毕业论文外文翻译-桥梁工程(箱梁).doc

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1、 毕业设计外文资料翻译 第20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,

2、 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 requi

3、red 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 progressivel

4、y 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 Gene

5、ralOne 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 secti

6、on 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 rs

7、t 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

8、 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

9、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 th

10、e 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

11、 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 b

12、y 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 p

13、hase 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

14、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 design

15、ed 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 th

16、at 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

17、 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 concr

18、ete 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 i

19、s 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 cou

20、ld 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 l

21、ength 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 a

22、nd 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

23、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

24、; 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 cantil

25、evers, 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 assemb

26、lage 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 cantileve

27、rsSide 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

28、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 e

29、ssential 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. Thi

30、s 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

31、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 r

32、ectangular 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 det

33、ermined 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 logi

34、c 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 dis

35、tortion 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. Furthe

36、rmore, 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

37、 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 se

38、gments 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 n

39、umber 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 clos

40、ely 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

41、 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 meth

42、od 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

43、 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 produc

44、tion. 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

45、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 1

46、3.5. 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

47、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 continuou

48、sly 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 Depart