土木工程畢業(yè)設(shè)計外文資料翻譯--簡支梁布局-建筑結(jié)構(gòu)(參考版)
2025-05-17 13:49本頁面
【正文】 giving the limiting zone within which the c. g. s. must pass in order that no tensile stresses will be produced. Compressive stresses in concrete are not checked by this method. It is assumed that the layout of the concrete sections and the area of prestressing steel have already been determined. Only the profile of the c. g. s. is to be located. Referring to Fig . 89, having determined the layout of concrete sections, we proceed to pute their kern points, thus yielding two kern lines, one top and one bottom, ( c ) . Note that for variable sections, these kern lines would be curved, although for convenience they are shown straight in the figure representing a beam with uniform cross section. For a beam loaded as shown in ( a ), the minimum and maximum moment diagrams for the girder load and for the total working load respectively are marked as MG and MT in ( b ). In order that, under the working load, the center of pressure, the Cline, will not fall above the top kern line, it is evident that the c. g. s. must be located below the top kern at least a distance a1=MT/F (81) Fig 89. Location of limiting zone for c. g. s. If the c. g. s. falls above that upper limit at any point, then the Cline corresponding to moment MT and prestress F will fall above the top kern, resulting in tension in the bottom fiber. Similarly, in order that the Cline will not fall below the bottom kern line, the c. g. s. line must not be positioned below the bottom kern by a distance greater than which gives the lower limit for the location of c. g. s. If the c. g. s. is positioned above that lower limit, it is seen that the Cline will be above the bottom kern and there will be no tension in the top fiber under the girder load and initial prestress F0. Thus, it bees clear that the limiting zone for c. g. s. is given by the shaded area in Fig. 89( c ), in order that no tension will exist both under the girder load and under the working load. The individual tendons, however, may be placed in any position so long as the c. g. s. of all the cables remains within the limiting zone. The position and width of the limiting zone are often an indication of the adequacy and economy of design, Fig. 810. If some portion of the upper limit falls outside or too near the bottom fiber, in ( a ), either the prestress F or the depth of beam at that portion should be increased. On the other hand, if it falls too far above the bottom fiber, in ( b ), either the prestress or the beam depth can be reduced. If the lower limit crosses the upper limit, in ( C ), it means that no zone is available for the location of c. g. s. , and either the prestress F or the beam depth must be increased or the girder moment must be increased to depress the lower limit if that can be done. On the other hand, as will be discussed later, the case shown in Fig. ( c ) may be very satisfactory when are allowing tensile stress in concrete. Fig 810. Undesirable positions for c. g. s. zone limits. 。 ( b ) has a bent soffit, while ( c ) has a curved one. For both layouts, the c. g. s. at midspan can be depressed as low as desired, while that at the ends can be kept near the c. g. c. If the soffit can be varied at will, it is possible
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