D e si g n o f H y b rid Co n cre te B u ild in g s R . W h it tl e M A ( C an tab ) CE ng M ICE H. T a y lo r FR En g , BS c, P h D, C En g , F IC E, F ISt ru ct E
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 978-1-904482-55-0 Price Group P
© The Concrete Centre
Riverside House, 4 Meadows Business Park,
Station Approach, Blackwater, Camberley, Surrey, GU17 9AB
Tel: +44 (0)1276 606 800
www.concretecentre.com
UDC
624.072.33:624.012.3/.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 Buildings
A guide to the design of buildings combining in-situ and precast concrete
R. Whittle MA (Cantab) CEng MICE
H. Taylor FREng, BSc, PhD, CEng, FICE, FIStructE
Hybrid cov-.indd 1
Published by The Concrete Centre
Riverside 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-030 Published January 2009 ISBN 978-1-904482-55-0 Price Group P
© The Concrete Centre
Cement and Concrete Industry Publications (CCIP) are produced through an industry initiative to publish technical guidance in support of concrete design and construction.
CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777
All advice or information from The Concrete Centre is only intended for use in 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
Acknowledgements
The authors would particularly like to thank the following people for their support in the development of this design guide:
Tony Jones Arup
Ian 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’Rourke
Graham Hardwick John Doyle Construction Ltd Peter Kelly Bison Concrete Products Ltd Alex Davie Consultant
David Appleton Hanson Concrete Products Kevin Laney Strongforce Engineering Plc
Norman Brown British Precast Concrete Federation Ltd
Type 1
Precast twin wall and lattice girder slab with in-situ concrete
Type 2
Precast column and edge beam with in-situ fl oor slab
Type 3
Precast column and fl oor units with cast in-situ beams
Type 4
In-situ columns or walls and beams with precast fl oor units
Type 5
In-situ column and structural topping with precast beams and fl oor units
Type 6
In-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.
Hybrid cov-.indd 2
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 1
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
Design of Hybrid Concrete Buildi2 2
Worked example 4 Differential shrinkage 31
Worked example 5 Bearing of a hollowcore unit 41
Worked example 6 Vertical tie 56
Worked example 7 Anchorage length of longitudinal tie bar 65
Worked example 8 Dowel bar for connection of precast stairs 80
Worked example 9 Corbel design 84
Design of Hybrid Concrete Buildi3 3
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
Design of Hybrid Concrete Buildi5 5
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
6 6Design of Hybrid Concrete Buildi6 6
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 concrete
Economic 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 2.1
Benefi ts of concrete.
7 7
2.1
Type 1: Precast twin wall
and lattice girder slab with
in-situ concrete
Design of Hybrid Concrete Buildi7 7
8
Figure 2.1
Typical hybrid concrete options.
Please note this diagram is repeated on the inside back cover for ease of reference.
Type 1
Precast twin wall and lattice girder slab with in-situ concrete
Type 2
Precast column and edge beam with in-situ fl oor slab
Type 3
Precast column and fl oor units with cast in-situ beams
Type 4
In-situ columns or walls and beams with precast fl oor units
Type 5
In-situ column and structural topping with precast beams and fl oor units
Type 6
In-situ columns with lattice girder slabs with optional spherical void formers
Figure 2.2
Type 1 construction, twin wall erection.
Photo: John Doyle Construction Ltd
Design of Hybrid Concrete Buildi8 8
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
Design of Hybrid Concrete Buildi9 9
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
Design of Hybrid Concrete Buildi10 10
11
Figure 2.3a
Example of type 3 projects.
Paternoster Square and offi ce building. Photo: John Doyle Construction Ltd
Figure 2.3b
Example of type 3 projects.
Homer Road, Solihull. Photo: Foggo Associates
Design of Hybrid Concrete Buildi11 11
12
Figure 2.4
Example of type 4 project, car park, West Quay, Southampton
Photo: 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.
Design of Hybrid Concrete Buildi12 12
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 2.5
Example type 5 project, Home Offi ce Headquarters, London.
Photo: Pell Frischmann Consulting Engineers Ltd and Bouygues (UK) Ltd
Design of Hybrid Concrete Buildi13 13
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 2.6
Type 6: Lattice girder soffi t panels used as permanent formwork.
Photo: John Doyle Construction Ltd
Design of Hybrid Concrete Buildi14 14
15
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 ties
A tie may be considered effectively continuous if the rules for anchoring and lapping bars given in Eurocode 2, Cl. 8.4 and 8.7 are followed and the minimum dimension of any in-situ concrete section in which tie bars are provided is not less than the sum of the bar size (or twice the bar size at laps), twice the maximum aggregate size and 10 mm.
The tie should also satisfy one of the following conditions:
A bar or tendon in a precast member lapped with a bar in connecting in-situ concrete, bounded on two opposite sides, by rough faces of the same precast member, see Figure 3.1.
A bar or tendon in a precast concrete member lapped with a bar in in-situ structural topping or connecting concrete anchored to the precast member by enclosing links. The combined ultimate tensile resistance of the links should be not less than the ultimate tension in the tie, see Figure 3.2.
Bars projecting from the ends of precast members joined by any method conforming with Eurocode 2, Cl. 8.7.
Bars lapped within in-situ structural topping or connecting concrete to form a continuous reinforcement with projecting links from the support of the precast floor or roof members to anchor such support to the topping or connecting concrete, see Figure 3.3.
3.1
Robustness
Design of Hybrid Concrete Buildi15 15
16
Figure 3.1
Continuity of ties: Bars in precast member lapped with bar in in-situ concrete.
Figure 3.3
Continuity of ties: Bars lapped within in-situ concrete.
Tie
Tie Tie
Figure 3.2
Continuity of ties: Anchorage by enclosing links.
Tie
Peripheral ties
The peripheral tie should be capable of resisting a design tensile force:
Ftie,per = (20 + 4n0) ≤ 60 kN
where
n0 = number of storeys
Internal ties
The 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 kN
Maximum spacing of internal ties = 1.5lr
Design of Hybrid Concrete Buildi16 16
17
Horizontal ties to columns and/or walls
Edge 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 considered
Tying of external walls is only required if the peripheral tie is not located in the wall.
Vertical ties
For class 2B and 3 buildings Approved Document A (and similarly the Technical Handbooks for Scotland for risk group 2B and 3 buildings) has the following requirements:
a) Each column and each wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of carrying a tensile force equal to the design load carried by the column or wall from any one storey under accidental design situation (that is loading calculated using BS EN 1990, Eurocode: Basis of Structural
Design22, Expression (6.11b)).
b) Where ties described in a) are not provided a check should be carried out to show that upon notional removal of each supporting column and wall, and each beam supporting columns or walls (one at a time in each storey of the building) that the building remains stable and that the area of floor at any storey at risk of collapse does not exceed 15 per cent of the floor area of that storey or 70 m2, whichever is the smaller, and does not
extend further than the immediate adjacent storeys.
c) Where the notional removal of such elements would result in damage or is in excess of the limit above then these elements should be designed as ‘key elements’. A key element
should be capable of withstanding a design load of 34 kN/m2 at ultimate limit state
applied from any direction to the projected area of the member together with the reaction
from the attached components, which should also be assumed to be subject to 34 kN/m2.
The latter may be reduced to the maximum reaction that can be transmitted by the attached component and its connections.
Anchorage of precast 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.
Design of Hybrid Concrete Buildi17 17
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 3.4
Typical diaphragm action from precast fl oor systems.
3.3
Diaphragm action
b) Floor carrying horizontal forces from wind by virendal action a) Floor carrying horizontal forces from wind by beam action
3.2
Stability
Design of Hybrid Concrete Buildi18 18
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
Design of Hybrid Concrete Buildi19 19
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Project details
Worked example 1
Hollowcore 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 surface
s - Smooth surface vs s vs vs vs s A A Edge beam Hollowcore unit KEY RW OB TCC CCIP-030 WE 1/1 April 08
Design of Hybrid Concrete Buildi20 20
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Worked example 1
Hollowcore floor acting as a diaphragm
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 = 90Ñ 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, Exp.(6.25) Eurocode 2, Cl.6.2.5 (2) OB TCC WE 1/2 April 08
Design of Hybrid Concrete Buildi21 21
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Project details
Worked example 1
Hollowcore floor acting as a diaphragm
Calculated by Job No.
Checked by Sheet No.
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. RW OB TCC CCIP-030 WE 1/3 April 08
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Worked example 2
Interface shear between hollowcore slab
and edge beam
Client DateConsider 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 kNm
Maximum 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 OB TCC WE 2/1 April 08
Design of Hybrid Concrete Buildi23 23
24
Project details
Worked example 2
Interface shear between hollowcore slab
and edge beam
Calculated by Job No.
Checked by Sheet No.
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) RW OB TCC CCIP-030 WE 2/2 April 08
Design of Hybrid Concrete Buildi24 24
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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 to reinforcement required to carry shear 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.
Design of Hybrid Concrete Buildi25 25
26
Figure 3.7
Design 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
Design of Hybrid Concrete Buildi26 26
27
Gradual detensioning mechanism
Stressed strands
Unit moulds or continuously extruded units
Jack blocks and embedded cantilever upright in concrete strong floor Figure 3.8
The long-line pre-tensioning system.
Mould end plate Strand
Detail of gap between moulds Unit in mould Figure 3.9
Typical detail of placing of moulds on the long-line system.
Debonding tendons
The position of the strands in the section is normally determined by the length of the unit and the design loading at mid-span. Stress limits are set for the serviceability limit state (for further information see Precast Eurocode 2: Design 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.
Design of Hybrid Concrete Buildi27 27
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Figure 3.10
Typical detail showing the debonding of a strand.
Typically 7 - 8 protruding links
Extra links at debonding point
Debonded strand Fully 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 3.11
Comparison between straight bonded, debonded and defl ected tendons.
Design of Hybrid Concrete Buildi28 28
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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
Design of Hybrid Concrete Buildi29 29
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Project details
Worked example 3
Upwards camber on slab due to
temperature gradient
Calculated by Job No.
Checked by Sheet No.
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 εi φ Figure 3.12
The effect of differential shrinkage across a section. RW OB TCC CCIP-030 WE 3/1 April 08
Design of Hybrid Concrete Buildi30 30
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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 E
i Ai /(Ei Ii + Ep Ip)} (5)
εp = εfds /{1 + Ep Ap /Ei Ai + (yi,b + yp,t)2 E
i 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 4
Differential shrinkage
Calculated by Job No.
Checked by Sheet No.
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 flange 2 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 RW OB TCC CCIP-030 WE 4/1 April 08
Design of Hybrid Concrete Buildi31 31
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Project details
Worked example 4
Differential shrinkage
Calculated by Job No.
Checked by Sheet No.
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 Curvature
Using 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 ection
Deflection 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 RW OB TCC CCIP-030 WE 4/2 April 08
(
)
Design of Hybrid Concrete Buildi32 32
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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
Design of Hybrid Concrete Buildi33 33
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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 or
b) suitable lateral reinforcement in both the supporting and supported members or c) sufficient continuity reinforcement through the joint to resist the lateral forces. Where type a) bearings are used then conservatively the horizontal design force should be taken as 20 per cent of the vertical force. A more detailed 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
Design of Hybrid Concrete Buildi34 34
35
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
Design of Hybrid Concrete Buildi35 35
36 Expanding material to plug gap Friction can cause cracking Movement Rotation If no plug, hard material can prevent rotation
Rotation
Rotation can cause spalling Figure 4.1
Examples 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 restraints
4.4.1 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.
Design of Hybrid Concrete Buildi36 36
