Bridges are a very important part of transportation. They are used to span things such as water, valleys or other roads. The design of a specific bridge is directly dependent on what it has to cross, where it is being built, and the materials that are being used.
The original bridges were simply trees, which by chance had fallen over small rivers or streams. However, as time went on, bridges became much more advanced, including suspension bridges, arch bridges and beam bridges.
In suspension bridges, cables are suspended from support towers, which hold up the deck, or road. Suspension bridges first started appearing in the early 19th century, with the most famous probably being the Golden Gate Bridge in San Francisco. The way that suspension bridges work is the weight of the deck is transferred to the cables, which in turn transfer the weight to the support towers which hold the whole bridge up. The advantages of a suspension bridge are that they are able to span much longer lengths than other bridge types, they require fewer materials, and they are able to withstand earthquakes much better than other bridges, which is why it was used in San Francisco. However, suspension bridges tend to have a low deck stiffness, which makes it difficult to carry heavy loads.
Arch Bridges are obviously arch shaped, and have supports, called abutments on either side. Arch bridges have been around for thousands of years, dating back to around 1300 BC. As the weight pushes down on the middle of the arch, the ends would naturally be pushed outwards. However, due to the abutments on each side, the weight is carried outward along the curve, and to the abutments. In the end, the abutments carry the weight, and keep the ends of the bridge from spreading outward. Arch bridges were originally built with stone, or brick, but now reinforced concrete or steel is used. Although arch bridges are very strong, they require a very strong foundation. Also, they usually tend to be...
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