本科毕业设计
中英文翻译
专业名称：土木工程
年级班级：XXX
学生姓名：XXX
指导教师：XXX
土木工程学院
二○一二年六月一日
Design of arch bridges and the bridge crack
produced the reason to simply analyse
This chapter considers the full range of arch bridge types and a range of materials presenting several case studies and describing the design decisions that were made. A general treatment of the analysis of arches is presented, including the derivation of the basic equations that can be used to undertake hand calculations which may beused to validate computer analysis output. Detailed arch bridge design is outside thescope of this chapter so only general issues are discussed. Most of the chapter is devoted to masonry arch bridges. Masonry arch bridge construction is discussed in its historical context and the importance for engineers to take a holistic approach to bridge assessment and design is emphasized. There is a significant section on bridge assessment which includes guidance in the application of current and emerging assessment methods. This is underpinned with background information regarding the material properties of masonry. The chapter concludes with a treatment of repair and maintenance strategies including a comprehensive table which considers common remedial and strengthening measures.
The origins of the use of arches as a structural form in buildings can be traced back to antiquity (Van Beek, 1987). In trying to arrive at a suitable definition for an arch we may look no further than Hooke‘s anagram of 1675 which stated ?Ut p endet continuum flexile, sic stabat continuum rigidum inversum‘ –?as hangs the flexible line, so but inverted will stand the rigid arch‘. This suggests that any given loading to a flexible cable if frozen and inverted will provide a purely compressive structure in equilibrium with the applied load. Clearly, any slight variation in the loading will result in a moment being induced in the arch. It is arriving at appropriate proportions of arch thickness to accommodate the range of eccentricities of the thrust line that is the challenge to the bridge engineer. Even in the Middle Ages it was appreciated that masonry arches behaved essentially as gravity structures, for which geometry and proportion dictated aesthetic appeal and stability. Compressive strength could be relied upon whilst tensile strength could not. Based upon experience, many empirical relationships between the span and arch thickness were developed and applied successfully to produce many elegant structures throughout Europe.
The expansion of the railway and canal systems led to an explosion of bridge building. Brickwork arches became increasingly popular. With the construction of the Coalbrookdale Bridge (1780) a new era of arch bridge construction began. By the end of the nineteenth century cast iron, wrought iron and finally steel became increasingly popular; only to be challenged by ferro cement (reinforced concrete) at the turn of the century.
During the nineteenth century analytical technique developed apace. In particular, Castigliano (1879) developed strain energy theorems which could be applied to arches provided they remained elastic. This condition could be satisfied provided the line of thrust lay within the middle third, thus ensuring that no tensile stresses were induced. The requirement to avoid tensile stresses only applied to masonry and cast iron; it did not apply to steel or reinforced concrete (or timber for that matter) as these materials were capable of resisting tensile stresses.
Twentieth century arch bridges have become increasingly sophisticated structures combining modern materials to create exciting functional urban sculptures.
Types of arch bridge
The relevant terms that are used to describe the various parts of an arch bridge are shown in Figure 1. Arches may be grouped according to the following parameters:
1. the materials of construction
2. the structural articulation
3. the shape of the arch
Historically, arch bridges are associated with stone masonry. This gave way to brickwork in the nineteenth century. Because these were proportioned to minimise the possibility of
tensile stress, they tend to be fairly massive structures. By comparison the use of reinforced concrete and modern structural steel gives the opportunity for slender, elegant arches.
Nowadays, timber is restricted to small bridges occasionally in a truss form but more usually as laminated curved arches. Although timber has a high strength to density ratio parallel to the grain, it is anisotropic and strength properties perpendicular to the grain are relatively weak. This requires careful detailing of connections to ensure economic use of the material.
With regard to structural articulation the arch can be fixed or hinged. In the latter case either one, two or three hinges can be incorporated into the arch rib. Whilst the fixed arch has three redundancies, the introduction of each hinge reduced the indeterminacy by one until, with three hinges, the arch is statically determinate and hence, theoretically, free of the problems of secondary stresses. Figure 2 shows a range of possible arrangements. The articulation of the arch is not only dependent upon the number of hinges but is also fund amentally influenced by the position of the deck and the nature of the load transfer from the deck to the arch.
The traditional filled spandrel, where the vehicular loading is transferred through the b ackfill material onto the extrados of the arch, represents at first glance the simplest structural condition. As will be seen later this is not the case and has led to much research for the specific case of the masonry arch bridge in an attempt to improve our understanding of such structures.
The spandrel may be open with columns and/or hinges used to transfer the deck loads to the arch. In an attempt to minimise the horizontal thrust on the abutments, the deck may be used to ?tie‘ the arch. Tied arches are particularly appropriate when deck construction depths are limited and large clear spans are needed (particularly if ground conditions are also difficult and would require extensive piling to resist the horizontal thrusts).
Returning to Hooke‘s anagram, the perfect shape for an arch would be an inverted catenary – this would only be the case for carrying its own self-weight. Vehicle loading and varying superincumbent dead load both induce bending moments. Consequently the arch has to have sufficient thickness to accommodate the ?wandering‘ thrust line.
For ease of setting out and construction simpler shapes are adopted nowadays segmental or parabolic shapes are used. Although in situations where maximum widths of headroom have to be provided (say over a railway, road or canal) an elliptical shape may be required or its nearest ?easy‘ equivalent the three-centred arch.
It is worth commenting at this stage regarding the idealization of arch structures.
Traditionally arches are perceived as being two-dimensional structures; this, of course is not true – but the extent to which it is not true should be of concern to the designer/assessor. Even in the case of a three-hinged arch whi ch ostensibly is statically determinate the ?pins‘ are capable of transmitting shear even though they theoretically cannot transmit moments. In the case of non-uniform transverse distribution of loading the hinges will transmit a varying shear which will produce torsion in the arch. Moreover, in the case of skew arches or non-vertical ribs the structure has a much higher redundancy and hence will require greater attention to detail in regard to the releases which are engineered into the structure.
From an aesthetic point of view, arches have a universal appeal. In spite of this, care must be taken as the impact of even modest sized bridges is significant. Filled arches are invariably masonry (or widening of masonry) bridges. Cleanness of line, honesty of conception and the attention to detail are vital ingredients to a successful bridge. Certainly, simple stringcourses and copings are preferable to elaborate details which would be expensive and inappropriate for most modern bridges. Where stone is used it is important to be sensitive to the nature of the material. Modern quarrying techniques should be employed (laser cutting, diamond sawing, flame texturing and sandblasting) reserving traditional dressing to conservation schemes. If brickwork is used different textured or coloured bricks and mortar can be specified. Here stringcourses can be particularly useful to mask changes in bedding angle.
Historically abutments comprised either rock, or else were massive masonry supports relying on their weight to resist the thrust of the arch. In terms of structural honesty this is necessary as it is instinctive to expect such support.
Reinforced concrete and steel arches are altogether much lighter structures. ?The structure consists basically of the arch, the deck and usually some supports from the arch to the deck – in that order of importance. These elements should be expressed in both form and detail, and with due regard for their hierarchy‘ (Highways Agency, 1996).
It is important that the deck, if it rests on the crown of the arch, should not mask it in any way. Any support whether spandrel columns or hinges (in the case of the tied arch) should not be allowed to dominate. Preferably they should be recessed relative to the parapet and stringcourse.
Concrete arches can be either a full width curved slab or a series of ribs. Steel is almost
always a series of ribs. Where ribs are used thought should be given (if they are going to be seen from underneath) to the chiaroscuro of the soffit.
The ratio of span to rise should generally be in the range 2:1 to 10:1. The flatter the arch the greater the horizontal thrust; this may affect the structural form selected, i. e. whether or not a tie should be introduced, or the stiffness of the deck relative to the arch.
In recent years, the traffic capital construction of our country gets swift and violent development, all parts have built a large number of concrete bridges. In the course of building and using in the bridge, relevant to influence project quality lead of common occurrence report that bridge collapse even because the crack appears The concrete can be said to " often have illness coming on " while fracturing and " frequently-occurring disease ", often perplex bridge engineers and technicians. In fact, if take certain design and construction measure, a lot of cracks can be overcome and controlled. For strengthen understanding of concrete bridge crack further, is it prevent project from endanger larger crack to try one's best, this text make an more overall analysis, summary to concrete kind and reason of production, bridge of crack as much as possible, in order to design, construct and find out the feasible method which control the crack, get the result of taking precautions against Yu WeiRan.
Concrete bridge crack kind, origin cause of formation In fact, the origin cause of formation of the concrete structure crack is complicated and various, even many kinds of factors influence each other, but every crack has its one or several kinds of main reasons produced. The kind of the concrete bridge crack, on its reason to produce, can roughly divide several kinds as follows :
First, load the crack caused Concrete in routine quiet.
Is it load to move and crack that produce claim to load the crack under the times of stress bridge, summing up has direct stress cracks, two kinds stress crack onces mainly. Direct stress crack refer to outside load direct crack that stress produce that cause. The reason why the crack produces is as follows: (1) Design the stage of calculating, does not calculate or leaks and calculates partly while calculating in structure; Calculate the model is unreasonable; The structure is supposed and accorded with by strength actually by strength ; Load and calculate or leak and calculate few; Internal force and matching the mistake in computation of muscle; Safety coefficient of structure is not enough. Do not consider the possibility that construct at the time of the structural design; It is insufficient to design the section; It is simply little and
assigning the mistake for reinforcing bar to set up; Structure rigidity is insufficient; Construct and deal with improperly; The design drawing can not be explained clearly etc. (2) Construction stage, does not pile up and construct the machines, material limiting ; Is it prefabricate structure structure receive strength characteristic, stand up, is it hang, transport, install to get up at will to understand; Construct not according to the design drawing, alter the construction order of the structure without authorization, change the structure and receive the strength mode; Do not do the tired intensity checking computations under machine vibration and wait to the structure. (3) Using stage, the heavy-duty vehicle which goes beyond the design load passes the bridge; Receive the contact, striking of the vehicle, shipping; Strong wind, heavy snow, earthquake happen, explode etc.
Stress crack once means the stress of secondary caused by loading outside produces the crack. The reason why the crack produces is as follows, (1)In design outside load function, because actual working state and routine, structure of thing calculate have discrepancy or is it consider to calculate, thus cause stress once to cause the structure to fracture in some position. Two is it join bridge arch foot is it is it assign " X " shape reinforcing bar, cut down this place way, section of size design and cut with scissors at the same time to adopt often to design to cut with scissors, theory calculate place this can store curved square in, but reality should is it can resist curved still to cut with scissors, so that present the crack and cause the reinforcing bar corrosion. (2)Bridge structure is it dig trough, turn on hole, set up ox leg, etc. to need often, difficult to use a accurate one diagrammatic to is it is it calculate to imitate to go on in calculating in routine, set up and receive the strength reinforcing bar in general foundation experience. Studies have shown, after being dug the hole by the strength component, it will produce the diffraction phenomenon that strength flows, intensive near the hole in a utensil, produced the enormous stress to concentrate. In long to step prestressing force of the continuous roof beam, often block the steel bunch according to the needs of section internal force in stepping, set up the anchor head, but can often see the crack in the anchor firm section adjacent place. So, if deal with improper, in corner of component form sudden change office, block place to be easy to appear crack strength reinforcing bar of the structure. In the actual project, stress crack once produced the most common reason which loads the crack. Stress crack once belong to one more piece of nature of drawing, splitting off, shearing. Stress crack once is loaded and caused, only seldom calculate according to the routine too, but with
modern to calculate constant perfection of means, times of stress crack to can accomplish reasonable checking computations too. For example to such stresses 2 times of producing as prestressing force, creeping, etc. , department's finite element procedure calculates levels pole correctly now, but more difficult 40 years ago.
In the design, should pay attention to avoiding structure sudden change (or section sudden change), when it is unable to avoid, should do part deal with, corner for instance, make round horn, sudden change office make into the gradation zone transition, is it is it mix muscle to construct to strengthen at the same time, corner mix again oblique to reinforcing bar, as to large hole in a utensil can set up protecting in the perimeter at the terms of having angle steel. Load the crack characteristic in accordance with loading differently and presenting different characteristics differently. The crack appears person who draw more, the cutting area or the serious position of vibration. Must point out, is it get up cover or have along keep into short crack of direction to appear person who press, often the structure reaches the sign of bearing the weight of strength limit, it is an omen that the structure is destroyed, its reason is often that sectional size is partial and small. Receive the strength way differently according to the structure, the crack characteristic produced is as follows: (1) Central tension. The crack runs through the component cross section, the interval is equal on the whole, and is perpendicular to receiving the strength direction. While adopting the whorl reinforcing bar, lie in the second-class crack near the reinforcing bar between the cracks. (2)The centre is pressed. It is parallel on the short and dense parallel crack which receive the strength direction to appear along the component. (3) Receive curved. Most near the large section from border is it appear and draw into direction vertical crack to begin person who draw curved square, and develop toward neutralization axle gradually. While adopting the whorl reinforcing bar, can see shorter second-class crack among the cracks. When the structure matches muscles less, there are few but wide cracks, fragility destruction may take place in the structure. (4) Pressed big and partial. Heavy to press and mix person who draw muscle a less one light to pigeonhole into the component while being partial while being partial, similar to receiving the curved component. (5) Pressed small and partial. Small to press and mix person who draw muscle a more one heavy to pigeonhole into the component while being partial while being partial, similar to the centre and pressed the component. (6) Cut. Press obliquly when the hoop muscle is too dense and destroy, the oblique crack which is greater than 45 degrees
direction appears along the belly of roof beam end; Is it is it is it destroy to press to cut to happen when the hoop muscle is proper, underpart is it invite 45 degrees direction parallel oblique crack each other to appear along roof beam end. (7) Sprained. Component one side belly appear many direction oblique crack, 45 degrees of treaty, first, and to launch with spiral direction being adjoint. (8) Washed and cut. 4 side is it invite 45 degrees direction inclined plane draw and split to take place along column cap board, form the tangent plane of washing.
(9) Some and is pressed. Some to appear person who press direction roughly parallel large short cracks with pressure.
Second, crack caused in temperature change.
The concrete has nature of expanding with heat and contract with cold, look on as the external environment condition or the structure temperature changes, concrete take place out of shape, if out of shape to restrain from, produce the stress in the structure, produce the temperature crack promptly when exceeding concrete tensile strength in stress. In some being heavy to step foot-path among the bridge, temperature stress can is it go beyond living year stress even to reach. The temperature crack distinguishes the main characteristic of other cracks will be varied with temperature and expanded or closed up. The main factor is as follows, to cause temperature and change. (1) Annual difference in temperature. Temperature is changing constantly in four seasons in one year, but change relatively slowly, the impact on structure of the bridge is mainly the vertical displacement which causes the bridge, can prop up seat move or set up flexible mound, etc. not to construct measure coordinate, through bridge floor expansion joint generally, can cause temperature crack only when the displacement of the structure is limited, for example arched bridge, just bridge etc. The annual difference in temperature of our country generally changes the range with the conduct of the average temperature in the moon of January and July. Considering the creep characteristic of the concrete, the elastic mould amount of concrete should be considered rolling over and reducing when the internal force of the annual difference in temperature is calculated. (2) Rizhao. After being tanned by the sun by the sun to the side of bridge panel, the girder or the pier, temperature is obviously higher than other position, the temperature gradient is presented and distributed by the line shape. Because of restrain oneself function, cause part draw stress to be relatively heavy, the crack appears. Rizhao and following to is it cause structure common reason most, temperature of crack to lower the temperature suddenly. (3) Lower the
temperature suddenly. Fall heavy rain, cold air attack, sunset, etc. can cause structure surface temperature suddenly dropped suddenly, but because inside temperature change relatively slow producing temperature gradient. Rizhao and lower the temperature internal force can adopt design specification or consult real bridge materials go on when calculating suddenly, concrete elastic mould amount does not consider converting into and reducing. (4) Heat of hydration. Appear in the course of constructing, the large volume concrete (thickness exceeds 2. 0), after building because cement water send out heat, cause inside very much high temperature, the internal and external difference in temperature is too large, cause the surface to appear in the crack. Should according to actual conditions in constructing, is it choose heat of hydration low cement variety to try one's best, limit cement unit's consumption, reduce the aggregate and enter the temperature of the mould, reduce the internal and external difference in temperature, and lower the temperature slowly, can adopt the circulation cooling system to carry on the inside to dispel the heat in case of necessity, or adopt the thin layer and build it in succession in order to accelerate dispelling the heat. (5) The construction measure is improper at the time of steam maintenance or the winter construction, the concrete is sudden and cold and sudden and hot, internal and external temperature is uneven, apt to appear in the crack. (6) Prefabricate T roof beam horizontal baffle when the installation, prop up seat bury stencil plate with transfer flat stencil plate when welding in advance, if weld measure to be improper, iron pieces of nearby concrete easy to is it fracture to burn. Adopt electric heat piece draw law piece draw prestressing force at the component, prestressing force steel temperature can rise to 350 degrees Centigrade, the concrete component is apt to fracture. Experimental study indicates, are caused the intensity of concrete that the high temperature burns to obviously reduce with rising of temperature by such reasons as the fire, etc. , glueing forming the decline thereupon of strength of reinforcing bar and concrete, tensile strength drop by 50% after concrete temperature reaches 300 degrees Centigrade, compression strength drops by 60%, glueing the strength of forming to drop by 80% of only round reinforcing bar and concrete; Because heat, concrete body dissociate ink evaporate and can produce and shrink sharply in a large amount.
Third , shrink the crack caused.
In the actual project, it is the most common because concrete shrinks the crack caused. Shrink kind in concrete, plasticity shrink is it it shrinks (is it contract to do ) to be the main
reason that the volume of concrete out of shape happens to shrink, shrink spontaneously in addition and the char shrink. Plasticity shrink. About 4 hours after it is built that in the course of constructing, concrete happens, the cement water response is fierce at this moment, the strand takes shape gradually, secrete water and moisture to evaporate sharply, the concrete desiccates and shrinks, it is at the same time conduct oneself with dignity not sinking because aggregate,so when harden concrete yet,it call plasticity shrink. The plasticity shrink producing amount grade is very big, can be up to about 1%. If stopped by the reinforcing bar while the aggregate sinks, form the crack along the reinforcing bar direction. If web, roof beam of T and roof beam of case and carry baseplate hand over office in component vertical to become sectional place, because sink too really to superficial obeying the web direction crack will happen evenly before hardenning. For reducing concrete plasticity shrink,it should control by water dust when being construct than,last long-time mixing, unloading should not too quick, is it is it take closely knit to smash to shake, vertical to become sectional place should divide layer build. Shrink and shrink (do and contract). After the concrete is formed hard, as the top layer moisture is evaporated progressively, the humidity is reduced progressively, the volume of concrete is reduced, is called and shrunk to shrink (do and contract). Because concrete top layer moisture loss soon, it is slow for inside to lose, produce surface shrink heavy, inside shrink a light one even to shrink, it is out of shape to restrain from by the inside concrete for surface to shrink, cause the surface concrete to bear pulling force, when the surface concrete bears pulling force to exceed its tensile strength, produce and shrink the crack. The concrete hardens after-contraction to just shrink and shrink mainly. Such as mix muscle rate heavy component (exceed 3% ), between reinforcing bar and more obvious restraints relatively that concrete shrink, the concrete surface is apt to appear in the full of cracks crackle. Shrink spontaneously. Spontaneous to it shrinks to be concrete in the course of hardenning, cement and water take place ink react, the shrink with have nothing to do by external humidity, and can positive (whether shrink, such as ordinary portland cement concrete), can negative too (whether expand, such as concrete, concrete of slag cement and cement of fly ash). The char shrinks. Between carbon dioxide and hyrate of cement of atmosphere take place out of shape shrink that chemical reaction cause. The char shrinks and could happen only about 50% of humidity, and accelerate with increase of the density of the carbon dioxide. The char shrinks and seldom calculates. The characteristic that the concrete shrinks the crack is that the
majority belongs to the surface crack, the crack is relatively detailed in width, and criss-cross, become the full of cracks form, the form does not have any law.
Studies have shown, influence concrete shrink main factor of crack as follows. (1) Variety of cement, grade and consumption. Slag cement, quick-hardening cement, low-heat cement concrete contractivity are relatively high, ordinary cement, volcanic ash cement, alumina cement concrete contractivity are relatively low. Cement grade low in addition, unit volume consumption heavy rubing detailed degree heavy, then the concrete shrinks the more greatly, and shrink time is the longer. For example, in order to improve the intensity of the concrete, often adopt and increase the cement consumption method by force while constructing, the result shrinks the stress to obviously strengthen. (2) Variety of aggregate. Such absorbing water rates as the quartz, limestone, cloud rock, granite, feldspar, etc. are smaller, contractivity is relatively low in the aggregate; And such absorbing water rates as the sandstone, slate, angle amphibolite, etc. are greater, contractivity is relatively high. Aggregate grains of foot-path heavy to shrink light in addition, water content big to shrink the larger. (3) Water gray than. The heavier water consumption is, the higher water and dust are, the concrete shrinks the more greatly. (4) Mix the pharmaceutical outside. It is the better to mix pharmaceutical water-retaining property outside, then the concrete shrinks the smaller. (5) Maintain the method. Water that good maintenance can accelerate the concrete reacts, obtain the intensity of higher concrete. Keep humidity high, low maintaining time to be the longer temperature when maintaining, then the concrete shrinks the smaller. Steam maintain way than maintain way concrete is it take light to shrink naturall. (6) External environment. The humidity is little, the air drying, temperature are high, the wind speed is large in the atmosphere, then the concrete moisture is evaporated fast, the concrete shrinks the faster. (7) Shake and smash the way and time. Machinery shake way of smashing than make firm by ramming or tamping way concrete contractivity take little by hand. Shaking should determine according to mechanical performance to smash time, are generally suitable for 55s / time. It is too short, shake and can not smash closely knit, it is insufficient or not even in intensity to form the concrete; It is too long, cause and divide storey, thick aggregate sinks to the ground floor, the upper strata that the detailed aggregate stays, the intensity is not even, the upper strata incident shrink the crack. And shrink the crack caused to temperature, worthy of constructing the reinforcing bar againing can obviously improve the resisting the splitting of
concrete, structure of especially thin wall (thick 200cm of wall). Mix muscle should is it adopt light diameter reinforcing bar (φ8~14 dia) to have priority, little interval assign (whether @ 10 construct @ 15cm ) on constructing, the whole section is it mix muscle to be rate unsuitable to be lower than 0.3%, can generally adopt 0.3%~0.5%.
Fourth, crack that causes out of shape of plinth of the ground.
Because foundation vertical to even to subside or horizontal direction displacement, make the structure produce the additional stress, go beyond resisting the ability of drawing of concrete structure, cause the structure to fracture. The even main reason that subside of the foundation is as follows. (1) Reconnoitres the precision and is not enough for, test the materials inaccuratly in geology. Designing, constructing without fully grasping the geological situation, this is the main reason that cause the ground not to subside evenly. Such as hills area or bridge, district of mountain ridge,, hole interval to be too far when reconnoitring, and ground rise and fall big the rock, reconnoitring the report can't fully reflect the real geological situation. (2) The geological difference of the ground is too large. Building it in the bridge of the valley of the ditch of mountain area, geology of the stream place and place on the hillside change larger, even there are weak grounds in the stream, because the soil of the ground does not causes and does not subside evenly with the compressing. (3) The structure loads the difference too big. Under the unanimous terms, when every foundation too heavy to load difference in geological situation, may cause evenly to subside, for example high to fill out soil case shape in the middle part of the culvert than to is it take heavy to load both sides, to subside soon heavy than both sides middle part, case is it might fracture to contain. (4) The difference of basic type of structure is great. Unite it in the bridge the samly, mix and use and does not expand the foundation and a foundation with the foundation, or adopt a foundation when a foot-path or a long difference is great at the same time, or adopt the foundation of expanding when basis elevation is widely different at the same time, may cause the ground not to subside evenly too. (5) Foundation built by stages. In the newly-built bridge near the foundation of original bridge, if the half a bridge about expressway built by stages, the newly-built bridge loads or the foundation causes the soil of the ground to consolidate again while dealing with, may cause and subside the foundation of original bridge greatly. (6) The ground is frozen bloatedly. The ground soil of higher moisture content on terms that lower than zero degree expands because of being icy; Once temperature goes up,
the frozen soil is melted, the setting of ground. So the ground is icy or melts causes and does not subside evenly. (7) Bridge foundation put on body, cave with stalactites and stalagmites, activity fault, etc. of coming down at the bad geology, may cause and does not subside evenly.
(8) After the bridge is built up, the condition change of original ground. After most natural grounds and artificial grounds are soaked with water, especially usually fill out such soil of special ground as the soil, loess, expanding in the land, etc. , soil body intensity meet water drop, compress out of shape to strengthen. In the soft soil ground, season causes the water table to drop to draw water or arid artificially, the ground soil layer consolidates and sinks again, reduce the buoyancy on the foundation at the same time, shouldering the obstruction of rubing to increase, the foundation is carried on one's shoulder or back and strengthened. Some bridge foundation is it put too shallow to bury, erode, is it dig to wash flood, the foundation might be moved. Ground load change of terms, bridge nearby is it is it abolish square, grit, etc. in a large amount to put to pile with cave in, landslide, etc. reason for instance, it is out of shape that the bridge location range soil layer may be compressed again. So, the condition of original ground change while using may cause and does not subside evenly produce the structure thing of horizontal thrust to arched bridge, etc. it is the main reason that horizontal displacement crack emerges to destroy the original geological condition when to that it is unreasonable to grasp incompletely, design and construct in the geological situation.
拱桥的设计和桥梁裂缝产生的原因分析
本章涵盖了许多桥型的拱桥和一系列关于材料的研究案例，还有介绍了所做的设计决策是怎么形成的。介绍了分析研究拱桥的一般处理方法，包括推导可用于进行手工计算的基本方程以用来验证计算机分析的结果。详细拱桥的设计在本章的范围之外，因此只进行了一般性问题的讨论。本章的大部分内容介绍的是砌体拱桥。砖石拱桥施工中讨论了它的历史背景和强调了工程技术人员采取一种全面的方法来评估和设计桥梁的重要性。桥梁评估有一个重要原则，就是一种在现在和未来都能够有指导作用的评估方法。这也是建立在以砌体材料的背景资料的基础之上的。本章最后总结得出了一种修补的方法和用一张包含了常用的补救措施和加强措施的综合图表来说明的维护策略。
拱作为一种建筑结构形式的起源历史可以追溯到古代(凡比克，1987年)。在试图为拱寻找一个适当的定义时，我们可以借鉴一下不久前胡克在1675年说的―刚性拱‖字谜。这表明任何给定负载量的柔性电缆如果在冻结并且倒转之后在外部荷载下将是一个平衡的抗压结构。显然，加载时任何轻微的变化都会对拱产生诱发突变。达到合适比例的拱厚度以符合推力线的偏心率的适用范围是桥梁工程师所面临的挑战。即使在中世纪表示赞赏砌体拱桥,但其表现形式基本上是作为几何和比例决定的审美情趣和稳定的重力结构。其压缩强度可以依赖但是拉伸强度却不能。根据经验，大跨度和拱厚度的很多经验关系融合并成功地应用，在整个欧洲产生了很多优雅的结构。
铁路和运河的扩大使用导致了桥梁的爆炸式增长，砖砌拱桥成为越来越受欢迎的形式。随着施工的Coalbrookdale桥（1780年），标志着拱桥建设的新时代开始了。到十九世纪，铸铁、锻造铁和钢终于越来越受欢迎，只是世纪之交会受到钢筋混凝土的挑战。
在十九世纪分析技术迅速发展，特别是，卡斯蒂利亚诺(1879)的应变能量定理，能适用于砌体拱桥使他们保持弹性。提供推力线属于中间第三，从而确保没有拉应力诱导，能满足这一条件。要求避免拉应力，因此只适用于砌体和铸铁，它并不适用钢或钢筋混凝土（或木材），因为这些材料有抗拉伸应力的能力。
二十世纪的拱桥已成为越来越多的与现代材料相结合的复杂的结构，以创造令人兴奋的功能性城市雕塑。
拱桥的类型
在图1中显示用来描述一个拱桥的各部分的有关术语。拱桥可以根据以下参数进行分组：
1．建筑材料
2．结构的衔接
3．拱桥的形状
从历史上看，拱桥都是与石头建筑有联系的，但是在十九世纪让位给了砖砌结构。因为他们的比例尽可能的减少了拉伸应力的可能性，这使得他们有成为巨大结构的趋势。相比之下，使用钢筋混凝土和现代结构钢为拱桥提供了简约、典雅的机会。
现在，木材被限制在一些桁架形式的小桥上，但通常更作为复合曲线的拱桥。虽然木材相比于粮食作物的比例有一个高的强度和密度，但是它的各向异性和横纹的强度性能是相对比较弱的。这就需要注意的连接细节，以确保材料的经济使用。
至于衔接拱结构可固接或铰接。在后一种情况中一个、两个或三个铰链可以被使用到拱肋。虽然固结的拱桥存在三个多余约束，但是一个铰的作用就是减少一个多余约束，而对于三个铰来说的拱桥是静定的，因此，从理论上来讲，无二次应力问题。图2显示了一系列可能的布置情况。拱桥的衔接不仅仅取决于铰的数量，也从根本上受到桥面的位置和从桥面传向拱的荷载性质的影响。
传统的填充拱肩是车辆荷载通过其回填材料传递到拱背，这是第一眼能够看到的最简单的结构状况。但是稍后我们看到的就不是这种状况，这就导致了我们在试图提高对这种结构的认识时，将要对这种砖石拱桥的许多特定案例进行研究。
带有柱子的拱肩会被做成开放式的，铰用来传递桥面板的荷载到拱。在试图尽量减少对桥墩的水平推力时，桥面板就会和系杆拱配合使用。当桥面的建设深度受到限制时或者需要大跨度时（特别是如果地面条件也很困难，将需要大量的桩以抵抗水平推力时），系杆拱就会特别的适用。
回到胡克的字谜，拱的完美形状将是一个倒立的悬链线——这是一种抵抗自身重量的情况。车辆荷载和不同的自上而下的恒载是两种诱导弯曲的情况。因此，拱必须有足够的厚度以适应―蜿蜒‖的推力作用线。
为了便于施工放样，简单的建筑物形状现在被分段采用或者采用抛物线形状。虽然在最大宽度的情况下净空必须提供（比如在一个铁路，公路或运河），椭圆形状可能是其最接近的要求或其最相近的等价物是―简单‖的三个铰的拱。
这是值得在现阶段讨论的理想化的拱结构。传统意义上，拱被认为是二维结构。这当然是不真实的——但它在何种程度上是不正确的应该是设计师和评估员所关心的问题。即使在表面上看是静定的三铰拱的情况下，―插脚‖都能传递剪力，即使他们理论上不能传递。非均匀的荷载横向分布的情况下，铰链会传递一个会在拱内产生扭矩的不断
变化的剪力。此外，斜拱或非垂直拱肋的情况下，结构具有很高的冗余，因此将需要更多地注意发布的工程结构方面的详细信息。
从美学的角度来看，拱桥有一个普遍的吸引力。尽管这样，重视规模不大的桥梁的影响是十分有意义的。实体的拱桥总是砖石（或砖石扩大）桥梁。行清洁、诚信的理念和注重细节是成功的桥梁的至关重要的因素。当然，简单的层拱和顶层更适合详细描述一些昂贵的和与许多现代桥梁不相称的细节。使用石头是因为它材质重要的环境敏感性。应采用现代开采技术（激光切割、金刚石锯、火焰变形和喷砂)，保留传统加工保护计划。如果使用不同的砖砌体可以指定纹理或琉璃砖和灰泥。在这里，层拱可以很有效的掩饰层面方向上的变化。
从历史上看，桥墩是由岩石或者是大量的厚重砖石结构组成的，依靠自身的重量抵抗拱桥的推力。依靠结构的稳定性，这是必要的，因为这是一种本能去期待这种支持。
钢筋混凝土桥和钢拱桥有许多轻结构。结构基本上包括拱、桥面板和通常一些拱肋到桥面板的支撑结构——重要的顺序。这些元素应该表现在细节和形式上，并充分考虑其层次性（高速公路局，1996年）。重要的是，如果在拱顶处的桥面板，不应该用任何方法去掩饰它。无论是层拱还是铰链的任何支撑结构（在系杆拱桥的情况下）都不应该被允许占据主导地位。最好是他们相对于栏杆和层拱能够隐藏起来。
混凝土拱可以是一个完整的弯曲宽板或者是一系列的肋骨组成，钢拱桥大多是由一系列的肋骨组成。凡是使用肋骨的时候应当考虑（如果是从下面看的话）拱腹的明暗变化。对跨度上升的比例应控制在10:1到2:1的范围内。拱越平坦水平推力就越大，这可能影响结构形式的选择等等，不论是系杆是否应该被引进，还是桥面板相对于拱的刚度。
近几年来，我国交通基础建设得到迅猛发展，各地区修建了大量的混凝土桥梁。在桥梁建造和使用过程中，有关因出现裂缝而影响工程质量甚至导桥梁垮塌的报道屡见不鲜。混凝土开裂可以说是“常发病”和“多发病”，经常困扰着桥梁工程技术人员。其实，如果采取一定的设计和施工措施，很多裂缝是可以克服和控制的。为了进一步加强对混凝土桥梁裂缝的认识，尽量避免工程中出现危害较大的裂缝，本文尽可能对混凝土桥梁裂缝的种类和产生的原因作较全面的分析、总结，以方便设计、施工找出控制裂缝的可行办法，达到防范于未然的作用。
混凝土桥梁裂缝种类、成因实际上，混凝土结构裂缝的成因复杂而繁多，甚至多种因素相互影响，但每一条裂缝均有其产生的一种或几种主要原因。混凝土桥梁裂缝的种类，就其产生的原因，大致可划分如下几种：
1．荷载引起的裂缝
混凝土桥梁在常规静、动荷载及次应力下产生的裂缝称荷载裂缝，归纳起来主要有直接应力裂缝、次应力裂缝两种。直接应力裂缝是指外荷载引起的直接应力产生的裂缝。裂缝产生的原因有：（1）设计计算阶段，结构计算时不计算或部分漏算；计算模型不合理；结构受力假设与实际受力不符；荷载少算或漏算；内力与配筋计算错误；结构安全系数不够。结构设计时不考虑施工的可能性；设计断面不足；钢筋设置偏少或布置错误；结构刚度不足；构造处理不当；设计图纸交代不清等。（2）施工阶段，不加限制地堆放施工机具、材料；不了解预制结构结构受力特点，随意翻身、起吊、运输、安装；不按设计图纸施工，擅自更改结构施工顺序，改变结构受力模式；不对结构做机器振动下的疲劳强度验算等。（3）使用阶段，超出设计载荷的重型车辆过桥；受车辆、船舶的接触、撞击；发生大风、大雪、地震、爆炸等。
次应力裂缝是指由外荷载引起的次生应力产生裂缝。裂缝产生的原因有：（1）在设计外荷载作用下，由于结构物的实际工作状态同常规计算有出入或计算不考虑，从而在某些部位引起次应力导致结构开裂。例如两铰拱桥拱脚设计时常采用布置“X”形钢筋、同时削减该处断面尺寸的办法设计铰，理论计算该处不会存在弯矩，但实际该铰仍然能够抗弯，以至出现裂缝而导致钢筋锈蚀。（2）桥梁结构中经常需要凿槽、开洞、设置牛腿等，在常规计算中难以用准确的图式进行模拟计算，一般根据经验设置受力钢筋。研究表明，受力构件挖孔后，力流将产生绕射现象，在孔洞附近密集，产生巨大的应力集中。在长跨预应力连续梁中，经常在跨内根据截面内力需要截断钢束，设置锚头，而在锚固断面附近经常可以看到裂缝。因此，若处理不当，在这些结构的转角处或构件形状突变处、受力钢筋截断处容易出现裂缝。实际工程中，次应力裂缝是产生荷载裂缝的最常见原因。次应力裂缝多属张拉、劈裂、剪切性质。次应力裂缝也是由荷载引起，仅是按常规一般不计算，但随着现代计算手段的不断完善，次应力裂缝也是可以做到合理验算的。例如现在对预应力、徐变等产生的二次应力，不少平面杆系有限元程序均可正确计算，但在40年前却比较困难。
在设计上，应注意避免结构突变（或断面突变），当不能回避时，应做局部处理，如转角处做圆角，突变处做成渐变过渡，同时加强构造配筋，转角处增配斜向钢筋，对于较大孔洞有条件时可在周边设置护边角钢。荷载裂缝特征依荷载不同而异呈现不同的特点。这类裂缝多出现在受拉区、受剪区或振动严重部位。但必须指出，如果受压区出现起皮或有沿受压方向的短裂缝，往往是结构达到承载力极限的标志，是结构破坏的前兆，其原因往往是截面尺寸偏小。根据结构不同受力方式，产生的裂缝特征如下：（1）中心受拉。裂缝贯穿构件横截面，间距大体相等，且垂直于受力方向。采用螺纹钢筋时，