Bridge (structure)

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.

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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. In the late 19th century and the first half of the 20th century, many large-scale steel suspension bridges were constructed over major waterways. Also in the late 19th century, engineers began to experiment with concrete reinforced with steel bars for added strength. More recently, reinforced concrete has been combined with steel girders, which are solid beams that extend across a span. When the Interstate Highway System in the United States and similar road systems in other countries were constructed in the mid- to late 20th century, the steel-and-concrete girder bridge was one of the most commonly used bridge designs. The last three decades of the 20th century saw a period of large-scale bridge building in Europe and Asia. Current research focuses on using computers, instrumentation, automation, and new materials to improve bridge design, construction, and maintenance.

II

Types of Bridges

Bridge designs differ in the way they support loads. These loads include the weight of the bridges themselves, the weight of the material used to build the bridges, and the weight and stresses of the vehicles crossing them. There are basically eight common bridge designs: beam, cantilever, arch, truss, suspension, cable-stayed, movable, and floating bridges. Combination bridges may incorporate two or more of the above designs into a bridge. Each design differs in appearance, construction methods and materials used, and overall expense. Some designs are better for long spans. Beam bridges typically span the shortest distances, while suspension and cable-stayed bridges span the greatest distances.

A

Beam Bridges

Beam bridges represent the simplest of all bridge designs. A beam bridge consists of a rigid horizontal member called a beam that is supported at both ends, either by a natural land structure, such as the banks of a river, or by vertical posts called piers. Beam bridges are the most commonly used bridges in highway construction. Single-piece, rolled-steel beams can support spans of 15 to 30 m (50 to 100 ft). Heavier, reinforced beams and girders are used for longer spans.

B

Cantilever Bridges

Cantilever bridges are a more complex version of the beam-bridge design. In a cantilever design, a tower is built on each side of the obstacle to be crossed, and the bridge is built outward, or cantilevered, from each tower. The towers support the entire load of the cantilevered arms. The arms are spaced so that a small suspended span can be inserted between them. The cantilevered arms support the suspended span, and the downward force of the span is absorbed by the towers.

Cantilever bridges are self-supporting during construction. They are often used in situations in which the use of scaffolding or other temporary supports would be difficult. The Forth Bridge, a railway bridge across the Firth of Forth in Queensferry, Scotland, has two main spans of 521 m (1,710 ft) each. The Hāora (Howrah) Bridge in Calcutta (now Kolkata), India, was opened in 1943, with a main span of 457 m (1,500 ft). The Québec Bridge across the St. Lawrence River in Canada has a span of 549 m (1,800 ft).

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