Beam bridge

A beam bridge is a structure that consists of a horizontal beam supported by columns or piers at the end of each span. The horizontal beams are limited in length, as the supporting columns can only handle so much weight. Beam bridges can be made longer, however, by joining together a series of beams over a network of multiple supports. These types of structures are known as continuous beam bridges. Beam bridges are the oldest and simplest forms of bridges. The first were likely built by prehistoric humans, who placed a fallen tree across a stream or river. Beam bridges were built during the days of ancient Rome and later by twelfth-century Chinese architects. While more technologically advanced bridge styles allow for longer spans, beam bridges are still used in the modern era. The world's longest beam bridge is located in the southeastern United States and spans more than 20 miles (32 kilometers).

Background

Bridges are human-made structures designed to create a path over a river, stream, ravine, or other obstacle. While beam bridges may be the most basic form of bridge, they are just one of several types developed over the centuries. All bridges face problems caused by the forces exerted by gravity. These forces take the form of stress caused by tension and compression. Tension is a force that pulls outward, stretching the bridge platform or deck. Compression is a force that pushes or squeezes inward. Bridges buckle when compression becomes too great; they snap under too much tension. To maintain stability, bridges must channel these forces into their structural supports.

Arch bridges are supported by arches—curved, semicircular structures designed to support the weight above it. The bridge deck pushes down on the arch, distributing the compression force through the supports and making them stronger. Tension forces usually do not affect an arch bridge unless the arch is unusually long. One of the oldest surviving bridges in the world—Greece's Arkadiko Bridge, which was built about 1300 BCE—is an example of an arch bridge.

Suspension bridges support the bridge deck by a series of ropes or cables suspended from large support towers. The towers support the majority of the bridge's weight and channel the compression forces into the ground. The suspension cables run horizontally and are anchored at either end. These cables dissipate tension forces. The design allows suspension bridges to span longer distances than other types. The world's longest suspension bridge is the 6,532-foot (1,991-meter) Akashi-Kaikyo Bridge in Japan. San Francisco's Golden Gate Bridge and New York's Brooklyn Bridge are also examples of suspension bridges.

A variation of a suspension bridge is a cable-stayed bridge, a design in which two sets of diagonal support cables are connected directly to a tower rather than to suspension cables. Cable-stayed bridges are not required to be anchored at the ends.

Overview

Although earlier civilizations undoubtedly constructed beam bridges, ancient Greek historians mention a wood and stone beam-type bridge built over the Euphrates River around the eighth century BCE. About a century later, the Romans built the column-supported Pons Sublicius (bridge made of wooden beams) over the Tiber River. One of the world's oldest surviving beam bridges is the Anping Bridge, which was built in the twelfth century over the Shijing River in Quanzhou, China. The bridge was made with multiple stone beams supported by a series of stone piers. At 1.4 miles (2.3 kilometers), it is the world's longest stone beam bridge.

A beam bridge consists of a horizontal bridge deck—also called a beam or girder—supported by two columns or piers. As in an arch bridge, the support columns dissipate some of the compression forces. In a bridge beam, however, the top of the beam also experiences compression forces while the bottom is affected by tension forces. When weight is applied, the top side of the beam bends under compression; the bottom bends under tension. If too much weight is applied, the top of the beam would buckle and the bottom would snap. As a result, single-span beam bridges are limited in length, rarely exceeding 250 feet (76 meters). Modern beam bridges use reinforced concrete or steel beams to handle these stresses.

Simple beam bridges can be made with a single stone or wooden bridge deck placed over a stream or ravine. For larger bridges meant to carry automobile or train traffic over water, the support columns can be constructed by placing waterproof molds into shafts drilled into the rock below a river. A cage of reinforced steel is then inserted into the shafts before concrete is poured into the molds. This forms the foundation for the bridge supports. The columns can then be completed by pouring additional concrete into the foundation molds or attaching precast concrete columns to the foundation.

The bridge beams are usually I-shaped beams or box girders—box-shaped, hollow steel beams. They are lifted onto the supports by crane and bolted to the columns. To build a longer beam bridge—known as a continuous beam bridge—engineers must connect multiple spans with additional support piers. The world's longest beam bridge is the Lake Pontchartrain Causeway in Louisiana. The continuous span consists of multiple supports and bridge beams running for 23.87 miles (38.41 kilometers) over Lake Pontchartrain, which is north of New Orleans.

A variation of the beam bridge is the truss bridge—a design that adds a latticework of supports to the top of a beam bridge. The truss supports are typically a triangle-shaped network of steel or wooden posts that allow a beam bridge to support more weight and better dissipate additional stress. Truss designs can be a single vertical post supported on the sides by two diagonal posts or an interlocking series of two horizontal and two vertical posts supported by diagonal posts.

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