桥梁工程毕业设计外文翻译(箱梁)(共17页).docx

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1、精选优质文档-倾情为你奉上西 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 月专心-专注-专业外文资料原文：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 ma

2、y 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 struct

3、ural 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 product

4、ive 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 (

5、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

6、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

7、 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 disa

8、dvantages 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 pres

9、tressing 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

10、, 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 d

11、ifculty 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

12、 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 woul

13、d 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 w

14、ith 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 joi

15、nts 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.

16、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 s

17、lab 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 d

18、own 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 essen

19、tial.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 unde

20、r-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 ba

21、lanced 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 b

22、e 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

23、 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, lead

24、ing 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

25、 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; t

26、he 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

27、 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

28、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. Rec

29、tangular 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

30、 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 gener

31、al, 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 un

32、der 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

33、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.Fo

34、r 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 importa

35、nt 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 impo

36、rtant 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 d

37、epth 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

38、 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

39、 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,

40、 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 prob

41、lem 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 acce

42、pt. 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 si

43、gnicant 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, a

44、ssisting 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 c

45、riticality 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

46、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 b

47、y 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