Bridges

Bridges are structures that join two otherwise inaccessible points of land, such as the two shores of a river or lake, or the two sides of a canyon or deep gully. Bridges are designed to carry railroad cars, motor vehicle traffic, or foot travel by pedestrians and/or animals, or to support pipes, troughs, or other conduits used for the movement of goods and materials, such as an oil pipeline or a water aqueduct.

Humans have been constructing bridges since ancient times. The earliest bridges were probably nothing more than felled trees used to cross rivers or ditches. As civilization advanced, artisans discovered ways to use stone, rock, mortar, and other naturally occurring materials in the construction of longer and stronger bridges. Finally, as physicists and engineers began to develop the principles underlying bridge construction, they incorporated other materials such asiron, steel, and aluminum into the bridges they built.

Bridges can be classified in a number of different ways, according to their intended use (railroad bridge or pedestrian walkway, for example), according to the material of which they are made (steel, wood, or concrete for example), or according to whether they are fixed or moveable. Moveable bridges are used when the height of ships traveling on a waterway will be greater than the floor of the bridge. In such cases, the bridge is built so that the roadway can be raised or pivoted to allow marine traffic to pass under or through it. Probably the most useful way to classify bridges for technical purposes, however, is according to their structural form. There are three major types of bridges: arch, cantilever, and suspension.