建筑-桥梁工程-外文翻译-外文文献-英文文献-混凝土梁桥的结构形式.doc

出处：
安瑞克．混凝土桥梁结构形式[J]．建筑实录(美)，2010，33(36)：34—36
The Structure of Concrete Bridge
Pre-stressed concrete has proved to be technically advantageous, economically competitive, and esthetically superior bridges, from very short span structures using standard components to cable-stayed girders and continuous box girders with clear spans of nearly 100aft .Nearly all concrete bridges, even those of relatively short span, are now pre-stressed. Pre-casting, cast-in-place construction, or a combination of the two methods may be used .Both pre-tensioning and post tensioning are employed, often on the same project.
In the United States, highway bridges generally must-meet loading ,design ,and construction requirements of the AASHTO Specification .Design requirements for pedestrian crossings and bridges serving other purposes may be established by local or regional codes and specifications .ACI Code provisions are often incorporated by reference .
Bridges spans to about 100ft often consist of pre-cast integral-deck units ,which offer low initial cost ,minimum ,maintenance ,and fast easy construction ,with minimum traffic interruption .Such girders are generally pre-tensioned .The units are placed side by side ,and are often post-tensioned laterally at intermediate diaphragm locations ,after which shear keys between adjacent units are filled with non-shrinking mortar .For highway spans ,an asphalt wearing surface may be applied directly to the top of the pre-cast concrete .In some cases ,a cast-in-place slab is placed to provide composite action .
The voided slabs are commonly available in depths from 15 to 21 in .and widths of 3 to 4 ft .For a standard highway HS20 loading, they are suitable for spans to about 50 ft, Standard channel sections are available in depths from 21 to 35 in a variety of widths, and are used for spans between about 20 and 60 ft .The hollow box beams-and single-tee girders are intended for longer spans up to about 100 ft.
For medium-span highway bridges ,to about 120 ft ,AASHTO standard I beams are generally used .They are intended for use with a composite cast-in-place roadway slab .Such girders often combine pre-tensioning of the pre-cast member with post-tensioning of the composite beam after the deck is placed .In an effort to obtain improved economy ,some states have adopted more refined designs ,such as the State of Washington standard girders.
The specially designed pre-cast girders may be used to carry a monorail transit system .The finished guide way of Walt Disney World Monorail features a series of segments, each consisting of six simply supported pre-tensioned beams ,together to from a continuous structure .Typical spans are 100 to 110 ft . Approximately half of the 337 beams used have some combination of vertical and horizontal curvatures and variable super elevation .All beams are hollow, a feature achieved by inserting a styro-foam void in the curved beams and by a moving mandrel in straight beam production.
Pre-cast girders may not be used for spans much in excess of 120 ft because of the problems of transporting and erecting large, heavy units.
On the other hand ,there is a clear trend toward the use of longer spans for bridges .For elevated urban expressways ,long spans facilitate access and minimize obstruction to activities below .Concern for environmental damage has led to the choice of long spans for continuous viaducts . For river crossings, intermediate piers may be impossible because of requirements of navigational clearance.
In typical construction of this type, piers are cast-in-place, often using the slip-forming technique .A “hammerhead” section of box girder is often cast at the top of the pier, and construction proceeds in each direction by the balanced cantilever method. Finally, after the closing cast-in-place joint is made at mid-span, the structure is further post-tensioned for full continuity .Shear keys may be used on the vertical faces between segments, and pre-cast are glued with epoxy resin.
The imaginative engineering demonstrated by many special techniques has extended the range of concrete construction for bridges far beyond anything that could be conceived just a few years ago .In the United States, twin curved cast-in –place segmental box girders have recently been completed for of span of 310 ft over the Eel River in northern California .Preliminary design has been completed for twin continuous box girders consisting of central 550 ft spans flanked by 390 ft side spans.
Another form of pre-stressed concrete bridge well suited to long spans is the cable-stayed box girder .A notable example is the Chaco-Corrientes Bridge in Argentina .The bridges main span of 804 ft is supported by two A-frame towers, with cable stays stretching from tower tops to points along the deck .The deck itself consists of two parallel box girders made of pre-cast sections erected using the cantilever method .The tensioned cables not only provide a vertical reaction component to support the deck ,but also introduce horizontal compression to the box girders ,adding to the post-tensioning force in those members .
Stress-ribbon Bridge pioneered many years ago by the German engineer Ulrich Finsterwalder. The stress-ribbon bridge carries a pipeline and pedestrians over the Rhine River with a span of 446 ft .The superstructure erection sequence was to (a) erect two pairs of cables, (b) place pre-cast slabs forming a sidewalk deck and a U under each of the sets of cables, and (c) cast-in-place concrete within the two Us. The pipeline is placed atop supports at railing height, off to one side, which greatly increases the wind speed of the structure.
It is appropriate in discussing bridge forms to mention structural esthetics .The time is past when structures could be designed on the basis of minimum cost and technical advantages alone .Bridge structures in particular are exposed for all to see .To produce a structure that is visually offensive ,as has occurred all too often in the past, is an act professional irresponsibility .Particularly for major spans ,but also for more ordinary structures ,architectural advice should be sought early in conceptual stage of the design process.

混凝土梁桥的结构形式
事实证明，预应力混凝土结构是在技术上先进、经济上有竞争力、符合审美学的一种先进技术。从使用标准组成的小跨径桥梁到吊梁和跨径将近100英尺的连续箱梁桥，几乎所有的混凝土桥梁，甚至于相对短时间的桥梁都是预应力结构的。采用预制、现场浇筑或两种方法并用。在同一工程中经常同时使用先张法和后张法。
在美国，公路桥一般情况下必须满足荷载、设计和AASHTO规定的建设的要求。对于服务于其它目的步行街和桥梁的设计要求由当地的或地方的代码建立。ACI代码的备注也被纳入参考。
伴随最小交通中断的大约100英尺的跨径的桥梁由提供低的初级预算，最小量维修和养护费用和快速的简易的预制板组成。这种梁一般是用先张法。预制板一块挨一块的放置并且在相邻的预制板间受剪的缝隙填满不收缩的灰泥之后，经常在中间横膈膜的位置后张拉。对于公路，用沥青铺设的表面可以直接用在预制混凝土的上面。在某些情况下，一块放置在正确位置的现浇板提供复合作用。
空心板一般用于深度15英尺到21英尺，宽度3英尺或4英尺。对于一个标准HS20的公路，空心板适合于大约50 英尺的跨径。标准渠化区段在多种宽度，深度从21英尺到35英尺时是有利的，可用于大约20英尺到60英尺的跨径。中空的箱形梁和T形梁用于大约100英尺的长跨径。
对于中等跨径的大约120英尺的公路桥，一般使用AASHTO 标准梁。它们和一种复合现场预制行车道板一起使用。在板被安置之后，这样的梁经常在预浇梁的先张拉 与合成梁的后张拉后结合。试图获得改进经济，一些国家已经采用更精炼的设计，例如华盛顿州标准梁。
经过特别设计预制建筑梁可以用来携带一个单轨铁路系统。完成的沃尔特迪斯尼乐园单轨铁路的一系列的特征, 每个包括六个单独支持的预拉梁，一起形成连续结构。典型的跨距是100到110英尺。被使用的337根梁，大约一半有垂直与水平曲率和易变超级升高的一些结合。所有的梁是中空的，它的特征是通过在曲梁中插入泡沫和在直梁制作中移动形心轴获得。
由于运输问题和构件垂直的大而重的问题，预制梁不能用于跨径超过120英尺的桥梁。另一方面，桥梁有一种向大跨径发展的明显趋势。对于提高的都市高速道路来说，大跨径促进通路而且将对下面的活动妨碍减到最少。对环境的损害问题的关心，导致连续高架桥大跨径的选择。对于跨河桥，因为航行间隙的需求，中间可能不可以搭设桥墩。
在这一个类型的典型建筑中，桥墩（台）采用现场预制, 经常使用滑动的技术。一个箱梁的" 锤头"浇筑在桥墩的顶端, 并且通过平衡的悬臂方法控制每个方向的建筑进行。最后,在完成跨中结合处的浇筑之后, 这种结构对于完全连续性的桥梁来说是进一步的受拉的 。修剪钥匙可能被用在片段之间垂直面之上, 而且预制建筑与环氧基树脂黏在一起。
想象的工程学藉着许多特别的技术示范已经延长混凝土建筑的范围，就桥梁而言，已经远远超过几年以前可以想象的到的任何事。在美国，双向弯曲的现场浇铸的部分箱梁已经成功地完成 310 英尺 的跨距，跨越了北加州的鳗鱼河。该桥的中间跨度550英尺，侧跨跨度390英尺，初步设计已经完成。
预应力混凝土桥梁很适合于大跨径的另一种形式是悬索箱梁。一个值得注意的例子是阿根廷境内的Chaco-Corrientes桥。桥的主跨径804英尺，由两个A型构架塔支撑，悬索从塔顶到沿着底板伸展。受拉的悬索不仅提供一种垂直反应组成支撑底板，而且对于箱梁产生水平的压缩，增加构件中的张拉力。
许多年以前德国工程师 Ulrich Finsterwalder 提出适合于负担荷载和大跨径的一个结构类型是压带式桥。压带是桥运输跨越莱茵河的一个管道和人行道，跨径446英尺。上部结构的施工顺序是（a）垂直的一对悬索，(b)在每个悬索的下面，安置预制板形成一人行道甲板和 一U型 的平台, 并且 (c)在附近现场浇注混凝土。放置在顶部的管道支撑在扶手的顶部，向一边倾斜， 大幅地增加结构的主要风速。
讨论桥梁形式应适当的考虑结构的美学。当结构可能根据最小的费用和技术设计的时候，时间消逝了。特别是桥梁结构到处可见。生产视觉上不愉快的结构 , 过去经常发生, 是一种不负专业责任的行为。特别地对于主要的跨距，还有一些比较普通的结构来说，应该在设计过程的概念阶段寻找建筑的建议。
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