163 pages

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Bridge watching , livre ebook

Intellect Books - Edmund W. Jupp

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163 pages

English

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Extrait

Description

The aim of the Watching series is to draw attention to some of the very interesting items around us, things that perhaps we don't notice as much as we might. The first was Bridge Watching, and when this was put ''on the Net'' it produced, to the surprise of the author, such a pleasant flood of e-mail that another was written, called Water Watching. This, too, was kindly received. So it was tempting to continue with the theme.Wherever we go we seem to meet bridges. Mostly we tend to use them almost without noticing them, except when we see a particularly striking example like the suspension bridge over the river Tamar in Devon. There is no attempt to cover everything about bridges, just enough to make a bridge a more interesting object for you, or your camera, or your paint-box. I do hope it will help you to enjoy bridges, wherever you see them. They are such nice comfortable things to watch, especially when you know something about them. As either a hobby or an intellectual pursuit bridge-watching has much to commend it, for people of all ages and persuasions. You don't have to pay a subscription. You can enjoy it on your own or in company, and weather is relatively unimportant. It doesn't need any special clothing or equipment. (If you like, you can use field glasses or cameras, and note-books; but they aren't essential). You need no training, no practice, no coaching. From all angles, bridge-watching is an attractive pastime, all over the world. Go out and enjoy these fascinating structures. You may find them addictive, in the nicest possible way.

Sujets

Arts visuels

Architecture

Cultural Studies

General

General officer

Informations

Publié par	Intellect Books
Date de parution	01 janvier 2003
Nombre de lectures	0
EAN13	9781841508047
Langue	English
Poids de l'ouvrage	6 Mo

Informations légales : prix de location à la page 0,0600€. Cette information est donnée uniquement à titre indicatif conformément à la législation en vigueur.

Extrait

Bridge Watching

Edmund W. Jupp

intellect

Bridge Watching

Edmund W. Jupp

TM intellect Bristol, UK Portland OR, USA

First Published in Paperback in UK in 2002 by Intellect Books, PO Box 862, Bristol BS99 1DE, UK

First Published in USA in 2000 by Intellect Books, ISBS, 5804 N.E. Hassalo St, Portland, Oregon 972133644, USA

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 written permission.

Consulting Editor: Production and Cover Design: Production Assistant

Masoud Yazdani Peter Singh Vishal Panjwani

A catalogue record for this book is available from the British Library

ISBN 1841508047

Printed and bound in Great Britain by Cromwell Press, Wiltshire

Contents Preface iv Some Basics Forces and Elasticity Stifness and Strength Kinds of Bridges Beam Bridges Arch Bridges Suspension Bridges Pontoon Bridges Bending Ties 40 Struts Beam Bridges Arch Bridges Suspension Bridges Pontoon Bridges Materials Appendix Glossary Diagrams Photos

iii

1 7 13 20 20 23 25 27 30

47 30 64 74 85 93 100 112 122 146

Preface

Wherever we go we seem to meet bridges. Mostly we tend to use them almost without noticing them, except when we see a particularly striking example like the suspension bridge over the river Tamar in Devon. Yet there is much that is worth seeing in even the humblest specimen if we know where to look and, perhaps, how to look.

So the aim of this book is to help you to enjoy looking at bridges, by explaining in simple language some features of their design and construction. Although the technical aspects will be treated gently, and the mathematical content will be such as not to frighten the non-mathematician, maybe the serious student, too, will find something of interest here.

It is particularly difficult to steer a course between being unduly and patronisingly detailed on the one hand and skipping too lightly over complex matters on the other. If I have erred in leaning too much one way here and there, if you find some of this rather repetitive, I do hope you will bear in mind that bridge-watchers come in all sizes and great variety.

There is no attempt to covereverythingabout bridges, just enough to make a bridge a more interesting object for you, or your camera, or your paint-box. I do hope it will help you to enjoy bridges, wherever you see them. They are such nice comfortable things to watch, especially when you know something about them.

As either a hobby or an intellectual pursuit bridge-watching has much to commend it, for people of all ages and persuasions. You don't have to pay a subscription. You can enjoy it on your own or in company, and weather is relatively unimportant. It doesn't need any special clothing or equipment. (If you like, you can use field glasses or cameras, and note-books; but they aren't essential). You need no training, no practice, no coaching. From all angles, bridge-watching is an attractive pastime, all over the world.

Go out and enjoy these fascinating structures. You may find them addictive, in the nicest possible way. I do hope so.

E.W.

Some Basics

This is not a text-book, nor a highly technical treatise on bridges, though we shall try to cover the essentials, a minimum of what a bridge-watcher should know. Since bridge-watchers are to be found among people of all ages we aim to steer a delicate course to satisfy everyone from schoolboys to pensioners. We cannot deal with everything, though some aspects will be mentioned more than once where it seems appropriate.

In the pursuit of simplification, it may be that we shall treat some aspects rather sketchily, but not so as to introduce serious inaccuracy. For the reader who would like to go more deeply into the subject there are many excellent books dealing with more advanced work.

Many bridges were designed in the days when the Imperial system of units was almost universal; but British engineers used other systems when they built for some places overseas.

So they were in general quite happy to work in any system. Many modern bridges are designed using the metric system. We shall try to avoid using designations of units for forces, weights, lengths and so on, in order to minimise confusion.

The word "bridge" can refer to a card game, false teeth, part of a violin, a rest for a snooker cue, a part of your nose, the middle bit of a pair of spectacles, and a good many other things, too. We shall be thinking only about the structures that carry people, goods and vehicles over roads, canals, rivers, railways, or valleys.

People who understand something about bridges like to go about and look at them. Bridge-watchers sometimes make notes about the various bridges they see; they list them, photograph them, draw them and paint them. They get interested in the details, and the more they know about them the more they find of interest in every bridge they see. Some basic knowledge is essential to get full enjoyment, so in this first chapter we shall look at some of those fundamentals.

Every time a bridge is needed there is much grave chin-stroking. It costs a lot of money to build, and everyone wants to be sure about many things before picking the best plan. Once built, a bridge is there for a long time, and an unfortunate error of judgement is on view for all to see.

Much depends upon where the bridge is to be built, how much load it will have to carry, and so on. Will it cross a river, and have to stand up to rushing water and perhaps flooding? Will it be wide enough for the traffic? Are there approach roads to be built? Will it be high enough for ships to pass below? Will people like the look of it? Most important of all, will it be safe under all conditions?

There have been some terrifying failures in the past; and even today there are occasions when things can go wrong. So it sometimes takes a long time to come to a decision. 1

Bridge Watching

A very large bridge is so expensive that there has to be a whole series of meetings, for people to talk about it.

It can go something like this. Various committees have to meet and argue the good and bad points of the different ways in which the bridge might be built, and where.

When everyone agrees, a decision is made, and then they get someone to draw up a "specification", which describes what is needed. This specification is published, so that people who wish to build the bridge can say how they would do it, what it would cost, and so on.

Engineers are trained to solve problems, and their ethical training is towards providing their clients with the most economical solution. Designers of bridges are specialists in that field. They have built many bridges, solved many problems, and have wide experience. So when they have read the specification some preliminary ideas are sketched and discussed and then, drawing upon their experience they set about the calculations, designing and drawing the bridge.

Several firms may prepare designs, and they all offer their solutions to the client.

These tenders are then carefully considered (more committees!) and eventually one is selected.

The firm which has been awarded the contract then allocates the work to contractors who do the actual construction, under the supervision of the designers, sometimes the contractors themselves. The contractors appoint sub-contractors to do the work; and then there is the ordering of the materials, a complicated task in itself. It doesn't always happen like this, but this gives the general idea.

The delivery dates have to be so arranged that everything arrives on the site at the right time as far as possible. Quantities are very important, as the lack of cement, say, at some stage of the work may hold up the whole undertaking, and so add to the cost, for delay is expensive.

So it sometimes takes what might seem a long time, after the design is completed, before things get going. The design itself is the starting point for a long series of events.

Although the first thing is to make sure that the bridge will be strong enough allowing for all expected cases of loading, the engineers think about the looks of the structure, too. Luckily, there are some things that look good anyway, when they are properly designed. A bridge is one of those things.

A bridge design that didn't use material properly would look skinny and wobbly, or fat and clumsy. It would grate on your eyeballs; but a well-designed bridge is a lovely thing, a pleasure to see. Most of them are indeed beautiful. Occasionally you may see one which makes some people shudder. Perhaps they'll make you wince, too; but it

Bridge Watching

won't be often. An ugly bridge, rare though it is, is an obtrusive blotch on the environment, and perhaps sufficient local protest might persuade the authorities to do something about it.

There are some interesting primitive ones, well worth more than a passing glance, in far countries, where there aren't many modern industrial facilities. They are fewer nowadays, but you can find pictures of flimsy-looking but strong foot-bridges in old books about explorers.

To a keen eye they tell a story of clever use of the materials to hand, and are charming.

When you understand bridges they will give you much pleasure all your life. You find them wherever there is a need to cross a track or road or water. They carry people, wheelbarrows, trains, camels, cars, pack-horses, or even liquids in pipes or channels. Although some look pretty much the same as others there are many differences if you know where to look.

Bridges differ in type, material, age, decorative features, and so on. They are always interesting to a bridge-watcher, because they stand still, and you can see them from underneath, from the sides, and on top. You can study their methods of construction, and see how they look in different lights, and weather conditions. Once you get into the swing of bridge-watching you will never pass a bridge without examining it.

A bridge in bright sunshine looks very different from when it is under snow, or in pouring rain; and you can learn a lot about how it deals with heavy rain or storm water, both on and under it. If it gets very hot and very cold at the site you can often see how the designer has allowed for changes in dimensions due to this.

Lighting, too, is important. Lighting systems are costly, and we all want best value for money. You will see many solutions to this problem. Mostly it is pretty well standardised these days, as the factories turn out lamps and posts in large numbers to keep costs low; but now and then you might be lucky enough to spot some original lighting schemes.

The Victorians had some wonderful ideas. They built strongly and boldly, so that it would cost more than the councils can afford today to replace, with skinny modern stalks, those lovely monuments to the age of cast iron.

I doubt if you will come across many oil lamps now, but the soft glow of gas lamps can still be seen here and there. They are less efficient than some of the garish modern lighting and it is doubtful if they will last much longer, but they did have a charm of their own.

Perhaps the best thing of all about bridges is that however long you look bridge-watching is free. There aren't many things like that about nowadays.

Bridge Watching

The earliest bridges weren't built by man, but by nature. A tree would fall across a stream, and then people and animals would scramble across. Occasionally, too, some long trailing creeper might be blown across a gap and become tangled in plants on the other side; and there you would have a primitive suspension bridge.

At times a small crack in a rock would allow a trickle of water to squeeze through. In time, this would carve itself a tunnel through rock; and the arch so made would become a stone bridge over the stream.

So it was that from these kinds of beginnings came the fascinating structures of today, linking places with footpaths, roads, railways, and so on.

Perhaps the strangest form of bridge is that used by some ants. They make a crossing for themselves by clinging to one another in a chain, letting the other ants walk over them.

Over the centuries bridge construction has changed. Wages are higher now, and materials are dearer, too. So bridge builders need to be much more careful about how much wood, steel or stone is used, and how many men and machines are to be involved. Handling materials in the old days was a relatively quiet non-polluting matter of horses and carts, but now the Diesel engine has taken over, solving some problems and creating others.

We know much more about the best way to use men and materials now, so we can choose carefully to get a really good-looking structure. There are a few not-so-lovely bridges about, but for the most part they are very comely. Luckily, badly-designed bridges tend to fall down in time.

Starting with the specification, engineers need to put in a lot of mathematical work. This at one time was a branch called "statics", dealing with stationary things; but bridge parts do move; they sway in the wind, they give and vibrate. So statics alone won't solve all the problems. Nowadays the bridge designer's mathematical "tool-kit" is new software for the computer.

Fortunately, bridge-watchers don't need to understand all the advanced mathematical processes that go into the making of a bridge. We can just enjoy the results. Still, as we stare at the shapes and the separate parts it is interesting to realise how much juggling with numbers and equations lies behind it all.

Before electronic computers were invented most of the work was done with slide rules, ingenious little calculating devices that you could hold in your hand. For some very accurate work you might have to use a big one that looked like a rolling-pin. It doesn't seem very long ago, but no-one uses them now.

Bridge Watching

With the modern computer it is possible to speed up the mathematical side of the work, and produce pictures of the finished bridge to show what it will look like, long before the first orders are issued for any material.

When you see a bridge nowadays you are perhaps looking at a structure that has been seen on a computer screen before the first bit of earth was dug out for the foundations.

We shall try to avoid technical jargon in these chapters, but we may have to use a little here and there; and in any case a little knowledge of some technical expressions is valuable for the bridge-watcher, helping to bring to life some important features of the art and science of this noble contribution to a nation's assets.

Some words have different meanings when used as technical terms, so we must make clear these differences. It is important to understand the engineer's special use of some words, and in particular the words stress and strain. In everyday use they mean pretty much the same thing. To an engineer, however, they are quite separate in meaning.

When a force is applied to a piece of material we say it produces a stress, or discomfort, and the stress is the cause of a strain, or distortion. So force, stress and strain are linked in this way.

These words, then, require our attention in this first chapter, to "clear the air". Most other terms that we shall meet will be explained when we come to them.

A force produces a large stress if it is applied over a small area, but only a small stress when it is spread over a large area. This is why it is easier to bang a pointed nail into a piece of wood than it is to make a blunt nail go in. A pointed nail concentrates the force to produce a greater stress at the point of application.

In the same way, a blunt knife doesn't cut easily, because the force on the knife is taken by a large area of the material, and thus the stress is less.

Mathematically, stress is the force divided by the area, and we shall say more about this in a later chapter.

Strain is a measure of distortion. It tells us how much the material is deformed by a stress. If a piece of wire is pulled by a force which stretches it so that its length increases by, say, a tenth, then we say that the strain is one tenth. If the length doubles the strain is one.

Materials used in bridges don't deform to that extent, of course. So the strains with which we deal are comparatively tiny. When we speak of a strain of, say, 0.001 we mean that the material has changed by one thousandth of its original dimension.

At the end of this book is a glossary for reference should you come across a word which you don't understand. In this glossary the explanations of some words have been 5