Looking at Bridges
From DT Online
Most people may be surprised at the number and variety of bridges in their local area. Bridges tend to be taken for granted unless they are particularly spectacular (e.g. the Humber Bridge or the Millau Viaduct) or they fail dramatically (e.g. Tay Rail Bridge or Tacoma Narrows).
All designers have to think of the job which needs to be done, the materials available and the shapes and forms which are possible. Each of these factors influences the others and the bridge designers' challenge is to find the right compromise.
Features and Applications
The job, or purpose, of structures like bridges and tunnels is to allow all the load placed on them safely to flow through and down to the ground. They must stay rigid and not wobble about. They must allow space for the railway, road or canal and sometimes space underneath to allow ships, trains, vehicles, or animals to pass through.
Activity: Visit bridges in your local area and do the tasks: Asking About Bridges and Collecting Pictures of Bridges to find out enough about them to catalogue them and perhaps enter them into the DT Online Structures Gallery.
The great variety of structures and styles of bridges result from the choices the designer has had to make. The use and location of the bridge has had to be taken into account and the wide range of materials available has had to be considered - materials have individual properties which make some of them more suitable than others in a particular situation.
Stone and brick, for example, are only good at carrying loads which press down on them - they are strong in compression. Stone and brick, like sticks of chalk, are not strong if they are bent - they are brittle materials. Bridge and tunnel designers, therefore, have to use structures which keep the stone and brick in compression. A suitable structure is an Arch.
The Pont du Gard in France shows what was achieved by the Romans using this simple structure.
Materials other than stone and brick make possible different ways of designing bridge type structures. There are only about four different bridge designs in total and the other three each need materials which have some bending strength and can resist being pulled apart - materials which are strong in tension. These materials tend also to be tough or resistant to shock, which is the opposite of brittle.
As new materials have become available, they have been exploited by bridge designers. Looking at bridges, old and new, can give some insight into how materials and construction methods have changed and also how the demands placed upon them have altered as industry and transport has developed.
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