关于桥梁的英语简介

reat Bridges of the World.For those who work on the site, bridge is hard in other way. They cannot settle down in one house because they rarely work in one place for longer than a year or two, and the working site is usually unprotected and something dangerous.The builders of the Forth Road Bridge〈1965〉 were particularly prould of the fact no lives were lost in spite of the heavy winds,the snow and the cold rain. Word on bridge foundations,usually in cofferdams and something in caissons under comperssed air has often caused loss of life.One of the most intersting recent birdges, which will remain for many years the world′ largest, if not its longest, is the Verrazano-Narrows Bridge, New York, compieted in 1964, a suspension bridge of 4,260ft main span, having six traffic lanes on each of its two deck. It is also probably one of the world′s highest bridges since its main span has to be 216ft above the shipping channel whiah is 2,00ft wide. Its capacity is estimated at 13 million vehicles in the first year, rising eventually to 48 million vehicles, and it cost some 324million dollars to build.The main parts of a suspension bridge are the deck, the two cables and many suspenders transferring the deck load to the cables, the towers over which the cables passand the anchorages on the two banks. The Narrows Bridge has four cables, two on each side. In a self-anchored suspension bridge thereare no anchorages, the two ends of the cables being tied to the shore ends of the bridge structure.Most road bridges have pies rising out of the valley, on which the girders rest, with a deck carried by the girders and a wearing surface on top of the deck. Railway bridges sometimes differ in the wearing surface being unnecessary and therefore left out completely, so that wearing surface being unnecessary and therefore left out completely, so that there are then gaps between the bridge beams.The clear span of a bridge is athe gap from the near edge of one pier to the near edge of the next. The effective span is the spacing between the centres of bridge piers. The span, which is large than the clear span, is used in the design of the girders. At each shore, every bridge has foundation called abutments,an abutment being a foundation which carries horizontal load.An interesting and economical bridge which has become popular in Europe in the last twenty years and was first used in Germany is called the cable cantilever or bridle chord bridge, though other names have been used for it. It is always of steel, and like the self-anchored suspension bridge it has two towers over which its cables pass. A bridge of this type was completed in 1965 over the river Usk at Newport,Monmouthshire,England〈Fig.26〉.The 150-metre main span is supported at twelve point by ropes from the two towers, making it into a 13-span continuous beam with twelve elastic internal supports and two rigid end supports. The complete esign was analysed by a strain energy method and the analysis was checked by a plastic model. The cables pass over towers 52m high and are anchored on the shore side to the concrete approach spans. The towers are hollow concrete boxsection and the concrete in them is never under under tension, the maximum compressive stress being 100kg/〈cm*cm〉.The test cubes taken from the concrete of the towers were specified to have a minimum strength of 400/〈cm*cm〉. At high tide, the least height under the dridge is 13m, and the tidal range is 9.2m at the bridge. The main girder on this bridge are steel welded section and were mard by barge to the site and joined thre into a continuous structure. Because of the support from the cables, the main girder depth is remarkably small, only 1.3.Highway bridges and railway bridges are designed for dead load, live load, impact, temperature stresses, wind load, traction and braking forces, and on curves also for centrifugal force. Railway bridges have to be designed also for nosing, a sideways force from the flanges of the engine wheels on to the rails at curves.Americans generally use either the short ton or the kip in their stress calculations, the short ton being 2,000 pounds, considerably less than the metric ton, and the kip or kilo-pound being 1,000 pound. The kip is very widely used in the United States and many British engineers also find it convenient for calculations. The long ton of 2,240 pounds used in Britain is about 17 kg larger than the metric ton.Proplem involving moving loads are conveniently solved by influence line, which are used in all countries of the world. Shear diagrams and bending moment diagrams or envelopes show the effect of shear or bending moment at all points along the bridge span, but an infulence line shows the effect, at one point, of a load placed at any point on the structure. The base length of an influence line is the same as that of a bending moment diagram so it is important to state on the diagram what it it,otherwise misunderstandings may occur.

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