Bridges – an Introduction

13th July 2001

Gleaming serenely in the sunshine, this bridge across the Severn estuary looks as though nothing could move it.  But walk across the span, and it is another story.  Every passing vehicle shakes it up and down, making waves that can travel at hundreds of kilometres per hour.  Luckily the bridge is designed to damp them out, just as it is designed to ignore the efforts of the wind to turn it into a giant harp.  A bridge is not a dead mass of metal and concrete: it has a life of its own, and understanding its movements is as important as understanding the static forces.

To design a bridge like this you need to take into account the many forces acting on it – the pull of the earth on every part, the ground pushing up the supports, the resistance of the ground to the pull of the cables, and the weight of every vehicle.  Then there is the drag and lift produced by the wind, the turbulence as the air rushes past the towers, the forces produced by the second highest tides in the world, and even the forces produced by the movement of the traffic – all that engine power has to go somewhere, and it isn’t entirely into overcoming air resistance.   What happens for example, if there is an emergency on the bridge, and all the vehicles on one side stop suddenly.  On a mile of bridge, there could be a lot of momentum to be absorbed.

On a railway bridge, the forces produced by powerful modern locomotives can be far larger than those foreseen when the bridges were built.  This is no fault of the designers, who built many bridges in the age of steam, and could not know that alternating current could be supplied above the track, thus releasing the engines from carrying fuel and water.

These web-pages are not intended to replace a book: indeed, one page provides a list of good books about engineering.  Nor are the pages intended to provide all the answers: again, links to good sites are provided.  Like any other subject, bridges are more fascinating the deeper you go into the subject.

The pages also try to relate bridges to other structures, and to show that even quite humble buildings can be interesting for both the builder and the onlooker.  The world we live in is dominated by a larger number of small buildings and a small number of large ones. The effect of any one building on a town may be small, but the sum of all the buildings and street equipment makes our surroundings, and all are potentially important to the world we inhabit.

Some of the bridges shown in this web-site are quite small, but nevertheless very interesting, because they are elegant solutions to a problem or just because they look good.

Fact – Any structure or mechanism has to obey the laws of physics.

Fiction – Therefore design is just a matter of finding the right formula and plugging in the values, and is therefore rather boring.

The laws and rules of games such as chess, cricket and football are pretty uninteresting, and if they were shown to people from another planet, they would give no clues to the wealth of opportunities they offer to players with skill and imagination.  The conduct of a game can be dull or inspired, honest or dishonest, attacking or defensive.

You can study the rules – in fact you need to know them thoroughly – but they are not enough.  You need to be able to create.  Conversely, a determination to be original, come what may, is not a guarantee of success.

In a game you can break the rules if the referee or umpire is not looking.  In engineering, if you break the rules, your product will not work: not for long, at any rate.  As Rudyard Kipling wrote in a poem; if you disobey the laws, you die.  Or someone else dies.

An extreme example – a footbridge 30 metres above the ground, half a metre wide, with no handrails.  Technically it would work, but few people would use it.  That’s the other consideration – the end result of all technology has to be usable by people.