There is a distinct trend in current design practice toward the use of partially prestressed beams, in which flexural tensile stress or even cracking is permitted in the concrete?in the service load stage or for occasional overloads. Cracks, if they occur, are usually small and well distributed, and normally close completely when the load that produced them is removed.

在目前的設(shè)計(jì)實(shí)踐中，明顯地傾向于采用部分預(yù)應(yīng)力梁。對(duì)于這種梁，在使用荷載階段或者在偶爾的超載時(shí)，容許混凝土產(chǎn)生彎曲拉應(yīng)力，甚至容許開裂。如果產(chǎn)生裂縫，它們通常都比較小，分布也比較均勻，而且當(dāng)使裂縫產(chǎn)生的荷載卸去之后，裂縫通?？梢酝耆]合。

It is argued convincingly that cracking has long been an accepted feature of reinforced concrete members and that there is no reason to penalize prestressed concrete designs by requiring that cracks be eliminated completely, even though this is possible. Furthermore, the condition of no tension or limited tension in a prestressed structure rarely exists. If combined effects including shear and torsion are taken into account, the calculated principal stresses usually exceed the tensile strength of the concrete. In regions of concentrated loads, load transfer, or anchorage of tendons, tensile stresses cannot be avoided.?Also, in most cases, a structure is prestressed in only one direction, so that in the transverse direction it acts as ordinary reinforced concrete. In view of these facts, it is hard to justify a requirement for no flexural cracking.

可以令人信服地證實(shí)，長(zhǎng)期以來裂縫一直是鋼筋混凝凝土構(gòu)件所容許的特征，并且，即使有可能的話，也沒有理由在預(yù)混凝土設(shè)計(jì)中完全消除裂縫。而且在預(yù)應(yīng)力結(jié)構(gòu)中，很少存在沒有拉應(yīng)力或者拉應(yīng)力受到限制的情況。如果考慮到剪切和扭轉(zhuǎn)的組合作用，則計(jì)算的主應(yīng)力往往超過混凝土的抗拉強(qiáng)度。在集中荷載、荷載傳遞或預(yù)應(yīng)力筋錨固等區(qū)域，拉應(yīng)力是不可避免的。此外，在大多數(shù)情況下，結(jié)構(gòu)僅在一個(gè)方向施加預(yù)應(yīng)力，因此它在橫向同普通混凝土一樣工作。從這些事實(shí)來看，就難以證明沒有裂縫的要求是合理的。

The advantages of partial prestressing are important. A smaller prestress force will be required, permitting reduction in the number of tendons and anchorages. The necessary flexural strength may be provided in such cases either by a combination of prestressed tendons?and non-prestressed reinforcing bars, or by an adequate number of high-tensile tendons prestressed to a level lower than the permitted?limit. In some cases a combination of stressed and unstressed tendons is used. Since the prestressing force is less, the size of the bottom flange, which is required mainly to resist the compression when a beam is in the unloaded stage, can be reduced or eliminated altogether. This leads in turn to significant simplification and cost reduction in the construction of forms, as well as resulting in structures that are more pleasing esthetically. Furthermore, by relaxing the requirement for low service load tension in the concrete, a significant improvement can be made in the deflection characteristics of a beam. Troublesome upward camber of the member in the unloaded stage can be avoided, and the prestress force selected primarily to produce the desired deflection for a particular loading condition. The behavior of partially prestressed beams, should they be overloaded to failure, is apt to be superior to that of fully prestressed beams, because the improved ductility provides ample warning of distress.

部分預(yù)應(yīng)力有很大的優(yōu)點(diǎn)，它需要較小的預(yù)張拉力，因此可以減少預(yù)應(yīng)力筋和錨具的數(shù)量。在此種情況下，必要的抗彎強(qiáng)度或者由預(yù)應(yīng)力鋼筋和非預(yù)應(yīng)鋼筋共同提供，或者由預(yù)張拉至低于容許值的足夠數(shù)量的高強(qiáng)鋼筋來保證。在某些情況下，可以同時(shí)使用張拉的和非張拉的鋼筋。因?yàn)轭A(yù)張拉力較小，主要為承受梁在未加荷載階段所需的底面，翼緣尺寸就可以減小或完全取消。這樣又使得模板結(jié)構(gòu)得到顯著的簡(jiǎn)化和減少模板費(fèi)用，并且做成在美觀上更令人滿意的結(jié)構(gòu)。此外，由于放松了對(duì)混凝土中在使用荷載下的拉應(yīng)力要求，梁的撓度特性可以得到顯著的改善。構(gòu)件可以避免產(chǎn)生在未加荷載階段過大的上拱度，而且對(duì)于特定的荷載情況，可以通過選擇預(yù)張拉力來獲得所要求的撓度。部分預(yù)應(yīng)力梁如遇超載而破壞，其工作性能也往往優(yōu)于全預(yù)應(yīng)力梁，因?yàn)榈玫礁纳屏说难有阅軌驗(yàn)槭鹿侍峁┏浞值念A(yù)兆。

The design of structural members based on strength requirements is appealing, because in all but unusual cases the most important single characteristic of a structure is its strength, which establishes the degree of safety incorporated into its design. For reinforced concrete members, strength requirements usually provide the starting point in proportioning cross sections and determining steel areas. Only later is the design checked for satisfactory serviceability, with specific reference to cracking and deflection at the service load level.?Checking of service load stresses is often dispensed with.

以強(qiáng)度要求為依據(jù)的構(gòu)件設(shè)計(jì)是受人歡迎的，因?yàn)?/span>，除了極罕見的情況以外，結(jié)構(gòu)唯一最重要的性能就是結(jié)構(gòu)的強(qiáng)度，它確定了設(shè)計(jì)中所體現(xiàn)的安全度。對(duì)于鋼筋混凝土構(gòu)件，強(qiáng)度要求通?？勺鳛閿M定截面尺寸和確定鋼筋面積的出發(fā)點(diǎn)。其后才專門根據(jù)使用荷載下的裂縫和撓度，為滿足使用要求而做設(shè)計(jì)校核。通常不用進(jìn)行使用荷載應(yīng)力的驗(yàn)算。

An analogous approach is proposed for prestressed concrete, although there are some complications. For reinforced concrete,?consideration is usually limited to underreinforced beams, for which the steel is at the yield stress at failure. With the tensile force thus known, the compressive area of the cross section is easily calculated from the summation of horizontal forces. With the centroid of the compression area known, the internal resisting lever arm is known and an explicit equation can be written for the ultimate resisting moment.?This equation can be rearranged to permit direct solution for the required concrete dimensions and tensile steel area. For prestressed concrete, on the other hand, the stress in the steel at flexural failure is at some value ?usually less than the tensile strength . It may be more or less than the nominal yield stress . The compression concrete area, which is a function of steel stress at failure, is not easily established at the outset of the design process, so the internal lever arm between compressive and tensile resultants is not known.

對(duì)于預(yù)應(yīng)力混凝土，雖然復(fù)雜一些，也建議采用類似的方法。對(duì)于鋼筋混凝凝土，通常只限于考慮低筋梁，這種梁在破壞時(shí)，鋼筋達(dá)到屈服應(yīng)力。這樣，當(dāng)拉力為已知時(shí)，便可通過對(duì)水平力求和來計(jì)算截面的受壓面積。當(dāng)受壓面積的形心為已知時(shí)，就可以知道抵拉內(nèi)力偶臂，并可對(duì)極限抵抗彎矩寫出清楚的計(jì)算公式。這一公式可以被重新整理，使其能直接求解需要的混凝土尺寸和受拉鋼筋面積。另一方面，對(duì)于預(yù)應(yīng)力混凝土，彎曲破壞時(shí)的鋼筋應(yīng)力通常處于比抗拉強(qiáng)度小的某個(gè)值?，它可能大于或小于標(biāo)稱屈服應(yīng)力。作為破壞時(shí)鋼筋應(yīng)力函數(shù)的混凝土受壓面積，在設(shè)計(jì)過程開始時(shí)是不容易確定的，因而壓應(yīng)力與拉應(yīng)力兩合力之間的內(nèi)力偶臂也無法求得。

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However, in practical cases, a trial concrete section may be found by assuming that the tendon stress at failure is 0. 9 times the ultimate strength .?Refinement will be found necessary only in cases when there is an unusually large percentage of steel. For flanged sections, the internal lever arm at failure is very nearly equal to the distance from the tensile steel centroid to the middepth of the flange.

然而，在實(shí)際場(chǎng)合下，假定破壞時(shí)預(yù)應(yīng)力鋼筋的應(yīng)力為0.9倍的極限強(qiáng)度，即可求出混凝土的試算截面。只有在鋼筋百分率相當(dāng)大時(shí)，才需要進(jìn)行精確計(jì)算。對(duì)于有翼緣的截面，破壞時(shí)的內(nèi)力偶臂非常接近于受拉鋼筋重心至翼緣厚度中心的距離。