Design Manual for Composite Slab

ECCS EUROPEAN CONVENTION FOR CONSTRUCTIONAL STEELWORK CECM CONVENTION EUROPEENNE DE LA CONSTRUCTION METALLIQUE E K S EUROPAISCHE KONVENTION FR STAHLBAU

ECCS - Technical Committee 7 - Cold Formed Thin Walled Sheet Steel Technical Working Group 7.6 - Composite Slabs

Design Manual for Composite Slabs

FIRST EDITION

1995 N87

'I ECCS EUROPEAN CONVENTION FOR CONSTRUCTIONAL STEELWORK CECM CONVENTION EUROPEENNE DE LA CONSTRUCTION METALLIQUE E K S InI EUROPAISCHE KONVENTION FR STAHLBAU

ECCS - Technical Committee 7 - Cold Formed Thin Walled Sheet Steel Technical Working Group 7.6 - Composite Slabs


FIRST EDITION

1995 N87

2 Design Manual for Composite Slabs

ISBN : 92-9147-000-8

Copyright 1995 by the European Convention for Constructional Siceiwork All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the Copyright owner: ECCS General Secretariat CECM Avenue des Ombrages, 32/36 bte 20 EKS 8-1200 BRUSSEL (Belgium)

Tel. 32/2-762 04 29 Fax 32/2-7620935

ECCS assumes no liability with respect to the use for any application of the material and information contained in this publication.

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Summary - Rsum - Zusammenfassung 3

SUMMARY This design manual has been produced for engineers as well as project managers in design offices, for engineers in steel construction companies and for engineers concerned with the manufacture of profiled steel sheets for composite construction. It contains a collection of the current knowledge for the design, calculation and construction of composite slabs with profiled steel sheeting. The manual is based on Eurocode 4, part 1.1, chapters 7, 10 and Annexe E which deals with composite construction, as well as Eurocode 3, part 1.3 which considers the design of profiled steel sheeting. It also contains complementary information on certain aspects of composite construction not covered in the Eurocodes. After a general introduction to composite slabs, in Chapter 1, the manual presents Chapter 2 of the complementary document "Good Construction Practice for Composite Slabs" making the link between construction and design. Chapters 3 and 4 describe the conception, the predesign and the detailing of structures using composite slabs. The main part of the manual (Chapters 5-9) is devoted to the design approaches for profiled steel sheeting and composite slabs, giving, in particular, data relating to materials, to loads and to the verification of the limit states. Finally, Chapter 10 presents a series of numerical examples covering the predesign, the design of the profile at the construction stage, the design of composite slabs and designs for special situations.

RESUME Le present manuel de dimensionnement a t rdig pour les ingnieurs en tant qu'auteurs de projet dans les bureaux d'tudes, les ingnieurs des entreprises de construction mtallique et les ingnieurs des unites de production des tles profiles pour dalles mixtes. Ii constitue l'ensemble des connaissances actuelles dans le domaine de Ia conception, du calcul et de la construction des planchers mixtes avec tles profiles. Le manuel est base sur l'Eurocode 4, partie 1.1, chapitres 7, 10 et annexe E, pour ce qui concerne La construction mixte, ainsi que sur l'Eurocode 3, partie 1.3, pour ce qui concerne la tle profile. Ii contient galement des informations complmentaires sur les sujets non traits dans ces Eurocodes. Aprs une introduction gnerale sur les dalles mixtes (chapitre 1), le manuel reprend intgralement le chapitre 2 du document parallle "Good Construction Practice for Composite Slabs', faisant le lien entre construction et dimensionnement. Les chapitres 3 et 4 constituent une base de conception, de prdimensionnement et d'tude des details des structures comportant des planchers mixtes. La partie principale (chapitres 5 a 9) est consacre au calcul des tles profiles et dalles mixtes, comprenant en particulier les donnes relatives aux matriaux, aux charges et aux verifications des tats limites. Finalement le chapitre 10 prsente des exemples numriques couvrant le prdimensionnement, le dimensionnement de la tle au stade de btonnage, le dimensionnement des dalles mixtes et des dimensionnements particuliers.

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4 Design Maiusal for Composite Slabs

ZUSAMMENFASSUNG Dieser Leitfaden zur Bemessung von Verbunddecken wendet sich an Lngenieure und Projektleiter, die sowohi in IngenieurbUros und Stahibaufirmen als auch in der Herstellung von Profilbiechen fr den Verbundbau tAtig sind. Er enthAlt eine Zusammenstdllung des aktuellen Wissensstandes Uber Entwurf, Berechnung und Konstruktion von Verbunddecken mit Profilbiechen.

Der Leitfaden basiert auf den Regelungen des Eurocode 4 "Bemessung und Konstruktion von Verbundtragweiten aus Stahl und Beton", Teil 1.1, Kapitel 7, 10 und Anhang E sowie Eun)code 3, Tell 1.3, der sich mit der Bemessung von Profliblechen befaBt. Weiterhin sind erganzende Informationen enthalten. die nicht in den Eurocodes behandelt wenlen. Nach einer ailgemeinen Einftthrung in die Verbunddeckenbauweise (Kapitel 1), steilt der vorliegende Leitfaden das Kapitel 2 der ergnzenden Broschre "Good Constniction Practice for Composite Slabs" vor und vethindet daxnit Konstniktion und Bemessung. Die Kapitel 3 und 4 beinhalten den Entwurf, die Vorbemessung sowie die Betrachtung verschiedener Konstruktionsdetails bei der Anwendung von Verbunddecken.

Der Hauptteil dieses Leitfadens (Kapitel 5-9) ist den Nachweisverfahren fUr Profilbieche und Verbunddecken gewidmet. Dazu werden insbesondere Angaben zu Werkstoffen, Lastannabmen und dem Nachweis von Grenzzustnden gemacht. SchlieBlich steilt Kapitel 10 eine Reihe von Rechenbeispielen vor, die die Vorbemessung, den Nachweis der Proffibleche im Bauzustand, die Bemessung der Verbunddecke und sogar Nachweisverfahren fr verschiedene Sondeffitile beinhalten.

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Preface 5

Preface

The first edition of the EUROPEAN RECOMMENDATIONS FOR THE DESIGN OF COMPOSITE FLOORS WiTH PROFILED STEEL SHEET was published in September 1974 by the ECCS Committee 11 "Multi-Storey Buildings". This ECCS document No. 14 was subsequently used as a reference publication for Section 15 of the "Model Code for Composite Structures" prepared by the Joint Committee on Composite Structures (CEB-ECCS-FIP-IABSE) and published under the title COMPOSITE STRUCTURES by the Construction Press, London, in 1981. The Model Code was finally used as a draft format for the preparation of Eurocode 4 "Design of Composite Steel and Concrete Structures", 1985.

In 1987 a technical group TWO 7.6 "Composite Slabs" was created within the ECCS Technical Committee TC 7 (Cold-formed thin-walled sheet steel in building), with the following tasks:

- To propose comments to Eurocode 4 (1985). - To revise the document ECCS No. 14 (1974). - To coordinate research efforts in the field of composite slabs.

The first part of the revision has been published as ECCS document No 73, entitled "Good Construction Practice for Composite Slabs". It contains practical information for construction site personnel. The present document represents the second part of the revision of ECCS document No. 14, concerning the design of composite slabs. It will be completed by a separate document concerning the way how to present load tables and diagrams for practical design and will be entitled "Standard ECCS Product Presentation for Composite Slabs". The working group TWO 7.6 is at present composed of the following members:

BEGUIN Philippe France BLAFFART Henri Belgium BODE Helmut Germany CRISINEL Michel (Chairman) Switzerland KOUKKARI Hell Finland VELJKOVIC Milan Sweden OLEARY David (Tech. Sec.) Great Bntain SCHUSTER Reinhold Canada STARK Jan Netherlands TSCHEMMERNEGG Ferdinand Austria

Corresponding members are: BAEHRE Roif Germany BREKELMANS Jan Netherlands DANIELS Byron Netherlands ENGEL Pierre France JANSS Jos Belgium MAGNIEZ Georges France MELE Michele Italy MOREAU Gerard France PATRICK Mark Australia PORTER Max USA SAUERBORN Ingeborg Germany

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SOKOL Leopold France WOELFEL Eilhard Germany WRIGHT Howard Great Britain

Principal contributions were provided by the following members:

Philippe BEGUIN, CI1CM, St-Rezny-les-Chevreuse, France Henri BLAFFART, Metal Pmfil Belgium, Liege, Belgium Dr Byron J. DANIELS, TNO Building and Construction Research, Deift, Netherlands Dr Pierre ENGEL, PAB-Sollac, Nanterre, France Mrs Hell KOUKKARI, VTF Finland David OLEARY, Civil Engineering Department, University of Salford, Great Britain Dr Leopold SOKOL, PAB-Sollac, Nanteire, France Mrs Ingeborg SAUERBORN, University of Kaiserslautem, Germany. Thanks are also due to many more colleagues who took part in working group meetings or offered suggestions.

Michel CRISINEL Prof. Michael DAVIES Swiss Federal Institute of Technology (EPFL) Civil Engineering Department Institute for Steel Structures (ICOM) University of Salford, Lausanne, Switzerland Salford, Great Britain

Chairman of TWG 7.6 Chainnan of TC7

Lausanne and Salford, November 1995.

Figures The figures have been graciously placed at our disposal by the following companies and institutions: - Ecole polytechnique f&Irsle de Lausanne (EPFL), Construction mtallique (ICOM), Lausanne (CR):

1.1 1.3/3.1 + 3.4 / 7.1 / 8.1 / 8.4 + 8.6 / 8.9 / 8.11 /8.12/10.4.1 -'- 10.4.4. - Schweizerische Arbeitsgemeinschaft fr Holzftrschung (SAH), Lignuxn, ZUrich (CR):

3.20 - Umversitt Kaiserslautern, Bauingenieurwesen, Fachgebiet Stahlbau, Kaiserslautern (D):

8.13 + 8.15 / 10.3.1 + 10.3.10 / 10.5.1 + 10.5.5. - Produils Btiment de Sollac (PAB-Sollac), Nanteire (F):

3.15 + 3.19 / 3.21 + 3.25 / 3.29 / 4.7 / 7.2 /7.3 / 9.1 / 9.5 + 9.24 /10.1.1 / 10.2.1. - Centre Technique Industriel de la Construction M&allique (CTICM), Saint-Rmy-ls-Chevreuse (F):

4.14.6/4.8+4.11/4.13/8.3. - Steel Construction Institute (SC!), Ascot (UK):

2.1 /22 / 2.3 / 3.5 3.14 / 3.26 + 3.28 / 4.12(a). Schweizerische Zentralstelle fr Stahlbau (SZS), ZUrich (CR):

4.12 (b) HiBond by Metecno, London (UK):

8.2 - Comit Eumpeen de Nonnalisation (CEN), Bruxelles (B):

6.1 / 8.7 / 8.8 / 8.10. The manuscript of this document has been prepared at the Swiss Federal Institute of Technology (EPFL), Institute for Steel Structures (ICOM), Lausanne, Switzerland.

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Conteius 7

CONTENTS

Page

SCOPE OF THE PUBLICATION 9

NOTATION 10

1 INTRODUCTION 13 1.1 State-of-the-art 13 1.2 Behaviour 17 1.3 Design requirements 20

2 LIST OF ESSENTIAL CONSTRUCTION SITE INFORMATION 23 2.1 General 23 2.2 Decking bundle identification 23 2.3 Information for steel sub-contractors 24 2.4 Information for concrete sub-contractors 25 2.5. Construction loads 25

3 PRELIMINARY CONSIDERATIONS AND PRE-DESIGN 29 3.1 Introduction 29 3.2 Possible composite action with beams 29 3.3 Column layout and the various beam arrangements 31 3.4 Renovation and refurbishment schemes 39 3.5 Shallow floor construction 43 3.6 Pre-design 45

4 DETAILING REQUIREMENTS 49 4.1 General conditions for steel sheeting and composite slab 49 4.2 Construction stage 50 4.3 Composite stage 54

5 PROPERTIES OF MATERIALS 59 5.1 PrOfiled steel sheeting 59 5.2 Concrete 60 5.3 Reinforcing steel 61 5.4 Structural steel 61 5.5 Partial safety factors for resistance and material properties 62

6 LOADS AND ACTIONS 63 6.1 General 63 6.2 Loads for the construction stage 63 6.3 Loads for the composite stage 64

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7 BASIS OF DESIGN - CONSTRUCTION CONDITION .67 7.1 Design procedure 67 7.2 Cross-sectional design resistances 69 7.3 Ultimatelimitstate 71 7.4 Serviceabilitylimitstates 74

8 BASIS OF DESIGN - COMPOSITE CONDITION 77 8.1 Design procedure 77 8.2 Cross-sectional resistances 83 8.3 Deflections 89 8.4 Verification. 91

9 SPECIAL DESIGN CONSIDERATIONS 97 9.1 Diaphragm effect 97 9.2 Fire design 100 9.3 Openings and penetration holes 105 9.4 Concentrated loads 114 9.5 Sound insulation 116 9.6 Corrosion protection 119

10 DESIGN EXAMPLES 121 10.1 Preliminary design example 121 10.2 Verification of the sheeting as shuttering 123 10.3 First typical design example 131 10.4 Second typical design example 143 10.5 Special design example 152 10.6 Design example for moving concentrated load 158 10.7 Design of composite slab with additional reinforcement carrying moving concentrated

load 162

BIBLIOGRAPHY 167

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Scope of the publication 9

SCOPE OF THE PUBLICATION The purpose of this publication is to present information on the design of composite slabs carried Out in accordance with Eurocode 4. The design and construction process for these slabs involves basically two stages: the temporary stage - when the profiled steel sheeting (hereafter referred to as decking), acting as a

one-way spanning element, carries the weight of the wet concrete and associated construction loads, the permanent stage - when the one-way spanning composite slab carries the imposed loads and a

percentage of the dead load dependent on the mode of construction. The publication is intended to complement Eurocode 4 "Design of Composite Steel and Concrete Structures" (particulary Chapters 7, 9, 10 and Annex E) and has been produced by the ECCS Technical Committee 7, Working Group 7.6 "Composite Slabs". In addition to the presentation of the normal design criteria for the ultimate and serviceability limit states, attention is given to the special design considerations of fire resistance, the treatment of openings, in-plane bracing and the effects of concentrated loads. Further information particular to the implementation of good site practice for composite slabs is available in the ECCS document "Good Construction Practice for Composite Slabs" which lists amongst other things the information which should be passed on from the designer/architect to site personnel. Another reference is the ECCS publication No 72 "Composite Beams and Columns to Eurocode 4" produced by the ECCS Technical Committee 11 "Composite Structures".

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NOTATION

Notation is presented in detail, including subscripts to symbols. Reference should also be made to Eurocode 4 Part 1.1.

Symbols Latin letters A : Cross-sectional area B : width b : width C : perimeter, coefficient c : coefficient D : orthogonal bending stiffeness d : pitch of corrugation E : modulus of elasticity (Youngs modulus) e : distance F strength of fastener f : ultimate strength of a material 0 : self weight, permanent action g : self weight, permanent action h : thickness, depth, height I : moment of inertia, second moment of area k : factor, constant, coefficient L : span length, length 1,1, span length, length, horizontal distance M internal bending moment, bending resistance m : coefficient N : axial force n : number, ratio P point load, concentrated load p : pitch of fasteners, unifonn distributed load Q imposed load, variable action q imposed load, variable action, uniform load R : resistance, support reaction r : radius S action effect s : construction load t sheet thickness V : vertical shear, shear resistance, shear buckling strength W : section modulus w : beam spacing x,y,z : coordinates x : position of neutral axis z : leverarm

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Notation

Greek letters

a coefficient B coefficient T partial safety factor 6 deflection

strain Ti degree of shear connection o rotation

slenderness p factor, density, reinforcement ratio o normal stress sp web inclination

shear stress x buckling coefficient

Subscripts

1,2,3 : number a structural steel, bearing adm : admissible, allowable ap : decking steel b : bottom c : concrete, compression corn : compressive cr : critical d : design value e elastic, effective eff : effective end end support f : full shear connection, floor finishes G : permanent action g : permanent action, global h haunch i number mt intermediate k characteristic 1, : longitudinal, local M : material m mean, effective, constniction stage max maximum mm minimum o reference value, ovethang p plastic, profiled sheeting, plane element, point load, punching Q : variable action q : variable action R resistance r reduced, relative S internal forces or moments

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12 - Design Manual for Composite Slabs

s reinforcement, shear, shrinkage, stiffener ser service internal forces or moments span span sup superior, upper, suppoit T thermal t : tensile, total, top test experimental, test value u : ultimate, uncracked alt : ultimate v : vertical, steel - concrete connection, shear w : web x,y,z : coordinates y : yieldofsteel a : normal stress

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Introduction 13

1 INTRODUCTION 1.1 STATE-OF-THE-ART

1.1.1 The development of composite slabs A composite slab comprises steel decking, reinforcement and cast-in situ concrete (Figure 1.1). The combination of the different elements is such that both structural and economic advantages are achieved. Initially the decking acts as both a platform for construction and as shuttering for the wet concrete. Secondly, when the concrete has hardened the decking carries some or all of the tensile forces in the slab caused by a load which is subsequently imposed. The concrete carries the compressive and shear forces in the composite slab and provides the sound insulation and fire resistance for the structure. The surface and shape of the decking is formed in such a way that at the interface between the decking and concrete horizontal shear forces can be transmitted. This is necessary to ensure the composite action between steel and concrete.

Figure 1.1 - Composite slab

Composite slab systems were first developed in the late 1930's for tall building applications. At that tune the technique brought a considerable dead-load reduction and it was essentially seen as a substitute for traditional reinforced concrete slabs. Because of their efficiency and advantages, composite slabs were soon used for a wide range of construction projects invariably based on structural steel framing (high rise, low rise and industrial buildings). During the late 1980's the introduction of fastrack construction methods brought a new interest in steel design and consequently a logical use of composite flooring. This change in mentality, coupled with the search by the manufacturers to use composite slabs with other framing materials, marked a new period of expansion for the technique. Steel decking is now used in conjunction with steel frames but also with concrete, prestressed concrete and timber structures.

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secondary beam


Composite floors are employed in a great variety of applications. The overall depth of composite slabs generally varies between 80 mm and 250 mm with a bare metal thickness of steel sheet between 0.7 and 1.5 mm (Fig. 1.2). The robustness of composite floors identifies them for the construction of thin slabs (80 rum to 120 mm) with moderate loading or medium span requirements. Other regular types of slabs (130 mm to 250 mm) with heavy loading or long span requirements axe also possible.

4 150 = 600 4 z 183 732

38.1 1373 B 89

4 x 150 600 I- 5x200= 1000 -l A ___'

___ B L1_J - L'.-I B L_.J t.2.J i 55 4 x 150 95 750 x 150 600

4x183= 732 3x190570

_1122WT' 5xl76=880 l

3x1O4312

J\J'UEjkJfl1 Figure 1.2 - Examples of decking used in composite slabs

1.1.2 The use of composite slabs Decking and composite slabs predominantly carry imposed vertical loads in bending and shear. Because both the decking and the composite slab do not have the same geometry in each direction (non- isotropic) a two-way design is complicated. To simplify this situation, design procedures consider only the bending and shear resistances along the longitudinal axis (in the direction of the ribs). This results in conservative estimates of actual load carrying capacity. Decking used in combination with concrete (composite slabs) have been designed especially for this purpose. It is thus not advisable to use cladding or roofing profiles as composite slab decking. Most decking manufacturers have produced table or charts with all the necessary cross-sectional properties. This simplifies the designers task as decking geometries can be quite complicated. Standard protection against corrosion of decking is normally a thin layer of galvanizing. This protection is generally sufficient for the most common use of composite floors (dry interior atmosphere). For more severe applications, other types of protection are available and an adequate layer must be provided.

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!nLroduction 15

Composite slab design is normally both simple and straight forward. Minimum slab thicknesses have been established to ensure that significant two-way load distribution can occur. Non-standard bay geometries and large openings represent cases for which special consideration must be given. Lastly, heavy concentrated loads, cyclical and dynamic loads must be treated with caution. Some examples of the widespread use of composite slabs in various branches of the construction industry are now described.

a) Office and administrative buildings Long span steel structures associated with composite slabs offer architects and their clients a greater free space for offices, administrative and commercial buildings. The beams are usually of sufficient depth for the primary service ducts to be accommodated by providing holes in the beam webs. The services may be directly suspended, with possibly a false ceiling, from the deck which is generally provided with a convenient suspension system.

b) Renovation schemes Renovation schemes often require irregularly shaped slabs and access to the construction site is difficult. Often the low carrying capacity of the existing foundation requires a severe limitation of the dead load. Composite floors are lighter in weight than conventional reinforced concrete slabs by up to 1.0 kN/m2 and are therefore very economical for these applications.

C) Housing and community service buildings There are many examples of family houses, housing schemes, schools, hospitals and other community buildings whose construction is based on the use of composite flooring. The satisfactory performance of composite slab systems in terms of fire resistance, acoustic and thermal insulation properties provide the high performance criteria required for such premises.

d) Car park units Composite floors may be used for car park construction built either as underground structures (diaphragm walling) or as multi-storey aerial platforms (framed structures). In both cases the speed and ease of erection coupled with the good span/strength capacity and reliable composite action offered by these floors lead to very competitive solutions.

e) Warehouse and storage buildings Warehouse and storage facilities are essentially buildings which are purpose designed to store various types of goods. Generally the layout is made as open as possible to allow flexibility of use. They are invariably characterised by heavy loads applied to the floors. The distributed and point-loads transmitted to the floor by pallet racks and/or fork-lift trucks may require special design attention. Nevertheless, composite slabs may provide a solution. There is also the advantage that the sprinkler fire devices and other piping networks may be suspended.

f) Industrial buildings and processing plants Composite floors may be designed to carry loadings met in industrial buildings. High uniformly distributed loads, in conjunction with punching forces and/or fork-lift trucks axles up to 30 kN, can be accommodated. For these buildings the steel deck is most often associated with steel framed structures. The panels are quickly and easily fastened onto the steel beams with shot-fired pins. Slabs may be attached to the beams by mean of shear connectors in order to transmit the in-plane forces providing an

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efficient form of bracing to static and dynamic loading. The stiffness of composite slabs is also beneficial for testing laboratories where deflections and vibrations must be as small as possible.

1.1.3 The main advantages of composite slabs Composite floors are now the popular choice for a wide range of structures, offering the designer and his client the following advantages:

Working platform Before concreting, the decking provides an excellent safe working platform which speeds the construction process for other trades.

Permanent shuttering The steel deck spans from beam to beam, forming permanent formwork to the concrete, the need for temporary props is often not necessary. The decking constitutes a good vapour barrier. The soffit remains clean after concreting and the use of colour-coated steel sheets can give an attractive aesthetical aspect to the ceiling.

Steel reinforcement The steel reinforcement provided by the cross-section of the deck is usually sufficient to resist positive moments. Additional fabric reinforcement may be provided in the slab to resist shrinkage or temperature movements or to provide continuity over intermediate supports (hogging moments). Composite action is obtained by the profile shape or by mechanical means provided by indentation or embossment of the steel proffle.

Concrete and steel saving The hollow shape of the proffle steel decking produces a saving of concrete which is variable with the deck type (up to 40 litres/rn2). This reduction in of the slab self-weight produces a significant reduction of the dead load (up to 1.0 kN/m2) carried by the structure and the foundations. Composite slabs are usually thinner than conventional reinforced concrete slabs because the relatively high steel area in the deck (between 1000 to 1500 mm2/metre width) is working at lower stresses.

Speed and simplicity of construction The unique properties of the steel deck combining high rigidity and low weight, ease considerably the transportation and the storage of the material on site. Often one lorry is capable of carrying up to 1500 rn2 of flooring. A team of four men can set up to 400 m2 of decking per day. Panels are light, pre-fabricated elements that are easily transportated and set in place by two or three men.

Quality controlled products Steel deck proffles are manufactured under factory controlled conditions. This allows the establishment of strict quality procedures and less random work on the construction site. This results in a greater accuracy of construction, assisting the following trades.

Service and building flexibility Composite floors are adaptable. They may readily be modified during the life of the building. This is especially true when the slab is used with framed structures. It is then always possible to create a new staircase between two floors by just simply adding the necessary trimmer beams. ECCS N 87

!ntrodziction 17

Recent developments and changes in communications, information and computing technology have shown the importance of being able to modify quickly the building services arrangement. Because of the present rate of change, it is not possible to predict precisely what further developments may occur at the time the building is constructed.

Furthermore, in commercially let buildings or in multi-shared properties it must be possible to modify the services without violating the privacy of the other occupants. In order to solve this problem, engineers have to choose between several solutions. There are generally three: Accommodation in the ceiling Accommodation within a false floor Accommodation in coffer box running along the walls The two last solutions are limited to specific services and they may cause a loss of space or result in poor appearance. Composite floors are rarely used without a false ceiling beneath the beams usually for aesthetical reasons. The gap between the soffit and the bottom flange constitutes an ideal zone in which services may be hidden. Many "dove-tailed" decks have slots or pre-formed tags to connect hanger wires. It is therefore possible to suspend new cable networks and piping without undertaking costly and noisy drilling attachments during building maintenance.

Temporary bracing of the steel structures The fastening of the steel deck to the structure prior to concreting provides a stiff and reliable floor bracing. Diaphragm action, which is produced by the capacity of the steel deck to resist distorsion in its own plan readily obviates the need for temporaiy horizontal bracing during construction.

Composite beam construction Shear connectors are generally used to provide connection between the underlying steel beam and the composite slab. This composite beam configuration increases considerably the strength of the structure, using the same beams or more efficiently smaller beams. The beam height but also the weight of the steel beam (between 15 and 30%) are effectively reduced (see also ECCS publication No 72).

1.2 BEHAVIOUR

1.2.1 BehavIour of the steel decking (construction stage) At the construction stage, when the concrete is wet, the decking alone resists the external loads. The behaviour is then comparable to the behaviour of the profiles for roofing. The decking is subjected mainly to bending and shear. Compression due to bending of the profile may arise in either the flanges and in parts of the webs. Shear occurs essentially near the supports. The thin component plate elements which make up the decking may buckle prior to yield under these compressive and shear stresses, thereby reducing its load can-ying capacity and stiffness. The current design procedures rely on the concept of effective width to provide a method for the calculation of this type of thin walled members. Clearly, the effective width of the compression flange depends upon the maximum stress imposed on the flange, which in turn depends on the location of the neutral axis of the cross-section. As the effective area of the flange decreases under increasing bending moment, the neutral axis of the proffle is lowered and the extreme fibre stresses will change accordingly. Iterative design becomes necessary for strength and serviceability calculations. It is also possible to determine design characteristics and methods by tests.

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1.2.2 BehavIour of composite slabs (permanent stage) The behaviour of composite slabs is different to that of other similar fonns of composite construction, such as reinforced concrete and composite beams of steel and concrete. In reinforced concrete, composite action is achieved as a result of the bond resistance of the reinforcement due generally to the cross section of the deformed bars used. This bond resistance, verified by tests, is equal to the ultimate tensile resistance of the reinforcement which ensures that the slab may always develop the full flexural resistance. In composite beams, composite action is achieved by connectors fixed to the top flange of the steel beam. The design of such connections is based on the assumption that the beam attains ultimate bending resistance (full connection). If the number of connectors is smaller that required for full connection then the connection is partial. In this case the ultimate resistance to bending depends essentially on the number of connectors, the span of the beam and the method of construction. The composite slab with decking has elements of both systems. On one hand decking with embossments or anchorages compares to reinforcement, whereas on the other hand decking is an element with bending rigidity similar to steel beams. The difference results from the fact that decking, and similary the embossments, can be deformed. Also, unlike reinforcement, decking does not benefit from being totally embedded in concrete. Such deformation behaviour depends on numerous parameters, which makes the analysis of the actual behaviour of composite slabs more complicated. A composite slab behaves in normal loading conditions usually as a cracked structure bent in the longitudinal direction of the sheet. 1) When the loads are small, the slab might be uncracked. The composite action between the parts is

full, and the stresses of the sheet and concrete are linearly dependent on the strains. 2) The cracking in the concrete in tension reduces the stiffiess of the structure and increase of loads

causes greater deflections of the slab than in uncracked state. The adhesion between the sheet and the concrete is capable of transferring the shear force between the cracks. It may happen that in the ends of the slab the adhesion fails.

3) When a composite slab is experimentally or accidentally loaded by higher loads than the design loads, its behaviour greatly depends on the type of the steel sheet. In all composite slabs some relative slip may take place between the elements when the shear stresses between them is greater than the strength of the joint.

4) Composite slabs have different failure modes depending not only on the sheet type but also on the dimensions of the structure. There are types of proffle which fail quickly if the load is larger than the first slip load (brittle or non-ductile behaviour). Some types of sheet can undergo great deflections before failure when the loads are gradually increased, although the relative slip increases at the same time (ductile behaviour). The failure of a composite slab may occur at the interface between the steel sheet and the concrete as a shear bond failure or as material failure in one element.

For the case when the slab has been propped during construction, the slab will deflect instantly after the removal of the props. This initial loading can cause cracking in concrete. Permanent and transient moving loads on the slab cause instant changes in deflection. The concrete will also creep for several years which will gradually increase the deflections of the slab. The manner in which a composite slab behaves during a loading test enables the basic information for the design of a particular type of the sheet to be developped. Because there are a great variety of the sheet types and there are no common design formula, all sheet types must be subjected to tests. Two modes of behaviour can be identified using Figure 1.3 from a loading test where the load was gradually increased by displacement controlled jacks; At first the load-deflection curve is approximately linear for all types of slabs which corresponds to the behaviour of a composite element bonded at the interface by chemical adhesion and/or friction. ECCS N 87

Introduction 19

Mode 1 - brittle behaviour The load suddenly decreases at a certain point where the relative slip is such that the surface bond is broken. All the shear force must be taken up by friction and embossments. The decrease in load depends on the quality of the mechanical embossments. With further deformation of the slab the load increases again slightly without ever reaching the level of the initial phase. None of the mechanical connections in the slab are capable of assuring a composite effect superior to that of simple surface adhesion. It should be noted that the decrease of the load is not due to the sudden opening of tension cracks in the concrete, because this is prevented by the decking, but by relative slip between the concrete and the decking.

Load P (kN)

50

40

30

20

10

Deflection

Figure 1.3 - Two typical behaviour modes of composite slabs

Mode 2- ductile behaviour The mechanical connection is capable of transferring the shear force until failure occurs. Failure is produced either by bending, corresponding to total connection, or by longitudinal shear, corresponding to partial connection.

Acconling to the Eurocode 4, the behaviour is classified as ductile if the failure load exceeds the load causing first recorded end slip by more than 10%. The load causing first recorded end slip is the load at which the slip at any end of the slab is greater than 0.5 mm. Otherwise, the behaviour is classified brittle (or non-ductile). Eurocode 4 takes into account of the ductile or non-ductile behaviour of a composite slab by means of different partial safety factors applied to the failure load.

ECCS N 87

Slip at first end

2

P P

Slip at second end

20 30 40 6 [mm]


1.3 DESIGN REQUIREMENTS

1.3.1 Structural stages A distinctive characteristic of composite slabs is the two structural states that exist: firstly, the temporary stage of construction when only the decking resists the applied loads and secondly, the permanent stage when the concrete is bonded to the steel allowing composite action. For the both structural stages, it shall be verified that no relevant limit states are exceeded: Profiled sheeting as shuttering Verifications at the ultimate limit state and the serviceability limit state are required for the safety and the serviceability of the proffled sheeting acting as formwork for the wet concrete. The effects of props (if used) shall be taken into account in this design situation. Composite slabs Verifications at the ultimate limit state and the serviceability limit state are required for the safety and the serviceability of the composite slab after composite behaviour has commenced and any props have been removed.

1.3.2 Verification conditions for the ultimate limit states The resistance of the decking (temporary stage of construction) or the composite slab (pemianent stage) must be sufficient to resist the external actions. Each section or member must be capable of resisting the internal forces determined by the analysis of the structure. When considering a limit state of rupture or excessive deformation, it shall be verified that:

Sd Rd

Sd : design value of action effects Rd : design value of the resistance Combination of actions For each load case, design values for the effects of actions shall be determined from combination rules involving design values of actions, as identified by Table 1.1. The most unfavourable combinations are considered at each critical location of the structure, for example, at the points of maximum negative or positive moment, In Table 1.1 a combination factor of 0.9 is taken into account. Eurocodes permit the use of other combination factors, if reliable load data is available.

1.3.3 VerIfication conditions for the serviceability limit states The behaviour of the decking under its self-weight and the weight of the wet concrete must fall within accepted limits. The following verifications shall be made: deflection is within the admissible limit, marks on the sheet due to the props should be avoided. The behaviour of the composite slab under permanent loads and variable service loads must fall within accepted limits.

ECCS N 87

introduction

Table 1.1 - Combinations of actions for the ultimate limit state

21

2. 1G 1.35

Gk+0.9.'yQ.Qk (*) G + 0.9 1.50

The following verifications shall be made: Concrete cracking in hogging moment regions is within a limited width. Deflection, or variation of deflection, auairiing the admissible limit. Vibrations above a limiting value. Combination of actions For each load case, design values for the effects of actions shall be determined fmm combination rules involving design values of actions as identified by Table 1.2.

TabLe 1.2 - Combinations of actions for the serviceability limit state

Load combinations to be considered: Parameters defined in Table 1.1 1. GkQk,max

2. Gk + O.9Ql

ECCS N 87

Load combinations to be considered: YGGk+YQQkmaX

1. 1.35Gk+l.5OQk,max (*)

= permanent actions, eg. self weight

Qk = variable actions, eg. imposed loads on floors, snow loads, wind loads

(*) If the dead load G counteracts the variable action Q:

= the variable action which causes the largest effect at a given

= 1.00

location

If a variable load Q counteracts the dominant loading:

= partial safety factor for permanent actions

YQ actions

= partial safety factor for variable

Page blank in original

Consiruction site information 23

2. LIST OF ESSENTIAL CONSTRUCTION SITE INFORMATION 2.1 GENERAL This chapter contains the minimum amount of information that the designer and/or architect should supply to construction site personnel. Most of the information contained in this chapter is used by the designer and/or architect when calculating decking and composite slab resistances. Ignorance of this information by field personnel can lead to situations that the designer and/or architect has not forseen. Any variations from the conditions specified by the designer and/or architect should be brought to their attention.

2.2 DECKING BUNDLE IDENTIFICATION An identification tag should be attached to each decking bundle delivered to the job site. An example tag is shown in Figure 2.1. Tags may look somewhat different but should contain the following information: Total bundle weight Deck type, surface condition, thickness Bundle identification code The number, length and thickness of each panel The bundle identification code will also appear on the decking layout plan, and can thus be used to identify the bay(s) for which the bundle is designated. A product description including the following should be available on site or from the decking manufacturer's technical information service: Rib height Embossment depths The yield strength of the core material The type of coatings (if any) and coating thickness

Job No. Deck type Galvanised id1e identification / o 0 XYPD 01 43000 - MARK: AZI 0 Q GRD FLR LVL 1.00 mm Q

o 4x7295.0 o lOx 10075.0

o 3x3335.0 Bundle weight o 0.967 tonnes

0 0

" 0 V

Thickness (mm)

Figure 2.1 - Example decking bundle identification

ECCS N 87

Location

N I

No. of sheets Length (mm)


2.3 INFORMATION FOR STEEL SUB-CONTRACTORS The steel sub-contractor should be provided with a decking layout drawing which divides the floor into bays. A bay consists of panels from the same decking bundle that are to be laid Out and fixed to the underlying frame as one unit. Each bay of each floor with composite slabs should be contained in this drawing. Information not included in this chapter may also be specified in this drawing. Such information may be necessary because of variations from standard practices. All such variations should be clearly indicated (highlighted) by the designer and/or architect.

2.3.1 Decking layout drawing

Bay definition

Bays may be defined using dashed lines and a diagonal solid line, such as are shown in Figure 2.2. A reference number may be placed in a circle on the diagonal line to indicate that special bay instructions are given elsewhere on the drawing. The approximate location of the first panel to be placed in each bay and the direction in which layout should continue is indicated. Other information given for each bay is: Decking rib orientation The number of panels The bundle identification code The panel length

Columns and supports The location and orientation of each column should be indicated as shown in Figure 2.2. All supports (permanent or temporary) should be included. Permanent supports are drawn using a solid line, temporary supports are drawn using a dashed line and the letters TP (Temporary Prop-line). The minimum width of the temporary support in contact with the decking should be given (the minimum bearing width) together with the line load reaction [kN/m] on the props.

Openings and edges The location and orientation of all openings and edges with respect to permanent supports should be given. This includes both permanent and temporary edges. Such information should be indicated in boxes identified by the words "Edge trim", see Figure 2.2. There may be more than one reference box for each edge. The following information should be contained in each reference box: A reference letter (or number) for details which appears elsewhere The decking rib height The distance between the edge of the decking and the centreline of the nearest permanent support. Details should be available for all exterior edges and edges next to openings. Details may also be necessary for temporary edges. Temporary edges include changes in the orientation of the decking ribs and edges between concretings. Examples of support and edge details are given in the document "Good Construction Practice for Composite Slabs" (Figures 17 and 19 of Chapter 6, and in Figures 24 and 25 of Chapter 8).

Panel fastening Panels may be fastened only to permanent supports and to adjacent panels (seam fasteners). Fastening should be undertaken immediately after each panel or bay has being laid out. For each bay special fastener information may be given. Fastener information is indicated on the decking layout drawing using infonnation boxes identified by the word "Fasteners", as shown in Figure 2.3. Each information box should contain the following: ECCS N 87

Construction site information 25

Fastener type Number of fasteners needed to fix each panel to each support, or the minimum number of seam

fasteners per metre length.

2.3.2 Shear connectors Shear connectors are normally shown on structural drawings for composite beams. This information need only be included in the decking layout drawing if holes must be cut in the decking, or if shear connectors are to be installed using through deck welding or through deck shot-firing. In these cases the location, type and length of each shear connector should be indicated on the decking layout drawing. The orientation and location of the shear connector relative to decking ribs should be clearly indicated.

The minimum distance between the centreline of the shear connector and the edge of the decking should be given. Installation and quality control procedure information from the shear connector supplier should be available on site.

2.4 INFORMATION FOR CONCRETE SUB-CONTRACTORS A reinforcement layout drawing should be made available to the appropriate contractor for each bay of each floor. The location, length, minimum overlap and minimum concrete cover of all reinforcement in the composite slab should be indicated. The specified grade of all reinforcement should also be indicated on this drawing. This grade should be checked against the identification tag for each reinforcement bundle. Important reinforcement details (such as near supports, openings and edges) should be referenced and placed on this drawing or on the decking layout drawing. Any special preparation needed to ensure that excessive leakage does not occur during concrete should be indicated. The concreting work should be started above the permanent supports of the slab and proceed towards the middle areas of the sheets. The height from which concrete falls should be as low as possible. The order of the work should be clearly shown in the drawings for the building site. Information concerning the concrete mix should be provided in the same manner as for other reinforced concrete components. Minimum necessary concreting information includes the following: The minimum concrete compressive strength Maximum aggregate size Types of admixtures : it is necessary to check if the admixtures used are compatible with the coating

of the profiled sheets. For example, the use of antifreeze-type admixtures is prohibited because they are definitely not compatible with zinc coatings.

2.5 CONSTRUCTION LOADS The design load that may be carried by the decking as a temporary working platform, as shuttering and by the composite slab should be clearly indicated on the decking layout drawings and on appropriate concreting drawings (in kN/m2). Special loading limitations should be clearly indicated for each bay. In addition the following values may be necessary: The minimum concrete compressive strength at which temporary supports may be removed (can be

given in terms of days after concreting) The minimum concrete compressive strength at which temporary construction load may be applied

(can be given in terms of days after concerting) The maximum allowable vehicular axle weight.

ECCS N 87

I I

til (. z 0 00

Tem

pora

ry p

ropl

ine

Bund

le id

entif

icatio

n

0

I

Ref

eren

ce fo

r D

eck

edge

det

ail

nb

draw

ings

he

ight

ln

dica

tor s

tart

poi

nt fo

r la

ying

of p

anel

s

No m

ere

than

(4) w

ork

men

al

low

ed on

deck

ing.

A

ncho

rage

s pro

vide

d at

supp

orts.

No

mpo

rsry

pr

op.lo

.ds a

llow

ed pr

ior t

o co

ncr

etin

g.

Tape

joints

betw

een

Bay

s (1)

and (

2).

No s

peci

al lo

ad

rest

rictio

ns.

Construction site information

1.. U C

(1 Id

1., >.. U

27

Figure 2.3 - Exa,nple decking layout drawing (Fasteners details only)

ECCS N 87

UVC

03 z.g&

C U

. U

U 4)

.C U

Prelijninary considerations and pre-design 29

3 PRELIMINARY CONSIDERATIONS AND PRE-DESIGN

3.1 INTRODUCTION This chapter has been written for architects and engineers and more generally for all those who have to produce a quick but sound pre-design for a composite floor. This might be required for either a preliminary project or a cost estimation exercise. Experience often shows that the architects must be given a realistic estimation of the floor system depth including slabs, suppoiting beams and ceiling. This is necessary because the overall depth of the floor has a direct influence on the total building height. This parameter which is usually fixed by urban planners may be restricted for a specific area. The building height depends directly on the floor arrangement and therefore it is not an exageration to state that the simple pre-design estimate of the composite slab thickness is meaningless for a project if the designer does not consider the beam spacing, the beam span, the total acceptable depth of the floor system (beam + slab) and also the column layout.

3.2 POSSIBLE COMPOSITE ACTION WITH BEAMS The use of composite beams follows naturally from the use of composite slabs in the transverse direction. Shear connectors are generally used to provide shear connection between the underlying steel beam and the concrete slab. The resulting in-span behaviour of these components is an optimum use of the two materials where the concrete works in compression and the steel beam mainly in tension (see Figure 3.1). In a composite beani the resulting centroid of the composite section is usually positioned in the vincinity of the top flange of the steel beam. The area of the steel beam in compression is therefore significantly reduced, and may even be zero. This so called 'composite beam' arrangement increases considerably the strength of the element. The beam height but also the weight of the steel beam is effectively reduced by 15% to 30% (see also Figure 3.2). Recent fire tests carried out in France and the U.K. have shown that it is possible to obtain a fire stability of 30 minutes for unprotected composite steel beams. Composite beam arrangements also make the structure stiffer and more ductile. Finally this type of structure has a improved resistance to seismic forces. The numerous beam solutions applicable to the composite slabs are outhned in section 3.3.2a).

3.2.1 Composite beams with welded shear connectors Welded shear connectors can be attached on site or in the workshop. The studs are welded directly onto the beam (welded in the shop) or directly through the steel deck (welded on site). Welded stud connectors were initially used for composite bridges. These connectors have various diameters between 12 and 22 mm (see Figure 3.3). 3.2.2 Composite beams with nailed shear connectors Cold formed angle connectors made of light-gauge steel are connected to the beam through the deck by using shot fired nails. This technique is very practical on site because the tools are light and easy to use. The top flange can be painted and the presence of moisture between the deck and the beam flange does not affect the performance of the system. However the number and the capacity of this type of connector to resist the shear forces is limited compared to the welded headed studs. Nailed shear connectors (Figure 3.4) are mainly used for small and medium size contracts or when the access to site for a generator is difficult.

ECCS N 87

30 Design Manual for Compose Slabs

Without connection : M = Mp,a fy Za

Figure 3.1 - The principle of composite beams

Self-weight (slab and profile) : 91 + 92 Weight of floor finishes : g = 1 kN/m2 Variable imposed load : q = 4 kN/m2

140 t999WW 9299 J h L 2500

With connection

Plastic design Connection :40% Connection: 100%

ECCS N 87

Figure 3.2 - Possible weight saving with composite beams

I iE74 IY

With connection: M = fy Z

I

I

7500 _

Without connection

Plastic design Elastic design

Depth h[mm] ::

:500 J_ 440 j 410

Section IPE 400 IPE 360 IPE 300 IPE 270

Weight of profile [kg/m] 66.3 57.1 42.2 36.1 Number of studs

&g[mmj

10

13019 12

12019 24

25019 33

shnnk (mm) 6 7 7

q[mmJ 8 3 10 6

Preliminary consideralions and pre-design 31

111 .. :j

* + + -.i_

Figure 3.3 - Welded shear studs

.i i Figure 3.4 - Nailed shear connectors

3.3 COLUMN LAYOUT AND THE VARIOUS BEAM ARRANGEMENTS The pattern of the column layout is certainly the first parameter to consider when designing a building structure. The distance between the columns and the beam/column arrangement define directly the beam spans and consequently the depth of the floor members.

3.3.1 Short to medium span structures Short/medium span structural arrangements are used when the building flexibility is not critical and/or when the beam depth is limited. These structures do not always use the composite beam approach as discussed earlier in this chapter. The decision whether a beam should or should not be composite (i.e connected to the slab) depends of national practice and can result in savings in beam weight in regards with the cost of connection between the slab and the beam. These siructures are usually based on "square grid" have a higher number of columns. Figure 3.5 shows a possible option.

3.3.2 Long span structures using composite beams The initial development of the use of composite construction was as a substitution for the traditional reinforced concrete frame. Thus grids are square or nearly square with column spacing in the range of 6 to 9 metres. Such span layout does not take full advantage of some of the composite beam's inherent benefits. In particular it does not recognize that a composite floor is essentially an overlay of one way structural elements. The floor spans between the secondary beams, which span transversely on to the primary beams; the latter in turn span to the columns. This set of loads paths lends itself to rectangular grids and it becomes feasible to increase the span in at least one direction to 12, 15 or even 20 metres and more.

ECCS N 87

Figure 3.5 - First type of column layout structure

32 Design Manual for Conzposue Slabs

The depth of the long span beams will clearly increase in order to achieve economy but now the beams are of sufficient depth for the primary service ducts to be accomodated readily within their depth, so that the overall floor depth does not necessarily increase significantly. The ducts may be accomodatecj by providing holes in the beam webs, or, by tapering the beams near their ends. Modem buildings are generally designed to have a life of not less than 50 years. Recent developments and changes in communications, information technology and manufacturing methods have already had a profound influence on commercial and industrial practice and consequently on various type of building arrangements. Therefore it is not possible to predict precisely what further developements may occur during the life of a building that is designed currently. Today there is no evidence that the rate of change of office and industrial technology or social habits will slacken and developers must expect profound changes in requirements for modern building during their life. While many of these changes will influence services requirements, others will primarily affect the partition layout. The best way to maximise flexibility of internal planning is to minimise the number of columns. Figure 3.6 shows typical examples of ways in which "long span" primary beams can reduce or eliminate the number of internal columns. The cost of the floor will increase but this can be partly offset by saving from the reduction in the number of foundations and some savings in speed and cost of erection. In any case the net increase in the structural cost may well be no more than 10 % and this represents a much smaller proportion of the total development cost. This is a very small premium to pay if proper account is taken of the potential future benefits because the structure is less likely to become obsolete.

a) Structural options for long span beams This section describes the various options for achieving the twin aims of long spans and ready incorporation of services within normal floor zones.

Beams with web openings In this method of construction, the beam depth is selected so that sufficiently large, usually rectangular- shaped openings can be cut into the web (see Figure 3.7). For general guidance, it is suggested that the openings should form no more than 70% of the depth of the web, where horizontal sliffeners are welded above and below the opening. Typically, the length of the openings should be no more than twice the beam depth. The best location for the openings is in the low shear zone of the beams. A modified form of construction is the notched beam where the lower part of the web and flange of the section is cut away over a short distance from the support. This method is not usually practical unless the cut web is stiffened.

ECCS N 87

Figure 3.6 - Long span primary beams

Casteflated beams can be used effectively for lightly serviced buildings or for aesthetic reasons where the structure is exposed (see Figure 3.8). Composite action does not significantly increase the strength of the beam but does increase their stiffness significantly. Castellated beams have limited shear capacity and ate best used as secondary beams.

db'6boo Composite tnisses

Figure 3.8 - Castellated floor beams

Trusses are frequently used in multi-stoity building in North America and are best suited for very long spans, where the truss is designed to occupy the full depth of the floor zone (see Figure 3.9). The cost of fabrication can be high in relation with the material cost. Little benefit is gained for composite action apart improving the stiffness of the truss. The modified Warren truss is the most common form as it offers the maximum zone for service between bracing members.

Stub girders Architectural demand for square column-grids with spacings of 10 to 12 metres led to the development of stub girder construction in North America. The stub girder comprises a bottom chord which acts in tension and a series of short beam sections (or stubs) which connect the bottom chord to the concrete slab. Secondary beams span across the bottom chord and can be designed as continuous members. Voids are created adjacent to the stubs for services. This is illustrated in Figure 3.10.

ECCS N 87

Preliminary considerations and pre-d esign 33

Reinforcern1nt

Stiffener Opening for services

Castellated beams

Figure 3.7 - Web openings in floor beams

Figure 3.9 - Composite trusses with composite floors

Parallel beam grillage systems This system is different from the other previouly described in that continuity can be developed in both the secondary and primary beams. The secondary beams are designed to act compositely with the concrete slab, and are made continuous by passing over the primary beams. The primary beams are arranged in pairs and pass on either side of the columns to which they are attached by shear resisting brackets. These primary beams are non-composite. The method of construction is illustrated in Figure 3.11. Dual beam systems are ideally suited to accomodated large service ducts in orthogonal directions.

Haunched beams Haunched beams are designed by forming a rigid moment connection between the beams and columns. The depth of the haunch is selected primarily to provide an economic method of transferring moment into the column; the length of the haunch is selected to reduce the depth of the beam to a practical minimum. The extra service zone created beneath the beam between the haunches offers flexibility in service layout. At edge columns, it would be normal to develop additional continuity through the slab reinforcement, but this is only an option at internal columns. This form of construction can be used for sway frames, i.e. where vertical bracing or concrete shear walls or cores are not provided. This is practical for buildings up to 5 storeys in height. An example of a haunched composite beam is shown in Figure 3.12.

34 ____ ___


Figure 3.10 - Stub girder system with composite slab

secordary beam

Figure 3.11 - Composite slab and parallel beam grillage systems

4- Shear connectors r, 7-.Tr-r,---r

C(mposite secondary bea"ms

Figure 3.12 - Composite floor with haunched beams

ECCS N 87

Preliminary considerations and pre-design 35

b) Structural arrangements with built-up sections Figure 3.13 shows three typical floor arrangements for a one-bay long-span structure. Wider, multi-bay buildings would simply be repetitions of these single-bay arrangements, although the 6 in column spacing that is shown along the building would be likely to increase for internal columns. Figure 3.13a shows the fabricated beams acting as the primary beams, supporting light hot-rolled, composite, secondary beams between 2 and 5 m centres, which depends on whether the sheet is supported or not during concreting. In Figure 3. 13b the fabricated sections are themselves placed at 2.4 to 3.6 m centres and are supported directly by the columns or by composite haunched beams. In multi-bays schemes the haunched internal beams would be replaced by primary beams or internal columns lines. Figure 3.13c is only applicable to one-bay structures, with beams on the center-lines of the mullion columns. Choice of arrangement will depend on the overall structural form. Type Cc) would only be used if mullion columns were required at centres of between 2.4 and 3.6 in to support the building envelope. Where the column spacing is greater than 3.6 in along the building, some form of grillage is required if conventional composite decking is used. Propping of the steel decking may also be used in this case. The choice between a) and b) is not clear cut. For conventional construction, b) would be generally favoured. However, layout b) does have a greater number of fabricated sections, which inherently more expensive per tonne than rolled sections. In addition the lightly loaded fabricated sections of b) are likely to be less efficient than the heavier fabricated sections of a). For example the webs of the former may be governed by minimum thickness criteria. Even if that is not the case their greater slenderness will reduce strength. Conversely, the number of connections in a) are greater than b) thus increasing erection and fabrication costs for the former.

Figure 14 shows a range of profiles for tapered beams. 3.3.3 Structural options with other materials Composite slabs are now widely used with reinforced, prestressed concrete and timber structures. They may lead to material saving and increased speed of erection. These applications are now discussed.

a) Reinforced and prestressed concrete structures The use of composite slabs in conjunction with reinforced concrete beams appeared in the mid 1980's. A large vanety of buildings ranging from an underground car park (a typical detail is given in Figure 3.15) to a tail building with a tubular core (a typical detail is given in Figure 3.16) were built using composite slab flooring. Single span sheets are used at each support, the slab and the beams may be linked using mesh reinforcement to ensure longitudinal shear connection. There are several possible ways to use these techniques, one of which is shown in Figure 3.17. Steel decks are easily adapted to the concrete support providing the minimum edge distances can be satisfied. The stability of the steel sheet must be assured during construction. The fastening of the steel deck may be carried out by various ways as shown in Figure 3.18. For prestressed concrete beams the deck is locked/clamped or fixed using shot fired fasteners. A minimum edge distance must be respected in order to avoid splitting or a steel plate should be inserted in order to provide a fastener base. The Fdration internationale de la prcontrainte (FIP) is preparing a Guide to good practice Precast composite floor structures, which gives general rules and recommendations for construction composite structures with prefabricated elements.

ECCS N 87


-6m-- hot-rolled secondary beam

fabricated primary beam

fabricated beam

composite haunched beam

mullion column

Figure 3.13 - Structural arrangements with built-up sectio

Lj111111111111 (a) Straight taper

(b) Semi-taper

ECCS N 87

(C) Cranked taper

Figure 3.14 - Fabricated beams alternative shapes

(a) Type A

(b) Type B

(C) Type C

Preliminary considerations and pre-dthgn ____________________________________ 37

/.

Figure 3.15 - Composite slab and reinforced concrete beams

ECCS N 87

5cm

temporary prop

Figure 3.16 - Europe's tallest building has composite floor (Messeturm in Franlfurt/Main, Germany)


steel mesh / steel decking

b) Timber structures

Figure 3.17 - Other construction details for concrete frames

P ' length 150 mm e=5h

Figure 3.18 - Various way to connect the deck on concrete

The timber beam option for flooring can be either a structural or an architectural choice. Floors made with composite slabs are well suited to this type of structure because of the reduced slab weight The fastening of the sheets onto the beams is carried out by mean of nails or screws (see Figure 3.19). When adequately fasten, the deck can be used to improve the structural stability for both the temporary and permanent stages. Typical construction details for timber structures are given in Figure 3.20.

ECCS N 87

.

shear bars

h

L L HI

Pre1iminay considerazwnsandpre-design 39

Figure 3.19 - Fcteners for timber structures

3.4 RENOVATION AND REFURBISHMENT SCHEMES

V

I

Composite slabs are versatile and can very often be used for renovation of existing structures. The use of composite slabs in this particular context is not very different from current applications but the uniqueness of such projects may lead to problems compared with conventional design. This section outlines briefly the various possibilities in these situations. Depending with the nature and/or the importance of the renovation scheme composite floors can be placed on various type of beams including steel beams (Figure 3.21) reinforced concrete beams (Figure 3.22)

wooden or timber beams (Figure 3.23) prestressed concrete beams (Figure 3.24) reinforced concrete walls (Figure 3.25)

ECCS N 87

Figure 3.20 - Typical construction details on timber structures


ECCS N 87

+

concrete slab

Fi2ure 3.21 - Renovation and refurbishment schemes steel beams

tile

steel joist

/1 composite __/' slab with NWC

Connected steel joist

/ composite I' slab with LWC

5000 to 8000 mm 5000 to 8000 mm

S S I

Figure 3.22 - Renovation and refurbishment schemes reinforced concrete beams

Preliminary considerations and pred esign 41

Existing floor Renovated floor

unsawn timber beam

composite slab tire bar

timber beam level

Figure 3.23 - Renovation and refurbishment schemes on wooden or timber beams

Figure 324 - Renovation and refurbishment schemes on prestressed concrete beams

ECCS N 87

timber floor 100 - 140 mm thick slab I h stable to fire

SLA 4O

p.

grout

shutter

prestressed concrete beam

slab 4 composite existing

wall


The number of renovation projects where composite slabs can be used is very large. Examples range from very simple projects with a new slab cast onto the existing steel beams to major refurbishment schemes where only the facade of the building has been retained. It is difficult to produce here a complete list of these multi-purpose applications but they include the following: Re-modelling Strengthening Extension

Composite flooi avoid the setting of costly and voluminuous formwork which slows down construction progress. Generally the low or unknown carrying capacity of the existing foundations places a severe limitation on the dead load. Lightweight composite slabs are beneficial because they are easy to handle and up to 1.0 1rN/m2 lighter than conventional reinforced concrete flooring. The installation of composite floor panels for renovation schemes does not require the use of a procedure which is different to new construction. Steel decking available on the market offers a large variety of solutions to tackle the problems of partial or total renovation. Renovation schemes are often characterised by the irregular shape of the slabs. The use of the conventional slab techniques (i.e.: cast-in place, pre-cast or hollow core slabs) is often difficult. Composite floors are generally useful in these situations, the pre-cut panel elements are cut on site to the exact shape of the building using simple tools such as grinders and nibblers. The flexibility and the lightness of the panels allows quick but efficient installation of the elements. The steel sheets may be manually positioned by 2 or 3 men. When the access to the construction site is difficult with the conventional lifting equipment the passage of the panels through the door or existing windows is possible without the need to dismantle the roof.

ECCS N 87

Figure 3.25 - Example of floor construction

Preliminary consideraJions and pre-desgn

3.5 SHALLOW FLOOR CONSTRUCTION

43

Shallow floor construction incorporates light gauge steel decking as part of a composite slab (see Figure 3.26). The system has the benefits of slim floor construction but possesses additional advantages over the traditional concrete option (i.e. lighter weight of the slab, ease and speed of erection). Shallow floors made of composite slabs usually span between 5 to 9 metres, the total thickness of the slab is usually set between 180 and 350 mm. All types of steel deck can be used providing they can achieved the required deflection and strength criteria. However, deep steel decking presents another advantage, the size of the corrugations allows light services to run within the floor depth parallel to the corrugation and through the beams webs (see Figure 327).

600

= 210

210 deep deck x 1.25 thk.

Figure 3.26 - Shallow floor arrangement with composite slab

3.5.1 Shallow floor with deep steel decking The slab is hidden within the beam height which is generally a compact section. The use of deep steel decking is therefore an efficient solution, because the deck rests directly on a plate welded to the bottom flange of the beam. The deck openings are closed by stop end accessories which prevent concrete leakage. The top flange of the beam is covered by a concrete topping (70 to 100mm) which houses the steel fabric for hogging moments and the shear connectors when the beam is designed for composite action. This topping constitutes the compression part of the slab within the current span. The fire stability of the beam is brought up to 1 hour and the use of lightweight concrete allows to span up to 6 metres without props.

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B = span/4 Transverse reinforcement

DI'

A4JL Cross-section through composite 'Slimfior' beam - Type B

Section A - A

I

600


Services

3.5.2 Shallow floor for long-span beams The steel beams for longer spans are rarely made of compact sections. The beam depth is clearly increased in order to achieve economy and the slab is not supported on the bottom flange of the beam. However it is still advantageous to use the shallow floor technique in order to reduce the total floor height and improve the fire stability of the beams. In this arrangement the slab rests on brackets or packings whose purpose are to maintain the steel sheet during the construction stage. All or part of the slab is contained within the beam depth as is shown in Figure 3.28. Any conventional steel decking can be use for this arrangement, providing that the design and propping requirements are met. The fire stability of the beams varies with the way they have been integrated within the slab.

ECCS N 87

In-situ concrete

Figure 3.27 - Floor service with shallow floor

Figure 3.28 - Shallow floor arrangement for long span structures

Preliminary consideradns and pre-design 45

3.6 PRE-DESIGN

3.6.1 Major information In order to carry out a realistic pre-design exercise it is necessary for the designer to gather a minimum amount of information about the project. This vital pre-design information will include all or a number of the following: Slab depth compatible with the fire rating Limiting slenderness ratio for slab Slab depth compatible with the sound insulation Accepted propping conditions on site The possible span of the slab and the beams Permanent or short temi imposed load applied to the floor.

a) Slab depth compatible with the fire rating This criterion fixes the minimum slab thickness necessary to achieve satisfactorily the required fire rating. The minimum slab thickness is variable and depends on the type of decking and the technique used to solve the fire problem. Two main techniques are available: Fire protection of the soffit using either applied or screened material (no minimum slab thickness

imposed by the fire design) The use of steel bar reinforcement for fire resistance (a minimum slab thickness is compulsory, see

also Section 8.2) The minimum slab depth for this fire resistance is a direct consequence of this choice. For a fire rating of two hours the steel reinforcement solution is efficient and cost effective. The first solution is generally preferred when the fire rating has been set above two hours.

b) Limiting slenderness ratio for slab The slenderness ratio is given by the span length (L) divided by the effective depth of the slab (dp). This number usually lies between 20 (heavy loads) and 40 (light loads). For typical structures this slenderness ratio may be taken as not greater than 32 to comply the serviceability requirement. Such limitation of the slenderness ratio also influences the dynamic response of the slab.

C) Slab depth compatible with the sound insulation The acoustic performance of composite slabs is described by the "mass law" equation. The performance of slabs are normally given in manufacturer's brochures. In certain circumstances the composite slab is not sufficiant for sound insulation and in this case a system involving an additional layer of insulation must be used. It is important to estimate the minimum thickness of the concrete slab in conjunction with the technique of insulation chosen for the construction as described in Section 8.6.

d) Accepted propping conditions on site The use of props may either depend on the method of construction and/or the conditions on site. Their use always induces extra-cost for the setting and removal of these devices. However one or two lines of props enables larger spans to be achieved.

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e) The possible span of the slab and the beams The beam spacing in a floor using composite slab is a critical decision. There are two cases: Beam spacing and column layout are imposed Beam spacing and column layout are free In the first case the designer has little freedom only the spacing of the secondary members requires determination.

The early decision whether constiuction is with or without propping is critical because the criterion "no props" limits the choice of the deck type or forces the designer to use shorter spans. Therefore the mmiinum required thickness of the slab will be dictated by the spanning limits of the deck. In the second case the designer has more design choices. A good way to stait the pre-design is to use the minimum slab thickness as explain before and then estimate the maximum span with or without props. The spacing of the secondary beams is then known and therefore their section depth can be estimated.

f) Total dead and Imposed load applied on the floor The majority of design charts given by manufacturers gives permissible loads for maximum spans (or vice versa). For a typical range of buildings imposed loading varries between 2 kN/m2 and 5 kN/m2 for light loads and up to 10 kN/m2 for heavy loads.

3.6.2 Pre-design procedure Pre-design of floors using composite slabs by architects or engineers is greatly simplified by the manufacturers design charts. Recently these charts are sometimes accompagned by software packages whose level of refinement is variable. These packages may provide safe load tables or more refined analyses. Their use is particular to each manufacturer and therefore will not be considered here since the design charts are universal.

Most brochures produced by the manufactures have a double entry system considering both the construction stage (number of props) and the loading stage (total slab depth, steel reinforcement for hogging and sagging). These two stages will be considered separately.

a) Temporary or construction stage The designer must first consider the construction stage and select a deck type compatible with the site requirement (props or no props). In many cases the site layout decision influences the choice of profile and also the form of the slab construction.

Clearly the allowable deck span is influenced by: The support conditions Sheet lengths can be used to span one or more spans (single or multi span). When investigating larger unpropped span the designer is advised to arrange a multi-span layout of the deck. This is more economical for both the unpropped span length and the site layout. The decking strength The deck carrying capacity is mainly influenced by the inertia which is in turn directly linked to the rib depth of the profile and the thickness of the metal sheet (smaller influence). ECCS N 87

Preliminary considerations aizdpre-design 47

A crude stereotyped approach for a typical office building slab would be:: deck with rib height of 40 mm spans up to 2.70 m deck with rib height of 60 mm spans up to 3.30 m deckwithribheightoflOmmand+spansupto3.70m specific decking for large span up to 6.5 m (no prop) The possible span depends of course of the finished slab depth but also with the type of concrete (normal or light weight).

b) Permanent or service stage As a guide the minimum depth of the slab is given: the limiting slenderness ratio L/dp 32 (see also 3.6.1 b) the minimum depth for fire rating and/or sound insulation.

The decking shape and properties can be obtained once this minimum slab depth has been set together with the possible span layout (single or multi-span, number of props accepted). All the data are normally computed using conventionnal hypothesis such as the average concrete strength, the deflection at service and other current parameters. These values are always given with the data and should be specified for the final design. Special calculations for the fire reinforcement or other special loading condition are usually not relevant to this stage of the project. They are carried out later by the design office.

3.6.3 Summary Table 3.1 gives an overview of the different alternatives for the choice of a composite floor system.

Table 3.1 - Different alternatives for the choice of a composite floor system

Feature Alternatives

Structural system Single or continuous span beam.

Floor beam length 6 to 20 m. Floor beam centres or spacing (slab span length)

1.80 to 5.0 m.

Steel decking Trapezoidal profiles, re-entrant profiles,, types embossments end anchorage.

of indentations or

Fire protection Thickness of the slab, additional reinforcement, (suspended ceilings, sprayed material).

protection system

Shear connection Stud connectors, welded through the sheet or welded to the beam with holed sheet. Nailed shear connectors.

Degree of shear connection (beam)

Partial to full (40 to 100%)

Concreting (slab) Unpropped or propped slab.

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Detailing requirementr 49

4 DETAILING REQUIREMENTS The following detailing requirements should be respected whatever conditions of design are considered as a minimum. More information may be found in the ECCS document No 73: "Good Construction Practice for Composite Slabs". Two specific design situations must be considered: Construction stage Composite stage

4.1 GENERAL CONDITIONS ON STEEL DECKING AND COMPOSITE SLABS

4.1.1 Decking and slab It is recommended that the nominal thickness of the steel decking should not be less than 0.75 mm. Zinc coating should be provided on each face with a minimum of 0.02 mm per face. This rule is for corrosion protection. The depth of the steel sheeting t'p should not be less than 35 mm and the depth of the composite slab not be less than 80 mm. This is a minimum condition for fire resistance and sound insulation.

The span to effective depth ratio of the slab should be less than or equal to 32 for simple supported slabs and 36 for continuous slabs. This is a condition for slab rigidity and comfort (see EC 2). The thickness hc of concrete above the ribs of the decking shall be greater than 40 mm. If the slab acts compositely with a beam or is used as a diaphragm, the minimum total slab depth h is 90 mm and the minimum concrete thickness hc above the decking is 50 mm.

;: T ' . h ______

___f jh 4hp h bb bh

re-rentrant trough profile open trough profile Figure 4.1 - Minimum conditions : decking and composite slab

4.1.2 Concrete The minimum characteristic resistance in compression of the concrete is 20 N/mm2 (Class C20). Concrete may be Normal Weight Concrete (NWC) or Light Weight Concrete (LWC). The nominal size of agregate depends on the smallest dimension on the structural element within which concrete is poured, and shall not exceed the least of: 0.4 h where h is the depth of concrete above the ribs b>/3 where b0 is the mean width of the rib (minimum width for re-entrant profiles) 31.5 mm.

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50 Design Manualfor Composite Slabs

4.2 CONSTRUCTION STAGE The stage during the erection of the structure is one of the most critical. Specific details must be complied at this design stage.

4.2.1 Bearing During construction, the steel sheeting is acting as shuttering. It is placed on permanent supports and

supports, the props. Figure 4.2 shows the minimum values for bearing sometimes, on specific temporary lengths on permanent supports.

1

beanng on other materials such as brick or block

(c) jj F

0

Figure 4.2 - Minimum bearing lengths for permanent supports (7.3 of EC4) For design calculations, it is convenient to consider that the decking is supported on the centre line of the bearing. Temporary supports shall be checked in accordance with part 1.3 of EC 3. Figure 4.3 gives the minimum values for the temporary bearing lengths (props). All interior panel ends shall be centered over permanent supports. During construction cantilevers shall be temporary supported. Note: EC 4 allows reduction of minimum bearing lengths given above if special care is considered in the design (see EC 4 for more information). 4.2.2 Fasteners Each panel should be connected at least twice at each end to the permanent supports and the decking shall be butted to each other or overlapped. The longitudinal overlapping depends of the shape of decking. Generally profiles overlap on one or half of one rib. When used, the minimum transversal overlapping on supports are 50 mm on steel supports and 70 mm on supports made of others materials (see Figure 4.2)

ECCS N 87

bearing on steel or concrete

- I

0 ., 3 - .0

(b) j L (d)

iooj.

Detailing requirements 51

SLAB DEPTH mm

SPAN m

MINIMUM TIMBER BLOCKSIZE

mm

HEIGHT WIDTH

120 130 150 200

3.25 3.75 4.25 4.75

175 200 225 225

50 50 50 75

Figure 4.3 - Minimum bearing lengths of temporary supports

Panels shall be seamed together. Minimum distance between seams is 500 mm for single spans and 1000 mm for continuous decking. Seam fasteners between panels are particulary important if heavy construction loads are expected or if the decking spans more than 3 metres. If the decking is acting as a diaphragm, the number and the placement o;f the fasteners must meet the relevant design specification. A 600 mm interval between fasteners is considered as a minimum. Figure 4.4 shows typical arrangement for fastening, overlapping and seaming. In any case, during construction, cantilevers shall be temporarely supported. Figure 4.5 shows typical cantilever situations.

Edge trims or angles shall be fixed to edges to contain the fresh concrete. The thickness of the trim depends on the expected slab thicknesses and are not specifically designed. Table 4.1 gives good practice values for trim thicknesses. Lateral edges trim deflection may be reduced by ties backs. Ties back spacing are typically between 250 mm and 1.0 metre.

Table 4.1 - Trim thicknesses

h [mm]

X [mm]

t [pjJ 200


ECCS N 87

r

Figure 4.5 - Cantilever situations

Sean, fe

Figure 4.4 - Fastening, overlapping and seaming

1 L.

M secUon

Detailing requirements 53

4.2.3 Edges treatment Edges are classif

Design Manual for Composite Slab

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