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1、Lesson 3 Prestressed Concrete 预应力混凝土,1,1.Concrete is strong in compression,but weak in tension:its tensile strength varies from(不同)8 to 14 percent of its compressive strength.Due to such a low tensile capacity(受拉承载能力),flexural cracks(挠曲裂缝)develop at early stages of loading.In order to reduce or prev
2、ent such cracks from developing,a concentric(adj.同中心的，轴心的)or eccentric force(偏心力)is imposed in the longitudinal direction(轴向)of the structural element.混凝土的抗压性能强而抗拉性能弱:它的抗拉强度仅仅是它抗压强度的8%-14%不等.由于它如此低的抗拉承载力,在荷载作用的初期挠曲裂缝就会出现。为了减小或阻止这种裂缝的开展,在结构杆件纵向施加一个轴心或偏心的压力.,2,3,This force prevents the cracks from dev
3、eloping by eliminating or considerably(adv.显著地,大大,相当(大，多)地)reducing the tensile stresses(拉伸应力，抗拉应力)at the critical(危险的,临界的)midspan(中跨 截面，部分)and support sections at service load(使用荷载),thereby raising the bending(弯曲,挠曲),shear(剪力，剪切),and torsional capacities(扭转承载力)of the sections.这个力(预应力)通过消除或大大减少在使用荷载
4、下跨中或支座的控制截面处产生的拉应力,阻止了(该处)裂缝的开展,因此提高了该截面的(抵抗)弯曲、剪切和扭转的承载能力.,4,The sections are then able to behave elastically(adv.弹性地),and almost the full capacity of the concrete in compression can be efficiently utilized(vt.利用)across the entire depth of the concrete sections when all loads act on the structure.然
5、后,这些截面表现为弹性,并且当所有荷载作用在结构上时,可以通过混凝土全截面受压有效地利用混凝土几乎全部的抗压性能。,2.Such an imposed longitudinal force(纵向力)is called a prestressing force,i.e.(也就是),a compressive force that prestresses(vt.给预加应力)the sections along the span of the structural element prior to(adv.在前,居先)the application of the transverse(adj.横向的
6、)gravity dead and live loads or transient(adj.短暂的,瞬时的)horizontal live loads.这样施加的一个纵向力叫做预应力,也就是说,在横(竖)向重力恒载和活载或短暂的水平活载(风,地震)作用之前,沿结构杆件跨度方向的截面预加的压缩力.,5,6,The type of prestressing force involved(adj.,有关的),together with its magnitude,are determined mainly on the basis of the type of system to be constr
7、ucted and the span length and slenderness(细长度)desired(想得到的).Since the prestressing force(预应力)is applied longitudinally along or parallel(v平行)to the axis of the member,the prestressing principle(n.原理,原则)involved is commonly known as(as v.被认为是，被称为)linear prestressing.相关的预应力的形式,包括它的大小,主要取决于被修建结构物的形式、杆件
8、跨度和理想的长细比。因为这个预应力被应用到纵向或者平行于杆件的轴向,所以相关的预应力原理普遍称为长线预加应力法（线性预应力法）.,3.Circular prestressing(环形预应力),used in liquid containment(容纳，容积)tanks(箱，槽),pipes(管),and pressure reactor vessels(反应堆容器),essentially(adv.本质上)follows(遵循，追随,跟随)the same basic principles(基本原理)as does linear prestressing.The circumferential
9、(adj.圆周的)hoop(箍筋),or“hugging”(n.拥抱)stress on the cylindrical(圆柱的)or spherical(adj.球的,球形的)structure,neutralizes(抵消；压制，中和)the tensile stresses at the outer fibers(外部纤维)of the curvilinear(adj.曲线的,由曲线而成的)surface caused by the internal contained(包含)pressure.用于液体容器箱、管道和压力反应堆容器的环向预应力结构,本质上遵循着和长线型预应力结构相同的基本
10、原理。环向(预应力)箍筋,或者圆柱或球形结构的”环向”应力,在曲线表面外部纤维下，抵消由内部包含物质(产生的)压力所引起的拉应力.,7,4.From the preceding(adj.在前的,前述的)discussion,it is plain(adj.清晰的,简单的,明白的,普通的,朴素的)that permanent stresses in the prestressed(adj.预应力的（混凝土）)structural member are created before the full dead and live loads are applied in order to elimi
11、nate or considerably reduce the net tensile stresses(净拉应力)caused by these loads.With reinforced concrete,it is assumed that the tensile strength of the concrete is negligible(可以忽略的,不予重视的)and disregarded(忽视).从上述讨论可知,在预应力(混凝土)结构的杆件中恒久预应力是在全部恒载和活载加载之前产生的,其目的是为了消除或大大减少由这些荷载产生的净拉应力。对于钢筋混凝土结构来说,假定混凝土的抗拉强度
12、是可以忽略的.,8,9,This is because the tensile forces resulting from the bending moments(弯矩)are resisted(vt.抵抗,反抗)by the bond created in the reinforcement process.Cracking and deflection are therefore essentially irrecoverable(不可恢复的)in reinforced concrete once the member has reached its limit state(极限状态)at
13、 service load(使用荷载).这是因为由弯矩产生的拉应力被(钢筋和混凝土)在加固过程中产生的黏结力所抵消。因此,在钢筋混凝土结构中,一旦杆件在使用荷载下达到它的极限状态时,挠度和裂缝是不可恢复的.,5.The reinforcement in the reinforced concrete member does not exert any force of its own on the member,contrary to the action(作用，动作)of prestressing steel.The steel required to produce the prestre
14、ssing force(预应力)in the prestressed member(预应力构件)actively preloads(预载，预加荷载)the member,permitting a relatively high controlled recovery(n.恢复,痊愈)of cracking and deflection(挠度).Once the flexural tensile strength(抗弯强度,挠曲强度)of the concrete is exceeded(超越，超过),the prestressed member starts to act like a rei
15、nforced concrete element.在钢筋混凝土构件中的钢筋没有完全发挥它的全部作用，而预应力钢筋正好相反。在预应力构件中的（要求产生）预应力筋积极的给构件产生预加荷载，允许对开裂和变形有较高的可控制的恢复。一旦混凝土的挠曲抗拉强度被超越，预应力混凝土构件就会像非预应力混凝土构件一样开始工作。,10,6.Prestressed members are shallower(adj.浅的,浅薄的)in depth than their reinforced concrete counterparts(配对物，对应物)for the same span(跨径)and loading c
16、onditions.In general,the depth of a prestressed concrete member is usually about 65 to 80 percent of the depth of the equivalent(相等的,相当的)reinforced concrete member.Hence,the prestressed member requires less concrete,and about 20 to 35 percent of the amount of reinforcement.在相同的跨度和荷载条件下，预应力构件（截面）的高度比
17、相应的非预应力构件的高度小。总的来说，预应力混凝土构件（截面）高度常常是相应的非预应力构件（截面）高度的6580。因此，预应力混凝土构件需要较少的混凝土，并且大概是非预应力构件所用混凝土量的2035。,11,12,Unfortunately,this saving in material weight is balanced by the higher cost of the higher quality materials needed in prestressing.Also,regardless of(adj.不管,不顾)the system used,prestressing oper
18、ations themselves result in an added(更多的,附加的,额外的)cost:formwork(模板)is more complex,since the geometry of prestressed sections is usually composed of(由.组成)flanged(翼缘)sections with thin webs(腹板,梁腹).不幸的是，在预应力构件中，所节省材料重量是和其所需高质量材料的花费相平衡（抵消）的。同时，更不用说（工艺）设备的使用，施加预应力自身会产生额外的费用：因为预应力构件的几何截面常常由翼缘和薄的腹板组成，所以模板是
19、非常复杂的,7.In spite of these additional costs,if a large enough number of precast units(预制混凝土构件)are manufactured(vt.制造,加工),the difference between at least the initial costs(最初成本，初建费用，初投资)of prestressed and reinforced concrete systems is usually not very large.尽管这么多附加费，但是如果制造大量的预制混凝土构件，至少在预应力构件和非预应力构件的最
20、初成本是相差不大的。,13,And the indirect(adj.间接的)long-term savings are quite substantial,because less maintenance(维护)is needed,a longer working life is possible due to better quality control(n.质量管理,质量控制)of the concrete,and lighter foundations are achieved due to the smaller cumulative(adj.累积的)weight of the su
21、perstructure(上部结构).并且，因为需要更少的维护，由于混凝土更高质量的控制，获得更长的生命周期是可能的，以及由于上部结构的轻质积累产生更轻的基础效应，所以长期的间接节省是非常实际的,14,8.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(adj.过多的,过分的),resulting in heavier(adj.沉重的,巨大的)members and,consequently(a
22、dv.从而,因此),greater long-term deflection and cracking.一旦钢筋混凝土梁的跨度超过7090英尺（21.327.4m），梁（上）的恒定重量变的非常大，从而导致更大的构件尺寸，并且因此产生更大的长期挠度和裂缝,15,Thus,for larger spans,prestressed concrete becomes mandatory(adj.命令的,强制的)since arches are expensive to construct and do not perform as well due to the severe(adj.严厉的,严格的,
23、严重的,严峻的)long term shrinkage(收缩)and creep(徐变)they undergo(vt.经历,遭受).Very large spans such as segmental bridges(分段拼装式桥)or cable-stayed bridges(斜拉桥)can only be constructed through the use of prestressing(预应力，预加应力).因此，对于大跨度结构来说，由于它们经受的严重的长期收缩和徐变，鉴于拱形结构在施工上昂贵的花费和不好完成，强制使用预应力结构。例如只能通过预应力这种技术建造分段拼装式桥或斜拉桥这种
24、大跨度结构。,16,9.Prestressed concrete is not a new concept,dating back to（从.时就有,回溯到,远在.(年代)）1872,when P.H.Jackson,an engineer from California,patented（n.专利权,专利品adj.专利的,特许的,vt.取得.的专利权）a prestressing system(n.系统,体系,制度,体制,秩序,规律,方法)that used a tie rod(拉杆)to construct beams or arches from individual(adj.个别的,单
25、独的)blocks.预应力混凝土不是一个新的概念，追溯到1872年，当P.H.Jackson，一个来自于加洲的工程师，通过使用拉杆来建造单块的梁或拱,从而申请了预应力系统专利。,17,After a long lapse of time(时光的流逝,一段时间)during which little progress was made known because of the unavailability(n.无效,不能利用)of high-strength steel to overcome prestress losses,R.E.Dill of Alexandria,Nebraska,re
26、cognized the effect of the shrinkage(收缩)and creep(徐变)(transverse(横向的)material flow)of concrete on the loss of prestress.随后很长一段时间，由于无法克服高预应力损失引起的高强钢筋的低利用率,从而使得该项技术没有得到很快发展。R.E.Dill,亚历山大市,内布拉斯加州,论证了混凝土的收缩和徐变给预应力损失所带来的影响.,18,He subsequently(adv.后来,随后)developed the idea that successive(adj.连续的)post-tens
27、ioning(后张拉)of unbonded rods(杆,棒，钢筋)would compensate for(vt.补偿,抵偿)the time-dependent loss of stress in the rods due to the decrease in the length of the member because of creep and shrinkage.In the early 1920s,W.H.Hewett of Minneapolis developed the principles of circular prestressing.他随后发明了一种方法,对于无黏
28、结筋的连续后张拉将会补偿由于混凝土的收缩和徐变造成的杆件长度随时间变短所带来的预应力损失。在20世纪20年代早期,W.H.Hewett 发展了环形预应力原理。,19,He hoop-stressed(环向应力,箍应力)horizontal reinforcement around walls of concrete tanks through the use of turnbuckles(n.螺丝扣,套筒螺母)to prevent cracking due to internal liquid pressure,thereby(adv.因此,从而,在那方面,在那附近)achieving wat
29、ertightness(水密性,不透水性).他通过借助套筒螺母锚具使用环向预应力筋水平缠绕混凝土池壁,从而阻止了由于内部液体压力引起池壁的开裂,因此,水池达到了密闭性.,20,21,Thereafter(其后,从那以后),prestressing of tanks and pipes developed at an accelerated pace in the United States,with thousands of tanks for water,liquid,and gas storage built and much mileage(n.英里数,英里里程)of prestress
30、ed pressure pipe laid in the two to three decades that followed.从此以后,桶和管的(环向)预应力(技术)在美国有了一个加速的发展,在随后的二三十年伴随着数以千记的水、液体、气体存储罐的建造和很长英里的预应力压力管道的铺设.,10.Linear prestressing continued to develop in Europe and in France,in particular through the ingenuity(天才的或有想象力的发明，独创性)of Eugene Freyssinet,who proposed in
31、 1926-28 methods to overcome prestress losses through the use of high-strength and high-ductility steels.In 1940,he introduced(vt.介绍,传入,引进,提出)the now well-known(adj.众所周知的,有名的,清楚明白的)and well-accepted Freyssinet system.在欧洲和法国,长线预加应力法得到了持续的发展,尤其是通过Eugene Freyssinet在1926-28年提出的克服预应力损失的天才办法,这种方法是通过使用高强和高
32、韧性钢筋得到的。在1940年,他发明了现在众所周知的和公认的Freyssinet(预应力)体系.,22,11.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 contributed a great deal to the art and science of
33、the design of prestressed concrete.英国的P.W.Abeles 在20世纪30年代到60年代之间发明并且发展了部分预应力(结构)的概念。德国的F.Leonhardt,俄罗斯的V.Mikhailov和美国的 T.Y.Lin 也对预应力混凝土设计的艺术和科技作出了很大贡献.。,23,Lins load-balancing method deserves particular mention in this regard(adv.在这点上),as it considerably(adv.相当（大,多）地,可观地)simplified the design proce
34、ss(设计程序),particularly in continuous structures.These twentieth-century developments have led to the extensive use of prestressing throughout the world,and in the United States in particular.在这点上,尤其Lin的荷载平衡法值得一提,因为它相当大的简化了设计程序,尤其在连续结构上。这些20世纪的发展导致了预应力在全世界,尤其是在美国的广泛应用。,24,12.Today,prestressed concrete
35、 is used in buildings,underground structures(地下构造物),TV towers,floating(漂浮的,浮动的,移动的,流动的,不固定的)storage and offshore structures(海上结构物),power stations(n.发电站),nuclear reactor vessels(核反应堆容器),and numerous(adj.众多的,许多的,无数的)types of bridge systems including segmental bridge(分段拼装式桥)and cable-stayed bridges(斜拉桥
36、).今天,预应力混凝土(结构)被用在建筑,地下地下构造物,电视塔,漂浮存储器和海上结构物,发电站,核反应堆容器和桥梁体系的众多形式上,其中包括分段拼装式桥和斜拉桥.,25,They demonstrate the versatility(n.多功能性，通用性)of the prestressing concept and its all-encompassing(v.包围,环绕,包含或包括某事物)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 short-and long-term losses in the prestressing forces.它们展示了预应力概念的通用性和全方位应用性。所有这些结构发展和建设的成功主要是由于材料技术的不断进步,尤其是预应力钢筋,以及对预应力短期和长期损失值估计的知识的积累.,26,
