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  • 关于桥梁的英语简介

    BridgesBridges are among the most important, and often the most spectacular,of all civil engineering works. The imposing bridges that have survived from ancient times are arched structures of heavy masonry, usually stone dr brick..Herodotus,the greek historian of the fifth cenury B.C., however mentions a wooden bridges across the Euphrates River at Babylon. In rome, the bridge of Fabricis, built in 62B.C.and named for its engineer,still carries traffic across the ataiber River,asdoes the Sant′Angelo Bridge, built in about 136A.D.Both of these bridges, and many other Roman bridges,have a series of arches supported by heavy piers that extend down to bedrock. Ancient sources also mention pontoon bridegs, usually in connection with military operation.A pontoon is a hollow drum that can float; a series of pontoons anchored to a riverbed can support a oadway. The Incas of per-Columbian Peru built remarkable suspension bridges, supported by cables of natural fibers, that crossed many of the deep gorges in their monutainous country.The sudden expansion in transportation systems that began in tha eighteenth century,and still continues in our own day, has enormously increaed the need for bridges as a part of highways and railroad, Better understanding of the forces that are exerted on structures and the improved materials that became available in the nineteenth century have made it possible to build increasingly longer ang stronger bridges.With the ability to span greater distances,the damlike effect of masonry arch bridges with several heavy piers that ; block he flow of a stream can be largely eliminaed.The simplest type of span is a beam bridge,consisting of a rigid beam between two supports. Today most simple beam bridges are strengthened by a truss, which is based on the triangle. Diagonal bams that extend between the horizontal and vertical beams give support against both comperssion and tension. Many early truss bridges were built of wood; one that was eected across the Susquehanna River in Pennslvania in 1815 had a span of 110 meters. Iron and then steel were later used in the consruction of truss bridges, still further increasing their stength. Trusses are not only strong but also light, because all unnecessary material has been eliminated in their design.Another type of beam bridge is the cantilever, in which a horizontal beam extends beyond its support.Cantilever bridges, like tusses, had also been built before iron and steel became available. Most cantilever bridges have wo arms of truss structure that meet or support a section between them. Cantilevers enabled bridges.During the ninteenth century, cantilevers were frequently used to build railroad bridges.The Quebec Bridge, which crosses the St. lawrence River in Canada, is the longest cantilever bridge in the world, with a span of 540 meters. It was completed in 1917, and until 1919 it was the longest bridge of any type in the world.A third type of moder bridge is he steel arch bridge, which can carry a roadway either above or below its arch of steel beams. An arch exerts strong downward and diagonal thrusts, so the piers that support it must be especially strong. Probably the most famous steel arch bridge is the Sydeny H arbor Bridge in Australia, wih a span of 495 meters. The Bayonne Bidge between New Jersey and Staten Island in New York has a span one meter longer.Suspension bridges span even longer distance than other types of bridge. The longest bridge of any type is the Verrazano-Narrows Bridge in New York, with a span of 1,280 meters. The deck or roadway of a suspension bridge is suspended from steel cables are supported by massive towers. The first modern suspension bridges used linked chains made of wroughe iron. Some of them survived for many years, like one across the Danube River in Budapest, Hungary. It was completes in 1849 and destroyed during World War, nearly a hunded years later.When steel became available, cables of steel wires replaced chains of wrought iron. Several suspension bridges built in this manner collapsed, however, as a result of storms or the movement created by the rhythm of the loads moving across them. It was later discoversd that these failures were caused by the lack of truss supports for the deck. The first major cable-type suspension bridge to overcomg these faults was designed by John A.Roebling at Niagara Falls. Its span of 250 meters was strengthened by trusses between the two decks. Roebling also used stays, inclined cables that ran from the main supporting cables to the deck, to stabilize the bridge. Roebling went on to design the Brooklyn Bridge in New York, which was completed in 1883 by his son, George Washington Roebling. The Booklyn bridge, with a span of 486 meters, is one of he most important-and one of the most esthetically satisfying-bridges ever built. The mothod devised by the Roeblings for laying the Brooklyn Bridge is essentially the same technique used today.He evelopment of reinforces and prestressed concrete has given engineers other important materials for bridge building. Concrete has been used particularly for relatively short-span bridges that are a part of freeway system. These bridges often use precast concrete beams. Many arch bridges have also been constructed of concrete. Currently, the longest concrete arch bridge is the Gladesville Bridge in Sydney, Australia. It has a span of 305meters, and its deck is above the arch.This is another example of an esthetically pleasing bridge.Many bridge that pass over rivers or canals must be movable so that shipping can pass under them. One type is the lift bridge, with towers that can raise the entire span between them by means of counterbablances and electric motors. Another type is the swing or pivot bridge, which pivots the span on a pier so that the bridge can swing openn parallel to the river or canal. A third type is the bascule bridge, which has one or two arms that can open upward at an angle by means of counterweights. A bascule with one arm is a single-leaf bridge, and with two arms it is a double-leaf bridge.Bridge constuction can present extraordinry difficulties. Usually the foundations for the piers must rest on bedrock, and often under water. One technique for working in these conditions is by means of a cofferdam. Piles usully made of interlocking steel plates are driven into the water bed. The water is then pumped out from within the use area that been enclosed.Anpther tachnique is the use of the pneumatic caisson. The caisson is a huge cylinder with a bottom sdge that can cut into the water bed. When compressed air is pumped into it, the water is forced out. Caissons must be used with extreme care. For one thing, workers can only stay in the compression chamber for short periods of time. For another, if they come up to normal atmospheric pressure too rapidly, they are subject to the bends,or caisson disease as it is also called, which is a crippling or even fatal condition caused by excess niteogen in he blood. When the Eads Bridge across the Mississippi River at St.Louis was under construction between 1867and 1874,at a time when the danger of working in compressed air was not fully understood, fourteen deaths were caused by the bends.When extra strength is necessary in the piers, they are sometimes keyd into the bedock-that is, that are extended down into the bedrock. This method was used to build the piers for he Golden Gate Bridge in San Francisco, which is subject to strong tides and high winds,and is located in an earthquake zone. The drilling was carried out under water by deep-sea divers.Where bedrock cannot be reached, piles are driven into the water bed. Tody, the piles in construction are usually made of prestressed concrete beams. One ingenious technique,used for the Tappan Zee Bridge across the Hudson River in New York, is to rest a hollow concrete box on top of a layer of piles. Whenthe box is pumped dry, it becomgs buoyant engough to support a large propotion of the weight of the bright of the bridge.Each type of bridge, indeed each individual bridge, presents special construction problem. With some truss bridge,the span is floated into possition after the piers have been erected and then raised into place by means of jacks or crames. Arch bridges can de constructed over a falsework, or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges. With steel arches,however,a technique has been developed where by the finished sections are held in place by wires that supply a cantilever support. Cranes move along the tension in the cables increases.With suspension bridges, the foundations and the towers are built first. The foundation and the towers are built first. Then a cable is run form the anchorage concrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage. A wheel that unwinds wire from a reel runs this cable. When the reel reaches the other side, anther wire is placed on it, and the wheel returns to its original position. When all the wires have been put in place ,anoth machine moves along the cable to compact and to bind them. Constryction begins on the deck when the cables are in place, with work progressing toward the middle from each end of the structure.Bridge design is probably the hardest brain work in civil engineering. Bridge designers have a long period of training, only the best of them succeed, and only a small fraction of the bridge they design are built. The time from the first proposal for a bridge to the provision of the money for its construction is usually long and many bridge have been built only after the beath of their designer. A fine bridge engineer, shirley smith, who was the contactor′agent for the Forth Road Bridge, has written very well about his love of bridge work in his book G

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