I INTRODUCTION
Bridge (structure), structure designed to provide continuous passage over an obstacle. Bridges commonly carry highways, railroad lines, and pathways over obstacles such as waterways, deep valleys, and other transportation routes. Bridges may also carry water, support power cables, or house telecommunications lines.
Some special types of bridges are defined according to function. An overpass allows one transportation route, such as a highway or railroad line, to cross over another without traffic interference between the two routes. The overpass elevates one route to provide clearance to traffic on the lower level. An aqueduct transports water. Aqueducts have historically been used to supply drinking water to densely populated areas. A viaduct carries a railroad or highway over a land obstruction, such as a valley.
The earliest bridges were simple structures created by spanning a gap with timber or rope. Designs became more complex as builders developed new construction methods and discovered better materials. The stone arch was the first major advance in bridge design. It was used by the ancient Greeks, Etruscans, and Chinese (see Arch and Vault). The Romans perfected arch design, using arches to build massive stone bridges throughout the Roman Empire. Stone arch construction remained the premier bridge design until the introduction of the steam locomotive in the early 19th century.
Between 1830 and 1880, as railroad building expanded throughout the world, bridge design and construction also evolved to carry these heavy vehicles over new obstacles. Designers experimented with a wide variety of bridge types and materials to meet the demand for greater heights, spans, and strength. Locomotives were heavier and moved faster than anything before, requiring stronger bridges. The basic beam bridge, a simple beam over a span, was strengthened by adding support piers underneath and by reinforcing the structure with elaborate scaffolding called a truss. During the period of railroad expansion, iron trusses replaced stone arches as the preferred design for large bridges.
In 1855 British inventor Sir Henry Bessemer developed a practical process for converting cast iron into steel (see Iron and Steel Manufacture). This process increased the availability of steel and lowered production costs considerably. The strength and lightness of steel revolutionized bridge building.