|There are many different bridge designs, but all bridges adopt
one of three basic structural forms, even though shape, size and construction
materials can vary considerably.
An arch remains in position because it is under compression. Gravity forces the bricks or stones forming the arch rings together. However, if the arch fractures, so as to hinge, it is likely to fail quickly. Different types of arch are described in the Brick pages.
A beam bridge comprises members supported from fixed points. The beam has to be sufficiently strong to resist downward force, resulting in compression on the top edge and tension in the lower one. Most such bridges on railways are simply supported ones, where the beams span between two fixed points. However some have continuous beams with multiple supports. A cantilever bridge is one where the beams are unsupported at the ends.
A suspension bridge is one where the deck is supported from above by cables. This form of bridge is rarely used for railways. Lyne bridge, Chertsey, a cable stay structure, is an unusual exception.
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This page was created 3 January 2010