【正文】 tween the two decks. John also used stays, inclined cables that ran from the main supporting cables to the deck, to stabilize the bridge. John went on to design the Brooklyn Bridge in New York, which was pleted in 1883 by his son, Gee Washington John. The Brooklyn bridge, with a span of 486 meters, is one of he most importantand one of the most esthetically satisfyingbridges ever built. The method devised by the John for laying the Brooklyn Bridge is essentially the same technique used today. He envelopment of reinforces and prostheses concrete has given engineers other important materials for bridge building. Concrete has been used particularly for relatively shortspan bridges that are a part of freeway system. These bridges often use recast concrete beams. Many arch bridges have also been constructed of concrete. Currently, the longest concrete arch bridge is the Glades Ville 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 bridges 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 counterbalances and electric motors. Another type is the swing or pivot bridge, which pivots the span on a pier so that the bridge can swing open 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 畢業(yè)設(shè)計 32 of counterweights. A bascule with one arm is a singleleaf bridge, and with two arms it is a doubleleaf bridge. Bridge construction can present extraordinary 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 usually 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. Another technique is the use of the pneumatic caisson. The caisson is a huge cylinder with a bottom sedge that can cut into the water bed. When pressed 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 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. When the Eads Bridge across the Mississippi River at Stylus was under construction between 1867and 1874, at a time when the danger of working in pressed air was not fully understood, fourteen deaths were caused by the bends. When extra strength is necessary in the piers, they are sometimes keyed into the bedecksthat 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 deepsea divers. Where bedrock cannot be reached, piles are driven into the water bed. Today, the piles in construction are usually made of priestesses concrete beams. One ingenious technique, used for the Tappan Zee Bridge across the Hudson River in New 畢業(yè)設(shè)計 33 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 large proportions 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 position after the piers have been erected and then raised into place by means of jacks or crams. Arch bridges can de 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 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 fastenedup 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, another machine moves along the cable to pact and to bind them. Construction 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 畢業(yè)設(shè)計 34 long and many bridges have been built only after the beat 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 Great 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 proud of the fact no lives were lost in spite of the heavy winds, the snow and the cold rain. Work on bridge foundations, usually in cofferdams and something in caissons under pressed air has oft