Other Pergamon Titles of Interest CHEUNG Finite Strip Method in Structural Analysis COHN and MAIER Engineering Plasticity by Mathematical Programming GIBSON Thin Shells: Computing and Theory HARRISON Structural Analysis and Design: Some Minicomputer Applications HENDRY Elements of Experimental Stress Analysis, SI Edition HORNE Plastic Theory of Structures, 2nd Edition LIVES LEY Matrix Methods of Structural Analysis, 2nd Edition NOOR and McCOMB Trends in Computerised Structural Analysis and Synthesis SACHS Wind Forces in Engineering, 2nd Edition WARBURTON Dynamical Behaviour of Structures, 2nd Edition Pergamon Related Journals International Journal of Solids and Structures Computers and Structures International Journal of Engineering Science Building and Environment Introducing Structures A TEXTBOOK FOR STUDENTS OF CIVIL AND STRUCTURAL ENGINEERING, BUILDING AND ARCHITECTURE A. J. FRANCIS Ph.D., M.Sc, M.C.E., F.I.C.E., F.I.Struct.E., F.I.E.Aust. Emeritus Professor of Civil Engineering, University of Melbourne, Victoria, Australia PERGAMON PRESS OXFORD • NEW YORK • TORONTO • SYDNEY • PARIS • FRANKFURT U.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 OBW, England U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. CANADA Pergamon of Canada, Suite 104, 150 Consumers Road, Willowdale, Ontario M2J 1P9, Canada AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., P.O. Box 544, Potts Point, N.S.W. 2011, Australia FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg Taunus, OF GERMANY Hammerweg 6, Federal Republic of Germany Copyright © 1980 A. J. Francis 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, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. First edition 1980 British Library Cataloguing in Publication Data Francis, A J Introducing Structures. - (Pergamon international library). 1. Structural engineering I. Title 624 TA633 79-41322 ISBN 0-08-022701-5 hardcover ISBN 0-08-022702-3 flexicover Printed and bound in Great Britain by William Clowes (Beccles) Limited, Beccles and London Preface MOST structural textbooks deal with the analysis of particular types of structure (by far the greatest emphasis being given to plane rigid-jointed frames, which are only one of many types) and do not aim to give the student a grasp of how structures in general act as organisms to transmit and resist forces, or of the principles governing the synthesis or putting together of structures so as to be efficient in the use of material. The in­ fluence on the behaviour of the finished structure of the properties of the different structural materials which the engineer has at his disposal is not usually explained very well, either. And even in textbooks on structural design, the designer's task is not always clearly described in simple terms. This book is an attempt to fill these gaps for the benefit of students of civil and structural engineering, building, and architecture. The main aim is to convey a physical understanding of the structural action of various kinds of structure (that is, how forces are transmitted through them and the stresses and deformations these forces produce) and how the engineer sets about the creative task of designing a structure for a particular function. The historical development of structural forms is touched on here and there in the discussion, and the main theme is also illustrated by reference to natural structures like plants, skeletons, and shells, and to structural failures and their causes. The presentation is almost entirely in non-mathematical terms, although an elementary knowledge of statics and structural mechanics is assumed. The general approach of the book grew out of the author's experience as a teacher of the subject of engineering structures. Since the aim is a general one, to explain, in as brief a compass as possible, structural behaviour, form, and design, rather than to discuss comprehensively the various structural types, the reader will not expect to find every kind of trussed framework, for example, referred to and illustrated. Rather the book should be regarded as complementary to the more rigorous viz viii Preface analytical and descriptive studies and the gradual initiation into the processes of creative design which the student normally undergoes. The author is particularly indebted to Professor L. K. Stevens and Dr. D. S. Mansell, of Melbourne University, and also to one or two other friends, for reading and commenting on the draft manuscript, and to Miss Raugi Tamekus for her careful preparation of the typescript. And finally he would like to thank his wife, without whose encouragement and support this book would probably never have been completed, and to whom it is dedicated. CHAPTER 1 Introduction ON 15 October 1970, at 11.50 a.m., a 112 m span of the Westgate bridge, under construction across the River Yarra in Melbourne, Australia, collapsed and fell into the river 40 m below. Thirty-five men working on the bridge were killed, completion was delayed by more than six years, and the cost of the acccident ran into many millions of dollars. Disasters like this are mercifully rare. But when one does occur, the public is suddenly made aware of the heavy social responsibility carried by the structural engineer, and to some extent, possibly, the high degree of professional expertise required of him. Generally, however, struc­ tures are well behaved, and people take them and their creators very much for granted. Yet they play an all-pervading role in modern life, and account for a large proportion of the capital investment in the na­ tional infrastructure. For we live and work in houses, office blocks, and factories, relax in theatres or sports centres, and worship in churches and temples. When we travel we cross bridges, or board ship from a jetty or wharf. The water we drink has probably been stored in a reser­ voir impounded by a dam, and may have reached us along an aqueduct. Tall chimneys, power transmission pylons, cranes, television masts, cooling towers and so on are all part of the modern scene. These are all building and civil engineering structures. A structure like the Eiffel Tower which overshadows Paris is so colossal that one wonders —especially if one stands on the top platform in a high wind — quite how safe it is, and how it was designed. Why should certain types of structure —suspension bridges, for example —have a characteristic and distinctive form? Why do modern structures look different from those in the past? How are they built, and what decides the choice of material used in them? Why, very occasionally, does one fail and make 1 2 Introducing Structures the headlines? This book is an attempt to introduce structures to the stu­ dent of structural engineering, building, and architecture by answering such questions as these in simple terms. Very broadly, a structure can be defined as something which forms a protective cover or casing for an object or living thing, or which carries weight, or supports or resists loads or forces of some kind. In this wide sense a snail's shell is a structure, and so is a man's skeleton, or the bran­ ches and trunk of a tree, or a spider's web. These are examples of natural structures, which have gradually evolved in the course of time to perform a particular function—and very efficiently they do so, as a rule. Taken in the broadest sense, man-made structures cover an extremely wide range of applications, including ships, aircraft, space vehicles, liquid and gas containers, car chassis, and even tables and chairs. In this book, however, we will be concerned mainly with the type of struc­ ture which is normally the responsibility of the structural engineer, working either on his own or in collaboration with an architect: these in­ clude buildings of all kinds, bridges, dams, jetties, deep-sea drilling platforms, masts, and so on. But we will be referring to other types of man-made structures as well as to natural structures in the course of the discussion, for the engineer can learn much about good design by study­ ing the solutions achieved by the aircraft or ship designer, or by looking at the shape of shells or the skeletons of animals or plants. Four main considerations or criteria govern nearly all structural design, and three of them were enunciated by Sir Henry Wooton, the Elizabethan poet, ambassador, and connoisseur, in his book The Elements of Architecture. Inspired by the writings of the Roman Vitruvius and the Renaissance architect Palladio, he gave as the requisites of good building commodity, firmness, and delight — or, as we would say in our less pungent modern phraseology, suitability for its function; serviceability and safety; and good appearance or aesthetic satisfaction. And today we would have to add a fourth criterion, economy, for though cost may have been no object to wealthy patrons of architecture like Sir Henry, it certainly is now. By commodity or function we simply mean that since the structure was built for some purpose, it should be the aim of the designer to see that it performs its job well. This is so obviously desirable that it should go without saying. But in fact designers do not invariably keep it well in Introduction 3 mind, and may be more preoccupied with the third criterion, ap­ pearance, than with making sure that their structure works well. We all know the handsome teapot that drips as it pours; and there are plenty of buildings and houses that are better to look at than to live and work in. On the other hand, some of the utilitarian structures designed by engineers, though they may function well, are ugly, because too little thought was given to what they would look like against their surround­ ings. The second criterion of good building, firmness, is the special respon­ sibility of the structural engineer. He must obviously make sure that his structure will not fall down. Further, it should not develop serious cracks or unsightly deflections, or vibrate alarmingly in the wind, or sink ignominiously into the ground. Good structural design calls for an understanding of the way in which structures sustain and transmit forces, and of the strength and other properties of structural materials. All structures deform under stress, and, unless they are built on rock, undergo some settlement. Cracking cannot be avoided in some materials. How much of this to allow, and how great a reserve of strength against failure there should be, inevitably affects the cost of the structure, and calls for engineering judgement. The engineer was once defined, rather unkindly, as a man who can do for one dollar what any fool can do for two. While this definition hardly does justice to the special skills of the professional engineer, it does underline the importance of the last criterion, economy, in all engineering work. Nearly all structures are built with some limit of ex­ penditure in mind, and this financial constraint has an important bear­ ing on the influence of the other criteria. Cutting the cost can endanger the structure. If, on the other hand, it has been made unnecessarily strong it may not be competitive, and the designer must try to refine the details or perhaps alter the whole structural conception, with possibly a change in material which gives the required strength and stability more cheaply. In the end the structure may be cheaper even though it weighs more: for example, reinforced concrete framed buildings of moderate height usually work out cheaper than steel-framed ones, even though they are heavier. Weight can, however, be a prime consideration. An aircraft must be capable of getting off the ground under its own power, which sets entirely new and difficult constraints on the design; and if it 4 Introducing Structures is to operate economically the ratio of the payload to its own weight must be as high as possible. Not surprisingly, aircraft design is easily the most precise and refined in the whole field of structures, and it has had a great if sometimes belated influence on the design of the building and civil engineering structures with which we shall be mainly concerned in this book. The true cost of a structure, of course, is not simply the cost of design­ ing and building it, but this first cost plus the capitalized cost of main­ taining it in good condition during its planned life. But designers — and clients —may not always care to face up to this, and prefer to opt instead for the choice that costs least to build, taking the biblical view that suffi­ cient unto the day is the evil thereof. Cost is linked not only with firm­ ness or safety but with the criterion of delight or good appearance, the visual impact which the structure makes on people. Canterbury Cathedral, regarded simply as a means of protecting worshippers from the weather, could have been built much more cheaply; but then it would probably not have had an atmosphere appropriate to worship, or such power to awe and entrance the beholder. The larger the structure, the more difficult it becomes to give it a particular appearance indepen­ dent of structural requirements; in fact, one of the remarkable features of Gothic churches is the success with which the demands both of aesthetics and strength were met in unprecedentedly large buildings made of such a difficult and limited material as stone, at a time when structural science was non-existent. In the greatest of all structures, suspension bridges, the general form is always the same, because the hanging cable is the only possible structural shape for very large spans. It is just as well that the shape of a hanging cable is generally thought to be pleasing. The materials used in structures have always had a strong influence on their form and appearance. Each structural material, especially if it is a natural product like stone or timber, has peculiarities and limita­ tions which determine structural form to an extent not often realized by the layman. We start our study of structures, therefore, by looking at the principal characteristics of building materials which have a bearing on strength and stability, in particular the properties of stone, brick, concrete, timber, iron, and steel. Introduction 5 We then introduce the reader to the simpler kinds of structural types—cables and arches, trusses and beams —before considering how structures in general collapse and how their strength can be calculated. The flow of stress through material of various shapes, and how different stress conditions affect strength, are then discussed, and there is an in­ troduction to the important and complicated question of flexural in­ stability or buckling, which has caused so many structural failures. We then look at some more recent structural developments such as prestressing, shell and cable structures, and the frameworks of modern buildings. Structural engineering is a technical skill which, like extractive metallurgy, was well developed as a practical art long before the underlying sciences were understood. Although the theory of structures is only about two centuries old, very remarkable structural feats had been achieved at least as long ago as 3000 B.C. Building, in fact, is one of the traditional crafts, in which practice at any time is strongly in­ fluenced by that of the past —sometimes the distant past. In an amusing poem of Kipling's, one of the Pharaohs suddenly appears on a modern building site—and finds everything very familiar I Within living memory, at least, there was much more than a grain of truth in this; methods of making bricks and building in brick, for example, had until recently hardly changed for many hundreds of years. In a study of struc­ tures their historical development is instructive as well as interesting, and throughout the discussion we will try to convey something of this—how man has made use of whatever materials were to hand, and developed new ones with better properties; how he has taken risks in design and construction in the absence of theory to guide him, and learned painfully from his mistakes; and how, gradually, he has evolved new structural systems to meet fresh and more exacting requirements. The structural designer's art is a highly skilled and difficult one. He must, as we have said, possess a grasp of structural action—how loads and forces are supported by and transmitted through structures, what stresses and deformations are set up, and how failure in one of its many manifestations finally occurs. But the designer is not merely an analyst: he is first and foremost a creator of structures. In Chapter 11 we try to convey an idea of the process of design, of the forces on structures, how the designer assembles his structure in imagination, how he ensures its