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This took the form of trees, which had fallen over naturally over sources of water like rivers and lakes.
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As time progressed, planks were then used. They were placed between stepping stones and were tied together.
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Slabs of granite and stones were placed over streams. If the river was too wide then they placed a pile of rocks in the centre for support. A slab would be laid to the pile of stone and another piece from the support to the opposite sides,
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Romans invented the viaduct, a road supported on a number of arches made from stone.
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Beams or girders of reinforced concrete are used.The simplest design is not supported at either sides. It has framework that is strengthened with metal supports or trusses as railings. It was originally made from steel was used. Today it is made from concreted and reinforced with steel.
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These are bridges with the longest spans. Majority of the support derives from a pair of huge cables that are draped above the gap by pillars at either ends.
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This type of bridge is only supported at one end, The other side hangs freely in mid air. It has two cantilever arms reaching into the centre from the piers at the sides.
