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Bridge

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The Capilano Suspension Bridge
A bridge is structure that is built for the purpose of providing easy passage over an obstacle, such as a body of water, railroad track, river, or road.

There are three major types of bridges: the beam bridge, the arch bridge, and the suspension bridge. Each is characterized by a specific design suited to a different bridge span, which is the distance between the two columns supporting the bridge.

A beam bridge typically spans a distance up to 200 feet (60 m); a modern arch, from 800 to 1,000 feet (240 to 300 m); and a suspension bridge can span up to 7,000 feet (2,100 m).

Contents

[edit] How it Works

[edit] Compression and Tension

The ability of a bridge to span a certain distance relies on two forces known as compression and tension. By definition, compression is the type of force that compresses or shortens the object it is acting on, while tension expands or lengthens it.

Compression is achieved when an object is pushed together or experiences downward pressure. Tension works the opposite way, when an object is pulled a part on two ends or pulled upon, resulting in a lengthening of the spring. Both compression and tension exist in all bridges and too much of either of these forces can result in a bridge buckling or snapping.

[edit] Types

[edit] Beam Bridge
Comstock Covered Bridge - A Howe Truss Bridge

A beam bridge makes use of compression by its design. It is defined as a rigid horizontal structure supported by a pier on each end. The weight of the traffic exerted on the bridge pushes downwards and creates compression. When compression is manifested on the topside of the beam bridge roadway, it results in the shortening of the upper deck; this causes tension in the lower portion of the deck and it reacts by lengthening.

Beam bridges also come in an assortment of styles, all determined by the design, location, and make-up of the truss. The different types include the Howe truss, the Through truss, the Deck truss, the Warren truss, and the Pratt. William Howe patented the Howe Truss in 1840.

[edit]
Arch Bridge over the Mississippi River
Arch Bridge

The arch bridge uses a semicircular structure with abutments on each end. Compression is exerted outward along the bridge’s curve toward the abutment. In turn, the curve of the arch aids in dissipating the force and reducing tension under the arch. While all arch bridges are architecturally similar, they can vary in design, ranging from Baroque, to Roman, to Renaissance type arches.

[edit] Suspension Bridge

The suspension bridge uses cables, ropes, or chains that are suspended from one end of the deck to the other. Compression occurs by the force exerted on the bridge’s deck, but as it is suspended, the cables absorb the compression and transfer it to the towers. Two anchorages that interconnect with the supporting cables receive the tension force. [1]

[edit] Famous Bridges

Some of the world’s famous bridges are San Francisco’s Golden Gate Bridge and the Brooklyn Bridge. [2]

[edit] References

  1. Bridge. Howstuffworks. 2008-11-03.
  2. Bridges. Department of Civil Engineering UK. 2008-11-03.