橋梁畢業(yè)設(shè)計(jì)英文翻譯(完整版)
【正文】 d concrete . The main disadvantage of prestressed concrete is that much more care is needed to make it than reinforced concrete and it is therefore more expensive ,but because it is of higher quality less of it needs to be used . It can therefore happen that a solution of a structural problem may be cheaper in prestressed concrete than in reinforced concrete , and it does often happen that a solution is possible with prestressing but impossible without it . Prestressing of the concrete means that it is placed under pression before it carries any working load . This means that the section can be designed so that it takes no tension or very little under the full design load . It therefore has theoretically no cracks and in practice very few . The prestress is usually applied by tensioning the steel before the concrete in which it is embedded . After the concrete has hardened enough to take the stress form the steel , some of the stress is transferred form the steel to the concrete . In a bridge with abutments able to resist thrust , the prestress can be applied without steel in the concrete It is applied by jacks forcing the bridge inwards form the abutments . This method has the advantage that the jacking force , or prestress , can be varied during the life of the structure as required . In the ten years from 1950 to 1960 prestressed concrete 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
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