土木工程 建筑 外文翻译 外文文献 英文文献 桥梁施工方法

土木工程 建筑 外文翻译 外文文献 英文文献 桥梁施工方法 桥梁施工方法 一座桥梁的最终造价是建造桥梁结构物的材料费用与这项工程相关的机械费用和临时工程及劳动力的费用的总和。材料的费用能够被预算的相当准确的，承包商们借助施工经验是能够估算施工机械和劳动力的费用的。但是劳动力的费用是不能够精确分析的。最近的一项比较有竞争力的试图通过减少临时工程和工程的工期来革新施工方法。 桥梁上部结构比较适宜的施工方法将随施工场地不同而不同，并主要取决于桥梁的长度和跨度、桥梁的类型、基础条件及其所使用的材料。例如:现场浇注混凝土的施工方法适用于40米以下的短跨径桥梁，如果河床在一年中的大部分时间是干燥的，那么预应力混凝土悬臂施工方法在有通航要求的大跨桥梁中是比较合适的。现在的发展趋势是尽可能避免使用台架施工，而在最大程度上采用预制构件;另一方面，起重机、滑拽梁等施工机械的应用范围也越来越广。这些都是从对施工方法的密切关注甚至是设计过程的密切关注而取得的巨大收获，而不是在善于解决永久材料。 小跨度桥梁 当桥梁的跨度小于40米左右时，它的上部结构可以通过支承在地面上的趾甲施工。其它的施工方法中的梁可以整体预制，然后利用顶推梁或起重机架设。如果这座桥梁是等跨的，在后一种施工方法中，由于架设设备而增加的费用和由于施工工期短而减少的费用之和将会比前一种施工方法的制作费用低。 大跨度桥梁 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，然后再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钢索控制就位(钢拱在安装过程中还没有合拢前，是两个悬臂，需要用钢索拉住两个悬臂以免倾倒)。当钢索中的拉力增加时，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。这时钢索从锚碇(一个固定钢索的大混凝土块)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钢索的轮子沿着钢索运动，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。当所有的钢索被放在固定的位置后，另一台机器沿着钢索移动并对其进行张拉锚固。当钢索施工完毕时，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。 LRFD规范将荷载按不同的方式划分为两种:永久作用和可变作用。 永久作用 荷载:包括所有桥梁构件、器件及辅助设备、道路面层的净重及未来铺装重量、填土恒载。AASHTO及LRFD规范都给出了表格，总结了桥梁工程重常用的单位重量。 可变作用 汽车荷载 小跨度桥梁的汽车荷载:美国和加拿大已致力于发展一种比H或HS AASHTO模型更实际的代表高速公路活荷载的模型。到目前为止，AASHTO模型仍被广泛采用。 Bridge Construction Techniques The final cost of a bridge is the sum of the cost of permanent materials,the proportionate cost to the project of plant and temporary works and the cost of labor .The cost of permanent materials can be estimated reasonably correctly.With experience,a bridge contractor can deal completely with cost of plant and temporary works .But the labor cost does not lend itself to exact analysis .Recent competitive designs have attempted to introduce innovations in construction methods with a view to effect economy in the cost on labor by reducing temporary works and by minimizing the duration of site work. The suitable techniques of construction of bridge superstructure will vary from site to site,and will depend on the spans and length of the bridge, type of the bridge,materials used and site conditions. For instance, cast-in-site concrete construction could be adopted for short spans up to 40 m, if the river bed is dry for a considerate portion of the year, whereas free cantilever construction with prestressed concrete decking would be appropriate for long spans in rivers with navigational requirements. The current trend is towards the avoidance of staging as much as possible and to use precast or prefabricated components to maximum extent.Also , construction machinery such as cranes and launching girders are coming into wider use . These are greater savings to be effected by paying attention to the method of construction even from the design stage than by attacking permanent materials . Short Span Bridges For bridges involving spans up to 40 m , the superstructure may be built on staging supported on the ground . Alternatively , the girders may be precast for the full span length and erected using launching girders or cranes,if the bridge has many equal spans.In the latter procedure , the additional cost on erection equipment should be less than the saving in the cost of formwork and in the labour cost resulting from faster construction . Long Span Concrete Bridges Long span concrete bridges are usually of post-tensioned concrete and constructed either as conditions beams types or as free ver cantile 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 developed by Baur-Leonhard team is shown schematically in Figure 22.1. 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 yilever 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 , ermits by repeated use the construction of large bridges . The avoidance of scaffold from below ,the speed of work and the saving in labour cost result in the construction being very economicdal . 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 ar 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 compresed 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 buoyantenough to support a large proportion of the weight of the bridge (see Fig.22.3). 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 falsework ,or temporaryscaffolding.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 foundions and the towers are built first .Then a cable is run from the anchorage-aconcrete block in which the cable is fastened-up to the tower and across to the opposite tower and anchorage .Awheel that unwinds wire from a reel quns 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 campact 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 . Permanent loads Dead Load : this includes the weight DC of all bridge components , appurtenances and utilities, wearing surface DW and future overlays , and earth fill EV. Both AASHTO and LRFD specifications give tables summarizing the unit weights of materials commonly used in bridge work . Transient Loads Vehicular Live Load (LL) Vehicle loading for short-span bridges :considerable effort has been made in the United States and Canada to develop a live load model that can represent the highway loading more realistically than the H or the HS AASHTO models . The current AASHTO model is still the applicable loading.