58559土木工程专业英语鲁正电子课件-普及版concrete

1、Unit 2 Introduction to Prestressed Concrete English for Civil EngineeringTeacher: Prof. Zheng LuE-mail: （School of Civil Engineering, TONGJI UNIVERSITY）Unit 2 Introduction to Prestressed Concrete 2.1 Introduction2.2 Effects of Prestressing 2.2.1 Concrete Stress Control by Prestressing 2.2.2 Equivale

2、nt Loads 2.2.3 Prestressed Concrete as a Variation of Reinforced Concrete2.3 Sources of Prestress Force2.4 Prestressing Steels2.5 Concrete for Prestressed ConstructionLogic1.Introduction why we develop prestressed concrete economic structures, use higher strength materials, reinforced concrete probl

3、em: cracking and deflection development of prestressed concrete2. Three alternative ways to look at the prestressing of concrete(分析方法) (1) concrete stress control example: a rectangular beam cross-section (2) equivalent Loads example: a simple span (简支跨) beam (3) special variation of reinforced conc

4、rete3. Sources of Prestress Force methods: pretensioning , post-tensioning 4. Steels stress lose problem(应力损失) example：超张拉5. why use High-strength Concrete 4 reasons 2.1 IntroductionModern structural engineering tends to progress toward more economic structures through gradually improved methods of

5、design and the use of higher strength materials. This results in a reduction of cross-sectional dimensions and consequent weight savings. Such developments are particularly important in the field of reinforced concrete, where the dead load represents a substantial part of the total design load. Also

6、, in multistory buildings, any saving in depth of members, multiplied by the number of stories, can represent a substantial saving in total height, load on foundations, length of heating and electrical ducts, plumbing risers, and wall and partition surfaces.structural engineering 结构工程；cross-sectiona

7、l dimensions 截面尺寸；reinforced concrete 钢筋混凝土；dead load 恒载；foundations 基础；ducts 管线；plumbing risers管道吊组绳2.1 IntroductionThese limiting features of ordinary reinforced concrete have been largely e by the development of prestressed concrete. A prestressed concrete member can be defined as one in which th

8、ere have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loading are counteracted to a desired degree. Concrete is basically a compressive material, with its strength in tension a low and unreliable value. Prestressing applies

9、a pression to the member that reduces or eliminates undesirable tensile stresses that would otherwise be present. Cracking under service loads can be minimized or even avoided entirely. Deflections may be limited to an acceptable value; in fact, members can be designed to have zero deflection under

10、the combined effects of service load and prestress force.预应力混凝土构件介绍be defined as 定义为；magnitude and distribution 大小和分布； tensile stresses拉应力；Cracking 开裂；deflection 变形2.2 Effects of Prestressing2.2.1 Concrete Stress Control by PrestressingMany important features of prestressed concrete can be demonstra

11、ted by simple examples. Consider first the plain, unreinforced concrete beam shown in Fig. 2-1a. It carries a single concentrated load at the center of its span. The self-weight of the member will be neglected here. As the load W is gradually applied, longitudinal flexural stresses are induced. If t

12、he concrete is stressed only within its elastic range, the flexural stress distribution at midspan will be linear, as shown. At a relatively low load, the tensile stress in the concrete at the bottom of the beam will reach the tensile strength of the concrete fr, and a crack will form. Because no re

13、straint is provided against upward extension of the crack, the beam will collapse without further increase of load.concentrated load 集中荷载；the center of its span 跨中；self-weight 自重; longitudinal flexural stresses 纵向弯曲应力；elastic range 弹性范围； tensile strength 抗拉强度2.2 Effects of Prestressing2.2.1 Concrete

14、 Stress Control by Prestressing（1）But it would be more logical to apply the prestressing force near the bottom of the beam, to compensate more effectively for the load-induced tension. A possible design specification, for example, might be to introduce the maximum compression at the bottom of the be

15、am without causing tension at the top, when only the prestressing force acts. It is easily shown that, for a beam with a rectangular cross section, the point of application of the prestressing force should be at the lower third point of the section depth to achieve this. 预应力施加过程prestressing force 预应

16、力； design specification 设计方案；rectangular cross section 矩形截面；section depth 截面高度2.2 Effects of Prestressing2.2.1 Concrete Stress Control by Prestressing（2）The force P, with the same value as before, but applied with eccentricity e = h/6 relative to the concrete centroid, will produce a longitudinal co

17、mpressive stress distribution varying linearly from zero at the top surface to a maximum of 2fc= P/Ac + Pec2/Ic at the bottom, where fc is the concrete stress at the concrete centroid, c2 is the distance from the concrete centroid to the bottom of the beam, and Ic is the moment of inertia of the cro

18、ss section. This is shown in Fig. 2-1c. The stress at the bottom will be exactly twice the value produced before by axial prestressing.预应力施加过程Eccentricity 偏心；concrete centroid 混凝土中部；longitudinal compressive stress distribution 纵向压应力分布；the moment of inertia of the cross section横截面的惯性矩2.2 Effects of P

19、restressing2.2.1 Concrete Stress Control by PrestressingFor each characteristic load distribution, there is a best tendon profile that produces a prestress moment diagram that corresponds to that of the applied load. If the prestress countermoment is made exactly equal and opposite to the load-induc

20、ed moment, the result is a beam that is subject only to uniform axial compressive stress in the concrete all along the span. Such a beam would be free of flexural cracking, and theoretically it would not be deflected up or down when that particular load is in place, compared to its position as origi

21、nally cast. Such a result would be obtained for a load of 1/22Q=Q, as shown in Fig. 2-le，for example.基于荷载分布的最佳钢筋束布置方案best tendon profile 最佳钢筋束布置方案；prestress moment diagram 预应力弯矩图；Countermoment 反弯矩，恢复力矩；all along the span 沿着跨长；flexural cracking 弯曲开裂2.2 Effects of Prestressing2.2.1 Concrete Stress Con

22、trol by Prestressing2.2 Effects of Prestressing2.2.1 Concrete Stress Control by Prestressing2.2 Effects of Prestressing2.2.2 Equivalent LoadsIn Fig. 2.2a, for example, a tendon that applies force P at the centroid of the concrete section at the ends of a beam and that has a uniform slope at angle be

23、tween the ends and midspan introduces a transverse force 2P sin at the point of change of slope at midspan. At the anchorages, the vertical component of the prestressing force is Psin and the horizontal component is Pcos. The horizontal component is very nearly equal to P for the usual flat slope an

24、gles. The moment diagram for the beam of Fig. 2-2a is seen to have the same form as that for any center-loaded simple span.预应力的力等效过程uniform slope 等斜率，等坡度；midspan 跨中； transverse force 横向力，剪力；anchorages 锚固端；moment diagram 弯矩图2.2 Effects of Prestressing2.2.2 Equivalent LoadsIf a straight tendon is used

25、 with constant eccentricity, as shown in Fig. 2-2c, there are no vertical forces on the concrete, but the beam is subject to a moment Pe at each end, as well as the axial force P，and a diagram of constant moment results.The end moment must also be accounted for in the beam of Fig. 2-2d, in which a p

26、arabolic tendon is used that does not pass through the concrete centroid at the ends of the span. In this case, a uniformly distributed upward load plus end anchorage forces are produced, as shown in Fig. 2-2b, but in addition, the end moments M = Pecos must be accounted for.预应力的力等效过程straight tendon

27、 直线型预应力筋；vertical forces 竖向力；axial force 轴向力；constant moment 固定弯矩；end moment 端弯矩；parabolic tendon 抛物线型的预应力钢筋；uniformly distributed 均布2.2 Effects of Prestressing2.2.2 Equivalent Loads2.2 Effects of Prestressing2.2.3 Prestressed Concrete as a Variation of Reinforced ConcreteIn the descriptions of the

28、effects of prestressing in the paragraphs above, it was implied that the prestress force remained constant as the vertical load was introduced, that the concrete responded elastically, and that no concrete cracking occurred. These conditions may prevail up to about the service load level, but if the

29、 loads should be increased much beyond that, flexural tensile stresses will eventually exceed the modulus of rupture and cracks will form. Loads can usually be increased much beyond the cracking load in well-designed prestressed beams. 作为钢筋混凝土的变形体flexural tensile stresses 弯曲拉应力；exceed the modulus of

30、 rupture and cracks will form 超过断裂模量而产生裂缝；well-designed 设计良好的2.2 Effects of Prestressing2.2.3 Prestressed Concrete as a Variation of Reinforced ConcreteEach of the three viewpoints describedconcrete stress control, equivalent loads, and reinforced concrete using prestrained steelis useful in the ana

31、lysis and design of prestressed concrete beams, and none of the three is sufficient in itself. Neither an elastic stress analysis nor an equivalent load analysis provides information about strength or safety margin. However, the stress analysis is helpful in predicting the extent of cracking, and th

32、e equivalent load analysis is often the best way to calculate deflections. Strength analysis is essential to evaluate safety against collapse, but it tells nothing about cracking or deflections of the beam under service conditions.总结三类观点equivalent loads 等效荷载；strength or safety margin 强度和安全界限；extent

33、of cracking 开裂的程度；Strength analysis 强度分析；evaluate safety against collapse 评估抵抗倒塌的安全度2.3 Sources of Prestress ForcePrestress can be applied to a concrete member in many ways. Perhaps the most obvious method of pressing is the use of jacks reacting against abutments, as shown in Fig. 2-4a. Such a sche

34、me has been employed for large projects. Many variations are possible, including replacing the jacks with compression struts after the desired stress in the concrete is obtained or using inexpensive jacks, that remain in place in the structure, in some cases with a cement grout used as the hydraulic

35、 fluid. The principal difficulty associated with such a system is that even a slight movement of the abutments will drastically reduce the prestress force. 预应力的施加 pressing 先张法；Jacks千斤顶；abutments 台座；cement grout 水泥浆；hydraulic fluid 液压流体2.3 Sources of Prestress ForceIn most cases, the same result is m

36、ore conveniently obtained by tying the jack bases together with wires or cables, as shown in Fig. 2-4b. These wires or cables may be external, located on each side of the beam; more usually they are passed through a hollow conduit embedded in the concrete beam. Usually, one end of the prestressing t

37、endon is anchored, and all the force is applied at the other end. After attainment of the desired prestress force, the tendon is wedged against the concrete and the jacking equipment is removed for reuse. Note that in this type of prestressing, the entire system is self-contained and is independent

38、of relative displacement of the supports.施加预应力wires or cables 钢线或锚索；a hollow conduit embedded in the concrete beam 嵌入混凝土梁的空心管道；attainment of the desired prestress force 达到所需预应力；wedged 嵌入；self-contained 独立的；relative displacement 相对位移2.3 Sources of Prestress ForceMost of the patented systems for apply

39、ing prestress in current use are variations of those shown in Fig. 2-4b and c. Such systems can generally be classified as pretensioning or post-tensioning systems. In the case of pretensioning, the tendons are stressed before the concrete is placed, as in Fig. 2-4c. This system is well suited for m

40、ass production, since casting beds can be made several hundred feet long, the entire length cast at once, and individual beams cut to the desired length in a single casting. Fig. 2-5 shows workers using a hydraulic jack to tension strands at the anchorage of a long pretensioning bed. Although each t

41、endon is individually stressed in this case, large capacity jacks are often used to tension all strands simultaneously.先张法post-tensioning 后张法；mass production 工厂生产； casting beds 浇筑平台； a hydraulic jack 液压千斤顶2.3 Sources of Prestress Force2.3 Sources of Prestress ForceA large number of particular system

42、s, steel elements, jacks, and anchorage fittings have been developed in this country and abroad, many of which differ from each other only in minor details. As far as the designer of prestressed concrete structures is concerned, it is unnecessary and perhaps even undesirable to specify in detail the

43、 technique that is to be followed and the equipment to be used. It is frequently best to specify only the magnitude and line of action of the prestress force. The contractor is then free, in bidding the work, to receive quotations from several different prestressing subcontractors, with resultant co

44、st savings. It is evident, however, that the designer must have some knowledge of the details of the various systems contemplated for use, so that in selecting cross-sectional dimensions, any one of several systems can be modated.预应力设计原则和要求the magnitude and line of action of the prestress force 预应力的

45、大小和作用线； contractor 承包商；subcontractors分包商； modated 适合的2.4 Prestressing Steels2.4 Prestressing Steels2.4 Prestressing Steels（1）The tensile stress permitted by ACI Code 18.5 in prestressing wires, strands, or bars is dependent upon the stage of loading. When the jacking force is first applied, a stress

46、 of 0.80 fpu or 0.94 fpy is allowed, whichever is smaller, where fpu is the ultimate strength of the steel and fpy is the yield strength. Immediately after transfer of prestress force to the concrete, the permissible stress is 0.74 fpu or 0.82 fpy, whichever is smaller (except at post-tensioning anc

47、horages where the stress is limited to 0.70 fpu). The justification for a higher allowable stress during the stretching operation is that the steel stress is known quite precisely at this stage. 超张拉ACI Code 18.5 美国混凝土协会标准 American Concrete Institute jacking force 顶推力；ultimate strength 极限强度；yield str

48、ength 屈服强度；permissible stress 容许应力2.4 Prestressing Steels（2）Hydraulic jacking pressure and total steel strain are quantities that are easily measured. In addition, if an accidentally deficient tendon should break, it can be replaced; in effect, the tensioning operation is a performance test of the m

49、aterial. The lower values of allowable stress apply after elastic shortening of the concrete, frictional loss, and anchorage slip have taken place, when service loads may be applied. The steel stress is further reduced during the life of the member due to shrinkage and creep in the concrete and rela

50、xation in the steel.超张拉Hydraulic jacking pressure 液压顶升压力； performance test 性能测试；allowable stress 容许应力；elastic shortening 弹性收缩；frictional loss 摩擦损耗；anchorage slip 锚固滑移2.5 Concrete for Prestressed Construction 1. High-strength concrete normally has a higher modulus of elasticity (see Fig.2-3). This means a reduction in initial elastic strain