Composite construction

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In structural engineering, composite construction exists when two different materials are bound together so strongly that they act together as a single unit from a structural point of view. When this occurs, it is called composite action. One common example involves steel beams supporting concrete floor slabs.1 If the beam is not connected firmly to the slab, then the slab transfers all of its weight to the beam and the slab contributes nothing to the load carrying capability of the beam. However, if the slab is connected positively to the beam with studs, then a portion of the slab can be assumed to act compositely with the beam. In effect, this composite creates a larger and stronger beam than would be provided by the steel beam alone. The structural engineer may calculate a transformed section as one step in analyzing the load carry capability of the composite beam. Composite Construction Edited by David A Nethercot ©2004 Taylor & Francis First published 2003 by Spon Press 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Spon Press 29 West 35th Street, New York, NY 10001 Spon Press is an imprint of the Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2004 © 2003 Spon Press All rights reserved No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data Nethercot, D.A Composite construction / David A Nethercot p cm Includes bibliographical references and index ISBN 0-415-24662-8 (alk paper) Composite construction Composite materials I Title TA664 N48 2003 620.1′18—dc21 2002042805 ISBN 0-203-45166-X Master e-book ISBN ISBN 0-203-45733-1 (Adobe eReader Format) ISBN 0-415-24662-8 (Print Edition) ©2004 Taylor & Francis Contents Contributors Foreword Acknowledgments Fundamentals D A V I D A N E T H E R CO T 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 Introduction History Basic concepts Material properties Shear connectors Design for ULS Design for SLS Composite systems Current usage Concluding remarks References Composite Beams HOWARD D WRIGHT 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 Introduction Types of beam Basic behaviour Ultimate strength design Calculating the deflection Shear connector behaviour Continuous beams Beams with composite slabs Current design and future development References ©2004 Taylor & Francis Composite Columns YONG C WANG 3.1 3.2 3.3 Introduction Composite columns under axial load in cold condition Composite column under combined axial load and bending moments at ambient temperature 3.4 Effect of shear 3.5 Load introduction 3.6 Composite columns in fire conditions 3.7 Summary 3.8 Acknowledgement 3.9 References 3.10 Notations Instability and Ductility ALAN R KEMP 4.1 4.2 4.3 4.4 4.5 Introduction and elastic buckling theory Ultimate resistance of composite columns Continuous composite beams Ductility considerations for compact beams References Composite Floors J B UI C K D A V I S O N 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 Introduction Current practice Behaviour as formwork Composite behaviour Dynamic behaviour Concentrated loads and slab openings Fire resistance Diaphragm action Slim floor decking References Composite Connections DAVID B MOORE 6.1 6.2 6.3 Introduction Types of composite connections Design principles ©2004 Taylor & Francis 6.4 6.5 6.6 6.7 6.8 6.9 Classification of composite connections Capacity of composite connections Ductility of composite connections Stiffness of composite connections Summary References Composite Frames GRAHAM H COUCHMAN 7.1 7.2 7.3 7.4 7.5 7.6 Introduction Principles of frame behaviour Frame analysis and design Design using software Conclusions References ©2004 Taylor & Francis Contributors Graham H Couchman Steel Construction Institute Silwood Park Buckhurst road Ascot Berks SL5 7QN J Buick Davison The University of Sheffield Department of Civil & Structural Engineering Sir Frederick Mappin Building Mappin Street Sheffield S1 3JD Alan R Kemp Faculty of Engineering University of Witwatersrand Jan Smuts Avenue Johannesburg 2001 South Africa ©2004 Taylor & Francis David B Moore BRE PO Box 202 Watford Herts WD2 7QG David A Nethercot Imperial College Department of Civil & Environmental Engineering London SW7 2AZ Yong C Wang School of Civil Engineering University of Manchester Oxford Road Manchester M13 9PL Howard D Wright University of Strathclyde James Weir Building 75 Montrose Building Glasgow G1 1XJ Foreword Composite Construction has developed significantly since its origins approximately 100 years ago when the idea that the concrete fire protection around columns might be able to serve some structural purpose or that the concrete bridge deck might, with advantage, be made to act in conjunction with the supporting steel beams was first proposed Take-up in practice and began in earnest shortly after the end of the Second World War and progress has been particularly rapid during the past 20 years Indeed, it is now common to ask, “Why is this not acting compositely?” when looking to improve the efficiency of a structural steelwork design In those countries where steelwork enjoys a particularly high market share e.g for high-rise buildings in the UK and Sweden, the extensive use of composite construction is a major factor Early approaches to the design of composite structures generally amounted to little more than the application of basic mechanics to this new system However, it was soon realised that this particular medium possessed features and subtleties of its own and that effective usage required that these be properly understood and allowed for Composite construction is now generally regarded as a structural type in its own right, with the attendant set of design codes and guidance documents The most comprehensive and up to date of these is the set of Eurocodes—specifically EC4 that deals exclusively with composite construction It is not the purpose of this textbook to serve as a commentary on the Eurocodes Rather, it is an explanatory and educational document, presenting the technical basis for many of the newer concepts, design procedures and applications of composite construction in buildings Inevitably, it makes some reference to the Eurocodes but only in the sense that their procedures often represent formal statements of the most appropriate simplified implementation of our current understanding For convenience and consistency it adopts their notation The authors—each an acknowledged expert in the topic on which they have written—have selected their own way of presenting the subject matter In all cases the intent has been to share the technical basis and background to design so that extrapolation and intelligent use beyond the obvious is possible The book is not claimed to be comprehensive or to represent a full state of the art It should be regarded as helpful background reading for all those wishing to acquire a better appreciation and understanding of the major developments in the use of composite construction for building structures The first Chapter of this book traces the key historical steps in the development and understanding of Composite Construction and introduces the main fundamental features The next two deal with basic elements—horizontal beams and vertical columns—showing how the combined action of the concrete and the steel member may be synthesised to give a more efficient load resisting arrangement A relatively new development is the deliberate use of composite action in beam to column ©2004 Taylor & Francis connections, thereby requiring them to be treated as partial strength and semi-rigid for design purposes as explained in Chapter Because buckling is a key item when dealing with the response of steel members, its importance for composite elements—especially beams—is then considered in some detail Building floor systems now often comprise arrangements with two-way spanning composite action and several such arrangements are discussed in Chapter The final Chapter deals with the interaction of beams, columns and joints in presenting a complete treatment for the design of non-sway composite frames that recognises the actual behaviour more closely than does conventional treatments based on consideration of individual components This book is collaborative effort, with all the Chapter authors having made an equal contribution Its preparation has inevitably involved delivery against deadlines and the required instructions My thanks to Howard, Yong, David, Buick, Alan and Graham for their patience and cooperation Production has benefited from the firm but sympathetic guidance of the publishers—particularly Alice Hudson The coordination and final preparation of the manuscript was just one of the tasks handled so efficiently by my PA Alice Kwesu David A Nethercot ©2004 Taylor & Francis Acknowledgments Considerable effort has been made to trace and contact copyright holders and secure replies prior to publication The authors apologise for any errors or omissions Extracts from Eurocode 4, Eurocode and BS 5950 Part 3: 1990 are reproduced with the permission of BSI under licence number 2002SK/0204 Eurocodes and British Standards can be obtained from BSI Customer Services, 389 Chiswick High Road, London W4 4AL (Tel + 44 (0) 20 8996 9001) Figures from Steel Construction Institute publications are reproduced with kind permission from the Steel Construction Institute Acknowledgments are also required for the following: Chapter One—Fundamentals David A Nethercot Figure 1.1 reproduced with kind permission from the ASCE from: Moore, W.P., Keynote Address: An Overview of Composite Construction in the United States, Composite Construction in Steel & Concrete, ed C.D Buckner & I.M Viest, Engineering Foundation, 1988, pp 1–17 Figures 1.2 and 1.3 reproduced from: David A Nethercot, Limit States Design of Structural Steelwork, Spon Press Figures 1.4, 1.5, 1.6 and 1.8 reproduced from: Johnson, R.P., Composite Structures of Steel & Concrete Volume Beams, Slabs, Column & Frames for Buildings, 2nd edition, Blackwell Scientific Publications Figure 1.16 reproduced from: Lam, D., Elliott, K.S & Nethercot, D.A., Structures and Buildings, ICE Proceedings Chapter Two—Composite Beams Howard D Wright Figure 2.4 reproduced from: Mullett, D.L., Composite Floor Systems, Blackwell Science Ltd Chapter Three—Composite Columns Yong C Wang Tables 3.4, 3.5 and 3.6 are reprinted from Journal of Constructional Steel Research, 51, Kodur, V.K.R., Performance-based fire resistance design of concrete-filled columns, pp 21–36, 1999, with permission from Elsevier Science ©2004 Taylor & Francis Chapter Five—Composite Floors J Buick Davison Figure 5.20 reproduced from: Composite Slab Behaviour and Strength Analysis Part calculation Procedure, Daniels, Byron J., Crisinel, Michael, Journal of Structural Engineering, Vol 119, 1993—ASCE Figure 5.26 reproduced courtesy of Corus plc Chapter Six—Composite Connections David B Moore Figures 6.1, 6.13, 6.16, 6.17 and 6.18 reproduced with kind permission of Building Research Establishment Ltd ©2004 Taylor & Francis In order to calculate the distribution of moments around the frame - unless a simplified approach (such as the wind moment method, which is described later) can be adopted, the distribution of moments must be determined as a function of the relative stiffnesses of the frame members A problem with unbraced frames lies in establishing the lengths of beam in hogging and sagging Because the beams and columns in an unbraced frame must resist horizontal loads as well as vertical loads, the connections load and unload as wind loads vary in magnitude and direction This means that the hogging moments at the beam cnds vary, and the lengths of beam in hogging also vary (Figure 7.16) As a result of these variations in moment distribution the effective stiffnesses of the beams also vary Bending moment diagram for a beam subject to UDL plus wind load from the left Bending moment diagram for a beam subject to UDL plus wind load from the right Figure 7.16: Variations in bending moments as a function of wind load direction Despite this apparent complexity, simplified models to predict an acceptable constant effective beam stiffness that may be used in analyses have been developed: According to Leon et ul (1996), the effective second moment of area may be be derived taken as:from those of the uncracked (I1) and cracked (I2) sections Iefl= 0.412 + 0.611 (when there is semi-continuity at both ends) or Iefl= 0.2512+ 0.751, (when there is semi-continuity at one end) According to Hensman and Way (1999) a slightly more complicated formulation gives greater accuracy: 1~~ Ierf= ( ~ ~ ~ ) / (+921112) ©2004 Taylor & Francis Columns The design of the columns in an unbraced frame must take into account so-called second order effects due to geometric non-linearity These are also known as P-delta effects; a column node which sways by an amount delta will lead to a secondary moment in the column equal to the axial force P times the lever arm delta Such second order effects may be allowed for in one of three ways: explicitly in a second order analysis (some software offers this capability, or an iterative procedure could be implemented) using a simple method to increase the moments and forces due to the first order loading using a simple method to decrease the resistance of the columns The Amplified Sway method described in BS.5950-1 (1990) provides an easy way of increasing the applied moments and forces to represent second order effects The increase is simply a function of the frame's sway stiffness, which is quantified in terms of its sway This method may be applied to 'orthodox' frames that possess a reasonable degree of sway ~tiffness A simple method of artificially decreasing the resistance of the columns, so that this resistance can be compared with the 'known to be too small' first order applied moments and forces, is to consider an effective length in excess of the system length Procedures for determining appropriate values of effective length are fully described in BS5950-I (1990) The length is a function of the restraint provided to the column by adjoining members The so-called Wind Moment Method, which is described is Section 7.3.4, uses this approach for dealing with second order effects, and as a simplification adopts an effective length 1.5 times the system length, regardless of the frame details 7.3 FRAME ANALYSIS AND DESIGN In this section the analysis and design of braced and unbraced composite frames will be discussed The frame at both the construction (steel) and final (composite) stages will be considered Key points are discussed, including some in-depth guidance, but in the interests of concision reference is made to existing guidance where appropriate 7.3.1 Braced frames Construction condition The guidance given below relates to frames in which the beams are unpropped during construction, so that the construction loads are applied to bare steel beams and connections This is the most common practice for composite frame construction due to the speed and therefore economic benefits of the site process ©2004 Taylor & Francis Composite connections are generally based on relatively thin flush end plate steelwork details Such details are appropriate because: The lower part o f the end plate provides a direct load path for the often substantial compression loads to transfer from the lower beam flange into the column The thin end plate behaves in a ductile manner, so that the connection can achieve high rotations without failure o f any components The availability o f capacity tables for standard details greatly simplifies the design process In their bare steel state such flush end plate connections are 'semi-rigid' and 'partial strength' Typically their moment resistance is 40 to 50% o f that o f the adjacent bare steel beam The frame must therefore be analysed under construction loading recognising these attributes o f semi-rigidity and partial strength They lead to what is known as a semi-continuous frame;the connections are neither stiff enough nor strong enough to provide full continuity between the beams and columns The frame may be considered to be statically determinate i f plastic hinges are assumed to form in the connections, so the beams and columns can be considered as separate members Hogging moments in the connections, equal to their strength, should be allowed for when determining the sagging moments in the beams Depending on the construction details it may be necessary to design the beams considering them to be laterally unrestrained Columns should be designed for unbalanced moments when the opposing connections at a node are o f unequal strength It has been demonstrated by both testing and analysis that unbalanced moments due to
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