A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges will vary depending on the function of the bridge and the nature of the terrain where the bridge is to be constructed.
Types of bridgesEdit
There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges.
Beam bridges are horizontal beams supported at each end by 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 piers at either end of the bridge. 
Cantilever bridges are built using cantilevers — horizontal beams that are supported on only one end. Most cantilever bridges use two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. The largest cantilever bridge is the 549 feet (167 m) Quebec Bridge in Quebec, Canada.
Arch bridges are arch-shaped and 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 thrusted into the abutments at either side. Dubai in the United Arab Emirates is currently building the largest arch bridge in the world, which is scheduled for completion in 2012.
Suspension bridges 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 12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan.
Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter.  The first known cable-stayed bridge was designed in 1784 by C.T. Loescher. The longest cable-stayed bridge is the Tatara Bridge in the Seto Inland Sea, Japan.
Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed girders for the sides. Early truss bridges were made of wood, but modern truss bridges are made of metals such as wrought iron and steel. The Quebec Bridge, mentioned above as a cantilever bridge, is also the world's longest truss bridge.
Noted UK Road BridgesEdit
(Road nos needed for some)
- Avon Gorge Bridge - Bristol
- Barton Bridge - Manchester
- Dartford Crossing Bridge - Dartford, Greater London, A282 (M25)
- Erskin Bridge - Glasgow
- High Level Bridge - Newcastle Road & rail on two decks
- Humber Bridge - Hull
- Kingston Bridge - Glasgow
- Tyne Bridge - Newcastle
- Seven Bridge - M4
- Second Seven Crossing Bridge - M48
- Skye Bridge - Isle of Skye
- Taymar Bridge - Devon > Cornwall
- Tinsley Viaduct - Sheffield M1
|This page uses content from Wikipedia. The original article was at Bridge. The list of authors can be seen in the page history. As with UK Roads, the text of Wikipedia is available under the GNU Free Documentation License.|