Early in the development of prestressed concrete, the goal of prestressing was the complete elimination of concrete tensile stress at service loads. The concept was that of an entirely new, homogeneous material that would remain uncracked and respond elastically up to the maximum anticipated loading. This kind of design, where the limiting tensile stress in the concrete at full service load is zero, is generally known as full prestressing, while an alternative approach, in which a certain amount of tensile stress is permitted in the concrete at full service load, is called partial prestressing.

在預(yù)應(yīng)力混凝土發(fā)展的早期，預(yù)加應(yīng)力的目的是要完全消除在使用荷載作用下混凝土中的拉應(yīng)力。這種曾經(jīng)是一種全新的勻質(zhì)材料的概念，認(rèn)為這種材料能夠不開(kāi)裂并且彈性地工作，一直達(dá)到其最大的設(shè)計(jì)荷載。在全部使用荷載作用下，混凝土拉應(yīng)力的極限值為零的這種設(shè)計(jì)，通常稱之為全預(yù)應(yīng)力設(shè)計(jì)；而另一種在全部荷載作用下容許混凝土內(nèi)產(chǎn)生一定大小的拉應(yīng)力的方法，稱之為部分預(yù)應(yīng)力設(shè)計(jì)。

There are cases in which it is necessary to avoid all risk of cracking and in which full prestressing is required. Such cases include tanks or reservoirs where leaks must be avoided, submerged structures or those subject to a highly corrosive environment where maximum protection of reinforcement must be insured, and structures subject to high frequency repetition of load where fatigue of the reinforcement may be a consideration.

有些場(chǎng)合必須避免任何產(chǎn)生裂縫的危險(xiǎn)，此時(shí)需要采用全預(yù)應(yīng)力。這些場(chǎng)合包括：不能產(chǎn)生滲漏的容器或水庫(kù)，必須保證具有最大鋼筋保護(hù)層的水下結(jié)構(gòu)和在強(qiáng)腐蝕環(huán)境中的結(jié)構(gòu)，必須考慮鋼筋疲勞問(wèn)題的承受高頻反復(fù)荷載的結(jié)構(gòu)。

However, there are many cases where substantially improved performance, reduced cost, or both may be obtained through the use of a lesser amount of prestress. Fully prestressed beams may exhibit an undesirable amount of upward camber because of the eccentric prestressing force, a displacement that is only partially counteracted by the gravity loads producing downward deflection. This tendency is aggravated by creep in the concrete, which magnifies the upward displacement due to the prestress force, but has little influence on the?downward deflection due to live loads, which may be only intermittently applied. Also, should heavily prestressed members be overloaded and fail, they may do so in a brittle way, rather than gradually as do beams with a smaller amount of prestress. This is important from the point of view of safety, because sudden failure without warning is dangerous, and gives no opportunity for corrective measures to be taken. Furthermore, experience indicates that in many cases improved economy results from the use of a combination. of unstressed bar steel and high strength prestressed steel tendons.

但是，也有許多場(chǎng)合施加少量的預(yù)應(yīng)力就可以顯著地改善結(jié)構(gòu)的工作性能，降低造價(jià)，或者二者兼得。施加全預(yù)應(yīng)力的梁，由于偏心預(yù)張拉力作用，可能出現(xiàn)不希望有的，較大的上拱度，因?yàn)楫a(chǎn)生向下?lián)隙鹊闹亓奢d只能抵消一部分位移量。混凝土的徐變加劇了這種趨勢(shì)，它加大了由于預(yù)張拉力引起的向上位移，但是對(duì)于只可能間歇作用的活載引起的向下?lián)隙扔绊憳O小。而且，施加很大預(yù)應(yīng)力的構(gòu)件如果由于超載而導(dǎo)致破壞，則構(gòu)件會(huì)呈脆性破壞，這與具有較小預(yù)應(yīng)力的梁逐漸發(fā)生的破壞不同。從安全角度來(lái)說(shuō)這是很重要的，因?yàn)闆](méi)有預(yù)兆的突然破壞是危險(xiǎn)的，而且來(lái)不及采取補(bǔ)救措施。此外，經(jīng)驗(yàn)表明，在許多情況下，非預(yù)應(yīng)力鋼筋與高強(qiáng)度預(yù)應(yīng)力鋼筋的結(jié)合使用可以產(chǎn)生更好的經(jīng)濟(jì)效益。

While tensile stress and possible cracking may be allowed at full service load, it is also recognized that such full service load may be infrequently applied. The typical, or characteristic, load acting is likely to be the dead load plus a small fraction of the specified live load. Thus a partially prestressed beam may not be subject to tensile stress under the usual conditions of loading. Cracks may form occasionally, when the maximum load is applied, but these will close completely when that load is removed. They may be no more objectionable in prestressed structures than in ordinary reinforced concrete, in which flexural cracks always form. They may be considered a small price to pay for the improvements in performance and economy that are obtained.

盡管在全部使用荷載作用下可以容許出現(xiàn)拉應(yīng)力和可能的裂縫，但是也要認(rèn)識(shí)到全部使用荷載并不是經(jīng)常出現(xiàn)的。典型的或特征性的作用荷載可能就是恒載加上一小部分設(shè)計(jì)活載。因此部分預(yù)應(yīng)力的梁在一般荷載情況下不會(huì)承受拉應(yīng)力。當(dāng)最大荷載偶爾作用時(shí)可能產(chǎn)生的裂縫，在該種荷載移去時(shí)，裂縫將完全閉合。比起始終帶有由于承受彎曲應(yīng)力而產(chǎn)生的裂縫的普通鋼筋混凝土來(lái)說(shuō)，預(yù)應(yīng)力結(jié)構(gòu)中的裂縫就不會(huì)有什么問(wèn)題了。偶爾的開(kāi)裂可以看作是為了得到工作性能上和經(jīng)濟(jì)上的改善所付出的小小代價(jià)。

It has been observed that reinforced concrete is but a special case of prestressed concrete in which the prestressing force is zero. The behavior of reinforced and prestressed concrete beams, as the failure load is approached, is essentially the same.

可以說(shuō)，鋼筋混凝土不過(guò)是預(yù)應(yīng)力混凝土中預(yù)張拉力為零的一個(gè)特例。在接近破壞荷載時(shí)，鋼筋混凝土梁和預(yù)應(yīng)力混凝梁的工作情況基本上是相同的。

The Joint European Committee on Concrete establishes three classes of prestressed beams:

Class 1: Fully prestressed, in which no tensile stress is allowed in the concrete at service load.

Class?2:?Partially prestressed, in which occasional temporary cracking is permitted under infrequent high loads.

Class 3: Partially prestressed, in which there may be permanent cracks provided that their width is suitably limited.

歐洲混凝土委員會(huì)規(guī)定了三類預(yù)應(yīng)力梁：

第一類：全預(yù)應(yīng)力梁，在使用荷載作用下，混凝土內(nèi)不容許有拉應(yīng)力產(chǎn)生。

第二類：部分預(yù)應(yīng)力梁，在不經(jīng)常出現(xiàn)的大荷載作用下，容許出現(xiàn)偶然的暫時(shí)性裂縫。

第三類：部分預(yù)應(yīng)力梁，在裂縫寬度受到限制的情況下，容許有永久性裂縫。

The choice of a suitable amount of prestress is governed by a variety of factors.?These include the nature of the loading. (for?example, highway or railroad bridges, storage, etc.), the ratio of live to dead load, the frequency of occurrence of the full load, and the presence of corrosive agents. With structures in which the direction of loading may be reversed, such as in transmission poles, a high uniform prestress would result in reduced ultimate strength and in brittle failure. In such a case, partial prestressing provides the only satisfactory solution.

對(duì)于適量預(yù)張拉力的選擇取決于多種因素。它們包括：荷載性質(zhì)（例如，公路和鐵路橋梁，貯罐，等等），活載與恒載的比例，滿載的出現(xiàn)頻率以及腐他性介質(zhì)的存在。對(duì)于荷載方向可能變更的結(jié)構(gòu)物，例如在輸電線路中的電桿，高而且均勻的預(yù)張拉力會(huì)降低其極限強(qiáng)度和導(dǎo)致突然破壞。在這種情況下，部分預(yù)應(yīng)力提供了唯一滿意的解決方法。