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1、桥梁结构及其设计原理对于桥梁来说，从采用装配式标准构件的小跨径结构到净跨径接近于1000英尺的斜拉梁和连续箱梁，已经证实了预应力混凝土工艺上的方便性，经济上的竞争性，审美上的卓越性。目前，几乎所有的混凝土桥，甚至跨度较小的桥都是预应力结构的。人们可以采用预制的，现浇式的，或者混合式的施工方法。先张法和后张法预加应力通常在同一个工程中使用。桥梁结构在建桥之前，设计工程师要考虑各种因素，例如最适合场地的桥梁形式（悬索桥，梁桥或拱桥），所用的材料，桥梁所承受的荷载类别，环境因素例如强风或震动，和最合适的施工方法。桥梁施工中最普遍的施工形式是梁。跨在沟渠上的一块板就是一座简单的梁桥，荷载作用由材料的弯
2、曲应力承担。一个简支梁只有两个支座，任何一个向下的荷载作用均会使梁的底面受拉，顶面受压。这些应力对支撑荷载起到足够的力偶矩，即弯矩。根据静力学原理，保持梁竖直的反力必定等于将其下压的荷载作用力。因此，对于一种已知量的材料，厚梁比薄梁产生更大的杠杆作用，所以除了最小的梁-，梁都应作成工字形梁，T梁或中空的箱梁。在简支梁中，最大弯矩值出现在跨中，并向两端递减至0。如果想要建成一座外观修长，形态优美的桥梁，这一特点就会造成很多不便。悬臂和悬跨通过“铰”断开，铰点把弯矩分成两部分，一部分在跨中区域下垂，一部分在内力作用下上部受拉，下部受压。如有必要，可以将副弯矩配置到承担一般以上的总力矩，以便更好的满
3、足优美的曲面，伦敦的滑铁卢大桥就是一个例子。英格兰西部布里斯托附近的埃文毛斯大桥与之相似，但它是一座没有铰接的连续梁桥。这种桥已经很普遍，尽管一个桥墩下沉或许会使梁弯曲并增加应力。悬索桥可有较大跨径，但是有一个严重的缺陷，那就是它很柔韧，而且交通荷载可以产生大幅度挠度，尤其当荷载作用在近1/4跨径时。由于这个原因，几乎一直都用加劲梁或箱梁来加强缆索。即使这样，这种形式几乎不用于铁路桥上，因为火车比较重，而且它们的荷载要比公路荷载更集中。但是悬索桥解决了最大跨径的问题，因为作缆索的钢绳成本廉价，而且其强度比适宜于做梁结构的钢要大的多。目前中等跨度的桥梁一般均采用一种折中办法是斜拉桥，这种桥装有拉
4、索，将搭栈和桥面接了起来。桥梁结构中，拱桥可能被看作是翻过来的悬索桥。虽然拱所施加的是推力，不是拉力，但和悬索桥一样，拱施加在桥墩（端支撑）的力比简支梁大的多，这使地面承受一部分力，因此在结构中节省了材料，但是需要更可靠的基础。因为这种结构可以使用普通等级钢，并且将其作成相当大的拱肋或大梁，以保证压曲抗力，所以挠度问题不大。结构理论与设计（1）桥梁的结构理论中对桥面铺装层的计算分析论述接近于零，现在规范中只给定了厚度的推荐值7，工程界一直在各等级的公路中运用了几十年。随着交通量的增大，现在铺装与重型、超重型汽车的增多和车速的增快已不相适应。桥面铺装层承受车轮荷载的冲击，桥面铺装部分或全部结构的
5、变形，因此桥面铺装是一个受力复杂的动力体系，各种形式的主梁及铺装本身的构造均影响其应力的分布。（2）现行桥规第3.2.2条规定：。如无精确的计算方法，箱型梁也可参照T形桥进行处理的【9】。从从多箱梁的设计来看，大部分设计者对箱梁构件时按T形梁进行处理的。而箱梁的实际受力虽有近于T梁的一面，又有异于T梁的一面，对于连续箱梁差别更大。尤其是近年来箱梁的桥面越来越宽，桥跨与桥宽之比越来越小，箱梁仍按T梁那种长细杆件设计配筋，越来越不适宜了，导致T梁设计的箱梁骨架钢筋在实际受力状态下难以像T梁主筋那样发挥应有的作用。所以，设计的假设状态与箱梁的实际受力状态不一致。（3）随着材料工业的发展，桥梁承重结构
6、的改进，使桥梁主梁能以教柔的结构达到受力的要求，高等级公路大跨桥梁的横向越来越宽。特别在设计计算中侧重于主梁纵向的计算分析，对桥梁横向刚度重视不足，横向构造措施不力使桥面铺装分担了过多的次内力。（4）对于连续桥梁、拱桥及悬臂桥等桥型构造，由于荷载的作用而产生负弯矩或拉力，使桥面铺装层受到拉力的总用而产生负弯矩区裂缝，从而造成桥面铺装的损坏。（5）在对高速公路进行交通组织管理中，由于车道功能的不同，人为强制地使桥梁构造运营始终处于偏载承担了比超车道高得多（量值可达三至四倍）的运营应力水平，因此加快了主车道铺装层的疲劳。特别是随着私营运输业的发展，货运业主为追求短期经济利益，通过改变车厢构造如加长
7、车厢和加高车轴弹簧等使汽车的载重、轴重及轮载成倍增加。这些年车辆对铺装层具有严重的毁坏作用，并使桥梁结构局部超载，加快了主车道铺装层的病害发展。因此，在设计中应根据运营中荷载的实际分布情况，在明确了桥梁结构受力的基础上，对桥面铺装成进行受力计算。一般来说，在美国公路桥梁通常满足美国洲际公路及运输工作者协会规范的荷载、设计和施工要求。铁路中使用。桥梁的设计和施工以美国铁道工程师协会的铁路工程手册的规定为基准。对于人行立交桥和用于其他目的的桥梁的设计要求，可由当地或区域性的规范和规程确立。美国混凝土协会规范的条款常常为参考文献所采纳，而且在大多数情况中作为其他指导性文件的规范规定。在欧洲，以及近年
8、来在西半球，这些要求导致了分段施工的大跨度预应力混凝土箱型梁桥的发展。在这种桥梁的典型施工中，往往应用滑模技术就地浇注桥墩。然后在桥墩顶部浇注“榔头”形箱梁节段，进而在每一个方向用平衡悬臂梁法进行施工。施工或者采用现浇节段，或者采用预制节段向前推进，每一节段都与前面已经完成的结构用后张法连接。最后，待跨中闭合的现浇接头作好以后，再用后张法张拉整个结构以达到完全的整体性。Bridges Structure theory and designPretested concrete has proved to be technically advantageous, economically comp
9、etitive, and esthetically superior for bridges, from very short span structures using precast standard components to cable-stayed girders and continuous box girders with clear spans of nearly 1000ft.Nearly all concrete bridges, even those of relatively short span, are now priestesses. Precutting, ca
10、st-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.Bridge StructureBefore a bridge is built the engineers designing it will have to consider various points such as the best type of bridge for the
11、site-suspension, beam or arch the materials to be used, the sort of loads it must carry, environmental conditions such as high winds or earthquakes, and the most suitable construction methods.The commonest form of bridge construction is the beam .A plank placed across a ditch is a simple beam bridge
12、, and the action of the load is resisted by bending stresses in the material. For a simple supported beam, which are rests on two supports only? Any load acting downwards puts the bottom part of the beam in tension and the top part in compression. These stresses develop sufficient lee rage (the tech
13、nical term is bending moment) to support the load. The forces keeping the beam straight must, by a fundamental law of statics, equal the load tending to fold it up. Therefore for a given amount of material, a deep beam will give more leverage than a thin one, and material near the top and bottom fac
14、es works to better effect than material in the middle. So for anything more than the smallest bridges, the beam will be fashioned into I-or T-sections or into a hollow box section.In a simple supported beam the maximum bending moment occurs at the centre, diminishing to zero at the ends. This is inc
15、onvenient if a slender, graceful appearance is wanted. The cantilever and suspended span arrangement has the CANTILEVERS and suspended spans separated by hinges , which divides the bending moment into two parts, sagging as before in the midspan regions and hogging (tension at the top ,compression at
16、 the bottom ) over the inner supports . The If desired, the hogging may be arranged to take more than half the total moment, which fits well with a graceful curved profile-Waterloo Bridge in London is an example . The Avon mouth Bridge near Bristol in western England is similar, but is a continuous
17、girder bridge with no hinges. This is now common practice, despite the fact that if one pier sinks slightly this bends the beam and adds to the stresses.The SUSPENSION bridge can cover a vast span but has serious drawback: it is very flexible and traffic loading may produce a large deflection, parti
18、cularly when it acts near the quarter point of the span. For this reason a stiffening beam or box girder is almost always provided to supplement the cables. Even so, this type is rarely used for railway bridges as trains are heavier, and their loading more concentrated, than road traffic. Sustention
19、 bridges are, however, the solution for the longest spans, since wire for the cables can be produced cheaply with much greater strength than the steel suitable for beam construction. A compromise which is now popular for intermediate spans is the cable stayed bridge which has stays joining the tower
20、s and deck.In bridge construction the ARCH could be considered to be a sustention bridge turned upside down. Like the sustention bridge the arch puts a much larger force on the abutments (the end supports) than a simple beam does, although it pushes rather than pulls. This makes the ground do part o
21、f the work, thereby saving material in the structure, but it requires reliable foundations. Flexibility is less of a problem in this type of construction because ordinary grade steel is used and built up into a relatively large arched rib or girder to ensure resistance to buckling.Structure theory a
22、nd design(1)bridge structure holds tier of calculation with in theory to bridge floor shop , Gives the recommendation value having fixed thickness7 , The project boundary has applied several tens year to every grade highway always middle . Already is out of keeping with the fact the volume of traffi
23、c enhancing, active shop hold increasing with the heavy type, increasing by and speed of a vehicle surpassing the heavy type automobile with quickly. The bridge floor lays the impingement pretending bear carriage wheel load tier directly, the bridge floor lays the deformation having loaded the part
24、or all participation betokening girder construction, bridge floor shop pretends to be a main beam accepting the strenuously complicated driving force system, various form and the distribution laying the structure pretending to be self affect whose strain equally therefore.（2）Bridges currently in eff
25、ect:.The regulation also may consult the T shape beam if there being accurate reckoning ;the box shape beam handle 9 .Judging from a lot of box beams design , the most designer is to handle to box beam component be in progress according to T shape beam . Box beam reality accepts but a force though h
26、aving one aspect bordering on the T beam, one aspect having being different from the T beam, the with regard to continuous box beam difference are bigger. Especially box beam bridge floor is in recent years more and more broad ,ratio of bay and bridge width is more and more small , does that long th
27、in member still design the box beam according to T beam matching tendon , push down with regard to more and more no proper , leading to ? The box beam framework reinforced bar that the beam designs is difficult to bring the due effect into play like T beam host tendon under actual strenuously circum
28、 stances .Therefore, the hypothesis states designing that box beam reality accept force state disaccords.（3）With material, development of industry, architectural bearing bridge improvement, make bridge host Liang Neng more and more broad reaching the request accepting a force, big advanced level hig
29、hway transverse direction across bridge with softer structure. Stiffness attaches importance to bridge transverse direction deficiency , disadvantageous messenger of transverse direction structure measure bridge floor shop pretends to have shared more than the correct or required number time of inte
30、rnal forces especially in the analysis designing that the middle secretly scheming against puts particular emphasis on the calculation in main beam direction .（4）To bridge type structure such as continuous beam bridge , arch bridge and cantilever beam bridge , effect because of load but produce nega
31、tive curved regulation or pulling force , make the bridge floor lay the effect pretending to accept pulling force tier but produce the breakdown shouldering the curved regulation area rift , bringing about bridge floor shop dress thereby .（5）Manages middle in the organization who carries out traffic
32、 on the express highway , different since the lane is functional , person has been to compel a field to make bridge structure be in motion and do business all the time in insisting to be loaded with state , has made host lane shop horizontal holding the strain having born ratio overtaking Daogaodedu
33、o (magnitude but having amounted to three till quitter) to be in motion and do business with , has accelerated the host lane therefore laying the strain installing a tier .Especially with the development of private carrying trade , the freight proprietor is to run after short-term economy benefit ,
34、the load , axle load on rail and wheel by changing compartment structure if lengthening a compartment and heightening axle spring and so on uses an automobile are loaded with to increase by several times . These vehicles are had to shop dress tire destroying an effect gravely, are make bridge struct
35、ure part overload and, have accelerated the host lane shop the disease installing a tier development. Therefore, should distribute condition according to the reality that the vehicle loads in being in motion and doing business in design, pretend to be in progress tier being secretly schemed against
36、by the force on the basis having made clear that bridge structure accepts a force, to bridge floor shop.In the Unit States, highway bridges generally must meet loading, design, and construction requirement of the AASHTO specification. The design and construction railway bridges are governed by provi
37、sions for the AREA Manual for Railway Engineering. 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 and in most cases serve as model provisions for
38、 other governing documents.Have led to the development in Europe and more recently on the western hemisphere, of long span segmental prestressed concrete box girder bridges. In typical construction of this type, piers are cast-in-place, often using the slip-forming technique. The Ahammerhead section
39、 of box girder is then cast at the top of the pier and construction proceeds in each direction by the balanced the cantilever method, the construction the advanced the using either cast-in-place or precast segments, each post-tensioned to the previously completed construction. Finally, after the closing cast-in-place joint is made at midspan, the structure is further post-tensioned for full continuity
