Bridge design

We will begin by looking at the three categories of bridge design that solve the problem of carrying a load across a gap. Theoretical principles and suitable building materials will be discussed. Within each category, we will see how the basic type has been developed or 'hybridised' over the centuries to give diversity in style and function. Finally, we will look at current trends in bridge design and try to identify some likely directions that bridge building could take in the future.










The Golden Gate bridge, San Fransisco

The beam bridge

This is the earliest and simplest kind of bridge: a fallen tree lying over a stream, perhaps, or a log lodged across a narrow chasm? We still come across these when walking in the hills, and there is no way of dating the first time some hominid straightened or stabilised such a 'beam' and thereby became the world's first bridge engineer.

The design

The design is as simple as a single rigid 'beam', resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade.

nantua Viaduct From The Autoroute A40, franco

The beam need not be of any particular shape and there are no other elements besides the piers to help dissipate the load. Hence the piers take the full weight of the load and are said to be in 'compression'. This means that they are being squashed by the forces at the top and bottom, and must be built from materials that can resist such forces without