1、 外文翻译 INTRODUCTION TO PRESTRESS CONCRETE Basic Concepts of Prestressing Prestressing concrete is basically concrete in which internal stresses of suitable magnitude and distribution are introduction so that the stresses resulting from external loads are conteracted to a desired degree .in reinforced
2、 concrete members, the prestress is commonly introduced by tensioning the steel reinforcement. The earliest examples of wooden barrel construction by force-fitting of metal tyres on wooden wheels indicate that the art of prestressing has been practiced from ancient times .the tensile strength of pla
3、in concrete is only a fraction of its compressive strength and the problem of it being deficient in tensile strength appears to have been the driving factor in the development of the composite material known as “ reinforced concrete” . The development of early cracks in reinforced concrete due to no
4、n-compatibility in the strains of steel and concrete was perhaps the starting point for the development of a new material like “ presstressed concrete “ .the application of permanent compressive stress to a material like concrete ,which is strong in compression but weak in tension ,increases the app
5、arent tensile strength of that material ,because the subsequent application of tensile stress must first nullify the compressive prestress .in 1904 Fresyssinet attempted to substitute permanently acting forces in concrete to resist the lastic forces developed under loads and this idea was later deve
6、loped under the name of “ prestressing” . the significant observation which resulted from pioneering research work on pretressed concrete were Need for High Strength and Concrete 1. Nessity of using high steel and concrete. 2. Recognition of losser of prestress due to various causes. he early attemp
7、ts to use mind steel in prestressed concrete were not successful since the working stress in mild steel of 120 N/MM2 more or less completely lost due to elastic deformation ,creep and shrinkage of concrete. The nomore loss of stress of stress in steel is generally about 100 to 240 N/MM2 and it is ap
8、parent that if this loss of stress is to be a small portion of the intitle stress, the stress in stress in steel in the intitle stages must be very high , about 1200 to 2000N/MM2.there high stress ranges are possible only with the use of high strength steel. High strength concrete is necessary in pr
9、estressed concrete since the materirail offers high resistance in tension , shear ,bond and bearing stresses being higher ,high strength concrete is invariably preferred to minimize cost.high strength concrete is less liable to shrinkage cracks ,and resulting in a smaller loss prestress in steel .th
10、e use of high strength concrete result in a reduction in the cross sectional dimensions of prestressed concrete structural elements . Whith reduced dead weight of the material ,longer spans become technically and ecnomically practicable. ADVANTAGES OF PRESTRESSED CONCRETE Prestressed concrete member
11、s posses improve resistance to shearing forces, due to the effect of compressive ,which reduces principal tensile stress .the use of curved cables ,particularly in long span members helps to reduce the shear forces developed at the support section. A prestressed concrete flexural member is stiffer u
12、nder working loads than a reinforced concrete member of the same depth. However, after the onset of cracking, the flexural behavior of a prestressed member is similar to that of a reinforced concrete member. The use of high strength concrete and steel in prestressed members results in lighter and sl
13、ender members than could be possible by using reinforced concrete. the two structural features of prestressed concrete ,namely high strength concrete and freedom from cracks , contributes to the improved durability of the structure under aggressive environmental conditions .Prestressing of concrete
14、improves the ability of the material for energy absorption under impact loads. The ability to resist repeated working loads has been proved to be as good in reinforced concrete. The economy of prestressed concrete is well established for long span structures. According to Dean ,standardlized precast
15、 bridge beams between 10 and 30 m long and precast prestressed piles have proved to be economical than steel and reinforced concrete is economical for floor , roofs and bridges of spans up to 30 and for cast in suit work , it applies to spans up to 100 m. in the long span range , prestressed concret
16、e is generally economical in comparison with reinforced concrete and steel construction. Prestressed concrete has considerable resilience due it its capacity recovering from substantial effect of overloading without undergoing any serious damage. Leonhardt points out than in prestressed concrete ele
17、ments, cracks which temporarily develop under occasional overloading will close up complete when the loads are removed .since the fatigue strength of prestressed concrete is comparatively better than of other materials, chiefly due to the small stress variations in prestressingsteel , it is recommen
18、ded for dynamically loaded structures , such as railway bridges and machine foundations. Due to the utilization of concrete in the tension zone . A saving of 15 to 20 percent in concrete is possible in comparison with reinforced concrete. The saving in steel are even higher, 60 to 80 percent , mainl
19、y due to the high permissible stresses allowed in the high tensile wires. Although there is considerable saving in the quantity of materials used in prestressed concrete member in comparison with reinforced concrete member , the economy in cost is not that significant due to the additional costs inc
20、urred for the high strength concrete high tensile steel , anchorages , and other hardware required for the tensile steel , anchorages , and other hardware required for the production of pretressed members , however , there is anoverall economy in using prestressed concrete .as the decrease in dead weigh reduces the design loads and the cost of foundations.
