Full Versus Partial Prestressing Concrete

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.

在预应力混凝土发展的早期，预加应力的目的是要完全消除在使用荷载作用下混凝土中的拉应力。这种曾经是一种全新的匀质材料的概念，认为这种材料能够不开裂并且弹性地工作，一直达到其最大的设计荷载。在全部使用荷载作用下，混凝土拉应力的极限值为零的这种设计，通常称之为全预应力设计；而另一种在全部荷载作用下容许混凝土内产生一定大小的拉应力的方法，称之为部分预应力设计。

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.

有些场合必须避免任何产生裂缝的危险，此时需要采用全预应力。这些场合包括：不能产生渗漏的容器或水库，必须保证具有最大钢筋保护层的水下结构和在强腐蚀环境中的结构，必须考虑钢筋疲劳问题的承受高频反复荷载的结构。

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，也有许多场合施加少量的预应力就可以显著地改善结构的工作性能，降低造价，或者二者兼得。施加全预应力的梁，由于偏心预张拉力作用，可能出现不希望有的，较大的上拱度，因为产生向下挠度的重力荷载只能抵消一部分位移量。混凝土的徐变加剧了这种趋势，它加大了由于预张拉力引起的向上位移，但是对于只可能间歇作用的活载引起的向下挠度影响极小。而且，施加很大预应力的构件如果由于超载而导致破坏，则构件会呈脆性破坏，这与具有较小预应力的梁逐渐发生的破坏不同。从安全角度来说这是很重要的，因为没有预兆的突然破坏是危险的，而且来不及采取补救措施。此外，经验表明，在许多情况下，非预应力钢筋与高强度预应力钢筋的结合使用可以产生更好的经济效益。

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.

尽管在全部使用荷载作用下可以容许出现拉应力和可能的裂缝，但是也要认识到全部使用荷载并不是经常出现的。典型的或特征性的作用荷载可能就是恒载加上一小部分设计活载。因此部分预应力的梁在一般荷载情况下不会承受拉应力。当最大荷载偶尔作用时可能产生的裂缝，在该种荷载移去时，裂缝将完全闭合。比起始终带有由于承受弯曲应力而产生的裂缝的普通钢筋混凝土来说，预应力结构中的裂缝就不会有什么问题了。偶尔的开裂可以看作是为了得到工作性能上和经济上的改善所付出的小小代价。

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.

可以说，钢筋混凝土不过是预应力混凝土中预张拉力为零的一个特例。在接近破坏荷载时，钢筋混凝土梁和预应力混凝梁的工作情况基本上是相同的。

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.

欧洲混凝土委员会规定了三类预应力梁：

第一类：全预应力梁，在使用荷载作用下，混凝土内不容许有拉应力产生。

第二类：部分预应力梁，在不经常出现的大荷载作用下，容许出现偶然的暂时性裂缝。

第三类：部分预应力梁，在裂缝宽度受到限制的情况下，容许有永久性裂缝。

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.

对于适量预张拉力的选择取决于多种因素。它们包括：荷载性质（例如，公路和铁路桥梁，贮罐，等等），活载与恒载的比例，满载的出现频率以及腐他性介质的存在。对于荷载方向可能变更的结构物，例如在输电线路中的电杆，高而且均匀的预张拉力会降低其极限强度和导致突然破坏。在这种情况下，部分预应力提供了唯一满意的解决方法。