一個簡單的預(yù)應(yīng)力混凝土梁由兩個危險截面控制：最大彎矩截面和端截面。這兩部分設(shè)計好之后，中間截面一定要單獨檢查，必要時其他部位也要單獨調(diào)查。最大彎矩截面在以下兩種荷載階段為控制情況，即傳遞時梁受最小彎矩MG的初始階段和最大設(shè)計彎矩MT時的工作荷載階段。而端截面則由抗剪強度、支承墊板、錨頭間距和千斤頂凈空所需要的面積來決定。所有的中間截面是由一個或多個上述要求，根它們與上述兩種危險截面的距離來控制。對于后張構(gòu)件的一種常見的布置方式是在最大彎矩截面采用諸如I形或T形的截面，而在接近梁端處逐漸過渡到簡單的矩形截面。這就是人們通常所說的后張構(gòu)件的端塊。對于用長線法生產(chǎn)的先張構(gòu)件，為了便于生產(chǎn)，全部只用一種等截面，其截面形狀則可以為I形、雙T形或空心的。在第5 、 6 和7章節(jié)中已經(jīng)闡明了個別截面的設(shè)計，下面論述簡支梁鋼索的總布置。
梁的布置可以用變化混凝土和鋼筋的辦法來調(diào)整?；炷恋慕孛嬖诟叨?、寬度、形狀和梁底面或者頂面的曲率方面都可以有變化。而鋼筋只在面積方面有所變化，不過在相對于混凝土重心軸線的位置方面卻多半可以有變化。通過調(diào)整這些變化因素，布置方案可能有許多組合，以適應(yīng)不同的荷載情況。這一點是與鋼筋混凝土梁是完全不同的，在鋼筋混凝土梁的通常布置中，不是一個統(tǒng)一的矩形截面便是一個統(tǒng)一的T形，而鋼筋的位置總是布置得盡量靠底面纖維。
首先考慮先張梁，如圖 8-7，這里最好采用直線鋼索，因為它們在兩個臺座之間加力比較容易。我們先從圖（a）的等截面直梁的直線鋼索開始討論。這樣的布置都很簡單，但這樣一來，就不是很經(jīng)濟的設(shè)計了，因為跨中和梁端的要求會產(chǎn)生沖突。通常發(fā)生在跨度中央的最大彎矩截面中的鋼索，最好盡量放低，以便盡可能提供最大力臂而提供最大的內(nèi)部抵制力矩。當(dāng)跨度中央的梁自重彎矩MG相當(dāng)大時，就可以把c.g.s布置在截面核心范圍以下很遠(yuǎn)的地方，而不致在傳遞時在頂部纖維中引起拉應(yīng)力。然而對于梁端截面卻有一套完全不同的要求。由于在梁端沒有外


The layout of a simple prestressed-concrete 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 working-load 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 common 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 commonly referred to as the end block for posttensioned members. For pretensioned members, produced on a long line process, a uniform I, double-T, 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 combinations of layout are possible to suit different loading conditions. This is quite different from the design of reinforced-concrete beams, where the usual layout is either a uniform rectangular section or a uniform T-section and the position of steel is always as near the bottom fibers as is possible.