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Alcántara Bridge, a Roman arch bridge in Spain

A bridge is a structure designed to span an obstacle, such as a river or valley, allowing vehicles, pedestrians, and other loads to pass across. Simple bridge structures include beam bridges made from logs, and suspension bridges made of ropes or vines. The Romans and ancient Chinese built major arch bridges (example pictured) of timber, stone, and brick. The longest spans use suspension or cable-stayed designs. Bridge design is an important discipline within the field of civil engineering. A bridge must be strong enough to support its own weight as well as the weight of the traffic passing over it. It must also tolerate violent, unpredictable stresses imposed by the environment, including winds, floods, and earthquakes. Many bridges are admired for their beauty, and some spectacular bridges serve as iconic landmarks that provide a sense of pride and identity for the local community. In art and literature, bridges are frequently used as metaphors to represent connection or transition. (Full article...)