橋梁畢業(yè)設計英文翻譯(已修改)
2024-12-17 06:32 本頁面
【正文】 Properties of Prestressed Concrete The rapid growth from 1945 onwards in the prestressing of concrete shows that there was a real need for this highquality structural material .The quality must be high because the worst conditions of loading normally occur at the beginning of life of the member ,at the transfer of stress from the steel to the concrete .Failure is therefore more likely then than later， when the concrete has bee stronger and the stress in the steel has decreased because of creep in the steel and the concrete ,and shrinkage of the concrete . Faulty members are therefore observed and thrown out early , before they enter the structure , or at least before it bees inconvenient and expensive to remove them . The main advantages of prestressed concrete in parison with reinforced concrete are : (a) The whole concrete crosssection resists load . In reinforced concrete about half the section , the cracked area below the neutral axis , does no useful work . Working deflections are smaller . (b) High working stresses are possible . In reinforced concrete they are not usually possible because they result in severe cracking which is always ugly and may be dangerous if it causes rusting of the steel. (c) Cracking is almost pletely avoided in prestressed 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 concret
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