土木工程专业英语讲稿Lesson_8 Bridge Design and Construction.doc

Lesson 8 Bridge Design and Construction桥梁设计和施工Planning 规划 The first step leading to the construction of a modern major bridge is a comprehensive study to determine whether a bridge is needed. 建一座现代化大型桥梁的第一步就是广泛研究并确定建桥的必要性。 If it is to be a highway bridge, in the United States for example, a planning study is initiated by a state bridge authority, possibly in cooperation with local governments or the federal government. 比如在美国，如果要建一座公路桥梁，州桥梁专家发起规划研究，且需要与当地政府或联邦政府合作。 Studies are made to estimate the amount of bridge traffic, the relief of jammed traffic in nearby highway networks, the effects on the regional economy, and the cost of the bridge. 要评估过桥的交通量，能否缓解附近公路网交通拥堵情况，对区域经济的影响以及桥的造价。 The means for financing the project, such as public taxes or sale of revenue bonds repaid by toll charges, are considered. 还要考虑工程资金的筹措，如公共税收，或通过收取过路过桥费偿还借贷的国债。 If the studies lead to a decision to go ahead with the project, the land needed for the bridge and its approaches is acquired at the selected site. At this point, field engineering work is started. 若经过研究决定要实施路桥工程，那么就需要征用选址处的建桥用地和建设通向桥址的道路占地。此后，现场工作便开始了。 Accurate land surveys are made. Tides, flood conditions, currents, and other characteristics of the waterway are carefully studied. Boring samples of soil and rock are taken at possible foundation locations, both on land and under the water. 现场要做好精确的勘测工作。潮汐、洪水状况、水流以及航道特点等都要进行仔细研究。尽量在基础施工现场（包括岸上和水下）进行土体和岩石的钻孔取样。 Selection of bridge design 桥梁设计的选择 The chief factors in deciding whether a bridge will be built as a girder, cantilever, truss, arch, suspension, or some other type are：(1) location； for example, across a river; (2) purposes；for example, a bridge for carrying motor vehicles；(3) span length；(4) strength of available materials；(5) cost；(6) beauty and harmony with the location. 决定将桥建成梁桥、悬臂桥、桁架桥、拱桥、悬索桥还是其他类型桥的主要因素有：（1）桥梁位置；如跨越河流；（2）用途，如汽车运输；（3）跨距；（4）可用材料强度；（5）造价；（6）美观以及与场地环境的协调性。 Each type of bridge is most effective and economical only within a certain range of span lengths, shown in the following table： 每类桥梁都只在某个跨距范围内才省钱、有用，如下表所列： As indicated in the table, there is a considerable overlap in the range of applicability of the various types. 表中数据表明，不同类型桥的跨距值存在相当大的交叠。 In some cases, alternative preliminary designs are prepared for several types of bridge in order to have a better basis for making the final selection. 有时，为了使最终方案论据充分，通常要准备多种类型桥梁设计方案进行初步比较。Selection of materials 选材 The bridge designer can select from a number of modern high-strength materials, including concrete, steel, and a wide variety of corrosion-resistant alloy steels. 桥梁设计人员可以选择多种现代高强材料，如混凝土、钢材和不同类型的抗腐蚀合金钢。 For the Verrazano-Narrows Bridge, for example, the designer used at least seven different kinds alloy steel, one of which has a yield strength of 50, 000 pounds per square inch (psi) (3, 515 kgsq cm) and does not need to be painted because an oxide coating forms on its surface and inhibits corrosion. 比如，维拉扎诺海峡大桥的设计者用了至少7种不同类型的合金钢，其中任何一种的屈服强度都达到3, 515 kg平方厘米, 而且无需喷涂，因为在其表面已经形成了氧化层可以抑制侵蚀。 The designer also can select steel wires for suspension cables that have tensile strengths up to 250, 000 psi (17, 577 kgsq cm). 设计人员还可以选择抗拉强度高达250000磅每平方英寸的钢丝束作悬索。 Concrete with compressive strengths as high as 8,000 psi (562.5 kgsq am) can now be produced for use in bridges, and it can be given high durability against chipping and weathering by the addition of special chemical agents and control of the hardening process. 如今用于桥梁的混凝土其抗压强度高达8000psi。通过添加特殊的化学添加剂可控制硬化过程；这种砼使用年限长，可抗剥落和抗风化。 Concrete that has been prestressed and reinforced with steel wires has a tensile strength of 250, 000 psi (17, 577 kgsq cm). 预应力混凝土和钢筋混凝土的抗拉强度为250000psi。 Other useful materials for bridges include aluminum alloys and wood. 其他建桥材料还包括铝合金和木材。 Modern structural aluminum alloys have yield strengths exceeding 40,000 psi (2,812 kgsq cm). 现代结构使用的铝合金其屈服强度超过40000psi。 Laminated strips of wood glued together can be made into beams with strengths twice that of natural timbers；glue-laminated southern pine, for example, can bear working stresses approaching 3,000 psi (210.9 kgsq cm). 多层胶合木材能制成强度比普通木材多一倍的梁。比如，层压胶合的南方松木，能承受接近3000psi的有效应力。 Analysis of forces 受力分析 A bridge must resist a complex combination of tension, compression, bending, shear, and torsion forces. 桥梁必须能抵抗拉、压、弯、剪和扭的合力作用。 In addition, the structure must provide a safety factor as insurance against failure. 此外，结构还必须保有抵抗破坏的安全系数。 The calculation of the precise nature of the individual stresses and strains in the structure, called analysis, is perhaps the most technically complex aspect of bridge building. 精确计算结构的应力和应变变化，称为受力分析，这也许是桥梁建造技术中最繁复的方面。 The goal of analysis is to determine all of the forces that may act on each structural member. 受力分析的目的是确定可能作用在每个构件上的所有力。 The forces that act on bridge structural members are produced by two kinds of loadsstatic and dynamic. 这些力分为两种：静荷载和动荷载。 The static loadthe dead weight of the bridge structure itselfis usually the greatest load. 静荷载桥梁结构本身的恒载通常是最大的荷载。 The dynamic, or live load, has components, including vehicles carried by the bridge, wind forces, and accumulations of ice and snow. 动荷载，或活荷载，有很多分力组成，包括桥梁承担的交通工具的作用、风力和冰雪堆积物的作用。 Although the total weight of the vehicles moving over a bridge at any time is generally a small fraction of the static and dynamic load, it presents special problems to the bridge designer because of the vibration and impact stresses created by moving vehicles. 尽管桥上移动车辆的总重任何时候都只是静、动荷载的一小部分，但因为车辆的移动会产生振动和冲击，所以对于桥梁设计人员而言这仍然是一个需要特别注意的问题。 For example, the severe impacts caused by irregularities of vehicle motion or bumps in the roadway may momentarily double the effect of the live load on the bridge. 比如，由公路上车辆移动或颠簸的不规则性产生的剧烈冲击可以瞬间使得动荷载对桥梁的影响倍增。 Wind exerts force on a bridge both directly by striking the bridge structure and indirectly by striking vehicles that are crossing the bridge. 风通过直接作用于桥梁结构和间接作用于穿过桥梁的车辆上而对桥梁施加荷载。 If the wind induces aeroelastic vibration, as in the case of the Tacoma Narrows Bridge, its effect may be greatly amplified. 如果风引起空气弹性振动，就像塔科马海峡大桥的情况一样，其效应可能会被无限放大。 Because of this danger, the bridge designer makes provisions for the strongest winds that may occur at the bridge location. 因此，桥梁设计人员采取措施对可能作用在桥梁上的最大风力做出防范。 Other forces that may act on the bridge, such as stresses created by earthquake tremors, must also be provided for. 其他可能作用在桥梁上的力，如地震作用，也须加以预防。 Special attention must often be given to the design of the bridge piers, since heavy loads may be imposed on them by currents, waves, and f1oating ice and debris. 设计时必须特别留心，因为桥墩要遭受水流、波浪、浮冰和碎块产生的重荷冲击。 Occasionally a pier may even be hit by a passing ship. 有时，桥墩甚至可能被一艘过往的船只撞到。 Electronic computers are playing an ever-increasing role in assisting bridge designers in the analysis of forces. 电子计算机在桥梁设计人员进行受力分析的过程中起到了越来越重要的作用。 The use of precise model testing, particularly for studying the dynamic behavior of bridges, also helps designers. 使用精确的模型进行测试对设计人员会有所帮助，尤其是在桥梁动力特性的研究过程中。 A scaled-down model of the bridge is constructed, and various gauges to measure strains, accelerations, and deformations are placed on the model. 有时还要建造桥梁的缩尺模型，并且在模型上安装能测量应变、加速度和变形的各种测量仪器。 The model bridge is then subjected to various scaled-down loads or dynamic conditions to find out what will happen. 然后，对模型桥梁施加不同的按比例缩减的荷载或动力条件来观测结果。 Wind tunnel tests may also be made to ensure that nothing like the Tacoma Narrows Bridge failure can occur. 可能还要进行风洞实验，以确保不再发生像塔科马海峡大桥一样的破坏事件。 With modern technological aids, there is much less chance of bridge failure than in the past. 在现代技术的帮助下，桥梁损坏的几率比起过去小得多。Construction the foundations 桥梁基础的施工 Construction starts with the foundations, which may cost almost as much as the superstructure. 桥梁施工是从基础开始，这部分的造价可能和上部结构差不多。 Foundations built in water usually present the greatest difficulties. 通常水中修建桥梁基础遇到的困难最多。 One of the older methods, which is still used in shallow waters, is to erect cofferdams similar to the ring of closely spaced piles that the Romans used. 在浅水区仍然使用较原始的方法，即修建围堰，这类似于罗马人使用过的环形密排桩。 For constructing foundations in deep water, caissons have long been used. 深水区内基础施工，一直都采用沉箱法。 The caisson, which is a huge box closed on all sides except the bottom, is lowered onto the river bed. 沉箱是指除了底面的所有面都被密封起来的巨大箱体，它被沉降到河床上。 Workers inside the caisson, which is filled with compressed air to keep out the water, dig deeper and deeper, and the caisson sinks as the digging proceeds. 箱体内充满压缩空气而将水排出。施工人员在箱体内开挖的越来越深，箱体随着开挖的进行而下沉。 When a suitable depth is reached, the caisson is filled with concrete and becomes part of the foundation itself. 当到达适当深度时，箱体内浇筑混凝土，而后沉箱就成为基础的一部分。 Another deep-water method, less hazardous and less costly than the caisson method, uses steel or concrete piles. 另一种比沉箱方法危险性更小、更经济的深水基础施工法是使用钢桩或混凝土桩。 With modern pile drivers, long heavy piles can be driven even in deep water. The piles can be cut off and capped either above the water level or below it. If they are capped below the water level, a prefabricated hollow pier case is floated out to the site, sunk on the piles, and then filled with concrete to form the pier. 现代打桩工具能把长重桩夯入深水中，在水面或水下截桩并套上桩帽。若在水下套上桩帽，那么要将预制好的中空墩架运到现场，沉到桩头上，然后浇筑混凝土形成桥墩。 Erecting the superstructure 上部结构施工 After all piers and abutments are in place, the erection of the superstructure begins. The method of construction used depends largely on the type of bridge being built. There are six construction methods：falsework, flotation, cantilevering, sliding, direct lifting, and suspension. 所有的墩台完工后，就开始上部结构的施工。施工方法很大程度取决于建造的桥梁类型。共有六种施工方法：临时支架法、浮船施工法、悬臂法、滑动施工法、直接起吊法和悬索法。 In falsework construction, mainly used in building concrete arch bridges, metal or wood supports are built temporarily to support the erection. 临时支架法，主要用于混凝土拱桥的施工，搭建临时金属支架或木支架支撑结构。 A great deal of ingenuity is often required just to erect the falsework, especially for structures over swift rivers or deep canyons. Temporary piles and trestles are commonly used in wide shallow rivers. 通常要求脚手架搭建要做工熟练，特别是对湍急河流和深山峡谷的结构施工有用。一般在宽浅的河流中搭设临时桩柱和脚手架。 In the floatation method, mainly used in building long bridges, large bridge sections are prefabricated on shore and floated out on barges to the bridge site. The sections are then hoisted into place, either by floating derricks or by winches placed on previously constructed sections of the bridge. 浮船施工方法，主要用于长桥施工。先在岸上预制好型的桥梁部件，用驳船运到施工地点。然后用移动的起重机或用桥上事先固定好的绞盘把这些部件吊装到位。 The cantilevering technique is used not only for cantilever bridges but also for steel arch bridges. Construction starts at an abutment and extends toward the center piece by piece. Moving derricks and cranes on the co