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2、ITY, COUNTRY,SOURCES OF PRESTRESS FORCE 预应力的来源,3,Lesson 15,main part,The most obvious method of pre-compressing预应力最常用的方法,Other means for introducing the prestressing force 使用预应力的其他方式,retensioning or post-tensioning systems先张法和后张法,For designer对设计者而言,Prestress can be applied to a concrete member in ma
3、ny ways. Perhaps the most obvious method of pre-compressing is to use jacks reacting against abutments, as shown in Fig.15-1a. Such a scheme has been employed for large projects. Many variations are possible, including replacing the jacks with compression struts after the desired stress in the concr
4、ete is obtained; or using inexpensive jacks that remain in place in the structure, in some cases with a cement grout used as the hydraulic fluid. The principal difficulty associated with such a system is that even a slight movement of the abutments will drastically reduce the prestress force.,5,预应力技
5、术可以通过很多方法应用于混凝土。可能预应力最普遍的方法就是用千斤顶压迫支座，如图15-1a。这种方案已经被用来做大项目。这种方法的变相使用都是可能的，包括用斜压杆取代千斤顶给结构施加所需的压力是可行的，或者把廉价的千斤顶扣在结构中，在某些情况下把水泥浆当做液压油使用。和这个系统相关的最重要的问题是支座的细微变化都可能降低预应力。,The most obvious method pre-compressing,预应力最常用的方法,Beam,Jack,Abutment,Fig.15-1(a) Post-tensioning by jacking against abutments,In most
6、cases, the same result is more conveniently obtained by tying the jack bases together with wires or cables, as shown in Fig.15-1b. 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, on
7、e end of the prestressing tendon is anchored, and all of the force is applied at the other end. After reaching the desired prestress force, the tendon is wedged against the concrete and the jacking equipment is removed for reuse. In this type of prestressing, the entire system is self-contained and
8、is independent of relative displacement of the supports.,7,在大多数情况下，把千斤顶和钢丝或钢索连接起来可以方便获得同样的数据，如图15-1b所示。这些钢丝或钢索可以放在梁的外边两侧；更常见的是使它穿过埋设在混凝土梁内的空心导管。通常来说，预应力钢筋的一端锚固，在另一端施加全部力。在达到所需的预应力值后，将钢筋束楔紧到结构中，千斤顶设备可以移走以便再次使用。在这种预应力下，整体结构是独立的而且不受支撑物将对位移的影响。,The most obvious method pre-compressing,预应力最常用的方法,Beam,Cabl
9、e,Jack,Fig.15-1(b) Post-tensioning with jacks recting against beam,Another method of prestressing, that is widely used is illustrated in Fig.15-1c. The prestressing strands are tensioned between massive abutments in a casting yard prior to placing the concrete in the beam forms. The concrete is plac
10、ed around the tensioned strands, and after the concrete has attained sufficient strength, the jacking pressure is released. This transfers the prestressing force to the concrete by bond and friction along the strands, chiefly at the outer ends. It is essential, in all three cases shown in Fig.15-1,
11、that the beam be supported in such a way as to permit the member to shorten axially without restraint so that the prestressing force can be transferred to the concrete.,9,另一种广泛使用的加预应力的方法，如图15-1c所示。预应力钢索在以梁的形式安装到结构前，在铸造的地方会被巨大的支座拉紧。结构会被放置在拉紧的钢索上，在结构达到足够的强度后，千斤顶的压力就会被释放。预应力通过粘接剂和钢索的摩擦力传给结构并主要在外部结束。 如图
12、15-1所示的所有三种情况下，梁的支撑条件必须允许其毫无约束地轴向缩短，以便将预应力传递至混凝土。,The most obvious method pre-compressing,预应力最常用的方法,Beam,Cable,Anchorage,Jack,Fig.15-1c Pretensioning with tendon stressed between fixed external anchorages,Other means for introducing the desired prestressing force have been attempted on an experiment
13、al basis. Thermal prestressing can be achieved by preheating the steel by electrical or other means. Anchored against the ends of the concrete beam while in the extended state, the steel cools and tends to contract. The prestress force is developed through the restrained contraction. The use of expa
14、nding cement in concrete members has been tried with varying success. The volumetric expansion, restrained by steel strands or by fixed abutments, produces the prestress force.,11,采用预应力的的其他方法正在通过试验性的原理尝试。用电或者其他方法预热钢可以获得加热预应力。当钢材处于广延态，逐渐降温并趋向于收缩的固定到混凝土的末端。此时预应力通过约束收缩已经发展成熟。膨胀水泥在混凝土构件中的应用已经取得很大成功。它体积的
15、膨胀受钢绞线和固定支座的限制而产生预应力。,Other means for introducing the derired,使用预应力的其他方式,细颗粒膨胀水泥,粗颗粒膨胀水泥,Most of the patented systems for applying prestress in current use are variations of those shown in Fig.15-1b and c. Such systems can generally be classified as pretensioning or post-tensioning systems. In the c
16、ase of pretensioning, the tendons are stressed before the concrete is placed, as in Fig.15-1c. This system is well-suited for mass production, since casting beds can be made several hundred feet long, the entire length cast at once, and individual beams can be fabricated to the desired length in a s
17、ingle casting.,13,目前使用的绝大多数的施加预应力的方法就像如图15-1b和15-1c所示。这些方法普遍都被归类为先张法或者后张法。对于先张法，钢束在混凝土浇筑之前施加应力，如图15-1c所示。这种体系非常适合批量生产，浇筑长度可以达到数百英尺，沿全长一次浇筑混凝土。再把长的浇筑体分割成所需长度的单个构件。,先张法,Pretensioning,先张法预应力高强度混凝土管桩的生产,乌海项目先张法预应力空心板预制超百,16m先张法预应力混凝土空心桥板,In post-tensioned construction, shown in Fig.15-1b, the tendons ar
18、e tensioned after the concrete is placed and has gained its strength. Usually, a hollow conduit or sleeve is provided in the beam, through which the tendon is passed. In some cases, tendons are placed in the interior of hollow box-section beams. The jacking force is usually applied against the ends
19、of the hardened concrete, eliminating the need for massive abutments In Fig.15-2, six tendons, each consisting of many individual strands, are being post-tensioned sequentially using a portable hydraulic jack.,16,后张法施工在混凝土浇筑并达到一定强度才能张拉钢束，如图15-1b所示。通常，梁内有空心导管或套筒，从而钢束可以穿过去。在有些情况下，钢索被安置在中空的箱型断面梁的内部。千斤顶
20、的力量经常用来压迫硬化的混凝土的两端，以消除对大量支座的需要，在图15-2中，有许多独立的钢绞线组成的六根钢索通过一台便携式液压千斤顶被依次张拉。,后张法,post-pretensioning,后张法预应力混凝土吊车梁施工(实例),后张法预应力箱梁桥施工组织设计,A large number of particular systems, steel elements, jacks, and anchorage fittings have been developed in this country and abroad, many of which differ from each other
21、 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 technique that is to be followed and the equipment to be used. It is frequently best to specify only the magnitude and line of actio
22、n of the prestress force. The contractor is then free, in bidding the work, to receive quotations from several different prestressing subcontractors, with resultant cost savings.,19,对于设计者而言,For designer,许多独特的方法，像钢材元素、千斤顶和装配锚固等国内外都很成熟，其中有很多彼此只在小细节上不同。就预应力混凝土结构的设计者而言，其实不需要甚至通过细节列举那些被模仿的方法和被使用的设备。最好只通过
23、预应力功能的量级和路线来指定。承包商将从投标工作中解放出来，从不同的分包商获得报价单并整合以节约成本。,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 accommodated.,20,对于设计者而言,For designer,显而易见，设计者必须拥有考虑到使用中的不同方法的细节知识，以便在选择横截面尺寸上，几个方法的任何一个都能够被接纳。,SEE YOU SOON!,THANK YOU!,
