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【正文】 圖 810 限制 附件 2：外文原文 82, Simple Beam Layout The layout of a simple prestressedconcrete beam is controlled by two critical sections: the maximum moment and the end sections. After these sections are designed, intermediate ones can often be determined by inspection but should be separately investigated when necessary. The maximum moment section is controlled by two loading stages, the initial stage at transfer with minimum moment MG acting on the beam and the workingload stage with maximum design moment MT. The end sections are controlled by area required for share resistance, bearing plates, anchorage spacings, and jacking clearances. All intermediate sections are designed by one or more of the above requirements, depending on their respective distances from the above controlling sections. A mon arrangement for posttensioned members is to employ some shape, such as I or T, for the maximum moment section and to round it out into a simple rectangular shape near the ends. This is monly referred to as the end block for posttensioned members. For pretensioned members, produced on a long line process, a uniform I, doubleT, or cored section is employed throughout, in order to facilitate production. The design for individual sections having been explained in Chapters 5, 6, and 7,the general cable layout of simple beams will now be discussed. The layout of a beam can be adjusted by varying both the concrete and the steel. The section of concrete can be varied as to its height, width, shape, and the curvature of its soffit or extrados. The steel can be varied occasionally in its area but mostly in its position relative to the centroidal axis of concrete. By adjusting these variables, many binations of layout are possible to suit different loading conditions. This is quite different from the design of reinforcedconcrete beams, where the usual layout is either a uniform rectangular section or a uniform Tsection and the position of steel is always as near the bottom fibers as is possible. Consider first the pretensioned beams, Fig. straight cables are preferred, since they can be more easily tensioned between two abutments. Let us start with a straight cable in a straight beam of uniform section, (a).This is simple as far as form and workmanship are concened, But such a section cannot often be economically designed, because of the conflicting requirements of the midspan and end sections. At the maximum moment section generally occurring at midspan, it is best to place the cable as near the bottom as possible in order to provide the maximum lever arm for the internal resisting moment. When the MG at midspan is appreciable, it is possible to place the c. g. s. much below the kern without producing tension in the top fibers at transfer. The end section, however, presents an entirely different set of requirements. Since there is no external moment at the end, it is best to arrange the tendons so that the c. g. s. will coincide with the c. g. c. at the end section, so as to obtain a uniform stress distribution. In any case, it is necessary to place the c. g. s. within the kern if tensile stresses are not permitted at the ends, and not too far outside the kern to avoid tension stress in excess of allowab
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