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Bridges have been one of the most important piece of infrastructure since the dawn of civilization. A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.

The first bridges were made by nature itself — a log fallen across a stream or stones in the river. The first bridges made by humans were probably spans of cut wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Some early Americans used trees or bamboo poles to cross small caverns or wells to get from one place to another. A common form of lashing sticks, logs, and branches together involved the use of long reeds or other harvested fibers woven together to form a rope which was capable of binding and holding in place materials used in early bridges.

Types of Bridges

Beam bridges
Beam bridges are horizontal beams supported at each end by abutments. When there is more than one span the intermediate supports are known as piers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on top of the beam pushes straight down on the abutments at either end of the bridge. They are made up mostly of wood or metal. Beam bridges typically do not exceed 250 feet (76 m) long. The longer the bridge, the weaker. The world's longest beam bridge is Lake Pontchartrain Causeway in southern Louisiana in the United States.

Cantilever bridges
Cantilever bridges are built using cantilevers—horizontal beams that are supported on only one end. Most cantilever bridges use a pair of continuous spans extending from opposite sides of the supporting piers, meeting at the center of the obstacle to be crossed. Cantilever bridges are built using much the same materials & techniques as beam bridges. The difference comes in the action of the forces through the bridge. The largest cantilever bridge is the Quebec Bridge in Quebec, Canada.

Arch bridges
Arch bridges have abutments at each end. The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments at either side. Dubai is currently building the Sheikh Rashid bin Saeed Crossing which is scheduled for completion in 2012. When completed, it will be the largest arch bridge in the world

Suspension bridges
Suspension bridges, such as the Golden Gate Bridge, are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river. The longest suspension bridge in the world is the Akashi Kaikyo Bridge in Japan.

Movable bridges
Movable bridges are designed to move out of the way of boats or other kinds of traffic, which would otherwise be too tall to fit. These are generally electrically powered.

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