大型橋梁及施工外文翻譯--大跨度橋梁-橋梁設(shè)計-在線瀏覽
2024-07-24 04:22本頁面
【正文】 k in sections generally of m .The cantilever truck ,whose cost is relatively small and which is attached firmly to permanent construction , emits by repeated use the construction of large bridges . The 1 avoidance of scaffold from below, the speed of work and the saving in labor cost result in the construction being very economical. The freecantilever system is ideally suited for launched girders with a large depth above the pier cantilever system is ideally suited for launched girders with a large depth above the pier cantilevering to the middle of the span. Another technique is the use of the pneumatic caisson .The caisson is a huge cylinder with a bottom edge that can cut into the water bed. When pressed 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 pression chamber for short periods of time .For another , if they e 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 nitrogen in the blood .When the Eads Bridge across the Mississippi River at was under construction between 1867and 1874 , at a time when the danger of working in pressed air was not fully understood ,fourteen deaths was caused by the bends . When extra strength is necessary in the piers, they sometimes keyed into the bedrockthat is ,they are extended down into the bedrock .This method was used to build the piers for the Golden Gate Bridge in San Francisco ,which is subject to strong tidies and high winds ,and is located in an earthquake zone .The drilling was carried out under water by deepsea divers . Where bedrock cannot be reached ,piles are driven into the water bed .Today ,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 .When the box is pumped dry ,it bees buoyant enough to support a large proportion of the weight of the bridge . Each type of bridge indeed each individual bridge presents special construction problems. With some truss bridges , the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes .Arch bridges can be constructed over a false work ,or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges .With steel arches ,however ,a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support .Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases . With suspension bridges ,the foundations and the towers are built first .Then a cable is run from the anchorageconcrete block in which the cable is fastenedup to the tower and across to the opposite tower and anchorage .A wheel that unwinds wire from a reel puns along this cable .When the reel reaches the other side ,another wire is placed on it ,and the wheel returns to its original position .When all the wires have been put in place ,another machine moves along the cable to
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