大型桥梁及施工外文翻译--大跨度桥梁

Large Span Bridge

1.Suspension Bridge

The suspension bridge is currently the only solution in excess of 600 m, and is regarded as competitive for down to 300. The world’s longest bridge at present is the Verrazano Narrows bridge in New York. Another modern example is the Severn Bridge in England.

The components of a suspension bridge are: (a) flexible cables, (b) towers, (c) anchorages, (d) suspenders, (e) deck and ，(f) stiffening trusses. The cable normally consists of parallel wires of high tensile steel individually spun at site and bound into one unit .Each wire is galvanized and the cable is cover with a protective coating. The wire for the cable should be cold-drawn and not of the heat-treated variety. Special attention should be paid to aesthetics in the design of the rowers. The tower is high and is flexible enough to permit their analysis as hinged at both ends. The cable is anchored securely anchored to very solid anchorage blocks at both ends. The suspenders transfer the load form the deck to the cable. They are made up of high tensile wires and are normally vertical. The deck is usually orthotropic with stiffened steel plate, ribs or troughs，floor beam, etc. Stiffening trusses, pinned at the towers, are providing. The stiffening system serves to control aerodynamic movements and to limit the local angle changes in the deck. If the stiffening system is inadequate, torsional oscillations due to wind might result in the collapse of the structure, as illustrated in the tragic failure in 1940 of the first Tacoma Narrows Bridge.

The side span to main span ratio varies from 0.17 to 0.50 .The span to depth ratio for the stiffening truss in existing bridge lies between 85 and 100 for spans up to 1,000m and rises rather steeply to 177. The ratio of span to width of deck for existing bridges ranges from 20 to 56. The aerodynamic stability will have be to be investigated thoroughly by detailed analysis as well as wind tunnel tests on models.

2.The cable-stayed bridge

During the past decade cable-stayed bridges have found wide application, s\especially in Western Europe, and to a lesser extent in other parts of the world.

The renewal of the cable-stayed system in modern bridge engineering was due to the tendency of bridge engineering in Europe, primarily Germany, to obtain optimum structural performance from material which was in short supply-during the post-war years.

Cable-stayed bridges are constructed along a structural system which comprises an

orthotropic deck and continuous girders which are supported by stays, i.e. inclined cables passing over or attached to towers located at the main piers.

The idea of using cables to support bridge span bridge span is by no means new, and a number of examples of this type of construction were recorded a long time ago. Unfortunately the system in general met with little success, due to the fact that the statics were not fully understood and that unsuitable materials such as bars and chains were used to form the inclined supports or stays. Stays made in this manner could not be fully tensioned and in a slack condition allowed large deformations of the deck before they could participate in taking the tensile loads for which they were intended.

Wide and successful application of cable-stayed systems was realized only recently, with the introduction of high-strength steels, orthotropic decks, development of welding techniques and progress in structural analysis. The development and application of electronic computers opened up new and practically unlimited possibilities for exact solution of these highly statically indeterminate systems and for precise stoical analysis of their three-dimensional performance.

Existing cable-stayed bridges provide useful data regarding design, fabrication, erection and maintenance of the mew system. With the construction of these bridges many basic problems encountered in their engineering are shown to have been successfully solved. However, these important data have apparently never before been systematically presented.

The application of inclined cable gave a new stimulus to construction of large bridges. The importance of cable-stayed bridges increased rapidly and within only one decade they have become so successful that they have taken their rightful place among classical bridge system. It is interesting to note now how this development which has so revolutionized bridge construction, but which in fact is no new discovery, came about.

The beginning of this system, probably, may be traced back to the time when it was realized that rigid structures could be formed by joining triangles together. Although most of these earlier designs were based on sound principles and assumptions, the girder stiffened by inclined cables suffered various misfortunes which regrettably resulted in abandonment of the system. Nevertheless, the system in itself was not at all unsuitable. The solution of the problem had unfortunately been attempted in the wrong way.

The renaissance of the cable-stayed, however, was finally successfully achieved only

during the last decade.

Modern cable-stayed present a three-dimensional system consisting of stiffening girders, transverse and longitudinal bracings, orthotropic-type deck and supporting parts such as towers in compression and inclined cables in tension. The important characteristics of such a three-dimensional structure is the full participation of the transverse construction in the work of the main longitudinal structure. This means a considerable increase in the moment of inertia of the construction which permits a reduction in the depth of the girders and economy in steel.

Long span concrete bridges are usually of post-tensioned concrete and constructed either as conditions beams types or as free versatile structures. Many methods have been developed for continuous deck construction. If the clearance between the ground and bottom of the deck is small and the soil is firm, the superstructure can be built on staging. This method is becoming obsolete. Currently, free-cantilever and movable scaffold systems are increasingly used to save time and improve safety.

The movable scaffold system employs movable forms stiffened by steel frames. These forms extend one span length and are supported by steel girders which rest on a pier at one end and can be moved from span to span on a second set of auxiliary steel girders.

An economical construction technique known as incremental push-launching method is developed by Baur-Leonhard team. The total continuous deck is subdivided longitudinally into segments of 10 to 30 m length depending on the length of spans and the time available for construction. Each of these segments is constructed immediately behind the abutment of the bridge in steel framed forms, which remain in the same place for concreting all segments .The forms are so designed as to be capable of being moved transversely or rotated on hinges to facilitate easy stripping after sufficient hardening of concrete. At the head of the first segment, a steel nose consisting of a light truss is attached to facilitate reaching of the first and subsequent piers without including a too large can yielder moment during construction . The second and the following segments are concreted directly on the face of the hardened portion and the longitudinal reinforcement can continue across the construction joint . The pushing is achieved by hydraulic jacks which act against the abutment .Since the coefficient of friction of Teflon sliding bearings is only about 2 percent, low capacity hydraulic jacks would suffice to move the bridge even over long lengths of several hundred metres . This method can be used for straight and continuously curved bridges up to a span of about 120 m .

The free-cantilever system was pioneered by Dyckerhoff and Willmann in Germany .In this system , the superstructure is erected by means of cantilever truck in sections generally of 3.5 m .The cantilever truck ,whose cost is relatively small and which is attached firmly to permanent construction , emits by repeated use the construction of large bridges . The

avoidance of scaffold from below, the speed of work and the saving in labor cost result in the construction being very economical. The free-cantilever system is ideally suited for launched girders with a large depth above the pier cantilever system is ideally suited for launched girders with a large depth above the pier cantilevering to the middle of the span.

Another technique is the use of the pneumatic caisson .The caisson is a huge cylinder with a bottom edge that can cut into the water bed. When compressed is pumped into it ,the water is forced out .Caissons must be used with extreme care .for one thing, workers can only stay in the compression chamber for short periods of time .For another , if they come up to normal atmospheric pressure too rapidly ,they are subject to the bends ,or caisson disease as it is also called , which is a crippling or even fatal condition caused by excess nitrogen in the blood .When the Eads Bridge across the Mississippi River at St.Louis was under construction between 1867and 1874 , at a time when the danger of working in compressed air was not fully understood ,fourteen deaths was caused by the bends .

When extra strength is necessary in the piers, they sometimes keyed into the bedrock-that is ,they are extended down into the bedrock .This method was used to build the piers for the Golden Gate Bridge in San Francisco ,which is subject to strong tidies and high winds ,and is located in an earthquake zone .The drilling was carried out under water by deep-sea divers .

Where bedrock cannot be reached ,piles are driven into the water bed .Today ,the piles in construction are usually made of prestressed concrete beams .One ingenious technique ,used for the Tappan Zee Bridge across the Hudson River in New York ,is to rest a hollow concrete box on top of a layer of piles .When the box is pumped dry ,it becomes buoyant enough to support a large proportion of the weight of the bridge .

Each type of bridge indeed each individual bridge presents special construction problems. With some truss bridges , the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes .Arch bridges can be constructed over a false work ,or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges .With steel arches ,however ,a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support .Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases .

With suspension bridges ,the foundations and the towers are built first .Then a cable is run from the anchorage-concrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage .A wheel that unwinds wire from a reel puns along this cable .When the reel reaches the other side ,another wire is placed on it ,and the wheel returns to its original position .When all the wires have been put in place ,another machine moves along the cable to compact and to bind them .Construction begins on the deck when the cables are in place ,with work progressing toward the middle from each end of the structure .

The loads to be considered in the design of substructures and bridge foundations include loads and forces transmitted from the superstructure, and those acting directly on the substructure and foundation.

AASHTO loads .Section 3 of AASHTO specifications summarizes the loads and forces to be considered in the design of bridges (superstructure and substructure). Briefly , these are dead load ,live load , impact or dynamic effect of live load , wind load , and other forces such as longitudinal forces , centrifugal force ,thermal forces , earth pressure , buoyancy , shrinkage and long term creep , rib shortening , erection stresses , ice and current pressure , collision force , and earthquake stresses .Besides these conventional loads that are generally quantified , AASHTO also recognizes indirect load effects such as friction at expansion bearings and stresses associated with differential settlement of bridge components .The LRFD specifications divide loads into two distinct categories : permanent and transient .

大跨度桥梁

1．悬索桥

悬索桥是现行的跨径超过600m大桥的唯一解决方案，而且对跨径在300m以上的桥梁它也是被认为是一种很有竞争力的方案。现在世界上最大跨径的桥梁是纽约的威拉查诺（Verrazano）海峡大桥，另一个是英国的塞温（Savern）大桥。

悬索桥的组成部分有：柔性，主塔，锚碇，吊索（挂索），桥面板和加劲桁架。主缆是有一组平行的单根高强钢丝在现场扭在一起并绑扎成型的钢丝束组成的。每根钢丝都是经过渡锌处理的，并且整个用保护层覆盖着。所用的钢丝应该是冷拔钢丝而不是经过热处理的各种钢丝。在进行主塔设计时应该特别注意其在美学上的要求。主塔很高而且具有足够的柔性，使其每一座塔顶都可认为是与主缆铰接。主缆的两端很安全的锚固在非常坚实的锚碇上。吊索把桥面板上的荷载传递到主主缆上。吊索也是有高强钢丝制成的而且通常是竖直的。桥面板通常是有加劲钢板，肋或槽型板，横梁制成的异性结构。提供一些加劲梁连接在其主塔之间，能够起到控制空气动力运动并限制桥面板局部倾角变化。如果加劲系统不适当，由于风引起的竖向振动也许会导致结构倾斜，就像塔科玛（Tacoma）海峡大桥的悲剧性的破坏所表明的那样。

边跨与主跨的跨径比的变化范围是0.17~0.50。在现有的采用加劲梁的桥梁上，当跨径高大1000米时跨径与桥梁的建筑高度之比为85与100之间。现有的桥梁的跨径与桥面板宽度之比约为20~56。桥梁结构的空气动力稳定性必须得通过对其模型的风洞试验及细部分析进行全面的研究。

2．斜拉桥体系

在过去的十年间，斜拉桥得以广泛的应用，尤其实在欧洲，而在世界其它地区，应用相对少一些。

在现代桥梁工程中，斜拉桥体系的重新兴旺起来是由于欧洲（主要是德国）的桥梁工程师有一种趋势，即从因为战争而短缺的材料上获得最佳的结构性能。斜拉桥是有按各向异性桥面板和由吊索支撑的连续梁构成的体系建造起来的，这些吊索是一些穿过或固定的位于主桥墩的索塔顶上的倾斜主缆。

用主缆来支撑桥跨并不是一种新思想，在很早以前就有大量此类结构的的记载。不幸的是这一体系只有很少成功的例子，这是由于人们对静力学的原理没有完全弄明白，并且还没有构成倾斜支撑或吊索的适当的材料，例如主缆和钢链等。这种吊索在它们能够按计划承担拉力之前不能完全被拉紧，而应处于允许桥面板产生较大变形的松弛状

态。

斜拉体系的广泛的成功的应用只在近年来，随着高强钢，各种异性桥面板的引入、焊接技术的发展和结构分析方法的进步才得以实现。电子计算机的发展和应用，导致了解决高次超静定体系的精确值及它们的三维空间性能的精确静力分析的新的无实际限制的可能性。

现有的斜拉桥提供了许多关于设计、制造、安装和维修的有用的数据。随着这些桥梁的建造，许多工程中遇到的基本问题已表明的到了成功的解决。然而这些重要的数据很显然在这以前决没有被系统的揭示出来。

斜吊索的应用对大型桥梁建造带来了一个新刺激。斜拉桥的重要性迅速增加起来，并且在仅三十年间这种桥梁类型就变的这样成功，已使其在古典桥梁体系中取得了它应有的地位。如果我们注意到这种桥梁建造带来如此彻底的变革的发展是怎样发生的话，我们都会感兴趣的，因为这一发展事实上并没有什么任何新发现。

这一体系的开始也许可以追溯到人们开始认识到通过三角形连接在一起能够构成刚性结构的时代。

尽管大多数这种设计是基于结构坚固的原理和假定的，但斜拉加劲梁还是遭受各种各样的不幸事故，而最终令人遗憾的导致了这一体系被放弃。尽管这样，这一体系并不是完全不适应，只是问题的解决被不幸的应用错误的方式去尝试。

然而，斜拉体系的复兴只是在近十年来才最终获得成功。

现代的斜拉桥提出了加劲梁、横向及纵向联系梁、各项异性桥面板和支撑部分（例如处于受压状态的索塔及受拉状态的斜吊索）组成的三维空间体系。象这种空间三维结构的重要特征是横向结构全部参与主要纵向结构的工作状态。这就意味着结构的惯性矩大大增加，这使的桥的建筑高度可以减少，并且使用钢材是最经济的。

大跨度桥梁经常是连续梁形式或悬臂梁形式的预应力混凝土桥梁。以前许多的施工方法已发展为连续梁桥的施工方法。如果模板和地面之间的距离较小并且土质坚硬，桥梁的上部结构可以使用支架施工方法。不过这种施工方法已经越来越过时了。目前，自由悬臂法和移动模架法的应用渐广并能节省时间和提高安全性。

移动模架法是利用固定在钢制台架上的移动系统而形成，这种系统能够达到一跨长并支承在一端支承在桥墩上并借助于第二根钢导梁逐跨移动的钢梁上。

一种经济的施工方法是被广泛知晓的由Baur-leonhard团队所发展的使用广泛的顶推法。整个的连续梁被划分成10-30米长度的节段，这种划分主要依据跨径和能够利用的施工时间。每个节段在桥台后面的钢模上能够快速浇注，钢模可以周转使用而浇注所有的节段。这样设计模板是为了能够横向移动或在铰上转动，以便在混凝土充分硬化后

脱模。在第一节段的顶端安装上一个由轻型桁架组成的钢导梁，以实现第一节段以后的节段顺利架设而防止在施工出现过大的悬臂部分。第二节段及以后的节段可以直接在第一节段的硬化面上浇注并在施工过程中将节段连接起来。顶推是通过支承在桥台上的液压千斤顶实现的，由于聚四氟乙烯的滑块的摩擦系数只有0.02，低效能的千斤顶就足够完成长度甚至达数百米的桥梁的顶推。这种方法可以应用在长度在120米左右的直线桥梁或曲线桥梁上。

自由悬臂法是由法国的Dyckerhoff和Willmann所创始。这种施工方法中，桥梁的上部结构是通过节段长度基本在3.5米的悬臂机上施工，悬臂机的费用相对比较低并且固定在桥梁承重结构上，由于它的重复利用性使它能在长桥上使用。由于施工速度的加快和时间的节省使得这种施工方法的费用比较低从而避免了使用台架施工，自由悬臂法比较适用于桥墩较高并且悬臂能伸到跨径中部的桥梁上。

另一种施工方法是整体沉箱法。沉箱是一种底边有刃脚的大型圆筒，其刃脚可以切入水底。当压缩空气进入沉箱内部时水就会被排出。沉箱的利用必须严加注意。首先，工人们只能在这种压缩空气的空间里呆很短的时间；另一方面，如果工人们从沉箱进入正常的大气压条件下过于迅速，他们将比较容易患上潜水病（也被称作沉箱病），这在能使人致残的甚至致命的环境中由于血液中氧气过多所引起的一种病。当St.louis市的密西西比河Eads上的桥在1867-1874年施工时，由于人们对在压缩空气中工作的危险性认识不足，最后由于患潜水病而导致14人死亡。

当在桥墩上有外力作用时，基桩经常需要嵌入基岩，也就是说它们的下部一直延伸到基岩。这种方法曾经用来建造位于强风和地震区域的旧金山金门大桥的桥墩。钻孔是在水下由深水潜水员进行的。在不能到达基岩的地方，桩通常被打进河床。今天，在施工的基桩基本上是预应力混凝土结构。在建造纽约哈德逊河上的泰平.吉桥时所采用的一种巧妙技术是将一个空心混凝土箱置于桥桩层上，当它里面的水被抽干时，它的浮力足够支承桥梁重力的一大部分。

每一种类型的桥梁实际上代表了特殊的问题。许多桁架桥的施工是先将桥上桁架运到已施工完毕的基桩位置，然后在利用千斤顶或起重机架设到适当位置。拱桥是在脚手架或临时脚手架上施工的，这种方法通常用于预应力混凝土拱桥。然而对钢拱桥来说已发展了一种技术，用这种技术将已装好的部分借助起支承作用的主缆控制就位（钢拱在安装过程中还没有合拢前，是两个悬臂，需要用主缆拉住两个悬臂以免倾倒）。当主缆中的拉力增加时，起重机就沿着拱桥的顶部移动以架设新的钢拱。

对悬索桥来说，需要首先施工基础和索塔。这时主缆从锚碇（一个固定主缆的大混凝土块）穿过直至索塔并且通过另一索塔而锚固在锚碇上，然后从卷线盘上放松主缆的轮子沿着主缆运动，当卷线盘到达另一面时，另一根钢丝又装进卷线盘并最终到达它的原位置。当所有的主缆被放在固定的位置后，另一台机器沿着主缆移动并对其进行张拉锚固。当主缆施工完毕时，逐渐开始在支架上从两端向中间施工。

在桥梁下部结构和基础设计中要考虑的荷载包括：从上部结构传下来的荷载和直接作用于下部结构的基础的荷载。

AASHTO荷载。 AASHTO规范第三部分总结了桥梁设计（上、下部结构）要考虑的荷载和作用力。主要有：恒载、活载、活载冲击力或动力作用、风荷载以及其他荷载——如纵向力、离心力、温度力、土压力、浮力、收缩及徐变、拱肋缩短、安装应力、冰及水流压力、冲撞力及地震应力。除了这些通常能够量化大的典型荷载外，AASHTO同样认识到诸如活动支座处产生的摩擦以及由于桥梁的沉降差而产生的应力等间接荷载效应。

出处：安瑞克．大跨径桥梁结构形式[J]．建筑实录(美)，2010，37—42