1、1附录 Bridge Design and ConstructionPlanning. The first step leading to the construction of a modern major bridge is a comprehensive study to determine whether a bridge is needed. If it is to be a highway bridge, in the United States for example, a planning study is initiated by a state bridge authori
2、ty, possibly in cooperation with local governments or the federal government. Studies are made to estimate the amount of bridge traffic, the relief of aimed traffic in nearby highway networks, the effects on the regional economy studied, and the cost of the bridge. The means for financing the projec
3、t, such as public taxes or sale of revenue bonds repaid by toll charges, are considered. 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, Accurate land
4、 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.Selection of bridge design. The chief factors in deciding whether a brid
5、ge will be built as a girder, cantilever, truss, arch, suspension, of 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 lo
6、cation.Each type of bridge is most effective and economical only within a certain range of span lengths, as shown in the following table :As indicated in the table , there is a considerable overlap in the range of applicability of the various types. In some cases, alternative preliminary designs are
7、 prepared for several types of bridge in order to have a better basis for making the final selection, Selection of materials. The bridge designer can select from a number of modern high-Best span RangeBridge Type (feet) (meters)GirderRigid FrameArchTrussCantileverSuspension20 to 1,00080 to 300200 to
8、 1,000200 to 1,400500 to 1,8001,000 to 5,0006.1 to 304.824,4 to 91.461.0 to 304.861.0 to 426.7152.4 to 548.6304.8 to 1,524.02strength materials, including concrete, steel, and a wide variety of corrosion-resistant alloy steels.For the Verrazano-Narrows Bridge, for example, the designer used at least
9、 seven different kinds of alloy steel, one of which has a yield strength of 50000 pounds per square inch (psi) (3515kg/sq cm) and dose not need to be painted because an oxide coating forms on its surface and inhibits corrosion. The designer also can select steel wires for suspension cables that have
10、 tensile strengths up to 250000 psi (17577 kg/sq cm).Concrete with compressive strength as high as 8000 psi (562.5 kg/sq cm) can now be produced for use in bridge, and it can be given high durability against chipping and weathering by the addition of special chemical agents and control of the harden
11、ing process. Concrete that has been prestressed and reinforced with steel wires has a tensile strength of 250000 psi (17577 kg/sq cm).Other useful materials for bridge include aluminum alloys and wood. Modern structural aluminum alloys have yield strengths exceeding 40000 psi (2812 kg/sq cm). Lamina
12、ted 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 3000 psi (210.9 kg/sq cm).Analysis of forces. A bridge must resist a complex complex combination of tension, compres
13、sion, bending, chear, and torsion forces. In addition, provide the structure must provide a safety factor as insurance against failure. 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 br
14、idge building. The goal of analysis is to determine all of the forces that may act on each structure member.The force that act on bridge structure members are produced by two kinds of loadsstatic and dynamic. The static loadthe dead weight of the bridge structure itself is usually the greatest load.
15、 The dynamic, or live, has components, including vehicles by the bridge, wind forces, and accumulations of ice and snow.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
16、designer because of the vibration and impact stresses created by moving vehicles. For example, the sevens impacts caused by irregularities of vehicle motion or bumps in the 3roadway may momentarily double the effect of the live load on the bridge.Wind exert force on a bridge both directly by strikin
17、g the bridge structure and indirectly by striking vehicles that are crossing the bridge. If the wind induces arroeladtic vibration, as in the case of the Tacoma Narrows Bridge, its effect may be greatly amplified. Because of this danger, the bridge designer makes provisions for the strongest winds t
18、hat may occur at the bridge location. Other forces that may act on the bridge, such as stresses created by earthquake tremors, must also be provided for.Special attention must often be given to the design of the bridge piers, since heave loads may imposed on them by currents, waves, and floating ice
19、 and debris. Occasionally a pier may even be hit by passing ship. Electronic computers are plying an even-increasing role in assisting bridge designers in the analysis of forces. The use of precise model testing, particularly for studying the dynamic behavior of bridge, also helps d signers. A scale
20、d-down model of the bridge is constructed, and various gauges to measure strains, acceleration, and deformations are placed on the model. The model bridge is then subjected to various to ensure that nothing like the Tacoma Narrows Bridge failure can occur. With modern technological aids, there is mu
21、ch less chance of bridge failure then in the past.Constructon the foundations .Construction starts with the foundations, which may cost almost as much as the super structure. Foundations built in water usually present the greatest difficulties. One of the older methods , which is still used in shall
22、ow waters, is to erect cofferdama similar to the ring of closely spaced piles that the Tomans used.For constructing foundations in deep water , caissons have long been used. The caisson , which is a huge box closed on all sides except the bottom , is lowerde onto the river bed. Workers inside the ca
23、isson, which is filled with comptressed air to keep out the water , dig deeper and deeper , and the caisson sinks as the digging proceeds. When a suitable depth is reached, the caisson is filled with conerrte and becomes part of the foundation itself.Another deep-water method, less hazardous and les
24、s costly than the caisson method, uses steel or concrete piles. With modern pile drivers, long heavy piles can be driber even in deep water. The piles can be cut off and capped either abobe the water level or below it . If they are capped below the water level, a prefabricated hollow pier case is fl
25、oated out to the site , sunk on the piles, and then filled with concrete to form the pier .4Erecting the superstructure. After all piers and abutments are in place, the etection of the superstructure begins. The method of construction used depends largely on the type of bridge being built. There are
26、 six construction methods: falsewrok, flotation, cantilevering, sliding, direct lifting, and suspension.In falsework construction, mainly used in buliding concrete arch bridges, metal or wood supports are bulit temporarily to support the erection. A great deal of ingenity is often required just to e
27、rect the falsework, especilally for structures over seift rivers or deep canyons. Temporary piles and trestles are commonly used in wide shallow rivers.In the floatation method, mainly used in building long bridge, large bridge sections are prefabricated on shore and floated out on barges to the bri
28、dge site. The sections are then hoisted into place, either by floating derricks or by wiches placed on previously constructed sections of the bridge.The cantilevering technique is used not only for cantilever bridges but also for steel arch bridges. Construction srarts at an abutment and extengs row
29、ard the center piece by piece. Moving derricks and cranes on the completed portion of the structure handle the heavy material.Sliding construction is used only rarely,. In this method , a prefabricated unit, such as a truss, is erected on shore and slid out over a temporary or permenent support unti
30、l it comes to rest on another support.In the direct lifting method, mainly used for light , short span highway bridges, a prefabricated bridge unit is lifted by a hoist and swung directly onto the bridge supports.In the construction of suspension bridges, the cables are strung between the bridge tow
31、ers and used as a support for the bridge deck. The deck erection starts at the ends of the bridge and progresses towers the center. A travelling derrick, moving on the completed part of the deck, is used to handle heavy material. Temporary suspension cables are occasionally used in the construction
32、of other types of bridges to convey material across the span. In all methods of construction , it is necessary to determine the stresses and deformations at every stage of construction. Steresses in a partly completed bridge-constructed by the cantilever method-can exceed the stresses in a completed
33、 bridge because of the totally diffeerern conditions of support and loading. 附录5桥的设计与构造规划 现代重要的桥梁建造的第一步是广泛地研究确定桥梁的必要性。比如：如果是高速公路桥，在美国则是由州桥管理局研究规划并确定，在程序上会同当地的政府或联邦政府一起，对主要公路桥梁进行评估研究。如在接近高速公路网上减少交通堵塞，对当地经济的影响和桥的造价。这就决定了工程的投资方式，如公众收费，发行债券或支付过桥费都被考虑进来。如果研究认为其可行信，那么桥选址和占地问题将着手处理。在这一点上，现场测绘工作开始进行，做好精确的实地
34、测量；潮汐，洪水因素，水流和水路上的其他的特征都要仔细研究，在陆地和水下的泥土和岩石的钻孔取样都尽可能地在基础处进行。桥梁设计的选择 决定把桥建成梁，悬臂，桁架，拱，悬索或其他类型结构的主要因素是：（1）地点，如跨越河流；（2）目的，如建桥为了方便交通；（3）跨度；（4）可用的材料；（5）花费；（6）美观和和谐性。在一定范围的跨度内，每种结构的都有最大的作用和经济。如下表所示：上表表明了许多类型的适用性有相当多的重叠。在一些实例中，在不同的初步设计中，用来比较不同类型的桥结构是为了在最后有最好的选择。材料的选择 桥梁设计者能选用大量的现代高强材料，包括混泥土，钢筋，和多种耐腐蚀的合金。拿 Va
35、rian-Narrows 大桥来说，设计者使用了七种不同的合金钢，其中之一的合金的屈服强度为 50000 英镑每平方英寸（3115kg/c）,而且不需要油漆保护，因为有一种氧化膜覆盖在它的表面而防止腐蚀。设计者还选用钢丝绳作为缆绳，它的抗拉强度超过 250000 英镑每平方英寸（17577 kg/c）抗压强度高达 8000 英镑每平方英尺（562.5kg/c）的混泥土现在被生产用作桥梁工程，而且它在增加特殊化学物质后具有很高的抗脆裂性能和抗风化性能，这种混最佳跨度桥的类型英尺 米梁桥 20 到 1000 6.1 到 304.8刚架桥 80 到 300 24.4 到 91.4拱桥 200 到 1
36、000 61.0 到 304.8桁架桥 200 到 1400 61.0 到 426.7悬臂桥 500 到 1800 152.4 到 548.6悬索桥 1000 到 5000 304.8 到 1524.06泥土被用作预应力砼，而且其加强了钢丝绳的抗拉强度，其强度达到 250000 英镑每平方英寸（17577 kg/c）桥梁的其它使用材料还铝合金和木材：现在的铝合金的屈服强度超过了 40000 每平方英寸（2818 kg/c）。把木材碾成细长的薄片，然后用胶水粘在一起而做成的梁是自然木材强度的二倍。例如用南部松树而胶结的梁能承受的工作应力达到了 3000 英镑每英寸（210.9kg/c）。应力分析
37、 一座桥要抵抗一系列的合力，如拉力，压力，剪力和扭力。另外，结构还需要一定的安全储备一保不足。对结构进行精确计算各种单独的压力和拉力，这就叫应力分析。这或许是桥梁建设中最复杂的技术。应力分析的目的是为了确定作用在结构上的里的数量。作用在桥梁结构的应力都可以分为二类荷载：动荷载和静荷载。静荷载即桥结构本身不变的重量它往往也是最大的荷载。动荷载或静荷载有很多，包括桥面上的机动车，风荷载，和积冰积雪荷载。虽然随时在桥面上移动的机动车的总重量相当于静荷载和动荷载来说是一个很少的部分，而对设计者来说，因为机动车辆产生的振动和冲击压力而会出现特殊问题。例如：在路面上机动车的不规则的运动或碰撞对桥面产生短暂
38、而影响加倍的活荷载而导致严重的影响。风在桥上的施加的里即直接敲打桥结构又间接的敲打在桥面上的通行的车辆。如果出现空气弹性振动，在这种情况下的 Tacoma Narrows 大桥的风作用被大大地增大，由于这种危险的存在，桥的设计者在桥址必须知道所能发生的最大的风。还有其它的力作用在桥上，如：地震产生的压力也必须注意。对桥墩的设计通要给予特殊的关注，因为桥墩承担水流，浮冰和漂浮物而产生的重荷，桥墩通常还有被船撞击的可能。电脑在应力分析上协助桥梁设计者，并扮演一个很重要的角色。用一个精确的模型试验，尤其对桥的动力的活动状态的研究也可以帮助设计者。一个小比例的桥模结构中，对桥模各处的应力，加速度和变形
39、都可以进行精确测量。桥模这时可以承受同样比例的荷载和动力条件来分析桥的变化。风洞试验也可以确保不再发生 Tacoma Narrows 大桥的失败。在现代技术的帮助下，桥梁事故出现的机会将大大少于以前。建筑基础 建筑物都是从基础开始的，基础的花费几乎大大超过上层建筑。水下基础通常会遇到很大的困难，有个古老的方法常被用于浅水中，即在小范围内垂直围堰而建桥墩。罗马人常用这种方法。7在深水中建基础一般用沉箱法。沉箱是一个底部开口其余封闭的大盒子而沉入河床上，工人们在为挡水而充满压缩空气的沉箱里，越挖越深，沉箱也跟着下沉。当达到合适的深度后在箱内填入混泥土而成为基础。在深水中建基础的另一种方法比沉箱法更
40、安全和更低的成本，用于钢或混泥土桥墩。在现代的打桩工具下把重桩打入深水中，桩可以在水面或水下截断或做成桩帽。如在水下把它们做成桩帽，可把一根预制空心桩浮运到做成承台桩的那一点，然后从空心桩套内灌入混泥土。 建设上层建筑 当所有的桩和支柱建好后，则上部结构开始建筑。结构的建设方法有很多种类，共有六类建造方法：脚手架，浮运，悬臂，滑移，直升和悬挂法。在用脚手架建造时，主要用来建混泥土拱桥。金属或木支撑都是临时搭设为竖直支撑。脚手架都是根据需要而灵活搭建的。尤其结构在激流回深谷上时，临时桥墩和站桥一般使用在宽而浅的河上。浮运法主要用来建很长的桥梁。主桥部分是在河岸预制的，然后用驳船浮移到桥梁位置。用
41、浮吊起重机或卷扬机把该部分精确吊到大桥的建设部位。悬臂技术不仅用于悬臂桥中，也用于刚拱桥上，先建成一个桥台，然后一步步延伸到中央，起重机和吊车可以完成着仪沉重物在结构上的操作。滑移法 建筑很少用到。这种方法，如一个预制构件或一个组合结构在竖立的支柱上，滑过临时或永久性的支撑，直到它进入安装的另一个支撑。直升法主要用于轻质小跨度的公路桥。每一个预制桥单元被垂直悬起并旋转到桥梁支撑点上。在由悬挂法建设的桥梁中，一串缆绳连接俩边的桥头堡，被用作桥面支撑点。开始的桥面施工却在在桥梁施工的最后，而且是由俩端向中央发展。移动吊车在已完成的桥面上移动，用来运送重材，悬挂钢缆，有时在其他类型的桥梁中被用来在全跨上运输材料。所有的建筑方法在施工阶段都需要验算应力和变形，在用悬臂梁法施工的桥梁中，因为完全不同的支撑和荷载条件，未竣工桥梁内的应力可能会超过已竣工桥梁内的应力。
