Interface shear between hollowcore slab and edge beam



Client Date

Consider Example 13.7 in the Precast Eurocode 2: Design Manual24. 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.

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 to 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.

fck = 35 MPa fy = 500 MPa Maximum sagging moment, MEd = 267 kNmMaximum design shear, VEd = 223 kN bi = 200 mm d = 540 mm

MEd/bd2fck = 267 x 1000000/(885 x 5402 x 35) = 0.0296

600 175 110

200

In-situ concrete

Shear interface

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Project details

Worked example 2Interface shear between hollowcore slab and edge beam



Client Date

From Figure B1 of the Precast Eurocode 2: Design Manual24 find value of z (alternatively find z by calculation or with any suitable design aid): z = 0.97

vEdi = βVEd /z bi

where β = 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 bi = 200 mm

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

vRdi = c fctd + μ σn + ρfyd (μ sinα + cosα) ≤ 0.5 υfcd

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

vRdi = 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)

vEdi ≤ vRdi ≤ 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 mm2

Using 3 No. voids each containing 1 No. H16 U bar.

As,prov = 3 x 2 x 162 π/4 = 1210 mm2 OK

Eurocode 2, Exp (6.24)

Eurocode 2, Exp (6.25)

Eurocode 2, Exp (6.6N)

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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.

3.6 Shear and torsion design

Figure 3.6‘Hang up steel’ requirement.

Slab shear strut

Beam shear strut

“Hang up steel” additional toreinforcement required to carryshear Eurocode 2, Cl 6.2.1 (9)

Slab shear strut

Types 2, 3 and 4 apply the fl oor 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-of-balance 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.


26


Figure 3.7Design for torsional restraint.

Centre of resistanceof 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.

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.

3.7 Long-line prestressing system


27


Gradual detensioningmechanism

Stressed strands

Unit moulds or continuouslyextruded units

Jack blocks and embeddedcantilever upright inconcrete strong floor

Figure 3.8The long-line pre-tensioning system.

Mould end plateStrand

Detail of gap between moulds

Unit in mouldFigure 3.9

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

Debonding tendonsThe 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 Manual24 and Post-tensioned

Concrete Floors Design Handbook25).

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 fi rst at 100 mm from the debonding point and the second 40 mm beyond that, are typically suffi cient. The proximity of the links to the bonding position ensures suffi cient restraint to bursting even if the transmission zone is less than that assumed in design in accordance with Eurocode 2.


28


Figure 3.10Typical detail showing the debonding of a

strand.

Typically 7 - 8 protruding links

Extra links atdebonding point

Debonded strandFully bonded stressed 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 defl ect 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.

Balance of moments

Unit with straight bonded tendons Unit with straight debonded tendons

Moments from quasi-permanent loading

Moments from prestress

Resulting camber

Figure 3.11Comparison between straight bonded,

debonded and defl ected tendons.


29


Prestressed units camber because of the hogging moment provided by the prestress. A pre-tensioned 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 defl ection 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 defl ection 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 fl oor 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.

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 Handbook25 describes the phenomena and the use of the equivalent load method.

The defl ection of a fl oor 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 uninsulated 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 Structures26.

When an in-situ screed is added onto a fi rst stage cast fl oor of either reinforced or pre-stressed construction, the shrinkage of the screed after its initial hydration will develop a compressive strain in the top of the fi rst stage cast and will induce a downwards defl ection in the span of the composite unit and, if the fl oor 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, εfds. The fi nal curvature, φ, is constant across the section. Design equations can be developed as follows:

3.8 Secondary effects of prestressing and the

equivalent load method

3.9 Temperature effects

3.10 Differential shrinkage


30


Project details

Worked example 3Upwards camber on slab due to temperature gradient



Client Date

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

Force equilibrium:εi Ei Ai = εp Ep Ap (1)

εp = εi Ei Ai /Ep Ap

Section equilibrium (φEI = M):φ (Ei Ii + Ep Ip) = εi Ei Ai ( yi,b + yp,t) (2)

Strain equilibrium:εfds = εi + εci + εcp + εp = εi + φ yi,b + φ yp,t + εp

φ = (εfds - (εi + εp))/(yi,b + yp,t)

φ = (εfds - (εi + εi Ei Ai /Ep Ap))/(yi,b + yp,t) (3)

In-situ

Precast

yp,t

yi,b

εfds

εcp

εp

εci

ε i

φ

Figure 3.12The effect of differential shrinkage across a

section.

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Combining (2) and (3):φ{yi,b + yp,t + (εfds - (Ei Ii + Ep Ip)) (1/Ei Ai + 1/Ep Ap)/(yi,b + yp,t)} = εfds

φ = εfds /{yi,b + yp,t + (Ei Ii + Ep Ip) (1/Ei Ai + 1/Ep Ap)/(yi,b + yp,t)} (4)

εi = εfds /{1 + Ei Ai /Ep Ap + (yi,b + yp,t)2 Ei Ai /(Ei Ii + Ep Ip)} (5)

εp = εfds /{1 + Ep Ap /Ei Ai + (yi,b + yp,t)2 Ei Ai /(Ei Ii + Ep Ip)} (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

Worked example 4Differential shrinkage



Client Date

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 fabric in in-situ concrete B283 fabric in precast concrete flange2 x 2 No. 7.9 mm super strand in precast rib

In-situ concrete

Precast concrete

150

1000

100

50

300

B785 mesh

B283 mesh

2 x 2 No 7.9 super strand

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Project details

Worked example 4Differential shrinkage



Client Date

In-situ concrete fck,in = 25 MPa, fcm,in = 33 MPa, creep coefficient, ϕ = 1.5 Ec,in,long = 22 [fcm,in/10]0.3/(1 + ϕ) = 22 x (33/10)0.3/(1 + 1.5) = 12.59 GPa

Section properties, including the reinforcement, are as follows: Ain = 112 x 103 mm2

Iin = bd3/12 = 1000 x 1003/12 = 87.5 x 106 mm4

yinbar,b = 52.1 mm zin,b = 1680 x 103 mm3

Precast concrete fck,p = 50 MPa, fcm,p = 58 MPa, Creep coeficient, ϕ = 1 Ec,p,long = 22 x (58/10)0.3/(1 + 1) = 18.64 GPa

Section properties, including the tendons and reinforcement, are as follows: Ap = 101.5 x 103 mm2

Ip = 1220 x 106 mm4

ypbar,b = 237.4 mm ypbar,t = 112.6 mm zp,t = 10900 x 103 mm3

CurvatureUsing expression (4) above:Curvature:

φ = 1000 x 0.0002

52.1 + 112.6 + (12.59 x 87.5 x 106 + 18.64 x 1.22 x 109) x (1/(12.6 x 112 x 103) + 1/(18.6 x 101.5 x 103))

50 + 112.6

= 0.00058/m

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

Eurocode 2, Table 3.1 and Cl.3.1.4

Eurocode 2, Table 3.1 and Cl.3.1.4

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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.

3.11 Designing for construction


34

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.

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 orb) suitable lateral reinforcement in both the supporting and supported members orc) 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 assessment 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 fl oor slabs, e.g. hollowcore slabs, spanning more than 8 m should be supported on elastomeric bearings, e.g. neoprene.

4.1 Horizontal forces at bearings


35

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.

For bearings that offer signifi cant 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 Structures26.

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 infl uence the design of the main reinforcement in such members. Such joints are not con-sidered 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 specifi c measures are taken to obtain a uniform distribution of the bearing pressure.

In the absence of other specifi cations, the bearing strength, fRd, of a dry connection should not exceed 0.4 fcd and the average bearing stress between plane surfaces should not exceed 0.3 fcd.

The bearing strength for joints with bedding material should not exceed the design strength of the bedding material, fbed ≤ 0.85 fcd where fcd is the lower of the design strengths for supported and supporting members.

4.2 Restrained bearings


36


Expanding materialto plug gap

Friction cancause cracking

Movement

Rotation

If no plug, hard materialcan prevent rotation

Rotation

Rotation cancause spalling

Figure 4.1Examples of potential failures at 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.

4.3 Movement joints

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.

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 fi lls 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.

4.4 Actions and restraints4.4.1 Action effects 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.


37


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 Structures26.

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.

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 fl oor, through shortening of the fl oor, if prestressed, and from thermal, live and further dead load movements, see also Section 4.1.

Prestressed members used for fl ooring 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.

di = ci + Δai with horizontal loop bars di = ci + Δai + ri with vertically bent barsci = nominal concrete coverΔai = a deviation (see Section 4.8)ri = radius of bend (see Table 4.1)

4.4.2 Restraints

4.5 Design considerations

4.6 Allowance for anchorage of reinforcement

at supports


38


c2 > +a a1 2� d3

r2

r3

d2 > +a a1 � 3 c3

Figure 4.2Effect of reinforcement on bearing

dimensions.

Table 4.1Minimum bend radii for reinforcement to

avoid damage to reinforcement.

Bar diameter Minimum radius of bendφ ≤ 16 mm 2 φ

φ > 16 mm 3.5 φ

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 defi nes the area of load transfer and thus has a direct effect on the detailed design of the ends of the supporting and supported members.

In the absence of other specifi cations, the bearing strength, fRd = fbed ≤ 0.85 fcd where fbed is the design strength of the bearing material may be used.

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 suffi cient 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.

4.7.1 Design of the bearing area

4.7.2 Bearing layout

4.7 Bearings that allow limited movement


39


The nominal length, a, of a simple bearing may be calculated as:

a = a1 + a2 + a3 + √(Δa22 + Δa3

2)

where a1 = net bearing length with regard to bearing stress = FEd /(b1fRd) but not less

than the values in Table 4.2 FEd = design value of the support reaction b1 = net bearing width fRd = design value of the bearing strength = 0.85fcd

a2 = distance assumed ineffective beyond outer end of supporting member (see Table 4.3)

a3 = distance assumed ineffective beyond outer end of supporting member (see Table 4.4)

Δa2 = allowance for distance between supporting members (see Table 4.5) Δa3 = allowance for deviation of the length of the supported member = ln /2500 ln = length of member in mm

b1

a1

> +� �a a2 3

a a3 3+ �a1

a

a a2 2+ �

Figure 4.3Critical dimensions for bearings.

Relative bearing stress, σEda/fcd ≤ 0.15 0.15 to 0.4 > 0.4

Line supports (fl oors and roofs) 25 30 40

Ribbed fl oors and purlins 55 70 80

Concentrated supports (beams) 90 110 140

Key:a σEd is the design bearing stress

Table 4.2Minimum value of a1 (mm).


40


Relative bearing stress, σEda/fcd ≤ 0.15 0.15 to 0.4 > 0.4

Reinforced concrete ≥ C30/37

LineConcentrated

510

1015

1525

Reinforced concrete < C30/37

LineConcentrated

1020

1525

2535

Keya σEd is the design bearing stress

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 barsexposed at end of member

5 15

Vertical loop reinforcement 15 End cover + inner radius of bend

Support material Δa2

Precast concrete 10 ≤ l /1200 ≤ 30 mm

Cast in-situ concrete 15 ≤ l /1200 + 5 ≤ 40 mmNote:l is clear distance between supports in mm

An example calculation is shown in worked example 5.

Table 4.4Distance a3 (mm) assumed ineffective from

outer end of supported member.

Table 4.5Allowance for deviations for the clear distance

between the face of the supports.

Table 4.3Distance a2 (mm) assumed ineffective from

outer end of supporting member.


41


Project details

Worked example 5Bearing of a hollowcore unit



Client Date

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 = 3.33 kN/m2

Variable load = 4 kN/m2

Partitions = 1 kN/m2

Finishes = 0.7 kN/m2

Bearing stress FEd = 9 x 1.2 x {1.35 (3.33 + 0.7) + 1.5(4 + 1)}/2 = 69.9 kNAssume a 30 mm wide neoprene bearing. σEd = 69.9 x 1000/(30 x 1200) = 1.94 MPa σEd/fcd = 1.94/(0.85 x 35/1.5) = 0.098

GeometryMinimum 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 Δa2 = 15 mm Δa3 = 9000/2500 = 4 mm say 5 mmThe reinforcement in the in-situ concrete nib is assumed to be 20 mm vertically bent with a nominal cover of 20 mm. d2 = c2 + Δa2 + r2

= 20 + 15 + 3.5 x 20 = 105 mm a2 + Δa2 ≥ d2

∴ a2 + Δa2 = 105 mm

Allowance for clearance at end of unit Δa2 + Δa3 ≥ 15 + 4 mm = 19 mm say 20 mm

The bearing stress should also be checked for the hollowcore unit.

Table 4.2Table 4.3Table 4.4

20

10 30 105

20

H20 bar

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Figure 4.4Typical 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 suffi cient to avoid contact, taking into account any long-term movements, defl ection, hogging and if the fl oor is laid to a fall for any reason, the difference in angle of the fl oor soffi ts 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 fi nal objective is to have a fully continuous connection.

Type 3 and 4 systems that use precast fl oors with in-situ beams do not always have a direct bearing since the in-situ concrete is often cast against the precast unit. The fl oor is propped and the formwork for the edge beam is fi xed. The steel protruding from the fl oor units is incorporated into the reinforcement of the edge beam that is then cast. The con-tinuity 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 fl oor 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.

4.8 Connections between precast fl oors and in-situ

concrete beams


43

Structural elements and connections 5

5. Structural elements and connections

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.

5.1 Twin wall construction (type 1)

Figure 5.1Typical example of a twin wall panel.

Photo: John Doyle Construction Ltd

The panels are supplied to site, erected and then fi lled with in-situ concrete to form a solid concrete wall. The trusses, therefore, also act to 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 specifi ed by the designer. However, starter bars and continuity reinforcement must be provided within the in-situ concrete.

The precast skins areconnected and spacedby steel lattice

Main horizontal andvertical reinforcementfor the wall is fittedwithin the precast skins

Figure 5.2Simple layout of a twin wall system.


44

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 fl oor, as the in-situ element of both the wall and fl oor can be combined to produce a monolithic structure.

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.

5.1.1 Manufacturing process

Figure 5.3Precasting 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 specifi ed 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 thicknessThe fi nal 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.


45


Precast concrete

Lattice reinforcement

In-situ concrete

Vertical reinforcement

Slab reinforcement

In-situ concrete

Vertical reinforcement

Tie reinforcement

Lap length

Figure 5.4Twin wall connection with a lattice girder slab.

With two layers of reinforcement an overall wall thickness less than 250 mm is diffi cult 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 suffi cient space for the concrete to fl ow 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.


46

Figure 5.5The available space for vertical continuity

reinforcement is restricted.Photo: Hanson Concrete Products

Overall panel sizesTypically 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.

10 m max.

3.5 m max. 10 m max.

3.5 m max.

Figure 5.6Typical twin wall maximum panel dimensions.



47

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 defi ned, 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-ordinationOften the twin wall manufacturer will require the following to be fully defi ned 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 design 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, soffi t levels and upstands/downstands must be fully defi ned and frozen; and the contractor must have defi ned a pour sequence so that the side to be propped can be identifi ed. The designer should allow for the additional time required to coordinate the work.

Detailing continuity rebar at jointsThe 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 drawingsIt 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 confl uence of site queries with panel drawing checking creating possible resourcing diffi culties at a key project stage.

5.1.2 Planning implications



48

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 fi xing 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).

5.1.3 Site erection and fi lling

Decide from which levelthe wall should spring

Consider tolerancesfor starter bars

Decide which side propsshould be positioned

Figure 5.7Typical issues to consider in the layout design.



49

Distance to 1st trussis typically 340 mm

Check sufficient lap lengthwith reinforcement in skin

Use of U-bars or links recommendedto ensure reinforcement remainsin correct position during concreting

Figure 5.8Plan view showing horizontal continuity

reinforcement.

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 diffi cult to remove all the air when concreting (‘blowing out’) and the contractor should provide specifi c proposals for this. Due to the very low volume of in-situ concrete required to fi ll 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 fi ll them. As the precast skins are functioning as permanent formwork, resisting the pressure of the wet in-situ concrete, the wall manu-facturer’s catalogue may have rate of rise limits – typically less than 1 m/hr. Coupled with the low fi ll volume, this leads to a relatively slow fi lling 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 unitsFigure 5.9 shows a typical section of a composite fl oor 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).

Lattice

Precastconcreteslab

In-situconcrete

Distribution steel

Main steel

Figure 5.9Section showing typical lattice girder fl oor.



50

Top bar 8 mm to 14 mm dia.

Diagonal bar 4.5 mm to 7 mm dia.

Bottom bar typically 5 mm dia.

Figure 5.10Typical details of a lattice girder.

The design of the lattice girder is dependant on the thickness of the composite fl oor, fi nal 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 specifi c bond, durability and fi re resistance requirements for individual contracts.

Design momentsDesign 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 designWhen 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)).

5.1.4 Design of panels



51

Minor axis bendingIf the decision to use a single row of starters has been adopted, minor axis bending on such walls should be checked. If signifi cant, 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.

5.1.5 Concrete and fi nishes Concrete mixThe nature of the in-situ concrete mix used to fi ll 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 diffi cult or impossible. The use of self-compacting concrete should be considered. A smaller aggregate size, for example 10 mm, may also be appropriate.

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

Figure 5.11Detail at an unrestrained horizontal panel

joint in compression.

The tendency to buckleunder compression at anunrestrained horizontaljoint, is resisted by thevertical continuityreinforcement, acting ata reduced lever arm.

C

d



52

Horizontal continuity reinforcementlowered into position after placingtwin walls. This must be detailed tomiss the wall trusses

Figure 5.12Horizontal continuity reinforcement to fi t

with twin wall reinforcement.

Vertical joints between adjacent panels at same levelAt 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 fi xing 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 signifi cant 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 defi ne 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 fi ve or six layers of reinforcement locally overlapping within the gap between the panels – a potential congestion issue, see Figure 5.12.

5.1.6 Detailing

The type 2 system uses precast columns and edge beams, often with a prestressed in-situ fl oor slab. The complex fi xing 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 fl oor plates. The precast edge strip is supported on the same shutter system that is used for the fl oor.

5.2 Precast columns, edge beams and in-situ slabs

(type 2)



53

The column to fl oor 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 diffi culty.

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 signifi cant. The designer should ensure that the detail can carry this load either by design or through testing.

Bearing under the precast columnIn 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 specifi c 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 fcd (= 0.85fck/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 suffi cient anchorage beyond the joint.

GroutingThe 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 fl oorFor axial load with moment transfer Eurocode 2, Cl. 6.7 limits the compression within the slab. Exp. (6.63) is modifi ed to:

FRdu = Ac0,eff fcd √(Ac1/Ac0,eff) ≤ 3.0 fcd Ac0,eff

where Ac0,eff = 0.9 x Ac0

Ac0 = area of precast column fcd = design strength of the slab Ac1 = (h/2 + b1) (h/2 + d1) h = depth of slab b1 = breadth of the precast column d1 = depth of the precast column

5.2.1 Column to fl oor joint



54

Figure 5.13Typical column fl oor connections.

a) Column shoe

b) Column bar coupler

c) Central dowel bar


Bars welded to doweltable and columnreinforcement

Grouting ringHole grouted beforeplacing column


55

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(d1/2 - e).

2 x ( - )d e1/2

fcd

e M/N=

C

C

L

L

of column

of compressionblock

d1

Figure 5.14Stress block in slab where moment is

transmitted from column.

For class 2B and 3 buildings (risk group 2B and 3 in Scotland) the vertical tie must be designed to take the full fl oor 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 NA.A1.3), see worked example 6.

If a central dowel bar system is considered for such a fl oor, 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.

Figure 5.15 shows a typical section of a composite fl oor 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, the cover to the reinforcement can be adjusted to meet the specifi c bond durability and fi re resistance requirements for individual projects.

5.2.2 Vertical tie

5.3 Biaxial voided slabs

Figure 5.15Typical layout of biaxial voided slab.

Photo: Cobiax Technologies Ltd



56

Project details

Worked example 6Vertical tie



Client Date

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.

ActionsThe total design force in vertical tie FEd = Gk + Ad

= 9 x 9 x {(25 x 0.3 + 1.0) + (0.5 x 4) = 851 kN.

ResistanceUsing a column shoe system: 4 No 25 mm bars will provide a resistance of FRd = γs fyk As

= 1.0 x 500 x 252 x 4 x π/4/1000 = 981 kN FRd > FEd → OK

Use 4 No 25 mm bars

Eurocode, Table NA.A1.3

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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 confi guration of the slab.

The benefi t of the reduced self-weight should be taken into account in the design. The design may assume a fl at 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 case, 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 fl exural strength.

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 Figure 5.16.

For punching shear it is recommended that the void formers are left out 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.

5.3.1 Slab geometry

5.3.2 Flexural design

5.3.3 Shear design



57

Figure 5.16Typical layout with fi nal reinforcement in

place.Photo: Cobiax Technologies Ltd

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 confi gurations the factor may be increased to 0.96. The manufacturers have data available to take advantage of these situations.

When using a fi nite 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.

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

Buoyancy of voidsWhilst 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 edgesVoids are not normally provided near slab edges to ensure a robust and continuous edge detail.

5.3.4 Defl ection control

5.3.5 General considerations



58

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.

5.4 Prestressed hollowcore units

Figure 5.17Typical hollowcore unit production.

Hollowcore units have lateral edges provided with a longitudinal profi le 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 specifi ed from manufacturers’ tables rather than designed from fi rst 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 fi re rating, detailed calculations should be made to verify such units are appropriate.

BS EN 11683 describes the requirements and the basic performance criteria and specifi es 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.

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.

5.4.1 Anchorage of prestressing tendons



59

The transmission length, lpt, for the prestressing wires or strands is that length required to transmit the full prestress, σp. lpt is defi ned in Eurocode 2, Cl. 8.10.2.3 where σpt1 and lpt1 are the values at ‘transfer’ and σpt2 and lpt2 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 σpt2 and σpd is less than that for the transmission length, lpt2, 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 fl exural stress in this region will tend to reduce the section size and nullify the wedge effects.

5.4.2 Transmission length

Tendon stress

Distance fromend of unit

Ipt1

Ipt2

Ibpd

pd

pt1

pt2

Figure 5.18Build-up of stress in prestressing wires or

strands from end of unit.

The cracking length, lcr, is the distance from the end of the unit to the point where the bottom fi bre stress resulting from all actions (bending, prestress and horizontal forces at the bearings) equals fctd. Figure 5.18 shows the components of actions and the net effect on the bottom fi bre stress. Note that if lcr is less than lpt2, 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 lcr is reached, see Figure 5.19(d).

If cracking does occur close to the support, the shear resistance is likely to be exceeded.

5.4.3 Cracking length

σ

σ

σ



60

a) Stress due to flexure b) Stress due to prestress

c) Stress due to horizontal force at support

d) Net bottom fibre stress showing cracking length, lcr

Support

CLof unit

Bottomfibrestress

CL

Support

of unit

Bottomfibrestress

CL

Support of unit

Bottomfibrestress

Bottomfibrestress

0

fctd

lcr

Tension

Compression

CLof unit

Possible overstress

near end of unit

fb,m

= Mx/Z

b

fb,P

= P/Ac + P

e/Z

b

fb,H

= H/Ac + Hy

b/Z

b

fb,Net

= fb, M

+ fb,P

+ fb,H

Figure 5.19Build-up of bottom fi bre stress in concrete

from end of unit.

The total anchorage length, lbpd, 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.

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.

5.4.4 Total anchorage length

5.4.5 Tendon slip at ends of units



61

Figure 5.20 shows the three typical types of end 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 to movement relative to its bearing. This may be the result of the effects 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 Structures26.

Cracked sectionsThe 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 lcr 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:

VRd,c = [0.12k(100ρl fck)1/3 + 0.15σcp]bwd

with a minimum of

VRd,c = (0.035k3/2 fck1/2 + 0.15σcp)bwd

5.4.6 Types of end failure

Large crackclose tosupport

Anchorageslip

Shear tensioncrack

Horizontal splitting cracks

a) Anchorage bond failure b) Shear tension failure c) Horizontal splitting cracks

Figure 5.20Types of end failure.

5.4.7 Anchorage bond failure

5.4.8 Shear resistance



62

where k = 1 + (200/d)0.5 ≤ 2.0 ρl = Asl /bwd ≤ 0.02 (normally = 0 since the distance to the end of the unit

< lbpd + d) σcp = NEd/Ac < 0.2fcd (NEd should be taken as γp times the prestress force)

Uncracked sectionsShear tension failure, see Figure 5.20, occurs 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.

Critical position

For circular core shapes = /2For oval core shapes say /3

hhs

Figure 5.21Critical section for shear tension failure.

The shear tension resistance is calculated in accordance with Eurocode 2, Exp (6.4):

VRd,c = I bw/S {(fctd)2 + αlσcp fctd }

0.5

where I = second moment of area bw = width of the cross-section at the centroidal axis S = first moment of area above and about the centroidal axis αl = lx/lpt2 ≤ 1.0 lx = distance of the section considered from the starting point of the transmission

length lpt2 = upper bound value of the transmission length of the prestressing element

according to Exp (8.18) of Eurocode 2 σcp = 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 VRd,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 with the Committee for BS EN 1168.)



63

BS EN 1168, Precast concrete products – Hollowcore slabs3 sets out further design checks that are required:

Prevention of horizontal splitting cracks (Cl. 4.3.3.2.1) Combined shear and torsion (Cl. 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) 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 satisfi ed, otherwise there is a danger of deterioration of the supporting nibs and ends of the hollowcore units that could lead to a shear and anchorage failure of the hollowcore units.

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 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 fl oors 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 barsHollowcore units should be connected to the supports or to the adjacent fl oor 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 fi xed in the correct position, as shown. If the latter method is used, note that it is essential that, after removing the top fl ange, the open core is thoroughly cleaned to allow good bonding of the new and old concrete.

5.4.9 Further design checks

5.4.10 Lateral distribution of vertical loads

5.4.11 Multi-span without structural topping



64

h/2

Limits to theplacing of tiebar

Limits to theplacing of tiebar

40 mm

a) With grouting key at top b) With grouting key at bottom

≤ 2φ,

≤ 25 mm

≤ 2φ,

≤ 25 mm

Figure 5.22Placing tie bars between hollowcore units.

h/2

Normal limits toplacing of tie bar

φ

Figure 5.23Placing tie bars in hollowcore unit.

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.

h/2

≤ 75 φ ≥ lcr

Figure 5.24Minimum length of tie bar between units.

The anchorage length of a tie bar should not be less than lcr (see Section 5.4.3). The anchorage length should normally be suffi cient 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 Δlb in accordance with Table 5.1 (see worked example 7).

Concrete grade

C20/25 C30/37Grout 13φ 10φ

Concrete 11φ 9φ

Table 5.1Additional anchorage length, Δlb, for ribbed tie bars with regard to design against progressive

collapse.



65

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, ladd, should be provided to ensure the shear transfer between the in-situ concrete or grout and the hollowcore unit.

ladd = Fst/fctu

where Fst = tensile capacity of the tie arrangement in one core or joint fct = tensile strength of the in-situ concrete or grout u = 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 lcr + lbd (see Eurocode 2, Cl. 8.4.4) (+Δlb) for anchorage in concreted cores and to lcr + 100φ for anchorage in grouted joints.

Connections to wallsIf the wall supports more than three fl oors, 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 fl oors, 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.

Project details

Worked example 7Anchorage length of longitudinal tie bar



Client Date

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

Minimum length of 10φ = 10 x 20 = 200 mm.

Use anchorage length = 1280 mm

For example 10 of Precast Eurocode 2: Worked Examples29

RW

OB

TCC

CCIP-030

WE 7/1

April 08



66

a) Edge support b) Intermediate support

Figure 5.25Connections to walls.

b) Internal supporta) Edge support

In-situ concrete

In-situconcrete

Figure 5.26Connections to beams.

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

Connections to ledge beamsThe continuity tie reinforcement should interlock with the reinforcement of the supporting beam. A typical detail is shown in Figure 5.27. The fl ange width of the supporting ledge beam should be limited to the continuous solid section at the ends of the hollowcore units or confi ned to the depth of their top fl anges.



67

Transverse tie bars

� � �/ (+ / ) /b b cr

Figure 5.27Typical detail for connections to ledge beam.

5.4.12 Multi-span with structural screed

Tie reinforcement within hollowcores

Tension lap length

Tie reinforcement within structural screed

Figure 5.28Typical detail showing the tie reinforcement

within the structural screed.

b) For Class 2B and over structuresa) For Class 2A structures

Tension lap length

Tying reinforcement within hollowcores

a) For Class 2A structures b) For Class 2B structures

Figure 5.29Typical detail showing connection of tying

reinforcement to an edge beam.

Figure 5.28 shows a typical detail where the tie/fl exural reinforcement is placed within the structural screed.

Where the structural screed is used to provide the tie and fl exural continuity reinforce-ment it should be adequately tied to the perimeter ties. Figure 5.29 shows a typical detail for this.

The permitted deviations are specifi ed 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.

5.4.13 Dimensions



68

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

The minimum cover cmin 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: cmin = 1.5φ; when the nominal centre to centre distance of the strands < 2.5φ: cmin = 2.5φ; cmin 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.

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.

5.4.14 Tolerances for construction purposes

5.4.15 Minimum concrete cover and axis distances of

prestressing steel

5.5 Double tee beams

a) Without structural screed b) With structural screed

Figure 5.30Typical double tee units.



69

The shape of the of the double tee unit is particularly suitable and economical for pre-stressing because of the high position of the neutral axis, which maximises the lever arm, and because the ratio of the top and bottom fi bre 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 offi ce 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 fl ange 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 fi xed mould. There are variations to dimensions as some manufacturers have fi xed moulds set for the full depth, e.g. 800 mm, and fi t 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 fi t 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 Examples29.

BS EN 13224, Precast Concrete Products – Ribbed Floor Elements5 provides the specifi cation for materials, production, properties, requirements and methods of testing for ribbed fl oor elements. This includes a section on permitted deviations and minimum dimensions.

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

Welding is commonly used in double tee construction as tests and experience show that the welded connection between fl anges 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 fl ange 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.

5.5.1 Self-stressing moulds

5.5.2 Welded joints



70

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 fi re 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 fl ammable 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 fl ange connectors usually allows inspection to be on an agreed statistical basis.

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

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)

Figure 5.31Typical double tee connection details.

5.5.3 Structural topping



71

Where the designer has designed the double tee fl oor 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 fl ange 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 fi nal structure. This is also used to augment the welded connections between units. The lateral shear con-nectors, 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 defi ned 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 fl oor to adjacent units.

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 specifi c design situation. It is not recommended that differences between the defl ection 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.

Lifter position decidedby supplier

May protrude intostructural topping

Steel end plate withinternal anchor fortie to support

Transverse bars forshear flow with flange

U-bars to close linksat end of web.

Upper strand layermay be debonded

Lower strand layershould never bedebonded

End U-bar or anchoredangle to restrain spallpotential at end of rib

Link cage nominally10 mm at 50, 100 &200 ctrs in end 2 ofunit to aid anchorageof strands

d

Figure 5.32Typical double tee end detail.

5.5.4 Transverse distribution of concentrated loads



72

It is recommended that the width of slab assumed to contribute to the support of concen-trated 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 con-siderations 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 fl oors 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 fl ange stiffness from the full fl ange depth in the other, even where the fl anges 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 fi re resistance, shear reinforcement for exceptionally heavily loaded cases and top steel for cantilever ends.

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 suffi cient 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 fi llet welds. This anchorage can be used as part of the transverse tying of the structure.

5.5.5 Tying requirements

End connector withwelded tie bar

Structuraltopping

Double tee

a) Standard double tee supportwith welded connection

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

End of web End of flange

Figure 5.33Typical double tee connection detail.



73

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.

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. Scarfi ng 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.

5.5.6 Half joints

Strut and tie (1) Strut and tie (2)

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

Chamfer allows inclined tie tobe in optimum position

Only additional reinforcement for the mechanism of strutThis figure is to be read with Figure 5.32

and tie is shown.

Strand must not be debonded

Strand must not be debonded

Strand must bepresent and mustnot be debonded

Figure 5.34Double tee with scarfed end.

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 soffi t. 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

5.5.7 Billet support of double tee units



74

fl oor 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 tor-sional fi xity. 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 fi tting is adequate, meets the specifi cation and is suitable for use in the UK. Galvanised fi ttings can have corrosion problems in chloride bearing environments so expert advice should be sought before the fi ttings are used in swimming pool roofs, car parks and exposed coastal locations.

Finally, the fi tting 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 fi tting. These may not have been considered in the development of the fi tting, particularly if it was manufactured overseas where the traditions of tying structures may not be the same as in the UK.

Two beams are usually supplied in a load and should be secured in such a way that holding-down straps do not bear on the top fl ange edges. The site access must be fi rm without irregularity. Careless handling and the loading of the top fl anges with site construction material can crack the top fl ange of a unit, typically at the ends at the interface between the fl ange 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 benefi t.

Tie

Tie

StrutDouble tee

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

Figure 5.35Support of double unit using billet connection.

5.5.8 Transportation of long double tee beams

5.6 Stairs



75

It should be noted that stairfl ights are the primary means of escape if a building is subject to fi re or explosion and thus the robustness of the structure is vital.

BS EN 14843, Precast Concrete Products – Stairs8 provides the specifi cation 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 defi nitions 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 fi nal 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.

Figure 5.36 shows the main features of a typical single stair fl ight.5.6.1 Single stair fl ights

Tread

Going

WaistRiser

Figure 5.36Typical single stair fl ight.



76

Production tolerancesThe tolerances are given in BS EN 13369, Common Rules for Precast Concrete Products2 and BS EN 148438, table 1, see Table 5.2. Unless stricter tolerances are given in the project specifi cation these should apply.

Target dimension of the cross-section in the direction to be checked

ΔLa (mm)

Δcb (mm)

L ≤ 150 mm +10

L ≥ 400 mm -5±15

± 5+15-10

Key:a The difference between two consecutive risers must not exceed 6 mm.b The minimum concrete cover defi ned in BS EN 14843, Cl. 4.3.7 must take into account the depth of any concrete removed by a fi nishing

process. The positioning of reinforcement shall ensure that the minimum cover defi ned in BS EN 14843, Cl. 4.3.7 is achieved.

Minimum dimensionsThe minimum dimensions given in Table 5.3 should apply.

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

Where precast stair fl ights are used supported on in-situ landings, the landings should be cast against the precast fl ight. This avoids the problems of tolerances where precast fl ights 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.

Table 5.2Tolerances for stairs.

Table 5.3Minimum dimensions for stairs.

5.6.2 Top and bottom supports with in-situ

connections

In-situ concrete

Precast stair flight

Temporary 2 waybraced props

In-situ concrete

Precast stair flight

Temporary 2-waybraced props

Figure 5.37Temporary support of precast stairs.

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



77

H12 barScreed

Screed

ScreedH12 bar

H12 bar

a) Connection with dowel bar only

b) Connection with hanging and dowel bar

Figure 5.38Preferred arrangements of reinforcement for

connection with in-situ concrete.

5.6.3 Top and bottom supports with precast

concrete

LayoutFigure 5.39 shows the preferred dimensions for the detailing of the top joint between a precast stair fl ight 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 defi ned: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 typeThe recommended bearing type for precast stairs to precast concrete supports is a 10 mm thick mortar bedding.



78

1590

100 min

9015

100 min

10

90 15

10

100 min

9015

90

90 min

15 90

100 min

15

10

a) Landing with sloping interface

b) Landing with square interface c) Wall with square interface

Figure 5.39Preferred dimensions for top joint between

stairfl ight and precast support.Design and supervision considerationsThe 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 connectionFigure 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.



79

Tie reinforcement in structural topping

Tie reinforcement in structural topping

375 15

15 375

Figure 5.40Preferred layout of reinforcement for precast

joints.

120

3540

Screed

70

120

375 15Screed

15 375

Dowel connectionTo provide suffi cient 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.

Figure 5.41Dimensions to allow for dowel hole.

Figure 5.42Reinforcement arrangement for dowel

connection.



80

Project details

Worked example 8Dowel bar for connection of precast stairs



Client Date

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, FEd required should be at least the permanent action of the stair flight.

ActionsAssume average vertical thickness of stair flight is (150 + 100)√2 = 350 mmSelf weight of stair flight FEd = 25 x 0.35 x 1.5 x 4 = 52.5 kN

ResistanceIt can be shown that the maximum dowel force, FRd, is FRd = φb

2.√(fcd.fyd).{√(1 + ε2) - ε} ≤ Asfyd/√3 (shear resistance of the dowel) where ε = 3(e/φb) x √(fcd/fyd) 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 FRd = 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 ∴ FRd = 33.5 kN

No req’d = 52.5/33.5 = 1.57

Use 2 No. 20 mm dia. dowel bars

Steel angles are used to allow the stair fl ight to rest directly onto walls or fl oor units, see Figure 5.43.

5.6.4 Top and bottom supports using steel angles

Figure 5.43Support using steel angles.

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Structural topping

Structuralreinforcement

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

Full strength weld toreinforcement to angle.Lap with structuralreinforcement.

Structural screed

Structuralreinforcement

In-situ structure

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 identifi ed in the construction sequence. One example is to provide a positive tie between the fl ight and the landing by reinforcement welded to the bearing angle (at the precast factory) to lap with the fabric in the structural topping.

Stair fl ights 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 fi nish to be laid uniformly 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.

Figure 5.44Bottom and top support details using steel

angles.

5.6.5 Stairs with integral landings



82


50 mm recess forfinish surface

In-situ wall

Bar inserted to lap withwall reinforcement

Two horizontal bars insertedwithin ‘U’ bar to lap withwall reinforcement

b) Stair flight with integral upper landing

Bar inserted to lap withwall reinforcement

50 mm recess forfinish surface

In-situ wall

Two horizontal barsinserted within‘U’ bar to lap withwall reinforcement


a) Stair flight with integral lower landing

Figure 5.45Stairs with integral landings.

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 defi nitions of hc and ac. Unless special provision is made to limit the horizontal forces on the support, a minimum horizontal force of HEd should be combined with the vertical force FEd. Reference should be made to Section 4.1 concerning the value of HEd.

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.

5.7 Corbels, nibs and half joints

5.7.1 Design by strut and tie model



83

Strut and tie model for a corbelFigure 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 σRd,max = 0.34fck(1-(fck /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 FEd and

HEd, and the depth to Ftd (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 231. It is recommended that z0 should not exceed 0.75d. The bearing stress under the load should not exceed 0.48 fck(1- (fck /250)), see Euro-

code 2, Exp (6.61). Check the tie force, Ftd = Ftd’ + HEd where Ftd’ is the horizontal component of the strut

force caused by FEd. The total area of secondary links should be at least 0.5 area required to resist Ftd, see

worked example 9.

hcdz0

aHHEd

FEd

ac

Ftd

ac’

x

�

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



84

Project details

Worked example 9Corbel design



Client Date

Design ultimate load FEd = 300 kN, fck = 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.

Actions HEd = 0.2 FEd = 0.2 x 300 = 60 kN

Geometry y = 175 + 60/300 x 45 = 184 mm z = √(1842 + 6052) = 632.4 mm α = sin -1 (120/(2 x 632.4)) = 5.44º β = tan -1 (184/605) = 16.92º γ = 90 – 5.44 – 16.92 = 67.6º z0/d = (184 tan 67.6)/605 = 0.73 < 0.75 ➝ OK

Strut designMaximum stress in the strut is: σRd,max = 0.34 fck(1-(fck 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: FEd = 300/sin 67.6º = 325 kNHence the stress: σEd = 326 x 1000/(120 x 300) = 9.1 MPa σRd,max > σEd ➝ OK

Note: Further iteration could be carried out to maximise the strut efficiency.


300 kN

175160

60 kN

bc = 300

45

650

400

300

x = 120

Z0

Ftd

67.64o 605

a) Chosen solution b) Geometry of solution

�

� �

605

yx/2

Z

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Worked example 9Corbel design



Client Date

ReinforcementThe tension force in the reinforcement: Ftd = Ftd’ + HEd = 300 cot 67.6º + 60 = 183 kNArea 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 mm2 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 Concrete32.

2 H20 bars

H32 bar

5 NoH8 links

Strut and tie model for nibsWhere 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, Ft2d in the link bar is FEd (zb + ac)/ zb. The value of zb may be taken as 0.8 db. Note this force is in addition to any shear force in the beam link.

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db

ac

FEd

HEd

Ft1d

Ft2d

Zb

Zn

Figure 5.47Nib connected to bottom of beam.

Strut and tie model for half jointsFigure 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).

Distance between edge of bearingand inside of bar to be a minimumof the bar diameter or 0.75 x cover,whichever is greater

Cranked bars improve crack control

Horizontal ‘U’ bar withstandard mandrel size

Tension lap

Nominal links at 150

Tension anchorage hh

Full depth links to resist totalreaction equally spaced

a) Section

b) Plan

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



87

Construction issues 6

6. Construction issues

The performance of an HCC structure may be affected signifi cantly 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 specifi cation 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 process, 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 constructionBS EN 1991-1-6 Eurocode 1: Actions on Structures – Part 1-6: General Actions – Actions

during Execution33 and BS 5975, Code of Practice for Formwork34 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 Design22, 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).

6.1 Method of construction

Overturning 1.35 + 1.5* G Qk,f k c,

Resisting 1.15* Gk,b

Gk,b

Qk,c Construction

Gk,f floor

Resistancebeam

* Check that using a factor of 1.0 for both favourable and unfavourable

does not give a more unfavourable effect

Figure 6.1Temporary loading during construction.


88

6 Construction issues

For type 5 form of construction (see Figure 2.1) the in-situ concrete is used to knit the precast fl oor and beams together. The support of the precast fl oor 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 benefi t from neoprene pads beneath the fl oor elements. The outer edge of the supporting beam should include a chamfer to eliminate spalling when the fl oor 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 fl oor 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 fl oor should be devised with back propping if necessary to support the fl oors above.

The defl ection of the shuttering of the in-situ edge beam during casting should be considered. If the fl oor 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 fl oor 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 fl oor 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 fl oor and edge beam shutter, see Figure 6.2.

A neat lower end to an embedded fl oor unit can be achieved by forming a small groove in the in-situ concrete. This allows the edge of the in-situ to fi ll properly, avoids the likelihood of spalling and masks any slight difference in the soffi t level, see Figure 6.3.

a) Separate support systems can cause cracking in precast unit. b) Common support of precast floor and insitu beam.

Figure 6.2Support for connection.


89

Construction joint

Groove detail

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

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.

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 manu-facture, 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 suffi ciently robust to be delivered and incorporated into the building successfully and to ensure that exposed soffi ts 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.

In situations where long and short units are side by side, for example where lift and stair cores shorten spans, differential cambers can produce diffi culties. 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.

6.2 Composite action between precast units and

in-situ structural topping

6.3 Specially shaped standard units


6.4 Long and short units adjacent to each other


90

Webbeneath

Key

Figure 6.4Specially shaped standard units.


15 mm camber

30 mm camber

Outer supports at same level

Inner supports set approx. 15 mmhigher to reduce camber stepbetween long and shorter unit

Figure 6.5Long and short unit adjacent to each other.


91


In some countries, it is considered good practice to jack double tee fl anges at mid-span to even out camber differences. This is sometimes carried out by casting loops of reinforcement that protrude from the fl anges, 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 not 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 fl anges, see also Section 5.5.4.

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 specifi ed and carried out. Otherwise bond checks should be carried out.

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.

On site, hollowcore units are often lifted into their fi nal 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 fi t 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 Components35.

6.5 Differences of camber in double tees

6.6 Method of de-tensioning double tee units

6.7 Checking strand or wire pull-in for hollowcore units

6.8 Placing hollowcore units into the correct

position


92


Production tolerances are specifi ed in BS EN 13369, Common Rules for Precast Concrete

Products2, Cl. 4.3.1.1. For cross–sectional dimensions L, the permitted deviation is ΔL, and for position of reinforcing steel, prestressing steel and for the design cover c the permitted deviation is Δc. 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

ΔL(mm)

Δc(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. ΔL and the positive values of Δc (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 Δc (lower permitted deviation) are given for durability purposes.4. In particular, functional specifi cities of the products may require tighter tolerances.5. The given values may be modifi ed 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, cmin, plus the permitted deviation , Δcdev, or the producer’s guaranteed deviation, whichever is lower.

For principal dimensions other than cross-sectional dimensions:

ΔL = ± (10 + L/1000) ≤ ± 40 mm

whereL 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 verifi cation 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.

6.9 Production tolerances

Table 6.1Permitted deviations of cross-section.


93

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.

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 defi ning 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 fl oor with a central atrium. The span of the fl oor was 16.2 m (9 x 1.8 m) with a fl oor-to-fl oor height of 4.5 m. The fl oor 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 Lloyd’s of London

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

7.1.1 Achieving a clean column appearance


94

7 Special structures - case studies

Figure 7.1aLloyd’s of London redevelopment, external view.Photo: Copyright Arup

Figure 7.1bLloyd’s of London redevelopment, internal view.Photo: Copyright Arup


95


Precast yoke

Structural topping Prestressed inverted U beam

Stub column

Steel permanent formwork panels

Precast bracket

Figure 7.2Layout of the fl oor components.

Steel dowel

Tolerance pocket withsteel inserts

Precast yoke

Precastbracket

Dip groove

Stainless steelflange

Steel plate withshear studs under

Elastomeric bearings

Waterproofing detail

In-situ node

Prefabricated bracing

Figure 7.3a leftPrecast concrete bracket connection.Schematic layout of brackets.

Figure 7.3b abovePrecast concrete bracket connection.Precast bracket and yoke.Photo: Copyright Arup


96


The design of the bearing had to fulfi l 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.

7.1.2 Precast brackets

a) Bearing allowing rotation between filterand bracket

b) Bearing restrains column c) Bearing transmits shear from buildinginto bracing

Figure 7.4Design of bearings.

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.


97


Connection of precast bearing to columnThe 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 columns 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 diffi cult to hold the fi ve tonne bracket and column form in place with suffi cient accuracy, since this took place outside the slab. The details of the bracket and column profi le 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 fi xed, 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.

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 fl oor was cast it was propped down through two levels to limit the amount of load applied to the bracket and hence rotation.

7.1.3 Connection of precast bearing to in-situ prestressed

inverted U-beams


98


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 constructionThe fl oor-to-fl oor height is 4.5 m, of which 1.5 m is the fl oor itself. Both the structure and the services are exposed, with no false ceilings. Air is supplied through the raised fl oor and extracted at high level through the light fi ttings. The return air is taken out through ducts at stub column level. The permanent formwork panels were made of profi led 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 fl oor is shown in Figure 7.7.

7.1.4 Stability

T3 T4

T5

T6

T1

T2

Figure 7.5Main building stability system.


99


Stub columns

Diagrid beams

1800

300

550

440 1500

60100

300

50Floor finishStructural slab

Permanent formwork& acoustic insulation

Figure 7.6Permanent formwork panel with acoustic trays.

Photo: Copyright Arup

Figure 7.7Typical section through fl oor.

The subcontractor developed a formwork system to produce the diagrid beams, see Figure 7.8. Their design was based on folded and welded steel frames with ply faces. Neoprene gaskets were built into the metal sections that also formed rebates at joint lines. The components were fi xed together with bolts and wedges with adjustment for tolerance. The reinforcement cages were supported on purpose-made plastic cradles bolted down to the soffi t 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 fi rst-class fi nish and could be put together and taken apart very quickly. It was the key to success of this subcontract.


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.

7.1.5 Points of interest

7.2 Bracken House

Figure 7.8Diagrid beam formwork system.



101


Structural organisation

Ceilingextract

The concept wheel

Extract

Supply

Floor services supplybetween structure

Figure 7.9Combined structure and services concept.

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.


102


Above: Figure 7.10Radial beams and quadrant-shaped column at

atrium corner.Photo: Copyright Arup

Above right: Figure 7.11Column beam connection.


The soffi t of the slab is above the soffi t 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 fl oor-based air supply, electrical power and communications. The raised fl oor is 300 mm above the beam.

Figure 7.13 shows the fl oor layout during construction.

Floor finish

False ceiling

Lighting, sprinkers and air extractionPrecast concrete radial beams

Air supply, electrical services and communications

950

300

650150

250

In-situ concrete slab on metal decking

Figure 7.12Typical section through fl oor zone.


103


Figure 7.13Floor layout during construction.


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.

7.2.1 Points of interest


104

References

References

1 BRITISH STANDARDS INSTITUTION, BS EN 1992-1-1, Eurocode 2: Design of concrete structures-

Part 1-1: General rules and rules for buildings, 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

fl oor systems, BSI, 2005.

5 BRITISH STANDARDS INSTITUTION, BS EN 13224, Precast concrete products - Ribbed fl oor

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 fl oorings, 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 Offi ce, 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 fl oors 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.


105

References

28 INTERNATIONAL FEDERATION FOR PRESTRESSING. FIP Recommendations: Precast prestressed

hollowcore fl oors. Thomas Telford, 1988.

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

30 TAYLOR, H. Strand defl ection 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

fl ooring 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.



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

CCIP-030Published January 2009 ISBN 978-1-904482-55-0Price 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 the UK by those who will evaluate the signifi cance 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 information is accepted by The Concrete Centre or their subcontractors, suppliers or advisors. Readers should note that the publications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Cover photo: Courtesy of Outinord International Ltd.Printed by Information Press Ltd, Eynsham, UK

AcknowledgementsThe authors would particularly like to thank the following people for their support in the development of this design guide:

Tony Jones ArupIan Feltham Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged:

John Stehle Laing O’RourkeGraham Hardwick John Doyle Construction LtdPeter Kelly Bison Concrete Products LtdAlex Davie ConsultantDavid Appleton Hanson Concrete ProductsKevin Laney Strongforce Engineering PlcNorman Brown British Precast Concrete Federation Ltd


in-situ concrete


fl oor slab


beams


fl oor units





Typical hybrid concrete options.Please note this diagram is a repeat of Figure 2.1, page 8.


CC

IP-030D

esign of Hybrid C

oncrete Buildings

R. W

hittle MA (Cantab) CEng M

ICE H. Taylor FREng, BSc, PhD

, CEng, FICE, FIStructE


This design guide is intended to provide the structural engineer with essential guidance for the design of structures that combine precast and in-situ concrete in a hybrid concrete structure. It introduces the options available for hybrid concrete structures, and goes on to explain the key considerations in the design of this type of structure.

Bearings, interface details, consideration of movement, composite action, robustness and the effects of prestressing are all explained in this guide and design examples are included where appropriate. The importance of overall responsibility and construction aspects are also described.

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0Price Group P

© The Concrete Centre

Riverside House, 4 Meadows Business Park,Station Approach, Blackwater, Camberley, Surrey, GU17 9ABTel: +44 (0)1276 606 800 www.concretecentre.com

CI/Sfb

UDC624.072.33:624.012.3/.4

Robin Whittle has extensive knowledge and experience of designing all types of concrete buildings. He regular contributes to concrete industry publications and is a consultant to Arup. He was a member of the project team which drafted Eurocode 2.

Howard Taylor has extensive knowledge and experience of designing precast concrete elements and buildings, including developing alternative production methods. He is a past president of the Institution of Structural Engineers and is currently chairman of the British Standards Institution Building and civil engineering structures Technical Committee B/525.

Design of Hybrid Concrete BuildingsA 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


Design Hybrid Concrete Buildings

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