by J E Gordon
Structures
____________________
or Why things don’t fall down
Structures
____________________
or Why things don’t fall down
J.E.Gordon
University of Reading
Reading, England
Copyright © 1978 by J. E. Gordon
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 written permission of the publisher. Printed in the United States of America.
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ISBN-10: 0-306-81283-5 ISBN-13: 978-0-306-81283-5
eBook ISBN: 9780786730353
Second Da Capo Press edition 2003
First Da Capo Press edition 1981
This Da Capo Press paperback edition of Structures: or Why Things Don’t Fall Down is an unabridged republication of the first edition published in London in 1978. It is reprinted by arrangement with Penguin Books Ltd.
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To my grandchildren,
Timothy and Alexander
Among the innumerable mortifications which waylay human arrogance on every side may well be reckoned our ignorance of the most common objects and effects, a defect of which we become more sensible by every attempt to supply it. Vulgar and inactive minds confound familiarity with knowledge and conceive themselves informed of the whole nature of things when they are shown their form or told their use; but the speculatist, who is not content with superficial views, harasses himself with fruitless curiosity, and still, as he inquires more, perceives only that he knows less.
Samuel Johnson, The Idler (Saturday, 25 November 1758)
List of Plates
1 Bent masonry column in Salisbury Cathedral.
2 Stress concentration at crack tip (courtesy Dr Richard Chaplin).
3 ‘Aneurism’ in cylindrical balloon.
4 Section of artery wall tissue (courtesy Dr Julian Vincent).
5 Corbelled vault at Tiryns.
6 Semi-corbelled postern gate at Tiryns.
7 Clare bridge, Cambridge (courtesy Professor. Adrian Horridge, F.R.S.).
8 Temple of the Olympian Zeus, Athens,
9 Skeletons of gibbon and gorilla.
10 Maidenhead railway bridge.
11 Menai suspension bridge (courtesy Institution of Civil Engineers).
12 Severn suspension bridge (courtesy British Steel Corporation).
13 King’s College Chapel, Cambridge.
14 H.M.S. Victory- (courtesy H.M.S. Victory Museum. Crown copyright).
15 American trestle bridge.
16 Britannia bridge (courtesy Institution of Civil Engineers).
17 and 18 Viomiet dresses (courtesy Mrs Nethercot and Vogue magazine).
19 Wagner tension field (courtesy The Fairey Company Ltd).
20 Tacoma Narrows bridge (courtesy Institution of Civil Engineers).
21 Portsmouth block-making machinery (Crown copyright. Science Museum, London).
22 Watson steam yacht (courtesy G. L. Watson & Co. Ltd).
23 The Parthenon.
24 The Lion Gate, Mycenae.
Foreword
I am very much aware that it is an act of extreme rashness to attempt to write an elementary book about structures. Indeed it is only when the subject is stripped of its mathematics that one begins to realize how difficult it is to pin down and describe those structural concepts which are often called ‘elementary’; by which I suppose we mean ‘basic’ or ‘fundamental’. Some of the omissions and oversimplifications are intentional but no doubt some of them are due to my own brute ignorance and lack of understanding of the subject.
Although this volume is more or less a sequel to The New Science of Strong Materials it can be read as an entirely separate book in its own right. For this reason a certain amount of repetition has been unavoidable in the earlier chapters.
I have to thank a great many people for factual information, suggestions and for stimulating and sometimes heated discussions. Among the living, my colleagues at Reading University have been generous with help, notably Professor W. D. Biggs (Professor of Building Technology), Dr Richard Chaplin, Dr Giorgio Jeronimidis, Dr Julian Vincent and Dr Henry Blyth; Professor Anthony Flew, Professor of Philosophy, made useful suggestions about the last chapter. I am also grateful to Mr John Bartlett, Consultant Neurosurgeon at the Brook Hospital. Professor T. P. Hughes of the University of the West Indies has been helpful about rockets and many other things besides. My secretary, Mrs Jean Collins, was a great help in times of trouble. Mrs Nethercot of Vogue was kind to me about dressmaking. Mr Gerald Leach and also many of the editorial staff of Penguins have exercised their accustomed patience and helpfulness.
Among the dead, I owe a great deal to Dr Mark Pryor – lately of Trinity College, Cambridge – especially for discussions about biomechanics which extended over a period of nearly thirty years. Lastly, for reasons which must surely be obvious, I owe a humble oblation to Herodotus, once a citizen of Halicarnassus.
Acknowledgements
We acknowledge with gratitude permission to quote from various authors. For Douglas English’s poem, Punch Publications Ltd; for quotations from Weston Martyr’s The Southseaman, Messrs. William Blackwood Ltd; for the quotation from Rudyard Kipling’s The Ship that Found Herself, Messrs. A. P. Watt & Son and the executors of the late Mrs Bambridge and the Macmillan Co. of London and Basingstoke. Also to Mr H. L. Cox for the quotation from his book The Design of Structures of Least Weight. The quotations from the New English Bible (Second Edition © 1970) are by kind permission of the Oxford and Cambridge University Presses.
We are also most grateful to all those named in the List of Plates who have so kindly provided illustrations and given permission to reproduce them.
We have received a great deal of help from many people and organizations with regard to both quotations and illustrations. If we have, in any instance, failed to make proper acknowledgement, we offer our apologies.
Chapter 1 The structures In our lives
-or how to communicate with engineers
As men journeyed in the east, they came upon a plain in the land of Shinar and settled there. They said to one another, ‘ Come, let us make bricks and bake them hard’; they used bricks for stones and bitumen for mortar. ‘ Come, ’ they said, ‘ let us build ourselves a city and a tower with its top in the heavens, and make a name for ourselves; or we shall be dispersed all over the earth.’ Then the Lord came down to see the city and tower which mortal men had built, and he said, ‘Here they are, one people with a single language, and now they have started to do this; henceforward nothing they have a mind to do will be beyond their reach. Come, let us go down there and confuse their speech, so that they will not understand what they say to one another.’ So the Lord dispersed them from there all over the earth, and they left off building the city. That is why it is called Babel (that is, Babylon), because the Lord there made a babble of the language of all the world.
Genesis 11.2–9 (New English
Bible)
A structure has been defined as ‘any assemblage of materials which is intended to sustain loads’, and the study of structures is one of the traditional branches of science. If an engineering structure breaks, people are likely to get killed, and so engineers do well to investigate the behaviour of structures with circumspection. But, unfortunately, when they come to tell other people about their subject, something goes badly wrong, for they talk in a strange language, and some of us are left with the conviction that the study of structures and the way in which they carry loads is incomprehensible, irrelevant and very boring indeed.
Yet structures are involved in our lives in so many ways that we cannot really afford to ignore them: after all, every plant and animal and nearly all of the works of man have to sustain greater or less mechanical forces without breaking, and so practically everything is a structure of one kind or another. When we talk about structures we shall have to ask, not only why buildings and bridges fall down and why machinery and aeroplanes sometimes break, but also how worms came to be the shape they are and why a bat can fly into a rose-bush without tearing its wings. How do our tendons work? Why do we get ‘lumbago’? How were pterodactyls able to weigh so little? Why do birds have feathers? How do our arteries work? What can we do for crippled children? Why are sailing ships rigged in the way they are? Why did the bow of Odysseus have to be so hard to string? Why did the ancients take the wheels off their chariots at night? How did a Greek catapult work? Why is a reed shaken by the wind and why is the Parthenon so beautiful? Can engineers learn from natural structures? What can doctors and biologists and artists and archaeologists learn from engineers?
As it has turned out, the struggle to understand the real reasons why structures work and why things break has been a great deal more difficult and has taken much longer than one might have expected. It is really only quite recently that we have been able to fill in enough of the gaps in our knowledge to answer some of these questions in any very useful or intelligent manner. Naturally, as more of the bits of the jig-saw puzzle are assembled, the general picture becomes clearer: the whole subject is becoming less a study for rather narrow specialists and more one which the ordinary person can find rewarding and relevant to a wide range of general interests.
This book is about modern views on the structural element in Nature, in technology and in everyday life. We shall discuss the ways in which the need to be strong and to support various necessary loads has influenced the development of all sorts of creatures and devices – including man.
The living structure
Biological structures came into being long before artificial ones. Before there was life in the world, there was no such thing as a purposive structure of any kind – only mountains and heaps of sand and rock. Even a very simple and primitive kind of life is a delicately balanced, self-perpetuating chemical reaction which needs to be separated and guarded from non-life. Nature having invented life – and with it individualism – it became necessary to devise some kind of container in which to keep it. This film or membrane had to have at least a minimum of mechanical strength, both to contain the living matter and also to give it some protection from outside forces.
If, as seems possible, some of the earliest forms of life consisted of tiny droplets floating in water, then a very weak and simple barrier, perhaps no more than the surface tension which exists at the interfaces between different liquids, may have sufficed. Gradually, as living creatures multiplied, life became more competitive, and the weak, globular and immobile animals were at a disadvantage. Skins became tougher and various means of locomotion were evolved. Larger, multicellular animals appeared which could bite and could swim fast. Survival became a matter of chasing and being chased, eating and being eaten. Aristotle called this allelophagia – a mutual eating – Darwin called it natural selection. In any case, progress in evolution was dependent upon the development of stronger biological materials and more ingenious living structures.
The earlier and more primitive animals were mostly made from soft materials because they not only make it much easier to wriggle and extend oneself in various ways, but soft tissues are usually tough (as we shall see), while rigid ones like bone are often brittle. Furthermore, the use of rigid materials imposes all kinds of difficulties in connection with growth and reproduction. As women know, the business of giving birth involves an engineering of high strains and large deflections. All the same, the development of the vertebrate foetus from conception onwards, like that of natural structures in general, is in certain respects from soft to hard, and the hardening process goes on after the baby has emerged.
One gets the impression that Nature has accepted the use of stiff materials rather reluctantly, but, as animals got bigger and came out of the water on to the land, most of them developed and exploited rigid skeletons, teeth and sometimes horns and armour. Yet animals never became predominantly rigid devices like most modern machinery. The skeleton usually remained but a small part of the whole, and, as we shall see, the soft parts were frequently used in clever ways to limit the loads upon the skeleton and thus to protect it from the consequences of its brittleness.
While the bodies of most animals are made preponderantly from flexible materials, this is not always true for plants. The smaller and more primitive plants are usually soft, but a plant cannot chase its food, nor can it run away from an enemy. It can, however, protect itself to some extent by growing tall, and, by doing so, it may also be able to get more than its fair share of sun and rain. Trees, in particular, seem to be extraordinarily clever at stretching out to collect the diffuse and fitful energy of sunlight and at the same time standing up to being bullied by the wind -and all in the most cost-effective way. The tallest trees reach a height of about 360 feet or 110 metres, being by far the largest and most durable of living structures. For a plant to reach even a tenth of this height, however, its main structure needs to be both light and rigid; we shall see later that it incorporates a number of important lessons for engineers.
It may seem obvious that questions like these about strength and flexibility and toughness are relevant in medicine and in zoology and botany, yet for a long time both doctors and biologists resisted all such ideas with considerable success and with the whole force of their emotions. Of course, it is partly a matter of temperament and partly a matter of language, and perhaps a dislike and fear of the mathematical concepts of the engineer may have had something to do with the matter. Too often biologists simply cannot bring themselves to make a sufficiently serious study of the structural aspects of their problems. Yet there can be no reason to assume that,, while Nature uses methods of infinite subtlety in her chemistry and her control mechanisms, her structural approach should be a crude one.
The technological structure
Wonders there are many, but there is no wonder
Wilder than man –
Man who makes the winds of winter bear him,
Through the trough of waves that tower about him,
Across grey wastes of sea;
Man who wearies the Untiring, the Immortal–
Earth, eldest of the Gods, as year by year,
His plough teams come and go.
The care-free bands of birds,
Beasts of the wild, tribes of the sea.
In netted toils he takes.
The Subtle One.
Sophocles, Antigone (440 B.C.; translated by F. L. Lucas)
Benjamin Franklin (1706–90) used to define man as ‘a tool-making animal’. In fact a good many other animals make and use rather primitive tools, and of course they quite often make better houses than do many uncivilized men. It might not be very easy to point out the exact moment in the development of man at which his technology could be said noticeably to surpass that of the beasts that perish. Perhaps it was later than we think, especially if the early men were arboreal.
However this may be, the gap both in time and in technical achievement between the sticks and stones of the earlies
t men – which were not much better than the tools used by the higher animals – and the sophisticated and beautiful artefacts of the late Stone Age is an immense one. Pre-metallic cultures have survived in remote places until only yesterday and many of their devices can be seen and admired in museums. To make strong structures without the benefit of metals requires an instinct for the distribution and direction of stresses which is by no means always possessed by modern engineers; for the use of metals, which are so conveniently tough and uniform, has taken some of the intuition and also some of the thinking out of engineering. Since the invention of Fibreglass and other artificial composite materials we have been returning at times to the sort of fibrous non-metallic structures which were developed by the Polynesians and the Eskimoes. As a result we have become more aware of our own inadequacies in visualizing stress systems and, just possibly, more respectful of primitive technologies.
As a matter of fact the introduction of the technological metals to the civilized world – probably between 2,000 and 1,000 B.C. – did not make a very large or immediate difference to most artificial structures, because metals were scarce, expensive and not very easy to shape. The use of metals for cutting tools and weapons and, to some extent, for armour had its effect, but the majority of load-bearing artefacts continued to be made from masonry and from timber and leather and rope and textiles.
Using the old mixed constructions, the millwright and the coachbuilder, the shipwright and the rigger, needed a very high degree of skill, though of course they had their blind spots and they made the sort of mistakes one might expect from men without a formal analytical training. On the whole, the introduction of steam and machinery resulted in a dilution of skills, and it also limited the range of materials in general use in ‘advanced technology’ to a few standardized, rigid substances such as steel and concrete.