外文翻译--斜拉桥的未来

中文1590字between towers increase if the number of cables increase and the angle of inclination of the cables remains the same. B.READING MATERIALFUTURE OF CABLE-STAYED BRIDGESI would like to begin with a view back on the development of cable-stayed bridges during the last 25 years.It started with Dischingers publication shortly after the end of Word War II. He pointed mainly to the necessity of gonging to high steel stresses in the stays to produce stiffness in the system.The first bridge following Dischingers recommendations,was built in Sweden,designed by Demag,a German steel construction firm,consulted by Dischinger.Then in 1953-54 the three Duesseldorf bridges were designed,all of them,with parallel stay cables but different tower arrangements,in order to have a family of similarly appearing bridges.The fundamental concept of these early designs was retained for over almost 20 years,which it took to built them. We learned by detailing and erecting these bridges.In these bridges,only a few stay cables were chosen;some engineers designed their bridges with even only one stay cable.This resulted in large cable forces causing difficulties to anchor the cables in the beam structure. Heavy cross beams were necessary, the ropes had to be formed.To gain sufficient space for the anchors, adjustment of cable lengths becomes difficult. In addition, the large distance between the stay cables complicate the erection requiring heavy equipment, auxiliary trusses,even auxiliary piers were necessary to build the Maracaibo bridge and the Kniebridge.Auxiliary stayes were needed for cantilevering the beam plate girder to the next stay cable. In addition, long spans between supports provided by stay cables,cause large bending moments in the continuous beam and hereby a considerable depth of the girders is needed.Form all of this experience, we concluded for our later and future designs ,thata) a large number of stay cables should be chosen in a way b) that one anchorage socket can be used to simplify the placing of the cable, c) by short spacing of the cables, bridge girder bending moments are low so that a depth of 1 to 2 m is sufficient, just providing a deflection line curvature satisfying traffic requirements and providing safety against buckling in the deflected stage. d) The spacing of the cables should be such, that no heavy erection equipment is needed to cantilever out for placing the next following stay cable. e) Feasible spacings may be between 6 and 12 m for concrete girders and between 8 and 16 m for steel girders.In order to satisfy these rules, my office developed a new type of cable anchorage in cooperation with BBR Switzerland, which allows ultimate cable forces up to almost 2,000 tons, using parallel wires or strands of very high strength, inside a polyethylene tube for perfect corrosion protection. The anchorage was developed to get high fatique strength, therefore called High Amplitude(HiAm) cable. These cables can be prefabricated and shipped on reels and allow a simple and inexpensive erection.Several bridges have been designed lately using these principles:The Pasco bridge, bridges in Parana, Argentina, and others.As we designed these bridges, I knew already the favorable effect of system damping in multi-stay cable bridges by experience which I had gained from the behavior of a pedestrian stay cable bridge in Stuttgart, but we had to prove the dynamic safety for these larges. A dynamic model test was made at the Ismes Institute of Profssor Oberti in Italy, 18 m long designed for full dynamic similitude. Short and long trains or just locomotives could run on the rails with different speeds-no adverse oscillations could be detected.Then the test engineers excited artificially oscillations going through all possible modes and frequencies and at many points of the bridge. Whenever they succeed in building up a small amplitude, it broke quickly again down to small amplitudes. It was impossible to find a mode of oscillation which would build up large amplitudes by resonance.Any mode of oscillation broke down as soon as the amplitudes starts to grow, because each of the cables has a different natural frequency and disturbs the oscillation of deck structure by interference so strongly, that large amplitudes cannot develop. We get a very effective system damping which does not allow resonance oscillation with dangerous amplitudes.Of course, this effect is only obtained with stiff and highly stressed cables and with a sufficient number of cables in close spacing. We must recognize that the dynamic behavior of the suspension bridge is perfectly different from that of a multi stay cabled bridge. In s suspension bridge without stiffening girder, there is full freedom for the dangerous first antimetric mode of oscillation, combining torsional and bending movement. Small force can excite this mode of oscillation and build up large amplitudes by resonance. These oscillations can be restrained by stiffening trusses with large bending and torsional rigidity and resonance can mainly be avoided by a large difference between the natural frequencies of bending and torsional oscillations. Sectional mode tests in wind tunnels and theories have been well developed for these suspension bridge problems.The multi stay cable bridge-on the other side-cannot oscillate in low order modes, it especially cannot move into combined torsional and flexural oscillations. With stays along the edges of the bridge, torsional oscillations are almost impossible and flexural oscillations assume quickly high order modes with only small amplitudes. The important fact is,that resonance is impossible for the reasons described.As a consequence, we must learn that the theories which were developed to check aerodynamic safety of suspension bridges are not valid for multi stay cabled bridges. Wind tunnel tests with sectional models must be made with realistic restraint by system damping which, however, is difficult to imitate for a sectional model. * * * Fritz Leonhardt, Dr. Ing. Stuttgart, Germany 斜拉桥的未来我想先简单回顾一下在过去25年间斜拉桥发展历史，二战结束后不久，迪斯钦格便在其著作中提出了斜拉桥这一概念。在著作中，迪斯钦格主要指出，在斜拉桥这个系统中，支索上所承受的巨大钢筋压力对维持桥的稳定性有重要作用。第一座依据迪斯钦格的建议建立的斜拉桥位于瑞典。这座桥是由德国一家名为德马格的钢铁建筑公司设计的，而这家公司正是由迪斯钦格担任指导。然后在1953-1954年间，共有3座杜塞尔多夫桥相继建成。这3座桥全部都是平行的钢筋支索。但支柱的设计却各不相同。这样做的目的是为了建造一个拥有相似造型的桥梁群。在将近20年的时间里，桥梁建设一直都是在应用这些早期的基本设计理念。我们通过研究这些桥梁的细节部位和建造桥梁模型来学习这些理念。这些桥只选择了几根钢缆，一些工程师设计的桥甚至只有一根钢缆。这跟钢缆在横梁结构的桥上的固定造成巨大缆绳压力。沉重的交叉横梁是必要的，绳子也是必要的。为了给支柱营造足够的空间，缆绳长度的调整变得困难起来。此外，钢缆之间的巨大空间使得需要沉重设备的建设变得复杂，辅助构架，甚至辅助墩对马拉开波桥和尼克桥的建造都是必要的。将支柱大梁连接到下一个钢缆需要用到辅助支索。此外，钢缆提供的支撑之间的长墩距造成连续支柱的巨大弯曲，因此，大梁需要有足够的深度。从所有这些经验我们得出这样的结论，我们今后或未来的设计A）多数钢缆应该选择这种形式；B）固定处的凹口可简化钢索的排列方法；C)通过缩短钢索的间距，桥梁弯曲几率降低，因此1到2米的深度已足够提供一条偏斜直线曲度，并在偏斜阶段保证交通安全；D）排列钢索应切记，桥梁悬臂上不能累加重物以影响接下来钢索的排列；E)可行的混凝土大梁的排列方法是6米到12米，而铁梁则需8米到16米。 为满足以上要求，我方与瑞士BBR公司合作研发出了一种新型斜拉桥，钢索可承重高达两千吨，在聚乙烯钢管中，采用了高强度平行结构金属线以防止腐蚀，新型斜拉桥可承受高压，因此被称为高振幅钢索。这些钢索可提前制造且造法简单，成本低。近期所造的一些大桥均采用了这些准则：毕加索大桥，巴西，阿根廷境内以及其他的一些大桥。由于我参与设计了这些桥梁，又加上在斯图加特市步行斜拉桥的经历，我已了解斜拉桥减震系统的优势。然而我们仍需证明这些大振幅的动力安全性。意大利Ismes研究所的Oberti教授进行了一项动力模型测试，为证明完全动力相似性设计了一个18米长的钢管。只有长短途火车和当地的摩托车能以不同车速在桥上行驶，至今未发现反振幅。然后，在桥的许多地方进行各种形式和频率的人为晃动后，测试工程师们很兴奋。每次当他们想要成功建成一个大振幅时，它就会很快垮掉变成一个小振幅，不可能找到一个震动模型可通过共振来加大振幅。任何震动模型在振幅开始增强时垮掉，因为每条缆索有不同的自然频率，木制结构受到的摆动力太强而不能成为大的振幅，我们发现一个很有效的系统，可抑制危险振幅共振的发生。当然，用坚实并可承受重压的缆索，在紧凑的间隔内安置足够数量的缆索可出现这种效果。我们必须认识到，吊桥的动态活动和斜拉桥的动态活动完全不同。没有坚固大梁的大桥，第一个非公制危险的震动模型很容易和扭转弯曲活动有关。很小的外力既可引起模型的晃动并通过共振增强大的振幅，加大幅度的弯曲，加大扭曲的坚固性可加固构架来减轻摆动浮动。用弯曲的自然频率和扭转摆动之间的不同，主要用来避免共振。装配模型测验和有关吊桥问题的理论，得到很好的发展。另一方面，吊桥在低层次的模型中不会摆动，尤其不会弯曲。因为在桥的边缘有牵索，使得斜拉桥几乎不会发生扭曲性的摇摆，且会出现一些很小幅度的弯曲性的摆动。重要的一点是，综上所述，吊桥不可能发生共振。因此，我们必须认识到，那些用来检查吊桥在空气动力性层面上的安全性而提出的理论，不适用于多元性斜拉桥。使用组合模型进行的风动测试，必须要在通过系统阻尼产生的实际的限制力下进行，然后仿造一个组合模型是非常困难的。