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Composite reinforced concrete Composite reinforced concrete R.TAYLOR BSc, DIC, MICE, MIStructE Senior Lecturer in Civil Engineering University of Manchester Simon Engineering Laboratories Thomas Telford Limited, London, 1979 Published by Thomas Telford Limited, Telford House, PO Box 101, 26-34 Old Street, London EC1P 1JH ISBN: 0 7277 0077 4 © R. Taylor, 1979 All rights, including translation, reserved. Except for fair copying, 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 written permission of the Managing Editor, Publications Division, Institution of Civil Engineers, PO Box 101, 26-34 Old Street, London EC1P 1JH Typeset and printed by Henry Ling Limited, Dorchester FOREWORD There is no doubt that none of the major structural materials is ideal. Despite the long-standing use of various combinations of steel and concrete, all methods of combining these materials have some draw backs. While ordinary reinforced concrete combines the use of the tensile strength of steel with the compressive strength of concrete, it cannot take full advantage of high tensile steels because of the limited capacity of concrete to undergo tensile strain without excessive crack ing. Although this problem is overcome in prestressed concrete, the economy resulting from the use of ultra-high strength steels is offset by the costly prestressing operation, at least for site construction. The other main traditional steel—concrete combination — composite con struction — has undoubted advantages in construction but is extravagant in its use of steel. It would be advantageous if the economic use of ultra high strength tensile steel could be combined with the convenience of composite construction. While composite reinforced concrete does not completely achieve this ideal, it goes some way towards it. The Author of this monograph, Mr R. Taylor, has developed his pro posals for composite reinforced concrete as a result of pursuing two other lines of research. The first arose from the aim of improving the efficiency of the use of steel in composite construction by incorporating deep haunches in the beams without sacrificing constructional advan tages. In the second investigation, Mr Taylor sought a means of improving the exploitation of reinforced concrete beyond the limits currently recommended by Codes of Practice. As a result of these two investigations, it was realized that the incorporation of deep haunches in composite beams could be achieved alongside the use of high tensile reinforcement to give a possible improvement in overall economy. It is hardly likely that innovatory methods of construction will be accepted unless they show distinct advantages. Composite reinforced concrete may well have such advantages in multi-storey structures when it is desired to use precast flooring units in conjunction with an in situ concrete frame in which the beams need to be of minimum construc tional depth. The present monograph is published with the intention of stimulating practical exploration of this proposed method of construction. Con siderable theoretical economies are possible; the realization of these economies in practice could lead to major steps forward in the more economic exploitation of steel—concrete composite construction. M. R. Home Manchester March 1979 PREFACE This research monograph on composite reinforced concrete is based on lectures given at a research seminar in April 1979 at the Simon Engineering Laboratories of the University of Manchester. At this stage composite reinforced concrete is only a research material and has not been used in practice. Indeed hitherto it has not been possible to use composite reinforced concrete as there has been insufficient information available to the designer. It was the purpose of the seminar, and is the purpose of this monograph, to make available the information that has been obtained at the University of Manchester from research over a period of several years, and also to indicate those areas in which more information is still needed. Although aimed essentially at engineers concerned with research in concrete structures, a wide cross-section of engineers from industry also attended the seminar. Such varying backgrounds of the participants of the seminar determined the material presented and its manner of presen tation. It could not be assumed, for example, that each had a good knowledge of reinforced concrete design and the requirements of CP 110, nor that each had a good knowledge of composite construction and the requirements of CP 117. For this reason somewhat fuller explanations are given than would normally be the case in a research report. Chapter 1 introduces the idea of composite reinforced concrete and presents examples of design for the case of simply supported beams. Chapter 2 discusses the problem of continuity and gives examples of design of continuous beams. Chapter 3 covers the problems of vertical and horizontal shear and the design of the shear connections. Chapter 4 discusses some miscellaneous problems such as deflexion, cracking, fatigue and fire resistance. CONTENTS Chapter 1. Simply supported beams 1 Background Composite reinforced concrete Experimental data for flexure Calculation of ultimate resistance moment Comparative designs Chapter 2. Problems of continuity 31 Compressive strength of concrete contained by channel Tests on some 'hogging type' beams Calculation of ultimate hogging resistance moment Design of continuous beams Constructional aspects Chapter 3, Problems of shear 67 Effects of vertical shear Problem of horizontal shear Alternative types of shear connectors Longitudinal shearing Chapter 4. Miscellaneous problems 92 Problem of deflexion Cracks at the service load Problem of fatigue Problem of fire resistance Summary and conclusions 109 Appendix 1. Calculations for the comparative designs in chapter 1 111 Appendix 2. Calculations of hogging resistance moments of test beams 115 CHAPTER 1 Simply supported beams The combination of steel and concrete for structural purposes normally falls into one of the traditional categories, such as reinforced concrete, prestressed concrete and composite construction. These are quite well defined categories but it is not unknown for the boundaries to be crossed and prestressed reinforced concrete (or partially prestressed concrete) and prestressed composite construction are examples of steel- concrete combinations where the traditional boundaries have been crossed. All these various modes of construction have their advantages and disadvantages which determine their use in practice. It will be suggested that a new combination of steel and concrete combines many of the advantages of the traditional forms whilst eliminating some of their attendant disadvantages. Nevertheless it is emphasized that this new material, designated composite reinforced concrete, is not suggested as a replacement of the traditional forms, but merely as an additional and alternative form to be used in appropriate circumstances. It is considered that composite reinforced concrete will be found to be most suitable for the multi-storey framed structure. Starting with a consideration of costs, it is shown that the traditional forms for concrete structures leave something to be desired. It will be shown for example that, although reinforced concrete is very efficient in its use of materials, one could reduce the cost of the basic materials by introducing a new philosophy of design. Moreover the constructional advantages of composite construction over fully in situ reinforced con crete are shown to be negated by the extra cost of the materials and also the cost of the shear connectors. These relative advantages and disad vantages of the traditional forms are shown to lead to composite reinforced concrete.