Cantilever and suspension bridges.

Cantilever bridges normally use pairs of cantilevers back to back with a short beam bridge in between the cantilevers. Modern motorways have cantilever bridges stretching across them, they have a cantilever coming out from each side and a beam bridge in between them. The outer cantilevers have counterweights at the ends to maintain balance.

Suspension bridges in their simplest form were originally made from rope and wood. Modern suspension bridges use a box section roadway supported by high tensile strength cables. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables. These cables rest on top of high towers and have to be securely anchored into the bank at either end of the bridge. The towers enable the main cables to be draped over long distances. Most of the weight or load of the bridge is transferred by the cables to the anchorage systems. These are imbedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

Truss and cable-stayed bridges.

Truss bridges are composed of connected elements. They have a solid deck and a lattice of pin-jointed or gusset-joined girders for the sides. Early truss bridges were made of wood, and later of wood with iron tensile rods, but modern truss bridges are made completely of metals such as wrought iron and steel or sometimes of reinforced concrete.

Cable-stayed bridges, like suspension 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.

Bridges by use.

A bridge is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed. A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic. To create a beautiful image, some bridges are built much taller than necessary. This type, often found in east-Asian style gardens, is called a Moon bridge, evoking a rising full moon.

A tunnel types by use.

A tunnel is an underground passageway. However, in general tunnels are at least twice as long as they are wide.

A tunnel may be: for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, for a canal.

Other uses include routing power or telecommunication cables, some are to permit wildlife, secret tunnels have given entrance to or escape from an area.

Channel Tunnel.

The Channel Tunnel between France and the United Kingdom under the English Channel is the second-longest, with a total length of 50 km, of which 39 km is under the sea. Its lowest point it is 75 metres deep. The Channel Tunnel has the longest undersea portion of any tunnel in the world. Its construction was completed in 1994. The entire project consists of three parallel tunnels. Two rail tunnels are 7.6 meters in diameter spaced about 30 meters; between the two-way mail is a service tunnel of 4.8 meters in diameter in which circulating road vehicles.

Date: 2016-01-03; view: 845