橋梁工程畢業(yè)設計方案外文翻譯(箱梁).docx

西南交通大學本科畢業(yè)設計（論文） 第II頁西 南 交 通 大 學本科畢業(yè)設計（論文）外文資料翻譯年 級:學 號:姓 名:專 業(yè):指導老師:2013年 6 月 畢業(yè)設計外文資料翻譯 第16頁外文資料原文：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.外文資料譯文：第13章 箱梁13.1概述箱梁橋是最具柔性的一種橋面形式。它的跨度范圍可以從25m到最大的跨度約為300m的非懸索結構混凝土橋。單箱室也可以做到承載30m寬的橋面。對于超過50m的較大跨度的主梁，箱梁幾乎是唯一可行的橋面形式。對于較小跨徑，它與大多數(shù)其他橋面形式的競爭也將在本書中進行討論。箱形梁的優(yōu)勢主要是其高效的結構性能，這最大限度地減少了所需的抵抗彎矩的預應力，其巨大的抗扭強度能力也減少了需要承載偏心活載所需的預應力。在過去的50年中，箱形梁已經(jīng)通過許多高效的橋梁生產(chǎn)建設方法得到日益完善，例如預制梁在接縫處需不需要加環(huán)氧樹脂，懸臂施工時要么在現(xiàn)場澆筑要么將預制構件連接起來，或者用頂推法（第15章）。13.2 現(xiàn)澆箱梁13.2.1 概述箱梁的主要缺點之一是由于施工時難以接近混凝土梁底面以及需要取出內(nèi)部澆筑模板造成的現(xiàn)場澆筑的難度。施工時要么模板已經(jīng)設計好因此整個節(jié)段可以一次連續(xù)澆筑，要么節(jié)段必須分階段施工。13.2.2分階段澆筑最常用的現(xiàn)場澆筑箱梁的方法是將橫截面劃分成幾個階段。首先澆筑底板，然后腹板和澆筑頂板一起澆筑，或者首先澆筑腹板和底板再以澆筑頂板結束。當首先澆筑底板時，施工接縫通常剛好位于底板面的上方，給腹板流出支模空間，如圖13.1所示。在這個位置的接縫有眾多的不利影響，具體在在11.7.1有詳細闡述?；蛘?，接縫可以在底板靠近腹板的位置，或者在梗腋開始的位置，圖中13.1的2的位置。接縫位于底板優(yōu)點在于它不會穿過預應力筋或大量配筋；也能防止受天氣的影響，視覺上也不突出。主要的缺點是底板只占了所有澆筑混凝土的一小部分，因此第二次澆筑時的澆筑量很大。接縫的位置也可以位于腹板的頂部，剛好在底板的底部，如圖13.1的3的位置。該位置具有位置1的很多缺點，即接縫被淺層的預應力管道穿過，由于腹板鋼筋的存在，在準備下一次次澆筑時也比較困難。此外，又引入了許多澆筑底板時的困難。其優(yōu)點在于該處的接縫更不顯眼，以及由于懸臂的保護不收天氣的影響，此外每次澆筑的混凝土量更小，更少的模具留在箱梁內(nèi)部。分階段澆筑會導致第二個階段的混凝土由于第一階段的混凝土硬化產(chǎn)生裂縫。雖然可以通過配筋來限制裂縫的寬度，但預應力混凝土在恒荷載下產(chǎn)生裂縫是不符合要求的。完全消除裂縫需要非常高成本的措施，例如冷卻第二階段的混凝土以限制在采用裂縫密封劑時的氣溫的上升。13.2.3 一次澆筑有兩種方法一次澆筑箱梁截面。底板可以在穿過頂板下部模板留的臨時孔洞的管道的幫助下首先澆筑。這就需要入口來使工人散布和振搗混凝土，并且一般只可能適用于至少2m高的梁。腹板的澆筑必須緊接著進行，以避免接縫冷卻。流態(tài)的混凝土必須設計好以避免混凝土溢出腹板的模板。如果有一條沿每個腹板從頂部模板到底板的寬約500mm的長條，這種情況將得到改善 。這種方法對于底板的寬度沒有限制，如圖13.2（a）.或者整個界面的形狀可由混凝土從腹板流進底板從而成型，這種施工方法需要一個完整的模板，如圖13.2（b）。這種澆筑方法特別適用底地板相對較窄的箱梁。底板混凝土的密實度需要外部的震蕩器來實現(xiàn)，這意味著需要使用鋼模板。混凝土可以從兩個腹板向下澆筑，同時模板上需要有檢視孔以確認空氣被排出以及底板混凝土填充完整?；蛘撸炷料葟囊粋€腹板往下澆筑待另一個腹板底部出現(xiàn)混凝土時再從此腹板往下澆筑，以此證明整個底板被填充完整。混凝土的配合比設計是具有決定性的，全面的實驗表示截面的幾何形狀和配筋率以及預應力鋼束同樣重要。然而，當整個截面澆筑完成之后，模板必須經(jīng)分解后再從頂板的洞中取出來，或是折疊后從墩頂橫隔板縱向取出。盡管有這些困難，但一次澆筑法仍然在使用。近期的自密實混凝土技術的發(fā)展將給這種澆筑方式帶來革命性的變化。這對于跨度介于40m至55m的中等跨度橋梁尤其重要。這種跨度對于雙肋梁式橋來說太長，而對于現(xiàn)澆平衡懸臂施工的箱梁橋來說太短了。對于總長度小于1000m的箱型梁橋，采用預制構件被證明是不合理的。目前，這種橋型使用鋼混結構最有利且具有競爭力。13.3 箱型梁的發(fā)展第11章和第12章闡述了為了在經(jīng)濟的前提下增加跨度，實心板式梁是如何向帶肋梁轉(zhuǎn)變的。肋梁主要的優(yōu)點在于它能簡單快速施工。然而它也有一些缺點，特別是： 跨度被限制在約45m； 活載不能有效地集中，導致兩根肋要承受大約1.7倍的集中荷載（如HB車輛荷載），這就需要額外的預應力。 截面承載力差，導致需要相對很大的預應力； 梁體高度不能做的太小； 橋墩上需要的承載肋梁的多種支座或是十字頭支承比較昂貴而且視覺上也很突出。箱梁能克服以上所有的缺點。13.4 箱梁的外形13.4.1 概述一個箱梁截面由端懸臂，頂板底板以及腹板組成。對于一個好的設計，橋面的總寬度與箱體的寬度必須有以合理的平衡關系。箱梁的的外形有一定的柔度；旁觀者不知道箱梁是由幾片薄板組合而成，還是就是實心混凝土。此外，混凝土大面積的平坦表面在澆筑完成時容易表現(xiàn)出缺陷，同樣顏色的改變也容易表現(xiàn)出缺陷。設計者應該意識到這些問題