Prestressed Concrete

1、 Text Prestressed Concrete Concrete is strong in compression, but weak in tension: its tensile strength varies from 8 to 14 percent of its compressive strength. Due to such a low tensile capacity, flexural cracks develop at early stages of loading. In order to reduce or prevent such cracks from deve

2、loping, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections at service load, thereby raising the be

3、nding, shear, and torsional capacities of the sections.The sections are then able to behave elastically, and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure.混凝土的抗壓性能強而抗拉性能弱:它的抗拉強度

4、僅 僅是它抗壓強度的8%-14%不等.鑒于它這么低的抗 拉承載力,撓曲裂縫就會出現(xiàn)在在荷載作用的初期. 為了減小或阻止這種裂縫的開展,在結(jié)構(gòu)桿件縱向施 加一個軸心或偏心的壓力.這個力(預(yù)應(yīng)力)通過消除 或大大減少在工況荷載下,跨中或支座的控制截面處 產(chǎn)生的拉應(yīng)力,阻止了(該處)裂縫的開展,因此提高了 該截面的(抵抗)彎曲,剪切和扭轉(zhuǎn)的承載能力.然后,這個截面(特性)表現(xiàn)為彈性,并且,當(dāng)所有荷載作用在結(jié) 構(gòu)上時,混凝土截面的全部深度(全截面)受壓,這樣有 效的利用了混凝土全部的抗壓性能. Lesson 2 Such an imposed longitudinal force is called

5、a prestressing force, i.e., a compressive force that prestresses the sections along the span of the structural element prior to the application of the transverse gravity dead and live loads or transient horizontal live loads. The type of prestressing force involved, together with its magnitude, are

6、determined mainly on the basis of the type of system to be constructed and the span length and slenderness desired. Since the prestressing force is applied longitudinally along or parallel to the axis of the member, the prestressing principle involved is commonly known as linear prestressing. 這樣施加的一

7、個縱向力叫做預(yù)應(yīng)力,也就是說, 在橫(豎)向重力恒載和活載或短暫的水平活載 (風(fēng),地震)作用之前,給沿結(jié)構(gòu)桿件跨度方向的截面預(yù)加的應(yīng)力,這樣一個壓縮力.相關(guān)的預(yù)應(yīng) 力的形式,包括它的大小,主要取決于被修建結(jié) 構(gòu)物的形式,桿件跨度,和想要得到的長細比.因為這個預(yù)應(yīng)力是被應(yīng)用到縱向或者平行于桿件的軸向,所以這個相關(guān)的預(yù)應(yīng)力原理被普 遍的稱為長線預(yù)加應(yīng)力法. Circular prestressing, Used in liquid containment tanks, pipes, and pressure reactor vessels, essentially follows the same

8、 basic principles as does linear prestressing. The circumferential hoop, or hugging stress on the cylindrical or spherical structure, neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure. 用于液體容器箱,管道,和壓力反應(yīng)堆容器的環(huán) 向預(yù)應(yīng)力結(jié)構(gòu),本質(zhì)上遵循著和長線型預(yù)應(yīng)力

9、結(jié)構(gòu)相同的基本原理.環(huán)向(預(yù)應(yīng)力)箍筋,或者 圓柱或球形結(jié)構(gòu)的”環(huán)向”應(yīng)力,抵消由內(nèi)部 包含物質(zhì)(產(chǎn)生的)壓力所引起的曲線表面外部 纖維所引起的拉應(yīng)力. Fig1.2.1 illustrates, in a basic fashion, the prestressing action in both type of structural systems and the resuling stress response. In a), individual concrete blocks act together as a beam due to the large compressive pr

10、estressing force P. Although it might appear that the blocks will slip and vertically simulate shear slip failure, in fact they will not because of the longitudinal force P(2). Similarly, the wooden staves in c) might appear to be capable of separating as a result of the high internal radial pressur

11、e exerted on them. But again, because of the compressive prestress imposed by the metal bands as a form of circular prestressing, they will remain in place.圖1.2.1舉例說明，在基本結(jié)構(gòu)中預(yù)應(yīng)力作用在兩種結(jié)構(gòu)體系中并且形成壓力。在a）圖中，獨立的混凝土塊由于大的壓縮預(yù)應(yīng)力P作為一個橫梁一起作用。盡管它可能出現(xiàn)混凝土塊滑動并且垂直的模擬剪應(yīng)力滑動不足，事實上，他們將不會由于縱向的力P（2）。同樣的，圖c)中的木棒可能會獨立分開，由于在他們上

12、作用了高的內(nèi)部徑向壓力。但是又由于作為一種環(huán)形預(yù)應(yīng)力的金屬環(huán)強加了抗壓預(yù)應(yīng)力，他將保持在適當(dāng)?shù)牡胤健?From the preceding discussion, it is plain that permanent stresses in the prestressed structural member are created before the full dead and live loads are applied in order to eliminate or considerably reduce the net tensile stresses caused by these

13、 loads. With reinforced concrete, it is assumed that the tensile strength of the concrete is negligible and disregarded. This is because the tensile forces resulting from the bending moments are resisted by the bond created in the reinforcement process. Cracking and deflection are therefore essentia

14、lly irrecoverable in reinforced concrete once the member has reached its limit state at service load. 從上述討論的可知,在預(yù)應(yīng)力(混凝土)結(jié)構(gòu)的 桿件中恒久預(yù)應(yīng)力是在全部恒載和活載加載 之前產(chǎn)生的,其目的是為了消除或大大減少由 這些荷載產(chǎn)生的靜拉應(yīng)力.對于鋼筋混凝土結(jié) 構(gòu)來說,假定混凝土的抗拉強度是可以忽略的. 這是因為由彎矩產(chǎn)生的拉應(yīng)力被鋼筋(和混凝 土)產(chǎn)生的黏結(jié)力所抵消.因此,在鋼筋混凝土 結(jié)構(gòu)中,一但桿件在使用荷載下達到它的極限 狀態(tài)時,撓度和裂縫是不可恢復(fù)的. The reinfor

15、cement in the reinforced concrete member does not exert any force of its own on the member, contrary to the action of prestressing steel. The steel required to produce the prestressing force in the prestressed member actively preloads the member, permitting a relatively high controlled recovery of c

16、racking and deflection. Once the flexural tensile strength of the concrete is exceeded, the prestressed member starts to act like a reinforced concrete element. 在鋼筋混凝土構(gòu)件中的鋼筋沒有完全發(fā)揮它 的全部作用，而預(yù)應(yīng)力鋼筋正好相反在預(yù) 應(yīng)力構(gòu)件中的（要求產(chǎn)生）預(yù)應(yīng)力筋給構(gòu)件 積極的產(chǎn)生一個預(yù)加荷載，相應(yīng)的對其撓度 和裂縫有一個很高的控制一但混凝土的撓 曲抗拉強度被超越，預(yù)應(yīng)力混凝土構(gòu)件就會 像一個非預(yù)應(yīng)力混凝土構(gòu)件一樣工作 Pres

17、tressed members are shallower in depth than their reinforced concrete counterparts for the same span and loading conditions. In general, the depth of a prestressed concrete member is usually about 65 to 80 percent of the depth of the equivalent reinforced concrete memher. Hence, the prestressed memb

18、er requires less concrete, and about 20 to 35 percent of the amount of reinforcement. Unfortunately, this saving in material weight is balanced by the higher cost of the higher quality materials needed in prestressing. Also, regardless of the system used, prestressing operations themselves result in

19、 an added cost: formwork is more complex, since the geometry of prestressed sections is usually composed of flanged sections with thin webs. 在相同的跨度和荷載條件下，預(yù)應(yīng)力構(gòu)件（截面）的高度比相應(yīng)的非預(yù)應(yīng)力構(gòu)件的高度小總的來說，預(yù)應(yīng)力混凝土構(gòu)件（截面）高度常常是相應(yīng)的非預(yù) 應(yīng)力構(gòu)件（截面）高度的因此，預(yù)應(yīng)力混凝土構(gòu)件需要較少的混凝土，并且大概是 的鋼筋（相對于非預(yù)應(yīng)力構(gòu)件）不幸的是，在預(yù)應(yīng)力構(gòu)件中，所需材料重量上的節(jié)省是 和其所需材料高質(zhì)量的花費相平衡（

20、抵消）的同 時，還沒有考慮（工藝）設(shè)備的使用，施加預(yù)應(yīng)力自身導(dǎo)致一個額外的花費：因為預(yù)應(yīng)力構(gòu)件的幾何 截面常常由翼緣和薄的腹板組成，所以模板是非常復(fù)雜 In spite of these additional costs, if a large enough number of precast units are manufactured, the difference between at least the initial costs of prestressed and reinforced concrete systems is usually not very large. And

21、the indirect long-term savings are quite substantial, because less maintenance is needed, a longer working life is possible due to better quality control of the concrete, and lighter foundations are achieved due to the smaller cumulative weight of the superstructure. 盡管這么多附加的花費，如果制造大量的預(yù) 制混凝土構(gòu)件，至少在預(yù)應(yīng)

22、力構(gòu)件和非預(yù)應(yīng) 力構(gòu)件的最初成本是相差不大的并且，因 為需要更少的維護，鑒于對混凝土更高質(zhì)量 的控制所使得結(jié)構(gòu)所獲得的一個更長的生命 周期，以及對于上部結(jié)構(gòu)其輕質(zhì)效果積累所 產(chǎn)生的更輕的基礎(chǔ)的效應(yīng)，其間接的長期節(jié) 省是非常實際的 Once the beam span of reinforced concrete exceeds 70 to 90 feet (21.3 to 27.4m), the dead weight of the beam becomes excessive, resulting in heavier members and, consequently, greater l

23、ong-term deflection and cracking. Thus, for larger spans, prestressed concrete becomes mandatory since arches are expensive to construct and do not perform as well due to the severe long term shrinkage and creep they undergo. Very large spans such as segmental bridges or cable-stayed bridges can onl

24、y be constructed through the use of prestressing. 一但鋼筋混凝土梁的跨度超過英 尺（m），梁（上）的 恒定重量變的非常的大，從而導(dǎo)致更大的構(gòu) 件尺寸，并且因此產(chǎn)生更大的長期撓度和裂 縫因此，對于大跨度結(jié)構(gòu)來說，鑒于拱形 結(jié)構(gòu)在施工上昂貴的花費和不好完成并且由 于它們長期經(jīng)受的嚴重的收縮和徐變，預(yù)應(yīng) 力結(jié)構(gòu)被強制使用例如分段拼裝式橋和斜 拉橋這種大跨度結(jié)構(gòu)只能通過預(yù)應(yīng)力這種技術(shù)被建造 Linear prestressing continued to develop in Europe and in France, in particular th

25、rough the ingenuity of Eugene Freyssinet, who proposed in 1926-28 methods to overcome prestress losses through the use of high-strength and high-ductility steels. In 1940, he introduced the now well-known and well-accepted Freyssinet system.在歐洲,長線預(yù)加應(yīng)力法得到了持續(xù)的發(fā)展,尤 其是在法國,通過Eugene Freyssinet在192628年提出的克

26、服預(yù)應(yīng)力損失的天才辦法,這種 方法是通過使用高強和高韌性鋼筋得到的.在 1940年,他發(fā)明了現(xiàn)在眾所周知的和公認的 Freyssinet( Freyssinet(預(yù)應(yīng)力)體系 . ) P. W. Abeles of England introduced and developed the concept of partial prestressing between the 1930s and 1960s. F. Leonhardt of Germany, V. Mikhailov of Russia, and T. Y. Lin of the United States also contr

27、ibuted a great deal to the art and science of the design of prestressed concrete. Lins load-balancing method deserves particular mention in this regard, as it considerably simplified the design process, particularly in continuous structures. These twentieth-century developments have led to the exten

28、sive use of prestressing throughout the world, and in the United States in particular.英國的P. W. Abeles 在20世紀30年代到60年 代之間發(fā)明并且發(fā)展了部分預(yù)應(yīng)力(結(jié)構(gòu))的概 念.德國的F. Leonhardt ,俄羅斯的V.Mikhailov 和美國的 T. Y. Lin 也對預(yù)應(yīng)力混凝土設(shè)計的 藝術(shù)的科技作出了很大貢獻.在這點上, Lin的 荷載平衡法尤其值得一提,因為它相當(dāng)大的簡 , 化了設(shè)計程序,尤其在連續(xù)結(jié)構(gòu)上.這些20世紀 的發(fā)展導(dǎo)致了預(yù)應(yīng)力在全世界,尤其是在美國 的廣泛應(yīng)用. To

29、day, prestressed concrete is used in buildings, underground structures, TV towers, floating storage and offshore structures, power stations, nuclear reactor vessels, and numerous types of bridge systems including segmental bridge and cable-stayed bridges. They demonstrate the versatility of the pres

30、tressing concept and its all-encompassing application. The success in the development and construction of all these structures has been due in no small measures to the advances in the technology of materials, particularly prestressing steel, and the accumulated knowledge in estimating the shortand l

31、ong-term losses in the prestressing forces. 今天,預(yù)應(yīng)力混凝土(結(jié)構(gòu))被用在建筑,地下地 下構(gòu)造物,電視塔,漂浮存儲器和海上結(jié)構(gòu)物, 發(fā)電站,核反應(yīng)堆容器和橋梁體系的眾多形式 上,其中包括分段拼裝式橋和斜拉橋.它們展示 了預(yù)應(yīng)力概念的通用性和包容性.所有這些結(jié) 構(gòu)發(fā)展和建設(shè)的成功是主要由于材料技術(shù)的 進步,尤其是預(yù)應(yīng)力鋼筋,并且對估計預(yù)應(yīng)力短 期和長期損失值的知識的積累. Text Prestressed Concrete Concrete is strong in compression, but weak in tension: its tensile strength varies from 8