橋梁畢業(yè)設(shè)計(jì)英文翻譯-文庫吧
2025-10-28 06:32 本頁面
【正文】 e ceased to be an experimental material and engineer won confidence in its use . With this confidence came an increase in the use of precast prestressed concrete particularly for longspan floors or the decks of motorways . Wherever the quantity to be made was large enough ,for example in a motorway bridge 500 m long , provided that most of the spans could be made the same and not much longer than 18 m , it became economical to use factoryprecast prestressed beams , at least in industrial areas near a precasting factory . Most of these beams are heatcured so as to free the forms quickly for reuse. In this period also , in the Untied States , precast prestressed roof beams and floor beams were used in many school buildings occasionally 32 m long or more . Such long beams over a single span could not possibly be successful in reinforced concrete unless they were cast on site because they would have to be much deeper and much heavier than prestressed concrete beams They would certainly be less pleasing to the eyes and often more expensive than the prestressed concrete beams . These school buildings have a strong , simple architectural appeal and will be a pleasure to look at for many years . The most important parts of a precast prestressed concrete beam are the tendons and the concrete . The tendons , as the name implies , are the cables ,rods or wires of steel which are under tension in the concrete . Before the concrete . has hardened (before transfer of stress ) , the tendons are either unstressed (posttensioned prestressing ) or are stressed and held by abutments outside the concrete (pretensioned prestressing ) . While the concrete is hardening it grips each tendons more and more tightly by bond along its full length . End anchorages consisting of plates or blocks are placed on the ends of the tendons of posttensioned prestressed units ,and such tendons are stressed up at the time of transfer , when the concrete has hardened sufficiently ,In the other type of prestressing , with pretensioned tendons , the tendons are released from external abutments at the moment of transfer . The factory precasting of long prestressed concrete beams is likely to bee more and more popular in the future ,but one difficulty will be road transport .As the length of the beam increases the lorry bees less and manoeuvrable until eventually the only suitable time for it to travel is in the middle of the night when traffic is at a minimum . The limit of length for road are straight or curved . Precasting at the site avoids these difficulties 。it may be expensive ,but it has often been used for large beams. There i
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