桥梁工程毕业设计方案外文翻译(箱梁).docx

西南交通大学本科毕业设计（论文） 第II页西 南 交 通 大 学本科毕业设计（论文）外文资料翻译年 级:学 号:姓 名:专 业:指导老师:2013年 6 月 毕业设计外文资料翻译 第16页外文资料原文：13Box girders13.1 GeneralThe box girder is the most exible bridge deck form. It can cover a range of spans from 25 m up to the largest non-suspended concrete decks built, of the order of 300 m. Single box girders may also carry decks up to 30 m wide. For the longer span beams, beyond about 50 m, they are practically the only feasible deck section. For the shorter spans they are in competition with most of the other deck types discussed in this book.The advantages of the box form are principally its high structural efciency (5.4), which minimises the prestress force required to resist a given bending moment, and its great torsional strength with the capacity this gives to re-centre eccentric live loads, minimising the prestress required to carry them.The box form lends itself to many of the highly productive methods of bridge construction that have been progressively rened over the last 50 years, such as precast segmental construction with or without epoxy resin in the joints, balanced cantilever erection either cast in-situ or coupled with precast segmental construction, and incremental launching (Chapter 15).13.2 Cast-in-situ construction of boxes13.2.1 GeneralOne of the main disadvantages of box decks is that they are difcult to cast in-situ due to the inaccessibility of the bottom slab and the need to extract the internal shutter. Either the box has to be designed so that the entire cross section may be cast in one continuous pour, or the cross section has to be cast in stages.13.2.2 Casting the deck cross section in stagesThe most common method of building box decks in situ is to cast the cross section in stages. Either, the bottom slab is cast rst with the webs and top slab cast in a second phase, or the webs and bottom slab constitute the rst phase, completed by the top slab.When the bottom slab is cast rst, the construction joint is usually located just above the slab, giving a kicker for the web formwork, position 1 in Figure 13.1. A joint in this location has several disadvantages which are described in 11.7.1Figure 13.1 Alternative positions of construction jointAlternatively, the joint may be in the bottom slab close to the webs, or at the beginning of the haunches, position 2. The advantages of locating the joint in the bottom slab are that it does not cross prestressing tendons or heavy reinforcement; it is protected from the weather and is also less prominent visually. The main disadvantage is that the slab only constitutes a small proportion of the total concrete to be cast, leaving a much larger second pour.The joint may be located at the top of the web, just below the top slab, position 3. This retains many of the disadvantages of position 1, namely that the construction joint is crossed by prestressing ducts at a shallow angle, and it is difcult to prepare for the next pour due to the presence of the web reinforcement. In addition, most of the difculty of casting the bottom slab has been re-introduced. The advantages are that the joint is less prominent visually and is protected from the weather by the side cantilever, the quantity of concrete in each pour is similar and less of the shutter is trapped inside the box.Casting a cross section in phases causes the second phase to crack due to restraint by the hardened concrete of the rst phase. Although the section may be reinforced to limit the width of the cracks, it is not desirable for a prestressed concrete deck to be cracked under permanent loads. Eliminating cracks altogether would require very expensive measures such as cooling the second phase concrete to limit the rise in temperature during setting or adopting crack sealing admixtures13.2.3 Casting the cross section in one pourThere are two approaches to casting a box section in one pour. The bottom slab may be cast rst with the help of trunking passing through temporary holes left in the soft form of the top slab. This requires access for labourers to spread and vibrate the concrete, and is only generally possible for decks that are at least 2 m deep. The casting of the webs must follow on closely, so that cold joints are avoided. The uidity of the concrete needs to be designed such that the concrete will not slump out of the webs. This is assisted if there is a strip of top shutter to the bottom slab about 500 mm wide along each web. This method puts no restriction on the width of the bottom slab, Figure 13.2 (a).Alternatively the deck cross section may be shaped so that concrete will ow from the webs into the bottom slab, which normally has a complete top shutter, Figure 13.2 (b). This method of construction is most suitable for boxes with relatively narrow bottom anges. The compaction of the bottom slab concrete needs to be effected by external vibrators, which implies the use of steel shutters. The concrete may be cast down both webs, with inspection holes in the shutter that allow air to be expelled and the complete lling of the bottom slab to be conrmed. Alternatively, concrete may be cast down one web rst with the second web being cast only when concrete appears at its base, demonstrating that the bottom slab is full. The concrete mix design is critical and full-scale trials representing both the geometry of the cross section and density of reinforcement and prestress cables are essential.Figure 13.2 Casting deck in one pourHowever the section is cast, the core shutter must be dismantled and removed through a hole in the top slab, or made collapsible so it may be withdrawn longitudinally through the pier diaphragm.Despite these difculties, casting the section in one pour is under-used. The recent development of self-compacting concrete could revolutionise the construction of decks in this manner. This could be particularly important for medium length bridges with spans between 40 m and 55 m. Such spans are too long for twin rib type decks, and too short for cast-in-situ balanced cantilever construction of box girders, while a total length of box section deck of less than about 1,000 m does not justify setting up a precast segmental facility. Currently, it is this type of bridge that is least favourable for concrete and where steel composite construction is found to be competitive.13.3 Evolution towards the box formChapters 11 and 12 described how solid slabs evolve into ribbed slabs in order to allow increased spans with greater economy. The principal advantage of ribbed slabs is their simplicity and speed of construction. However this type of deck suffers from several disadvantages, notably: the span is limited to about 45 m; live loads are not efciently centred, resulting in a concentrated load (such as an HB vehicle) being carried approximately 1.7 times for a deck with two ribs, requiring additional prestress force; the section has poor efciency, leading to the requirement for a relatively larger prestress force; the deck cannot be made very shallow; the piers need either multiple columns to carry each rib, or a crosshead that is expensive and visually very signicant.Box section decks overcome all these disadvantages.13.4 Shape and appearance of boxes13.4.1 GeneralA box section deck consists of side cantilevers, top and bottom slabs of the box itself and the webs. For a good design, there must be a rational balance between the overall width of the deck, and the width of the box. Box sections suffer from a certain blandness of appearance; the observer does not know whether the box is made of an assemblage of thin plates, or is solid concrete. Also, the large at surfaces of concrete tend to show up any defects in the nish and any changes in colour. The designer should be aware of these problems and do what he can within the constraints of the project budget to alleviate them.13.4.2 Side cantileversSide cantilevers have an important effect on the appearance of the box. The thickness of the cantilever root and the shadow cast on the web mask the true depth of the deck. If the deck is of variable depth, the perceived variation will be accentuated by these two effects, Figure 13.3 (15.4.2). In general, the cantilever should be made as wide as possible, that is some seven to eight times the depth of the root (9.2).13.4.3 The box cross sectionBoxes may be rectangular or trapezoidal, with the bottom ange narrower than the top. Rectangular box sections are easier to build, and are virtually essential for the longest spans due to the great depth of the girders. However, they have the disadvantages that their appearance is somewhat severe, and that their bottom slabs may be wider than necessary.The visual impact of the depth of the box is reduced if it has a trapezoidal cross section. This inclination of the web makes it appear darker than a vertical surface, an impression that is heightened if the edge parapet of the deck is vertical.The trapezoidal cross section is frequently economical as well as good looking. In general, the width of the top of the box is determined by the need to provide points of support to the top slab at suitable intervals. The cross section area of the bottom slab is logically determined at mid-span by the need to provide a bottom modulus sufcient to control the range of bending stresses under the variation of live load bending moments. For a box of rectangular cross section of span/depth ratio deeper than about 1/20, the area of bottom slab is generally greater than necessary, resulting in redundant weight. Choosing a trapezoidal cross section allows the weight of the bottom slab to be reduced. Close to the piers, the area of bottom slab is determined by the need to limit the maximum bending stress on the bottom bre and to provide an adequate ultimate moment of resistance. If the narrow bottom slab dened by mid- span criteria is inadequate, it is simple to thicken it locally.For a very wide deck that has a deep span/depth ratio, this logic may give rise to webs that are inclined at a very at angle. The designer should be aware of the difculties in casting such webs, and make suitable allowances in specifying the concrete and in detailing the reinforcement.Also, an important consideration in the design of box section decks is the distortion of the cross section under the effect of eccentric live loads (6.13.4). The effect of this distortion is reduced in a trapezoidal cross section.Boxes may have a single cell or multiple cells. In Chapter 8 it was explained how important it is for economy to minimise the number of webs. Furthermore, it is moredifcult to build multi-cell boxes, and it is worthwhile extending the single-cell box as far as possible before adding internal webs.Figure 13.3 River Dee Bridge: effect of side cantilever on the appearanceof a variable depth deck (Photo: Edmund Nuttall)13.4.4 Variation of depthOnce the span of a box section deck exceeds about 45 m, it becomes relevant to consider varying the depth of the beam. This is not an automatic decision as it depends on the method of construction. For instance, when the deck is to be precast by the counter- cast method (Chapter 14), if the number of segments is relatively low it is likely to be more economical to keep the depth constant in order to simplify the mould. On the other hand, if the deck is to be built by cast-in-situ balanced cantilevering, it is relatively simple to design the mould to incorporate a variable depth, even for a small number of quite short spans.Clearly, this decision also has an aesthetic component. The depth may be varied continuously along the length of the beam, adopting a circular, parabolic, elliptical or Islamic prole, Figure 13.4. Alternatively, the deck may be haunched. The decision on the soft prole closely links aesthetic and technical criteria.Figure 13.4 Variable depth decksFor instance, when the depth varies continuously it is often judged that an elliptical prole is the most beautiful. However, this will tend to create a design problem towards the quarter points, as at these locations the beam is shallower than optimal, both for shear resistance and for bending strength. As a result, the webs and bottom slab may need to be thickened locally, and the prestress increased. However, the economic penalty may be small enough to accept. The Islamic form is likely to provide the most exible method of optimising the depth at all points along the girder, but the cusp at mid-span may give a problem for the prole of the continuity tendons while for long spans the greater weight of the deeper webs either side of mid-span implies a signicant cost penalty. Also, the appearance may not be suitable for the particular circumstance.When the change in the depth of the box is not too great, haunched decks are often chosen for the precast segmental form of construction, as they reduce the number of times the formwork must be adjusted, assisting in keeping to the all-important daily cycle of production. However, here again there is a conict between the technical optimisation of the shape of the beam and aesthetic considerations. The beginning of the haunch is potentially a critical design section, both for shear and bending. This criticality is relieved if the haunch extends to some 2530 per cent of the span length. However, the appearance of the beam is considerably improved if the haunch length is limited to 20 per cent of the span or less.When variation of the depth is combined with a trapezoidal cross section, the bottom slab will become narrower as the deck becomes deeper, Figure 13.5. This has an important aesthetic impact, as well as giving rise to complications in the construction. When a deck is built by the cast-in-situ balanced cantilever method, such as the 929m long Bhairab Bridge 1 in Bangladesh designed by Benaim, Figure 13.6, the formwork may be designed to accept this arrangement without excessive additional cost. However for a precast deck it is better to avoid this combination, as the modications to the formwork increase the cost and complexity of the mould and interfere with the casting programme. It is easier to cope with a haunched deck than a continuously varying depth, as in the former case the narrowing of the bottom slab is limited to a relatively small proportion of the segments, and the rate at which it narrows is constant.Figure 13.5 Variable depth with trapezoidal cross sectionFigure 13.6 Bhairab Bridge, Bangladesh (Photo: Roads and Highways Department,Government of Bangladesh and Edmund Nuttall)If the bottom slab is maintained at a constant width, the web surfaces will be warped. For a deck that has a continuously varying depth, the timber shutters of a cast-in-situ cantilevering falsework can accept this warp, whereas this may not be the case for the steel shutter of a precast segmental casting cell. However, for a haunched deck the warp would be introduced suddenly at the beginning of the haunch, which would probably be impossible to build, and would look terrible.外文资料译文：第13章 箱梁13.1概述箱梁桥是最具柔性的一种桥面形式。它的跨度范围可以从25m到最大的跨度约为300m的非悬索结构混凝土桥。单箱室也可以做到承载30m宽的桥面。对于超过50m的较大跨度的主梁，箱梁几乎是唯一可行的桥面形式。对于较小跨径，它与大多数其他桥面形式的竞争也将在本书中进行讨论。箱形梁的优势主要是其高效的结构性能，这最大限度地减少了所需的抵抗弯矩的预应力，其巨大的抗扭强度能力也减少了需要承载偏心活载所需的预应力。在过去的50年中，箱形梁已经通过许多高效的桥梁生产建设方法得到日益完善，例如预制梁在接缝处需不需要加环氧树脂，悬臂施工时要么在现场浇筑要么将预制构件连接起来，或者用顶推法（第15章）。13.2 现浇箱梁13.2.1 概述箱梁的主要缺点之一是由于施工时难以接近混凝土梁底面以及需要取出内部浇筑模板造成的现场浇筑的难度。施工时要么模板已经设计好因此整个节段可以一次连续浇筑，要么节段必须分阶段施工。13.2.2分阶段浇筑最常用的现场浇筑箱梁的方法是将横截面划分成几个阶段。首先浇筑底板，然后腹板和浇筑顶板一起浇筑，或者首先浇筑腹板和底板再以浇筑顶板结束。当首先浇筑底板时，施工接缝通常刚好位于底板面的上方，给腹板流出支模空间，如图13.1所示。在这个位置的接缝有众多的不利影响，具体在在11.7.1有详细阐述。或者，接缝可以在底板靠近腹板的位置，或者在梗腋开始的位置，图中13.1的2的位置。接缝位于底板优点在于它不会穿过预应力筋或大量配筋；也能防止受天气的影响，视觉上也不突出。主要的缺点是底板只占了所有浇筑混凝土的一小部分，因此第二次浇筑时的浇筑量很大。接缝的位置也可以位于腹板的顶部，刚好在底板的底部，如图13.1的3的位置。该位置具有位置1的很多缺点，即接缝被浅层的预应力管道穿过，由于腹板钢筋的存在，在准备下一次次浇筑时也比较困难。此外，又引入了许多浇筑底板时的困难。其优点在于该处的接缝更不显眼，以及由于悬臂的保护不收天气的影响，此外每次浇筑的混凝土量更小，更少的模具留在箱梁内部。分阶段浇筑会导致第二个阶段的混凝土由于第一阶段的混凝土硬化产生裂缝。虽然可以通过配筋来限制裂缝的宽度，但预应力混凝土在恒荷载下产生裂缝是不符合要求的。完全消除裂缝需要非常高成本的措施，例如冷却第二阶段的混凝土以限制在采用裂缝密封剂时的气温的上升。13.2.3 一次浇筑有两种方法一次浇筑箱梁截面。底板可以在穿过顶板下部模板留的临时孔洞的管道的帮助下首先浇筑。这就需要入口来使工人散布和振捣混凝土，并且一般只可能适用于至少2m高的梁。腹板的浇筑必须紧接着进行，以避免接缝冷却。流态的混凝土必须设计好以避免混凝土溢出腹板的模板。如果有一条沿每个腹板从顶部模板到底板的宽约500mm的长条，这种情况将得到改善 。这种方法对于底板的宽度没有限制，如图13.2（a）.或者整个界面的形状可由混凝土从腹板流进底板从而成型，这种施工方法需要一个完整的模板，如图13.2（b）。这种浇筑方法特别适用底地板相对较窄的箱梁。底板混凝土的密实度需要外部的震荡器来实现，这意味着需要使用钢模板。混凝土可以从两个腹板向下浇筑，同时模板上需要有检视孔以确认空气被排出以及底板混凝土填充完整。或者，混凝土先从一个腹板往下浇筑待另一个腹板底部出现混凝土时再从此腹板往下浇筑，以此证明整个底板被填充完整。混凝土的配合比设计是具有决定性的，全面的实验表示截面的几何形状和配筋率以及预应力钢束同样重要。然而，当整个截面浇筑完成之后，模板必须经分解后再从顶板的洞中取出来，或是折叠后从墩顶横隔板纵向取出。尽管有这些困难，但一次浇筑法仍然在使用。近期的自密实混凝土技术的发展将给这种浇筑方式带来革命性的变化。这对于跨度介于40m至55m的中等跨度桥梁尤其重要。这种跨度对于双肋梁式桥来说太长，而对于现浇平衡悬臂施工的箱梁桥来说太短了。对于总长度小于1000m的箱型梁桥，采用预制构件被证明是不合理的。目前，这种桥型使用钢混结构最有利且具有竞争力。13.3 箱型梁的发展第11章和第12章阐述了为了在经济的前提下增加跨度，实心板式梁是如何向带肋梁转变的。肋梁主要的优点在于它能简单快速施工。然而它也有一些缺点，特别是： 跨度被限制在约45m； 活载不能有效地集中，导致两根肋要承受大约1.7倍的集中荷载（如HB车辆荷载），这就需要额外的预应力。 截面承载力差，导致需要相对很大的预应力； 梁体高度不能做的太小； 桥墩上需要的承载肋梁的多种支座或是十字头支承比较昂贵而且视觉上也很突出。箱梁能克服以上所有的缺点。13.4 箱梁的外形13.4.1 概述一个箱梁截面由端悬臂，顶板底板以及腹板组成。对于一个好的设计，桥面的总宽度与箱体的宽度必须有以合理的平衡关系。箱梁的的外形有一定的柔度；旁观者不知道箱梁是由几片薄板组合而成，还是就是实心混凝土。此外，混凝土大面积的平坦表面在浇筑完成时容易表现出缺陷，同样颜色的改变也容易表现出缺陷。设计者应该意识到这些问题