Bridges are used all over the world to connect places that would otherwise be inaccessible to each other. Some of the world's most famous bridges are symbols of the cities in which they are found, such as the Golden Gate Bridge in San Francisco, Sydney Harbor Bridge in Sydney, Australia, and the Tower Bridge of London, England.
With the most basic type of construction, beam bridges are the most commonly seen. They are found everywhere, ranging from standard bridges on highways to the most recognisable beam bridge, the Tower of London Bridge. Beam bridges are usually built from concrete or steel and are essentially just beams supported by weight-bearing columns or pillars.
Suspension and Cable-Stayed Bridges
The Golden Gate Bridge is a famous example of a suspension bridge, which is a flat surface suspended from cables. The cables link each end of the bridge to the towers, holding the weight and transferring it to the towers, which in turn transfer the weight to the ground. A cable-stayed bridge uses less cable; therefore the towers are closer together than in suspension bridges.
Unsurprisingly, arch bridges have arches in them that run lengthways. These arches force the weight into the supports at either end of the bridge. Arch bridges are made of either steel or concrete, and the arch itself can be either above or below the road surface. Structurally, a bridge with an arch above the roadway functions the same way as a bridge with an arch below the roadway: the arches force the weight into the columns at either end of the bridge. Only cosmetically are they different. One of the more recognisable arch bridges is the Ponte di Rialto in Venice, Italy.
A "cantilever" is a structure that is supported on one end and carries a load on another end, typically forming an extended X. Consequently, these bridges are very distinctive in appearance. Cantilever bridges depend on counterweights to function: the bridge is supported by two projecting beams or trusses that are supported on piers and are joined in the centre by a connecting member, which encompasses the road. Because these beams give only tenuous support, cantilever bridges must be anchored very well to prevent disaster. The Quebec Bridge of Canada is a well-known example of a cantilever bridge.