bridge (engineering)

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Structure that provides a continuous path or road over water, valleys, ravines, or above other roads. The basic designs and combinations of these are based on the way they bear the weight of the structure and its load. Beam, or girder, bridges are supported at each end by the ground with the weight thrusting downwards. Cantilever bridges are a complex form of girder in which only one end is supported. Arch bridges thrust outwards and downwards at their ends. Suspension bridges use cables under tension to pull inwards against anchorages on either side of the span, so that the span hangs from the main cables by a network of vertical cables. The cable-stayed bridge relies on diagonal cables connected directly between the bridge deck and supporting towers at each end. Some bridges are too low to allow traffic to pass beneath easily, so they are designed with movable parts; swing and draw bridges are examples.

The construction of a bridge is also affected by aesthetics (the preferred or fashionable style at the time of its building) and practicalities such as the materials at hand. The appearance and construction of a bridge reflects the technology and tastes of the era in which it was built; for example, the simple, stocky Pont Neuf constructed over the River Seine in Paris in the 16th century contrasts sharply with the ornate neo-Gothic style of Tower Bridge, built in London in 1894.

History
In prehistory, people used logs or wove vines into ropes that were thrown across the obstacle. Clapper bridges, made from flat stones simply laid across or supported by piles of stones, were some of the earliest bridges. By 4000 BC arched structures of stone and/or brick were used in the Middle East, and the Romans built long, arched spans, many of which are still standing. Cast iron bridges were introduced in 1779. The Bessemer process produced steel that made it possible to build long-lived framed structures that support great weight over long spans.

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