guide to long-span concrete floors - cement concrete

Guide to Long-Span Concrete Floors

C&CAA T36

Guid

e

Guide to Long-Span Concrete FloorsCement and Concrete Association of Australia

Guid

e

First published 1988

Second edition August 2003

C&CAA T36

© Cement and Concrete Association of Australia 2003

Except where the Copyright Act allows otherwise, no

part of this publication may be reproduced, stored in a

retrieval system in any form or transmitted by any

means without prior permission in writing from the

Cement and Concrete Association of Australia.

The information provided in this publication is intended

for general guidance only and in no way replaces the

services of professional consultants on particular projects.

No liability can therefore be accepted by the Cement

and Concrete Association of Australia for its use.

DESIGN AND LAYOUT Helen Rix Design

ILLUSTRATION TechMedia Publishing Pty Ltd

ISBN 1-877023-09-4

The Cement and Concrete Association of Australia isa not-for-profit organisation established in 1928 andcommitted to serving the Australian constructioncommunity.

The Association is acknowledged nationally andinternationally as Australia’s foremost cement andconcrete information body – taking a leading role ineducation and training, research and development,technical information and advisory services, andbeing a significant contributor to the preparation ofCodes and Standards affecting building and building materials.

The Association’s principle aims are to protect andextend the uses of cement, concrete and cement-based products by advancing knowledge, skill andprofessionalism in Australian concrete constructionand by promoting continual awareness of products,their energy-efficient properties and their uses, and of the contribution the industry makes towards abetter environment.

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Concrete floor systems offer the designer a widevariety of options from which to choose a floor systemfor a particular project. All of these systemsincorporate the many advantages which concretebestows. Most importantly it is a plastic material whenfresh and can be moulded into any shape thedesigner chooses. Thus it imposes almost norestriction on the plan of the floor although individualsystems may impose limitations. Further, concreteallows a variety of surface finish, colour and texture tobe used. Concrete is non-combustible and possessesgood insulating qualities, concrete floor systems canthus be designed to meet the requirements for fireresistance. Concrete is also a durable material andcan easily be designed to meet the durabilityrequirements for the particular exposure location, whilethe abrasion resistance of the floor surface can beadjusted to meet the most demanding requirements.This Guide concentrates on the structural design ofthe floor system and the designer should consultother manuals for advice on how to specify theconcrete mix and construction practices to achievethe desired performance (see Bibliography).

The permission of the National Precast ConcreteAssociation Australia to use material from its PrecastConcrete Handbook in the section on precast floorsystems is gratefully acknowledged.

The charts for the insitu floors have been preparedusing the computer program RAPT version 5.41 aslicensed by the Prestressed Concrete DesignConsultants (PCDC) to the Cement and ConcreteAssociation of Australia.

This Guide supersedes Design Guide for Long-spanConcrete Floors (T36) published by the Cement andConcrete Association of Australia and the SteelReinforcement Promotion Group in 1988.

Relevant Australian Standards

AS/NZS 1170.0 Structural design actions – Generalprinciples, Standards Australia, 2002.

AS/NZS 1170.1 Structural design actions –Permanent, imposed and otheractions, Standards Australia, 2002.

AS/NZS 1170.2 Structural design actions – Windactions, Standards Australia, 2002.

AS 1311 Steel tendons for prestressedconcrete – 7-wire stress-relieved steelstrand for tendons in prestressedconcrete, Standards Australia, 1987.

AS 1366.3 Rigid cellular plastics sheets forthermal insulation – Rigid cellularpolystyrene – Moulded (RC/PS – M)Standards Australia, 1992.

AS 1530.4 4 Methods for fire tests on buildingmaterials, components andstructures – Fire-resistance tests ofelements of building construction,Standards Australia, 1997.

AS 3600 Concrete structures StandardsAustralia, 2001.

AS 3610 Formwork for concrete StandardsAustralia, 1995.

Supplement 2 Formwork for concrete –Commentary (including Amdt. 1)Standards Australia, 1996.

AS/NZS 4671 Steel reinforcing materials, StandardsAustralia, 2001.

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PREFACE

CHAPTER 1 Scope 1

CHAPTER 2 Process of Floor Selection 2

CHAPTER 3 Architectural Considerations

3.1 General 3

3.2 Floor-zone thickness 3

3.3 Services 3

3.4 Penetrations 4

CHAPTER 4 Structural Considerations

4.1 General 6

4.2 Strength 6

4.3 Deflection 6

4.4 Cantilevers 7

4.5 Vibration 8

4.6 Crack control 8

4.7 Check-list for structural design procedures 8

CHAPTER 5 Construction Considerations

5.1 General 10

5.2 Formwork 10

5.3 Reinforcement 11

5.4 Joints 11

CHAPTER 6 Insitu Concrete Floor Systems

6.1 General 12

6.2 Flat slab 14

6.3 Flat plate 16

6.4 Beam and slab 19

6.5 Ribbed (waffle) slab 20

6.6 Band beam and slab 22

6.7 Slab and joist 26

CHAPTER 7 Precast and Composite Floor Systems

7.1 General 27

7.2 Hollowcore 28

7.3 Permanent formwork or soffit slabs 30

7.4 Composite floors – beam and infill 32

7.5 Solid slabs 34

7.6 Single and double T-beams 36

CHAPTER 8 References 39

CHAPTER 9 Bibliography 40

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CONTENTS

1 SCOPE

Concrete structures have, for many years, dominatedthe Australian multi-storey commercial and residentialbuilding scene. Landmark projects such as Sydney’sMLC building and Melbourne’s Rialto rank among thetallest reinforced concrete buildings in the world andtestify to the skills of Australian designers, buildersand tradesmen.

Traditionally, column spacings and floor spans in thesebuildings have been in the range of 6 to 9 metres, toboth contain costs and simplify construction.However, recently there is an increasing preference bybuilding owners and tenants for large floor areas withcolumn-free space and spans from 9 to 16 metres.This has focused the interest of designers andbuilders on methods of reducing costs and speedingconstruction of long-span floors. For the purposes ofthis Guide, long-span floor systems are generallyspanning greater than six metres for reinforcedconcrete systems or eight metres for prestressedsystems. Some systems are effective below thesearbitrary limits and their full range is included hereinfor completeness.

The aim of this Guide is to provide designers with anappreciation of the factors that should be taken intoaccount in selecting a floor system for a particularbuilding. A section on the major architecturalconsiderations is followed by another on the majorstructural design considerations and one onconstruction considerations. These are followed by adescription of the various floor systems, photographs/sketches of each showing the appearance of the soffitand a chart indicating the economical spans and loadcapacities to aid in their selection.

The Guide provides discussion on only the commonfactors to be considered in the choice of a floorsystem. Designers are responsible for identifying anddesigning for all the requirements specific to theirparticular project, eg attack by chemicals to be usedin the manufacturing process to be carried out in thebuilding or specific limits required on deflections orvibration.

It is emphasised that the graphs are not designcharts but aids to enable designers to quickly identifyappropriate floor systems to carry the applied loadingfor the desired span, and thus provide approximatedimensions for the preliminary design.

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C H A P T E R 1 S c o p e

2 PROCESS OF FLOOR SELECTION

The process of selecting a floor system can becomplex, covering architectural, structural andconstruction considerations and generally will involveseveral iterations or progressive refinements until thefinal choice is made and the detailed design can beundertaken. It encompasses a number of identifiablestages commencing with a conceptual design of thestructure and ending in the completed designapproved for construction. These stages can besummarised as:

Conceptual Design – when the space and usagerequirements, the architectural appearance, thestandard of quality and the broad requirements for thestructure are defined. It will also consider how thestructure will perform and how it is to be built. Inaddition, a preliminary cost estimate should be madeto confirm that the project is economically viable.Usually, a number of alternative schemes will beevaluated and perhaps a couple chosen for furtherevaluation at the next stage.

Preliminary Design – when the client requirementsfor the project are refined and the chosen alternativeschemes considered in more detail. In this stage, foreach scheme being evaluated, the following aspectsof the total structure have to be defined:

■ Lateral load-resisting systems.

■ Framing plans.

■ Preliminary member sizes, including floorthicknesses. These may be based on span-to-depth ratios, the charts in this Guide or apreliminary structural design carried out toconfirm proposed sizes.

■ Control of volume change deformations andrestraint forces.

■ Connection concepts.

Approximate member sizes for the alternative designsare used to develop more-precise costing for eachscheme to find the optimum solution. Planningapplication based on the preferred scheme is usuallysubmitted and, if required, a more detailed costestimate is prepared to confirm the project is onbudget.

Final Design – when the client requirements arefinalised and the chosen preliminary design schemeis fully analysed, designed and detailed for the wholedesign life, including construction and demolition,covering all limit states. Durability, fire resistance andother relevant design actions also have to beconsidered. At this stage, the project documentation,plans and specifications, etc are prepared andsubmitted for approval and, if required, a further costcheck carried out.

The effect of loads, forces and deformations on thejoints and the behaviour of the total structure underthe various design actions should always beconsidered. Restraint, by loadbearing walls andcolumns under the slab, of volume-changedeformation in floors, due to shrinkage or tensioningof the floor system, should always be consideredotherwise significant cracking is likely to occur in thefloor system and/or supporting elements.

For precast floors, the following may need to beconsidered during the design process:

■ The design of the member during handling,transport and erection.

■ The design of the structure during construction(sequence, support of individual members,bracing—including structural robustness requiredby AS/NZS 1170.0).

■ The design of the completed structure.

C H A P T E R 2 P r o c e s s o f F l o o r S e l e c t i o n

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3 ARCHITECTURAL CONSIDERATIONS

3.1 General

The numerous architectural considerations for anyparticular project range from space requirements forthe various processes and activities to be carried outinside the building to the overall appearance of thebuilding – both in isolation and in its context. By andlarge these are similar for floors of any span. Theaspects considered herein are those that aresignificant or peculiar to long-span floors (ie floor-zonethickness, services, penetrations through the floor)and generally have a direct impact on the design ofthe floor system. Nevertheless, other aspects notconsidered herein may be controlling factors, eg arequirement on limiting vibration, facade treatment, etc.

The requirements for the surface finish on floors areno different to those for normal-span floors exceptperhaps for drainage gradients because of theirsensitivity to deflection (see Section 4.3).

In most cases, the soffit finish is produced off formworkeven for precast elements. If the soffit is visible in thecompleted building, the effect of the formwork andthe pattern it makes on the surface needs to beconsidered. While the appearance of the floor structurewhen it is to be exposed to view is often decided bythe architect, the structural designer must be aware ofaesthetics and participate in achieving the desiredappearance for the completed building. If the soffit isto receive a plaster coating, then consideration needsto be given to providing a suitable texture to the soffit.It is extremely difficult, if not impossible, to get plastercoatings to adhere to surfaces of high strengthconcrete that have a high density, eg surfaces ofmembers cast against steel formwork.

3.2 Floor-zone thickness

The thickness of the floor zone is greater than theoverall depth of the floor system. It includes the depthrequired for any floor finishes, set downs or set ups,falls, and the depth required to accommodate below-floor building services Figure 1. In modern buildingsthese often include air conditioning ductwork and fancoil units, ventilation and exhaust ducts, sanitary floortraps and waste pipes, stormwater waste pipes, hotand cold water, fire water and sprinklers, smokedetectors, data and electrical cabling, lighting andmany other specialist services. Below these servicesthere is usually a suspended ceiling (often a tile

system to allow access to the services) supported ona tee bar system or similar with hangers to the floorover. Alternatively, an applied ceiling finish to the soffitof the floor can be used. The 'gravity' services ofsewer and stormwater usually have precedence overother services as they must be laid to nominated falls.

Minimising the floor-zone thickness can provide asignificant cost benefit. If there is a limit on the overallheight of the building, it will maximise the number ofstoreys that can be provided. Alternatively, if thenumber of storeys is fixed, it will minimise the height –and therefore the cost – of all vertical elements. Thisis particularly significant for the external walls inwhich relatively high-cost materials will often be used.

3.3 Services

Services need to move horizontally along and acrossthe floor and vertically from floor to floor in a buildingwithout clashing. The architect, structural designerand the services designer should liaise closely toensure that services such as ducts, cables and thepenetrations for the services do not jeopardise thestructural behaviour of the floor system or theoperation of the building.

Electrical, data and telephone ducts are sometimescast within the slab thickness. A single electrical ductdoes not reduce the slab capacity significantly.

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C H A P T E R 3 A r c h i t e c t u r a l C o n s i d e r a t i o n s

FLOO

R ZO

NEVa

ries

with

ser

vice

s

Ceiling

Floor level

Services

Structure

F IGURE 1 : Typical floor zone

However, two or more ducts tied to a reinforcing barmay significantly reduce its bond and effectiveness.Certainly the effect of a number of ducts brought to acentral distribution point must be evaluated bothstructurally and from a fire-resistance requirement.Concentrations of service ducts should be identifiedat the design stage and appropriate design strategiesused to accommodate them, eg locate service boxesinto deepened areas of the slab Figure 2. Locatingthese ducts under a false floor above the structuralslab will also avoid the problem.

Services located below the floor system may poseother problems of support and transverse distributionpast or through intervening beams or bands. It isnormal for the service contractor to support theservices with light drilled fixings into the soffit of thefloor within the cover zone. The location of heavy-dutyfixings which extend beyond the cover zone or drilled,cut or cored holes or penetrations through the floorsystem should have approval of the designer toensure that the structural performance is unimpaired.For example, a transverse duct with either dimensiongreater than, say, 0.25D through the web of a beamcan significantly reduce its shear and/or its flexuralcapacity.

The designer may also have to nominate fixing zonesand coring zones for services in prestressed andprecast floor systems.

Vertical services are usually accommodated byproviding penetrations through the floor structure.These are often in riser ducts in one location within

the building – frequently adjacent to stairs, lift shafts,rear walls and the like. Incorporating services exceptsmall electrical conduits within concrete columns andwalls should be avoided. Columns and walls arehighly stressed elements and reducing the concretecross-section available to carry the load maynecessitate increasing the dimensions or the requiredconcrete strength. Further, bringing the service ductthrough the reinforcing cage in a column, wall orfooting will involve loss of support area and caninvolve having to modify the reinforcing cage. Latermodification of the service due to changes over thelife of the building is also difficult, while with water,waste water or sanitary drainage services, there is thepotential for corrosion of the column reinforcement.

Set downs in the top of slabs are often required forbathrooms and toilets or areas open to the weather.These setdowns allow finishes such as floor tiles to belaid on the concrete to the required falls for drainage.In buildings such as hotels and hospitals, the extentof such setdowns can be significant. Set downs of20–100 mm may be required and this will need to beallowed in the floor system design, in terms of bothstructural performance and fire rating.

The requirements for services will change with timequite apart from any change implicit in change ofoccupancy. Thus at the design stage considerationshould be given to how future changes in therequirements for services are to be accommodated.The design should facilitate these future decisionseven though they cannot be foreseen in detail. Forexample, incorporating an access floor on top of thestructural slab provides a space where horizontalservices can be relocated or updated as necessary.Similarly, marking the positions of prestressing cableducts on the soffit of the slab by using plastic supports(or similar) to hold the ducts in the desired positionmeans that future penetrations through the slab canavoid the cables with a high degree of reliability.

3.4 Penetrations

Large penetrations through a floor for major ducts,non-loadbearing stairs or lift shafts are part of thestructural design of the floor system. Loads due tolarge non-loadbearing ducts, shafts, stairs and liftsand the loads of surrounding floor are usually carriedby bands or beams around the perimeter of thepenetration which in turn carry the loads back to thecolumns or adjacent beams.

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F IGURE 2: Cables placed within a slab

Vertical services such as downpipes tend to belocated adjacent to columns because columnsprovide lateral support and protection. However, froma structural point of view locating services adjacent tothe columns is not ideal. At this location, the shearand moment are usually a maximum and there istherefore a concentration of reinforcing steel (and/orprestressing tendons) to provide the necessaryflexural and shear strength. Furthermore, this may notbe a desirable location for a rainwater downpipe froma flat roof as the column is likely to be a high point ofa roof (the roof will tend to sag between columns).This may not be true for fully prestressed structuresas the floor system will tend to hog between columns;however, the comment regarding high shear andmoment still applies.

Vertical penetrations through floors over say 200 mm by 200 mm, or where many penetrations areanticipated, are best detailed on the design drawings.Consideration should be given to providing morepenetrations or larger penetrations than those initiallyrequired, so as to provide space for later expansionof services. Generally, these penetrations will requireto be designed to achieve the fire resistance requiredfor the floor system or be enclosed in a fire-rated duct.

Beams and slabs can be designed to accommodatefairly large horizontal penetrations reasonably close tocolumns but these need to be designed and detailedon the drawings and their detailing not left to theservice contractor. The possible rearrangement of thereinforcement layout to suit the service contractorshould be avoided at all costs. Consideration shouldbe given to reducing the depth of the beam or bandlocally by using a haunch, notch, etc to avoidpenetrations. Upstand beams can sometimes beincorporated around service ducts to allow services toexit below the floors easily. Small horizontalpenetrations up to say 50–100 mm in diameter canusually be accommodated in the middle third of abeam depth. However, this restriction may make itdifficult to accommodate pipes needing falls –particularly where there are long horizontal runs.Figures 3 and 4 show examples of various servicesand penetrations through slabs, beams and columns.

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F IGURE 4: Services in a column

F IGURE 3: Services and penetrations

through slabs

4 STRUCTURAL CONSIDERATIONS

4.1 General

The aim in the structural design of long-span floors isto provide a serviceable and economical floorstructure. The floor must meet the client’s needs as setout in the brief and also reflect community expectationsfor safety, amenity and environmental impacts.

A serviceable long-span floor is one that has sufficientstrength to carry the permanent and imposed actionsas well having adequate stiffness to limit deflectionand vibrations. It must have the required resistanceagainst fire, be durable, and be visually acceptable, if exposed to view. The floor may also have to resistactions other than vertical actions and meet otherdesign criteria such as watertightness or chemicalresistance.

The aspects of durability and fire resistance, whilerequiring careful consideration, are not peculiar tolong-span floor systems and are not discussedherein. Similarly, while the appropriate specification ofthe concrete and the reinforcing materials will have adirect impact on the performance and long-termbehaviour of the floor, this is no different to theirimpact on other concrete structures. Designers aretherefore referred to the bibliography for furtherinformation on these aspects.

An economical long-span floor is one that optimisesthe material and labour costs. Minimum weight doesnot necessarily result in the lowest cost. Structuraldesigners should review the design progressively as itproceeds to ensure the structural design is integratedwith other aspects of the design of the building, egbuilding services, and no significant errors have beenmade. The accuracy of structural design theories,applied actions and material properties is such thatthere is no point in determining actions or membersizes to an excessive degree of accuracy.

The designer should have a feel for how the floor willwork, understand how the actions will be resisted andwhere the forces will go. Importantly, designers shouldalso consider buildability and other constructionissues to ensure the floor can be built efficiently. It isalso important that the designer reviews progressivelyand checks the drawings of the floor prior toconstruction to ensure that the intent of the designhas been properly translated onto the drawings.

4.2 Strength

The design of long-span concrete floors has to conformto the requirements of the Building Code of Australia(BCA)1 and the principles of design as contained inAS/NZS 1170 and AS 3600. In essence these followlimit-state design principles and require that:

■ actions for each limit state are determined;

■ the structure and its parts are analysed for theappropriate actions using the specifiedcombinations of factored actions; and

■ the structural responses under the above actionsdo not exceed the appropriate member or sectioncapacity.

For example, for the design for the strength limit statethe design strength of the section shall not be lessthan the design action effect (derived from thecombination of factored actions), ie Rd = φRu ≥ S*.

Generally, section dimensions and properties areestimated initially and the member analysed for theapplied actions. Choosing appropriate dimensions isa matter of experience and using general sizing rulessuch as span-to-depth ratios or the charts in thisGuide. The dimensions of the member are adjusted ifthe section is either under-strength or significantlyover-strength, or if the serviceability or any other limitstate is exceeded. The fundamental concept of theductility of reinforced concrete structures has been re-emphasised in the most recent editions of AS 3600.

4.3 Deflection

Designers should appreciate that deflection is just asimportant as strength in the structural design ofconcrete floors, particularly in the design of shallowreinforced concrete floors systems. Special attentionshould be given to the proportioning of the floorsystem for deflection before the final design.

The rules for deflection control given in the currentedition of AS 3600 incorporate some changes fromthose given in previous editions. Table 2.4.2 of AS 3600suggests a limit of span/250 for the total deflectionand states that this applies to the idealised frame asset out in AS 3600. For long-span floors these limitsmay not be appropriate. As an example, a 60-mmdeflection in a 15-m span meets the span/250 rule butmay be visually unacceptable and affect partitions andwalls under. Designers need to carefully consider sucheffects and may need to design for a lesser deflection.

C H A P T E R 4 S t r u c t u r a l C o n s i d e r a t i o n s

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For two-way floor systems such as flat plates or flatslabs the deflection of the column strip may need tobe limited to, say, 25–30 mm so that the deflection inthe centre of the slab between the columns in themiddle strip is not too high. Designers will need toconsider if the column-strip deflection, an average ofthe column-strip and the middle-strip deflections orthe maximum deflection in the centre of the slabbetween the columns is the controlling deflection. It isstrongly recommended that the expected order ofdeflections be discussed with the architect, the client,and the user or owner of the building so that theyunderstand what orders of long-term deflections maybe expected and how this may affect the operation ofthe building. Excessive long-term deflections can leadto unserviceable floors and, in the extreme, the needfor demolition.

Reinforced floors can be precambered to counteractthe effects of long-term deflection but care is neededin applying such cambers. It is suggested camberingbe limited to a maximum of half of the total deflection.

Architectural and other components, both under andover (eg partitions including deflection heads, glasswalls, folding doors) can be affected by deflections,which can cause unintended load transfer to the wallsbelow. Adequate allowance for these effects shouldbe made, remembering that all elements above andbelow the floor will still be subject to the same long-term deflections irrespective of any cambers, exceptwhere the supporting floor is a slab on ground. Rigidwalls (eg masonry) should have control joints to allowfor such deflections and careful consideration shouldbe given if supported on flat plate or flat slab floorsystems. Wherever possible, rigid walls should besupported on a stiff floor system such as beamsystems with limited long-term deflections.

The selection of the level of prestress and the load tobe balanced are important in the design ofprestressed slabs. The level of average prestress istypically in the range of 1.5–3.5 MPa. Generally, theload to be balanced is the permanent actions plus aproportion (often a small proportion) of the imposedactions. Care is needed in determining this proportion.Most prestressed concrete floors will have a netpositive (upward) camber or hog at the time oftransfer of prestress caused by the eccentricity of theprestressing force.

Many inherent variables affect camber and deflection,eg early stripping of formwork for insitu floors,incorrect back propping, time of release of prestress,time of erection and application of imposed actionsfor precast members, and relative humidity.Calculated long-term values should therefore neverbe considered any better than estimates. Whiledetailed methods have been derived for predictingthe long-term deflection of concrete members, thedata on which they are based has a scatter of at least15 to 30% using laboratory-controlled specimens.

4.4 Cantilevers

Cantilevers are often used for balconies and alongthe external edge of a building to balance the internalspan and/or improve the shear capacity at edge andcorner columns Figure 5. Regardless of the floorsystem chosen they always need careful consideration.Cantilevers are usually limited to a span of about 25 to 35% of the back span and should be supportedby a stiff support such as a beam or column. Theyshould not be supported off another cantilever at rightangles. The deflection of a cantilever is sensitive tothe rotation at the support. In particular, a longcantilever with a short back span may deflect downsignificantly or a short cantilever with a longbackspan may deflect up significantly.

7

F IGURE 5: Cantilevered floor slabs

Where cantilevers, edges of floors and floors areexposed to weather, consideration will need to bemade for set downs for finishes, falls for drainage andcover for durability, as well as the spacing and amountof reinforcement for crack control. With thin floors, suchas flat slabs or flat plates, where cantilevers or theedges of the slab are exposed to view they will deflectbetween the column lines. This will possibly make thesupport of external walls difficult with differentialdeflections, and may result in an unacceptable visualprofile along the edge of the structure even though itmay be structurally adequate. Solutions include anedge beam or band beam or closely spacedsupports or similar to stiffen the edge.

With some precast floor systems such as hollowcore,cantilever spans can be difficult to achieve andspecial consideration and consultation with themanufacturer should be made if these are required.

4.5 Vibration

AS 3600 does not prescribe any design criteria for thecontrol of vibration. It merely requires that vibration beconsidered and appropriate action taken to ensurethat vibration does not adversely affect theserviceability of the structure.

Aerobics, dancing and other rhythmic human activitieshave caused annoying vibrations in a number ofbuildings in recent years (particularly in cantilevers) inboth insitu and precast construction. The two mainfactors behind these problems are resonance (wherethe natural frequency of the floor structure is equal toor close to a forcing frequency of the rhythmicactivity), and the presence of other occupancies inthe building such as offices or restaurants wherepeople are sensitive to the vibrations generated.Where such activities are anticipated, the designteam should obtain specialist advice.

4.6 Crack control

Concrete is a brittle material and even minor tensilestrain will cause it to crack to some degree, inservice. The various types of cracking are discussedin detail in Durable Concrete Structures 2.

Cracking may allow oxygen and moisture to reach theembedded steel, providing conditions where rustingof the steel can occur and spalling of the surface willresult. A sufficient amount of closely spacedreinforcement limits the width of cracks, henceminimising the intrusion of water, and maintaining theprotection of the steel. Prestressing may also be usedto limit, or eliminate, cracking. High-quality concreteprovides adequate corrosion protection forreinforcement for most conditions.

The structural and durability requirements of AS 3600are based on a nominal crack width of 0.3 mm. Cracksthat are not expected to exceed that width do notneed repairing. Crack control for flexure in reinforcedbeams is based on the provision and adequatedistribution of a minimum area of reinforcement.Restrictions are placed on either the bar diameter orthe centre-to-centre spacing, depending on thetensile stress in the steel in critical tensile zones.

4.7 Check-list for structural design procedures

The structural design of concrete structuresincorporating suspended long-span floor systemsneeds an appreciation of many factors and for thesereasons must be carried out by a designer (usually astructural engineer) experienced in such design. TheBCA provides a simplified and uniform set ofregulations designed to establish essential constructionstandards for structural adequacy, fire resistance,public health and general amenity. The technicalrequirements of this code refer to AS 3600, as well asto other standards such as AS/NZS 1170.

In any structural design of floors, one designer shouldbe responsible for the overall structural designincluding stability, robustness, movement joints, fireresistance and durability even if more than onedesigner is involved in the design process.

The following check list sets out the principal steps indesigning a suspended concrete floor system to meetthe requirements of the BCA and AS 3600. While it iswritten for insitu floors, a similar process applies forprecast floors.

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STEP 1 Member Arrangement

Determine a feasible arrangement for the suspendedconcrete floor system including slab thickness, bandrib or beam sizes, supporting columns, walls, etc. Theaim should be to arrive at a preferred option that iseconomical and structurally efficient. This may requirepreliminary design and assessment of two or morealternatives. Experience, the site constraints, thebuilder’s or other preferences will sometimes dictate a solution.

STEP 2 Establish the Basic Design Criteria

■ Occupancy of the structure (BCA)

■ Fire rating (BCA)

■ Sound transmission (BCA)

■ Exposure classification and durabilityrequirements (AS 3600)

■ Any other special design criteria that may berequired (eg heavy loads).

STEP 3 Floor System Sizing

Determine the minimum floor system thickness fromStep 2 above, and with the chosen concrete strengthand cover, use the span-to-depth ratios or charts inthis Guide or other information available to thedesigner to determine the initial suspended floorsystem thickness.

STEP 4 Floor Thickness

Select a suitable overall thickness of floor system tosatisfy expected deflection and which is equal to orgreater than the thickness of the slab determined inStep 3. If the floor system is not thick enough then thedesigner will need to choose a greater thickness andrepeat the design processes set out below until asuitable thickness is found.

STEP 5 Determine the Permanent and ImposedActions (AS/NZS 1170)

This includes permanent and imposed actions as wellas wind and earthquake loads where the floor systemis part of the overall structural system.

STEP 6 Calculate Design Bending Moments andShear Forces

Determine the design action effects at the criticalsections in accordance with the strength requirementsof AS 3600 for the chosen floor system.

STEP 7 Strength Design (AS 3600)

This requires detailed analysis of the floor system anddetailed design including calculations and, if the floorsystem is not adequate for strength or if the flexuralreinforcement or shear reinforcement is excessive,repeating the design from Step 4 until suitable sizesare determined. The final design calculations areusually required to be submitted to the local authorityor checked and certified by others.

STEP 8 Calculate Deflection

Check the calculated deflection or the span-to-depthratio using the simplified method in accordance withAS 3600 or by more-sophisticated analysis, or frommanufacturer’s information as appropriate. Estimaterequired camber, if any, or repeat the design fromStep 4 if deflection is excessive until a satisfactoryfloor system thickness is found.

STEP 9 Complete Detailed Designs andDocumentation

Complete the detailed calculations for all designaspects necessary for the particular project andfinalise the documentation, ie drawings, specification,etc. It is important that the designer remembers thatthe documentation is the means of communicatingthe design intentions to the contractor/builder and itshould be reviewed from this viewpoint before beingissued. Guidance should be provided on how thestructure is to be stabilised during erection of theelements until lateral stability is achieved by thecompleted structure. It may be necessary for thedesigner to describe the sequence for construction toensure the design concept is not compromised andthe structure remains stable during erection.

9

5 CONSTRUCTION CONSIDERATIONS

5.1 General

The considerations listed in this Section are general innature and do not provide a checklist for all mattersthat need to be addressed. They should besupplemented by considerations specific to theparticular project, eg the requirements for finish tofloors, the proposed construction method, eg precastconstruction, and the location of the project.

In general, minimising the amount of labour on site,measures to minimise the construction period (orfloor-to-floor construction time) and schedules thatgive early access for following trades and advancedoccupation to part of the project will reduce theconstruction time, the holding charges and thusimprove the economic viability of the project.

5.2 Formwork

Formwork for insitu concrete and propping of precastis a major consideration as it can directly affect thearchitectural appearance of the project and theconstruction cost and schedule. Formwork can be50–75% of the direct cost of a floor. Generally, aspecialist designer, employed by the contractor, willbe responsible for formwork design. It will take intoaccount the requirements of the specification and theconstruction process, eg construction loads, stripping,back propping, and specified finishes and formworkpatterns Figure 6.

To simplify formwork, member sizes should berationalised consistent with structural economy, egband-beam widths of 1200, 1800 and 2400 mm willenable standard plywood sheets to be used. Similarly,drop panels in flat slabs should be dimensioned tosuit plywood and timber sizes.

Because of the high cost of site labour, making soffitformwork on site is often avoided. Formwork is usuallydesigned to have a number of reuses and to facilitateeasy stripping and re-erection to keep the overall floorconstruction period as short as possible. Sophisticatedtable and flying forms have been used on multistoreyprojects, while, to minimise costs, precast floor unitsare sometimes preferred.

Early stripping of formwork and back propping allowsearly access to the floors by other trades. It also allowsgreater re-use of formwork and reduces the amount offormwork required. On the other hand, particularly forreinforced floors, if the formwork is removed too early

and appropriate back propping and reshoring notcarried out then the floor immediately below maycarry most of the load before its design strength isfully developed. This may result in overstressing thefloors and long-term deflection problems.

Back propping and reshoring of suspended floors inmulti-storey construction can be a complex issue. Insome circumstances the loads due to constructionmay exceed the design load of the floor as set out inAS/NZS 1170.2. As an example, a suspended floorover a reactive clay site with insufficient space forpropping may have to carry the full construction loadof the floor above. For further details on backpropping see McAdam and Behan3 and Beeby4.

Prestressed floor systems will reduce stripping timescompared to conventionally reinforced floor systems.Post-tensioned floors are usually fully stressed at 7 days allowing formwork to be stripped after thistime. Reshoring and back propping, however, stillneed to be carefully considered.

C H A P T E R 5 C o n s t r u c t i o n C o n s i d e r a t i o n s

10

F IGURE 6: Formwork is a major

consideration in a project

5.3 Reinforcement

Reinforcement details should be as simple aspossible to facilitate fixing, reduce the risk of errorsand to simplify checking. Cogs and hooks should belimited to those that are essential. Mesh should beused where possible, while consideration should begiven to using stock length bars for larger projects,and to using alternative bar sizes to distinguishcolumn strips from middle strips. More informationcan be found in the Reinforcement DetailingHandbook 5.

Prefabricated cages and mats expedite reinforcementfixing, while standardising the layout to accommodateservice ducts minimises problems likely to arise witharbitrary on-site adjustment of the layout to suitservices. On larger jobs, consideration should begiven to making a mock up of difficult areas to see if itis possible to simplify the reinforcement Figure 7.

In slabs, proprietary shear reinforcement should beused at columns instead of bent up bars and smallligatures which are difficult to fix and time consuming.

Mechanical splicing of reinforcement is the quickestmethod of developing continuity of the reinforcing baracross a construction joint without the need to damagethe formwork or where lap splicing is not appropriate.A number of different splicing systems are available.The majority involve some form of threaded couplerthat can be used to join two threaded bar endstogether. However, designers should evaluate eachcoupler type to ensure that it will not significantlyreduce the ductility of the reinforcement.

Post-tensioning should follow industry practice. Strandsize commonly used in building work is 12.7 mm,either four or five in slab cables and seven in beams.This allows the use of lightweight jacks capable ofstressing either individual strands or up to sevenstrands together. This eliminates problems associatedwith handling heavy jacking equipment within theconfined areas of a floor deck. Use uniform recesssizes and dimensions for anchorages. Avoidinterferences at intersection duct profiles especially atcolumns and slab corners. Consult a specialist post-tensioning company before finalising details.

5.4 Joints

Joints should be shown on the drawings as requiredby AS 3600, Clause 1.4.2. Construction joints shouldbe shown on the drawings as it gives thebuilder/contractor an understanding of what jointsneed to be allowed for. They occur in slabs, columns,stairs and other structural elements. In verticalelements they are usually at the underside and/ortopside of horizontal elements.

Where construction joints are exposed to view,designers should provide a rebate or similar to ensurethey read as a straight line.

11

F IGURE 7: Mock-up of complicated

reinforcement details

6 INSITU CONCRETE FLOOR SYSTEMS

6.1 General

There are many concrete floor systems from which thedesigner can select an economical and technicallysatisfactory solution.

Traditionally, concrete floor systems are reinforcedusing bars, fabric or using high-strength strand whichis stressed. The action of prestressing with a straightor a draped cable in concrete enables the appliedloads to be balanced by the uplift force so thatpractically no deflection results. This is a significantbenefit in long-span floors as it eliminates the need tocamber formwork or to provide deeper reinforcedconcrete sections.

In recent times, with the increasing popularity ofpartially prestressed concrete, the difference betweennormal reinforced concrete and traditional fullyprestressed concrete has become less clear-cut. Thisapproach combines prestressing’s benefits ofcontrolling deflection and cracking with the economyof reinforced concrete. For example, an economicalfloor system might comprise partially prestressedbands or beams (with both strand and reinforcement)in one direction combined with a normally reinforcedslab in the other direction.

Even with a total prestressed system, bar reinforcementwill be required for ductibility, crack control and forrestraint at anchors and the like.

Spacing and location of columns and loadbearingwalls need to be selected to provide the mosteconomical structure and to meet spatial and otherrequirements. In the case of a multiple-use buildingsuch as a carpark, retail and residential, differentcolumn spacing may be required for each use,leading to transfer floors and transfer beams in somecases. Care should always be taken with the initialframing of a structure to ensure the most economicalcolumn and loadbearing wall locations are used. Slabspans in the past were often in the order of 8–9 m,but longer spans have been used in recent years.

With the trend towards longer spans, the criterion ofstiffness has become more important, so that inpractice the dimensions of the floor are determinedby stiffness considerations rather than strength.

Longer span floors generally have a greater structuraldepth, higher self weight, possible problems with

vibration and deflection, and increased cost. However,when taken as part of the overall cost of the structurethese costs may be acceptable. In many cases theaddition of a few strategically placed columns orloadbearing walls can significantly reduce the slabspan. Sensible positioning of columns need notdetract from the flexibility of floor areas. The cost toprovide long-span column-free space may not bewarranted in all cases.

For continuous spans, reducing the outer span toabout 75–80% of the inner spans can minimise theneed for deeper floor systems in the external spans.Similarly, cantilevers may be used to balance theinner span, but see comments on the use ofcantilevers in Section 4.4.

Particular attention should always be given to thecorners and the edges of the floor system where two-way behaviour of the slab is likely to occur(depending on the stiffness of edge beams andcolumns) and the effects of lack of continuity shouldbe considered.

With all floor systems, careful consideration should begiven to shrinkage of floors and shortening due toprestress. Both processes, and particularly the latter,will generate significant forces in the floor system if itis restrained by walls or stiff columns, especially ifthey are located near or at each end of the floorsystem and parallel to the direction of stressing. Thisin turn can lead to significant cracking and otherproblems. Techniques to overcome these problemsinclude pour strips, slip joints and the like.

An examination of some of the common examples ofconcrete floor systems follows. These cover bothsingle and multiple spans.

A quick selection of possible floors can be made fromthe chart in Figure 8. The preliminary designthickness can then be assessed from the particularchart for each floor system. Note that these floorthicknesses/beam depths should be used only as thebasis for detailed design.

All of the charts in Sections 6.2 to 6.7 are based on aregular grid with equal or near-equal spans. They arefor initial sizing and preliminary design only beforecarrying out the detailed design of the selected floorsystem. Detailed design should be carried out inaccordance with the suggested design procedure setout in Section 4.7.

C H A P T E R 6 I n s i t u C o n c r e t e F l o o r S y s t e m s

12

The following assumptions have been made inpreparing the charts for insitu floors. Designersshould satisfy themselves that the particular floorsystem they are designing meets these assumptionsor make appropriate adjustments to the preliminarysizes to cover variations in the assumptions:

■ For continuous spans there are 3 or more spans(in each direction for two-way systems) with acantilever at each end so that equal or near equaldeflections and bending moments are achievedin each span

■ For a single span the floor spans one way from acore wall to a suitable edge beam or column. Thecore wall has been assumed to provide fullcontinuity to the floor system

■ Pattern loading is not considered for vertical loads.AS 3600 covers pattern loading when required

■ Slabs are regular and do not have spans inorthogonal direction greater than 2 to 1

■ Spans do not vary by more than 25%

■ Uniform loads are applied to all spans

■ Loads are in the range of 1.5 to 5.0 kPa, ie domestic to office type loadings.

Actual floor systems in buildings come in all sizes,shapes, and forms. There are so many variables toany floor system such as different spans, offset spans,cantilevers, and the extent of continuity, the effects ofbeams, columns and walls on the slab system, etcthat it is difficult to cover all situations in a limitedseries of charts. With the range of sophisticatedcomputer packages now available for concrete floordesign, it is strongly recommended that once a floorsystem and initial sizes are chosen then a series oftrial runs be carried out to determine the appropriatethicknesses and details as part of the preliminarydesign process.

13

F IGURE 8: Quick selection guide for

insitu floors

0 5 10 15 20

0 5 10 15 20

Comparative span (m)


Floor system

Single span Reinforced PrestressedMulti-span Reinforced Prestressed

FLAT PLATE

Multi-span Reinforced Prestressed

FLAT SLAB

Single span ReinforcedMulti-span Reinforced

RIBBED SLAB

Single span Reinforced Prestressed

BAND BEAMand SLAB

Single span PrestressedMulti-span Reinforced Prestressed

BAND BEAMSat 8.4 m

6.2 Flat Slab

A flat slab is a one-way or two-way system withthickenings in the slab at the columns and loadbearingwalls called 'drop panels' Figure 9. Drop panels actas T-beams over the supports. They increase theshear capacity and the stiffness of the floor systemunder vertical loads, thus increasing the economicalspan range. This form of construction has becomeless popular in recent years because of the limit oneconomical spans of about 9.5 m for reinforced slabsand about 12 m for prestressed slabs. Reinforced flatslabs may need to be sensibly pre-cambered (notoverdone) to control deflection.

The plan dimensions of the drop panels are aminimum of 1/3 of the span in the direction underconsideration, usually rounded to the nearest 100 mm.The overall depth of the drop panel is typically takenas 1.75 to 2 times the depth of the slab, againrounded to suit timber sizes or the nearest 25 mm.

The principal features of a flat slab floor are a flatsoffit, simple formwork and easy construction. Theeconomical span 'L' of a reinforced concrete flat slabis approximately D x 28 for simply supported, D x 32for an end span and D x 36 for an interior span.Prestressing the slab increases the economical spanto D x 35, D x 40 and D x 45 respectively, where D isthe depth of the slab excluding the drop panel.

Advantages:

■ Simple formwork

■ No beams—simplifying under-floor servicesoutside the drops

■ Minimum structural depth

■ Usually does not require shear reinforcement atthe columns.

Disadvantages:

■ Medium spans

■ Generally not suitable for supporting brittle(masonry) partitions

■ Drop panels may interfere with larger mechanicalducting

■ Vertical penetrations need to avoid area aroundcolumns

■ For reinforced flat slabs, deflection at the middlestrip may be critical.

FLAT SLAB

14

F IGURE 9: Flat slab floor system

15

FLAT SLAB

A

160

140

180

200

220

7SPAN, L (m)

REINFORCED FLAT SLAB and PRESTRESSED FLAT SLAB Multi-span

CONC

RETE

THI

CKNE

SS, D

(mm

)

8 9 10 11 12 13 146

240

260

280

300

320

340

360

380

400

420

440

460

120

NOTES:

1 For preliminary design and initial sizing only.

2 Imposed action of 2.0 kPa typical for domestic etc, 3.0 kPa typical for offices etc and 5.0 kPa typical for assembly areas without fixed seating

3 A 120/120/120 Fire Resistance Level assumed

4 The following additional permanent actions (dead loads) are included with the imposed actions (live load) when preparing this chart:

a For imposed action of 2.0 kPa an additional permanent action of 0.5 kPa has been allowed

b For imposed actions of 3.0 kPa and 5.0 kPa an additional permanent action of 1.5 kPa has been allowed

5 For typical square grid, interior span only

6 With reinforced flat slabs, for spans over 9 m, while the deflection for column strips is less than span/250 the total deflection at mid-span is likely to exceed 25 mm

Reinforced

Prestressed

Imposed action (kPa) = 5.0

Generally not economical

3.0

2.0


3.0

2.0

CURVES

SECTION A–A

1

2

1

2

L

D

L

A

0.75D

L/3

D

6.3 Flat Plate

A flat plate is a one- or two-way system usuallysupported directly on columns or loadbearing wallsFigure 10. It is one of the most common forms ofconstruction of floors in buildings. The principalfeature of the flat plate floor is a uniform or near-uniformthickness with a flat soffit which requires only simpleformwork and is easy to construct. The floor allowsgreat flexibility for locating horizontal services above asuspended ceiling or in a bulkhead.

The economical span of a flat plate for low to mediumloads is usually limited by the need to control long-term deflection and may need to be sensiblypre-cambered (not overdone) or prestressed. Aneconomical span for a reinforced flat plate is of theorder of 6 to 8 m and for prestressed flat plates is inthe range of 8 to 12 m.

The span 'L' of a reinforced concrete flat-plate isapproximately D x 28 for simply supported, D x 30 foran end span of a continuous system, to D x 32 forinternal continuous spans. The economical span of aflat plate can be extended by prestressing toapproximately D x 30, D x 37 and D x 40 respectively,where D is the depth of slab.

Advantages:

■ Simple formwork and suitable for direct fix orsprayed ceiling

■ No beams—simplifying under-floor services

■ Minimum structural depth and reduced floor-to-floor height.

Disadvantages:

■ Medium spans

■ Limited lateral load capacity as part of a momentframe

■ May need shear heads or shear reinforcement atthe columns or larger columns for shear

■ Long-term deflection may be controlling factor

■ May not be suitable for supporting brittle(masonry) partitions

■ May not be suitable for heavy loads.

FLAT PLATE

16

F IGURE 10: Flat plate floor system

17

FLAT PLATE

120

140

160

180

5SPAN, L (m)

REINFORCED FLAT PLATE and PRESTRESSED FLAT PLATE Single Span

CONC

RETE

THI

CKNE

SS, D

(mm

)

6 7 8 9 10 11 124

200

220

240

260

280

300

320

340

360

380

400

420

440

100

NOTES:







5 Full continuity at the core wall assumed

6 Unsupported edges of floors may require stiffening for support of external walls and/or visual reasons

7 For larger spans, shear heads or reinforcement may be required at the columns

8 Deflection to be the lesser of span/250 or 35 mm

Reinforced

Prestressed



3.0

2.0


3.0

2.0

CURVES

1

2

1

2

L

D

Service

core

wall

FLAT PLATE

18

160

180

200

220

7SPAN, L (m)

REINFORCED FLAT PLATE and PRESTRESSED FLAT PLATE Multi-span

CONC

RETE

THI

CKNE

SS, D

(mm

)

8 9 10 11 12 13 146

240

260

280

300

320

340

360

380

400

420

460

500

480

440

520

140

NOTES:







5 For typical square grid, interior span only

6 Deflection to be the lesser of span/250 or 35 mm

Reinforced

Prestressed



3.0

2.0Imposed action (kPa) = 5.0

3.0

2.0

CURVES

1

2

1 2

L

D

L

6.4 Beam and Slab

This system consists of beams framing into columnsand supporting slabs spanning between the beamsFigure 11. It is a very traditional system. The relativelydeep beams provide a stiff floor capable of longspans, and able to resist lateral loads. However, thecomplications of beam formwork, co-ordination ofservices, and overall depth of floor have led to adecrease in the popularity of this type of floor.

The traditional reinforced concrete beam-and-slabfloor has an economical span 'L' of D x 15 for a singlespan and D x 20 for a multi-span, where D is thedepth of the slab plus beam. The depth of slabbetween the beams can be initially sized using thespan-to-depth ratios for a flat plate. Prestressing isnot normally used with this system.

Advantages:

■ Traditional effective solution

■ Long spans.

Disadvantages:

■ Penetrations through beams for large ductsdifficult to handle

■ Depth of floor

■ Greater floor-to-floor height.

19

BEAM AND SLAB

FIGURE 11 : Beam and slab system

Span, L8400

typic

al

230

Typic

al

600 wide beams typical

6.5 Ribbed (Waffle) Slab

Ribbed floors consisting of equally spaced ribs areusually supported directly by columns Figure 12.They are either one-way spanning systems known asribbed slab or a two-way ribbed system known as awaffle slab. This form of construction is not verycommon because of the formwork costs and the lowfire rating. A 120-mm-thick slab with a minimum ribthickness of 125 mm for continuous ribs is required toachieve a 2-hour fire rating. A rib thickness of greaterthan 125 mm is usually required to accommodatetensile and shear reinforcement. Ribbed slabs aresuitable for medium to heavy loads, can spanreasonable distances, are very stiff and particularlysuitable where the soffit is exposed.

Slab depths typically vary from 75 to 125 mm and ribwidths from 125 to 200 mm. Rib spacing of 600 to1500 mm can be used. The overall depth of the floortypically varies from 300 to 600 mm with overall spansof up to 15 m if reinforced, longer if post-tensioned.The use of ribs to the soffit of the slab reduces thequantity of concrete and reinforcement and also theweight of the floor. The saving of materials will beoffset by the complication in formwork and placing ofreinforcement. However, formwork complication isminimised by use of standard, modular, reusableformwork, usually made from polypropylene orfibreglass and with tapered sides to allow stripping.

For ribs at 1200-mm centres (to suit standard forms)the economical reinforced concrete floor span 'L' isapproximately D x 15 for a single span and D x 22 fora multi-span, where D is the overall floor depth.

The one-way ribs are typically designed as T-beams,often spanning in the long direction.

A solid drop panel is required at the columns andloadbearing walls for shear and moment resistance.

Advantages:

■ Savings on weight and materials

■ Long spans

■ Attractive soffit appearance if exposed

■ Economical when reusable formwork pans used

■ Vertical penetrations between ribs are easy.

Disadvantages:

■ Depth of slab between the ribs may control thefire rating

■ Requires special or proprietary formwork

■ Greater floor-to-floor height

■ Large vertical penetrations are more difficult tohandle.

RIBBED [WAFFLE] SLAB

20

F IGURE 12: Ribbed (waffle) slab supported

directly by columns

21

RIBBED [WAFFLE] SLAB

300

250

350

400

450

7SPAN, L (m)

REINFORCED RIBBED SLAB Single and Multi-span

RIB

DEPT

H, D

(mm

)

AVER

AGE

CONC

RETE

THI

CKNE

SS (m

m)

8 9 10 11 12 13 146

500

550

600

650

700

750

800

200

150.0

141.7

158.3

166.7

175.0

183.3

191.2

200.0

208.3

216.7

225.0

233.3

133.3

NOTES:







5 For single-span, continuity at core wall assumed. For multi-span, typical interior span assumed

6 Deflection of rib to be the lesser of span/250 or 35 mm

Single span

Multi-span



3.0

2.0


3.0

2.0

CURVES

SECTION A–A

SINGLE SPAN

MULTI-SPAN

1

2

1

2

L8400

A

D

50 200 900 wide beam

Taper

12001200

Slab 120

A

Service

core

wall

L8400

A

A

6.6 Band Beam and Slab

This system consists of a series of parallel, wide,shallow beams (known as band beams or thickenedslab bands) with the floor slab spanning transverselybetween the bands Figure 13. The floor slab isdesigned as a continuous slab, with the shallow bandbeams carrying all loads from the slab. Band beamsor thickened slab bands are a two-way slab systemdesigned in accordance with Clause 7.5 of AS 3600and are not treated as beams, except for shear underClause 8.2 of AS 3600.

Band beams are commonly used for longer spanstructures often with the bands post-tensioned andthe slabs reinforced. Sometimes, compositeconcrete/metal decking is used for the slabs,provided the slab spans are not too large.

The band beam has a relatively wide, shallow crosssection which reduces the overall depth of floor whilepermitting longer spans similar to the traditionalconcrete beam. The concrete section simplifies boththe formwork and services which can pass under the beams.

Advantages:

■ Relatively simple formwork

■ Shallow beams to allow services to run under the floor

■ Minimum structural depth and reduced floor-to-floor height

■ Long spans

■ Good cost/time solution

■ Allows the use of flying forms.

Disadvantages:

■ Long-term deflection may be controlling factorand post-tensioning may be required

■ May need service penetrations through beamswhich are difficult to handle.

In a single-span floor, the spacing of the band beamsmay coincide with the columns, or the bands may bemore closely spaced to reduce the thickness of theslab spanning between walls or beams. For single-span reinforced concrete floors the economical span'L' of the band beam is D x 20 to D x 22 depending onthe width and spacing of the band beam, where D isthe depth of the slab plus band beam. Prestressingthe band beam gives economical band-beam spans

in the range of D x 24 to D x 28. In a multi-span floor,the spacing of the band beams is fixed by thetransverse spacing of the columns.

For initial sizing of the slab, the span-to-depth ratiosfrom Section 6.3 can be used. For internal spans theslab thickness is based on the clear span betweenband beams, and for an external bay is from the edgeof band to the column line of the external band. Thedepth of the band is typically 1.5 to 2 times the depthof the slab and the minimum economical span for aband beam is about 7–8 m.

In multiple spans using reinforced concrete, theeconomical slab of the band beam 'L' is approximatelyD x 22 for 1200-mm-wide band beams and D x 26 fora 2400-mm-wide beams at 8400-mm centres.Prestressing increases the economical span 'L' to D x 24 to D x 28 for similar beam widths. D is thedepth of slab plus band beam in each case.

The maximum span for reinforced concrete bandsshould not normally exceed 12 m. Above this span,bands should be prestressed. The slab band widthshould be between band-spacing/3 to band-spacing/4and, where possible, should be based on a module ofa standard sheet of ply of 2.4 m x 1.2 m.

Vertical sides should be used if possible to simplifyformwork. Sloping sides are sometimes used wherebands are exposed to view or where the effectivespan of the slab needs to be reduced.

BAND BEAM AND SLAB

22

F IGURE 13: Band beam and

composite/metal decking slab

23

BAND BEAM AND SLAB

A

300

250

350

400

450

7SPAN, L (m)

BAND

BEA

M D

EPTH

, D (m

m)

8 9 10 11 12 13 146

500

550

600

650

700

750

800

200

NOTES:


2 Imposed action of 3.0 kPa typical for offices etc and 5.0 kPa typical for assembly areas without fixed seating


4 For imposed actions of 3.0 kPa and 5.0 kPa, an additional permanent action (dead load) of 1.5 kPa has been included with the imposed actions (live load) when preparing this chart

5 Continuity at core wall assumed

6 Deflection of band beam to be the lesser of span/250 or 35 mm

Reinforced, 600 4200

Prestressed, 1200 4000



Bw S



3.0

5.0

3.0

Imposedaction(kPa) = 5.0

3.0

3.0

5.0

CURVES

SECTION A–A

1

2

3

4

2

4

1

3

L8400

A

D

70500 wide beam

120 slab

Service

core

wall

S

Band beam

Band beam

Bw

REINFORCED and PRESTRESSED BAND BEAMS and SLAB Single Span

BAND BEAM AND SLAB

24

300

250

350

400

450

7SPAN, L (m)

REINFORCED BAND BEAMS at 8.4 m CENTRES Multi-span

BAND

BEA

M D

EPTH

, D (m

m)

8 9 10 11 12 136

500

550

600

650

700

750

800

200



Bw Ds

Ds



3.0

5.0

3.0

CURVES

SECTION A–A

1

2

2

1

L8400

A

D

Band beam

Band beam

Bw

A

NOTES:


2 Imposed action of 3.0 kPa typical for offices etc and 5.0 kPa typical for assembly areas without fixed seating


4 For imposed actions of 3.0 kPa and 5.0 kPa, an additional permanent action (dead load) of 1.5 kPa has been included with the imposed actions (live load) when preparing this chart

5 Typical interior span assumed


25

BAND BEAM AND SLAB

300

250

350

400

450

8SPAN, L (m)

PRESTRESSED BAND BEAMS at 8.4 m CENTRES Single and Multi-span

BAND

BEA

M D

EPTH

, D (m

m)

9 10 11 13 1512 14 167

500

550

600

650

700

200

Single span, 1800 160

Single span, 2400 150

Bw Ds

Ds


CURVES

SECTION A–A

MULTI-SPAN

SINGLE SPAN

1

2

Multi-span, 1800 1603

Multi-span, 2400 1504

4

3

2

1

L84

00

A

D

Band beam

Band beam

Bw

A

L8400

A

Band beam

Band beam

A

NOTES:


2 Curves are for an imposed action of 5.0 kPa (typical for assembly areas without fixed seating) to which has been added a permanent action (dead load) of 1.5 kPa


4 For single span, continuity at core wall assumed. For multi-span, typical interior span assumed


Service

core

wall

6.7 Slab and Joist

This system consists of a slab spanning betweenbeams (which span between columns) and usually anintermediate joist in one direction.

Frequently, the slab thickness between the joists iscontrolled by requirements for fire-resistance. Forexample, a 2-hour fire resistance rating requires a120-mm slab thickness, which is capable of spanningapproximately 4 m. For this widely spaced rib or joistfloor the economical span 'L' is D x 18 for a singlespan and D x 22 for a multi-span. Prestressing thejoists permits the span to be increased to D x 24 andD x 28 respectively, where D is the depth of floorincluding the slab and joist.

Advantages:

■ Thin slab panels possible

■ Suits industrial structures

■ Long spans

■ Vertical penetrations between beams easilyaccommodated.

Disadvantages:

■ More formwork

■ Joists and beams intrude on services

■ Depth of floor

■ Greater floor-to-floor height.

SLAB AND JOIST

26

Span, L8400

typic

al

120

Typic

al

4000

typica

l

600 wide joists typical900 w

ide be

ams ty

pical

7 PRECAST AND COMPOSITE FLOORSYSTEMS

7.1 General

These systems consist of precast floor elementsspanning one way onto either insitu or precast beamsor precast beam shells or other suitable supports. Theprecast floor elements are usually simply supportedbefore a topping concrete is placed to complete thesystem.

Precasting offers the advantages of off-sitemanufacture under factory conditions and fasterection on site. When combined with prestressing,additional benefits of long span and high load-capacitycan be obtained. A number of different systems areavailable including:

■ Hollowcore planks, either with or without atopping

■ Composite flooring using precast permanentformwork panels

■ Composite flooring using precast beam and infill

■ Solid slabs either reinforced or prestressed

■ Single and double T-beams.

Manufacturers will usually deliver precast concretepanels to the builder’s time schedule, thus allowingpanels to be lifted from the transport and placeddirectly into position. A quick selection of possiblefloors can be made from the chart in Figure 14. Thepreliminary design thickness can then be assessedfrom the particular chart for each floor system. Notethat these floor thicknesses/beam depths should beused only as the basis for detailed design.Comprehensive information on precast floor systemscan be found in the Precast Concrete Handbook 6.

27

C H A P T E R 7 P r e c a s t a n d C o m p o s i t e F l o o r S y s t e m s

0 5 10 15 2520

0 5 10 15 2520



Floor system

60 mm topping Thickness (mm) 150 200 250 300 350 400

HOLLOWCORE

Multi-span Total slab thickness (mm) 160 190 230 270

SOFFIT SLABS

Temporary prop Beam types 130R 150R/130C 150C 200C 250C

BEAM AND INFILL

60 mm topping Thickness (mm) 100 150 200 250

SOLID SLABS

70 mm topping Depth (mm) 600 800 1000

SINGLE T-BEAMS

70 mm topping Depth (mm) 400 500 600 700

DOUBLE T-BEAMS

F IGURE 14: Quick selection guide for

precast floors

7.2 Hollowcore

Hollowcore floor planks are precast, prestressed unitsproduced on long-line casting beds using slideforming or extrusion methods Figure 15. Duringmanufacture, cores are formed throughout the unit,reducing its self-weight. Planks are usually 1200-mm-wide, though some manufacturers canproduce 2400-mm-wide units. These wider units mayrequire increased crane capacity but offer greaterspeed of placement, less joints, grouting and sealing.Thicknesses vary from 150–400 mm in 50-mmincrements. The thickness is determined by span,loading, fire rating and cover to reinforcement tosatisfy exposure conditions. Profiled edges formshear keys between units.

The economical typical span for a precast hollowcoreunit is approximately D x 30 to D x 35 where D is thedepth of the precast unit plus topping. Whereslenderness ratios fall between 35:1 and 45:1 panelsshould be checked for vibration-resonance effects.Spans exceeding 45:1 should not be used.

Planks may be used as plain sections or topped to givea composite unit. The topping increases plank capacityand fire rating. It provides a level surface or drainagefalls and is recommended for most building work.

For economy, the structure should be dimensioned toaccommodate the 1200- or 2400-mm modular plankwidth. If this is not possible, planks can be sawnlongitudinally by the manufacturer, or partial-widthswet cast. Planks can be supplied with block-outs andcored holes to suit openings, services, etc. Thepermitted core-hole shape and number in a plank willvary with the depth and the particular proprietaryforming machine.

Fire rating is a function of the effective concretethickness and the concrete cover to strand. Fire ratingcan be increased by the addition of a concretetopping and cover increased by application ofinsulating material to the soffit. Comprehensiveinformation on fire-resistance levels can be found inHollowcore Flooring Technical Manual 7.

HOLLOWCORE

28

F IGURE 15: Hollowcore planks floor system

29

HOLLOWCORE

15SPAN (m)

150/8-9.3/40

200/7-12.7/40

250/9-12.7/40

300/11-12.7/40

350/13-12.7/40

400/16-12.7/40

HOLLOWCORE PLANKS WITH 60-mm TOPPING Single Span

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

2

3

4

5

6

1200 nominalCURVES

60-mm topping

Strand diameter (mm)

Concrete cover (mm)

Number of strands

Plank thickness (mm)

200 /7-12.7/40

PLANK IDENTIFICATION

1

2

3

4

5

6

1 2 3 4 5 6

NOTES:

1 For preliminary design and initial sizing only. Refer to the specific manufacturer's design information for actual load capacities and design information for the chosen system






60

T

7.3 Permanent Formwork or Soffit Slabs

The system, known by several names depending onthe manufacturer (eg Transfloor, Humeslab)incorporates precast concrete slabs, usually 55-mm-thick, with embedded reinforcement and trussesFigure 16. To complete the floor, an insitu concretetopping acts compositely with the precast panels.

For long spans, soffit slabs require temporary propsfor support until the composite section is able to carrythe construction loads.

The bottom reinforcement embedded in the precastpanel may consist of a layer of mesh, the bottomchords of the trusses and additional reinforcing barsas required by the designer. The trusses providestrength and stiffness for handling and transport,allow panels to support construction loads with aminimum of temporary propping, contribute to the topand bottom reinforcement, and act as bar chairs tosupport the top reinforcement.

Polystyrene void-formers can be bonded to the topsurface of the panel between the trusses in place ofthe insitu concrete, resulting in a significant reductionin the self weight of the finished slab and reducingthe amount of insitu concrete required.

Since the product is cast in a steel mould using wet-mix process, considerable flexibility of plan shapeis possible, while openings for stairs and majorservices may be cast-in. Small penetrations for electricwiring and plumbing can be cut on site. Keywaysmay be cut into the polystyrene for service runs.

The system imposes few restrictions on designersbecause there are no standard panel sizes. The length,width, thickness, plan geometry and reinforcementcan be varied to suit the design requirements. Theonly restriction is transport limitations which generallylimit maximum width to 2.5 m and maximum length toabout 12 m. Special lifting frames are required forunits over 8 m long.

PERMANENT FORMWORK OR SOFF IT SLABS

30

F IGURE 16: Permanent formwork or

soffit slabs

31

PERMANENT FORMWORK OR SOFF IT SLABS

14SPAN (m)

PERMANENT FORMWORK or SOFFIT SLABS Multi-span

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

5 6 7 8 9 10

2

3

4

5

6

1

2

3

4

1

2 3 4

NOTES:







Precast panel(N40 concrete) 120 min. 120 min.

d

20clear

cover

t T

65

D

Panel width, 2500 maximum (nominal)

SL62 top reinforcementplus 500N bars as required

Polystyrene voidformers (EPS)

Insitu topping(N32 concrete)

SL62 bottom reinforcementplus 500N bars as required

Wire trusses (min. of 5 to 2500 wide panel)

Slabdepth,D (mm)Curve

Paneldepth,d (mm)

EPSthickness,t (mm)

Trussheight,T (mm)

Self-weight(kPa)

Topreinf.(kg/m2)

Bottomreinf.(kg/m2)

160 55 40 82 3.3 4.4 4.3

190 55 70 110 3.6 4.9 5.0

230 55 110 154 3.9 5.6 6.1

270 55 150 192 4.3 6.1 6.8

7.4 Composite Floors – Beam and Infill

The system, known as Ultrafloor and similar, comprisesprecast, prestressed concrete inverted T-beams,spaced apart with an infill material spanning betweenthe flanges of the beams Figure 17. This assemblyprovides the strength to support the weight of the(subsequently placed) wet insitu concrete topping.After the insitu concrete has hardened, its compressivestrength acts compositely with the tensile strength ofthe precast beams to efficiently carry the designloading on the floor.

In longer spans, where the load of the fresh concretewould be critical, a row of temporary props can beplaced under the centre of the beams duringconcreting. These remain in place for approximatelythree days until the prestressed beams are able to actcompositely with the insitu concrete topping.

In addition to offering the well-established advantagesof precast flooring, it has a number of further benefits,including ready accommodation of site tolerancesand service penetrations (with adjustment of beampositions) and enhanced acoustic performance withappropriate detailing.

COMPOSITE FLOORS – BEAM AND INF ILL

32

F IGURE 17: Ultrafloor composite

floor system

33

COMPOSITE FLOORS – BEAM AND INF ILL

14SPAN (m)

COMPOSITE BEAM AND INFILL Single Span (Temporally propped)

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

5 6 7 9 118 10 12

2

3

4

5

6

1

2

3

4

5

6

NOTES:







SPrestressed beams

130R 130 130 180 98 480 2.79

150R 150 130 200 88 480 2.98

130C 130 190 180 98 540 2.90

150C 150 250 200 88 600 3.34

200C 200 250 250 88 600 3.71

250C 250 250 300 138 600 4.89

12-mm formboardBeamCurve

Insitu topping

tT

B

T(mm)

t(mm)

B(mm)

D(mm)

Selfweight(kPa)

S(mm)

Shrinkage control mesh

D

651 2 3 4

7.5 Solid Slabs

Solid precast prestressed floor slabs are typically wet-cast on long-line beds in unit moulds or byhollowcore equipment using slipform or extrusionmethods. The hollowcore equipment will set the widthdimension, usually at 1200 mm. It is also possible tocast them on separate casting beds. A convenientmodule to suit the building layout may be selectedwhen using unit moulds. The cross-section has similarshear key details to hollowcore planks. Thickness isusually 150, 200 and 250 mm. Units over 250 mmdeep are likely to be too heavy for normal buildingpurposes.

The slenderness should not exceed an L/D of 45. Thismay need to be limited to 30 to 35 for applicationsthat are sensitive to vibration.

Solid slabs are chosen where:

■ the loading results in high shear or there areheavy point loads;

■ the environment is aggressive, for example insplash zones or where condensation may occurover water;

■ high cover or special concrete is required;

■ projecting reinforcement requires the units to bewet-cast, for example when used as a soffit beamto support hollowcore.

Prestressed units will develop hog, with inevitablysome differential between adjacent units. Where this isa concern, an insitu topping can be used to provide a level floor (usually normal-class N32 concrete). Atopping is recommended for most projects. A groutedshear key detail is required between planks to transferlocal loads to adjacent units. Typically, 50% of localloading on a 1200-mm-wide plank is transferred toadjacent units through the shear keys.

Un-topped units require grouted joints for fire integrity,sound insulation and to prevent long-term differentialmovement between adjacent slabs. Topped units arenot grouted separately, the width of the joint is selectedto allow filling during the topping operation.

The length of support at the end of the slab will bedictated by circumstances. A bearing length of 80 mmis a normal minimum and, preferably, the lengthshould be half the depth of the precast section. Planklengths of less than 3 m may not develop the tensilecapacity of the prestressing strand due to bondlimitations. The critical section for shear usually occursin the transmission zone of the strand.

In addition to service requirements, construction loadconditions should be checked. These should include:live load on the precast unit; the topping weight pluslive load on the precast section prior to developingcomposite action; and stacked materials on both theprecast and composite sections, if this is likely to occur.

SOLID SLABS

34

F IGURE 18: Lifting of solid slab unit

35

SOLID SLABS

14SPAN (m)

100/8-9.3/40

150/7-12.7/40

200/9-12.7/40

250/11-12.7/40

PRECAST PRESTRESSED SOLID SLABS WITH 60-mm TOPPING Single Span

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

65 7 8 9 10 11 12 13

2

3

4

5

6

CURVES


Concrete cover (mm)

Number of strands

Slab thickness (mm)

200 /7-12.7/40

SLAB IDENTIFICATION

1

2

3

4

1 2 3 4

NOTES:







60

T

1200 nominal

Shear key

60 mm topping

7.6 Single and Double T-Beams

Single and double T-beams cover the span rangebeyond slab-type members such as hollowcoreplanks Figure 19. They are a very efficient structuralshape. The typical range for double-tees is 12 to 24 mand single-tees from 15 to 30 m. Through the use ofadjustable moulds, the depth, and to a lesser extentwidth, can be varied to suit the application. Beamsare usually topped with a levelling or structural screedconcrete on-site.

The units are cast on a long-line pretensioning bedwith straight (preferred arrangement) or deflectedstrands. Long units are carried on articulated vehiclesand site access must take account of this. Erection isnormally by mobile crane and access for thesevehicles will also have to be taken into account in theplanning stage. Single and double T-beams are thebasis for design of economical, fire rated structureswhere construction time, long spans or heavyloadings are important cost influences.

The economical span 'L' between supporting beamsis approximately D x 15 to D x 20 (depending on thelevel of prestress) where D is the overall depth of theT-beam including topping.

SINGLE AND DOUBLE T-BEAMS

36

F IGURE 19: Double T-beams

37

SINGLE T-BEAMS

110SPAN (m)

600/6-12.7/50

800/6-12.7/50

1000/6-12.7/50

600/12-12.7/50

SINGLE T-BEAMS WITH 70-mm TOPPING Single Span

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

1211 13 14 15 16 17 18 19 20 21 22 23

2

3

4

5

6

CURVES


Concrete cover (mm)

Number of strands

Depth, D (mm)

800 /6-12.7/50

BEAM IDENTIFICATION

1

2

3

4

800/12-12.7/50

1000/12-12.7/50

5

6

5 61 2 3

4

NOTES:







D

70

50

50

Super low-relaxation strand

70 mm topping

2400 nominal

DOUBLE T-BEAMS

38

16SPAN (m)

400/4-12.7/50

500/4-12.7/50

600/4-12.7/50

700/4-12.7/50

DOUBLE T-BEAMS WITH 70-mm TOPPING Single Span

IMPO

SED

ACTI

ON (k

Pa) [

see

note

4]

87 9 10 11 12 13 14 15 16 17 18 19

2

3

4

5

6

CURVES


Concrete cover (mm)

Number of strands

Depth, D (mm)

800 /6-12.7/50

BEAM IDENTIFICATION

1

2

3

4

600/10-12.7/50

700/10-12.7/50

7

8

400/10-12.7/50

500/10-12.7/50

5

6

5 6 7 81 2 3 4

NOTES:







D

70

50Super low-relaxation strand

70 mm topping

2400 nominal

50

1 Building Code of Australia Volume 1 Class 2to Class 9 Buildings (including Amdts. 1–11)Australian Building Codes Board, 1996.

2 Durable Concrete Structures (Z7)Recommended Practice, Concrete Institute ofAustralia, 2001.

3 McAdam, P S and Behan, J E Multi-storeyFormwork Loading (Z35) Concrete Institute ofAustralia, August 1990.

4 Beeby, A W The Forces in Backprops DuringConstruction of Flat-Slab StructuresProceedings of the Institution of CivilEngineers, Structures and Buildings, August2001.

5 Reinforcement Detailing Handbook (Z6)Recommended Practice, Concrete Institute ofAustralia, 1988.

6 Precast Concrete Handbook National PrecastConcrete Association Australia and ConcreteInstitute of Australia, 2002.

7 Hollowcore Flooring Technical ManualNational Precast Concrete AssociationAustralia, July 2003.

39

C H A P T E R 8 R e f e r e n c e s

■ Economic Long-Span Concrete Floors, BritishCement Association, 1990

■ Various technical publications available fromthe Steel Reinforcement Institute of Australiaand the reinforcement suppliers

■ Various technical publications available frommanufacturers/suppliers.

■ Reinforced Concrete Design HandbookCement and Concrete Association of Australiaand Standards Australia, February 2002.

■ Warner, R F, Rangan, B V, Hall, A S, andFaulkes, K A Concrete Structures, Longman,Melbourne, Australia, 1998.

■ Manual for the design of reinforced concretebuilding structures The Institution of StructuralEngineers and The Institution of CivilEngineers, Second Edition 2002.

■ The Silverton Case Australian Journal ofStructural Engineering, IEAust, Vol 1 No 1 1998.

■ Concrete Structural Floor Systems and MorePortland Cement Association, Version 2, 2001.

■ Guide to Concrete Construction Cement andConcrete Association of Australia (T41) andStandards Australia (HB64), 2002.

■ Ferguson, S Limit State Formwork Design,Changes in AS 3610—2003 Paper presentedat CIA Biennial Conference, Brisbane, July2003.

C H A P T E R 9 B i b l i o g r a p h y

40

guide to long-span concrete floors - cement concrete

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