大型橋梁及施工外文翻譯--大跨度橋梁-橋梁設(shè)計-文庫吧在線文庫
【正文】 ilt on staging. This method is being obsolete. Currently, freecantilever and movable scaffold systems are increasingly used to save time and improve safety. The movable scaffold system employs movable forms stiffened by steel frames. These forms extend one span length and are supported by steel girders which rest on a pier at one end and can be moved from span to span on a second set of auxiliary steel girders. An economical construction technique known as incremental pushlaunching method is developed by BaurLeonhard team. The total continuous deck is subdivided longitudinally into segments of 10 to 30 m length depending on the length of spans and the time available for construction. Each of these segments is constructed immediately behind the abutment of the bridge in steel framed forms, which remain in the same place for concreting all segments .The forms are so designed as to be capable of being moved transversely or rotated on hinges to facilitate easy stripping after sufficient hardening of concrete. At the head of the first segment, a steel nose consisting of a light truss is attached to facilitate reaching of the first and subsequent piers without including a too large can yielder moment during construction . The second and the following segments are concreted directly on the face of the hardened portion and the longitudinal reinforcement can continue across the construction joint . The pushing is achieved by hydraulic jacks which act against the abutment .Since the coefficient of friction of Teflon sliding bearings is only about 2 percent, low capacity hydraulic jacks would suffice to move the bridge even over long lengths of several hundred metres . This method can be used for straight and continuously curved bridges up to a span of about 120 m . The freecantilever system was pioneered by Dyckerhoff and Willmann in Germany .In this system , the superstructure is erected by means of cantilever truck 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
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