土木外文文獻及譯文-簡支梁布局

1、土木外文文獻及譯文簡支梁布局本科畢業(yè)設計外文文獻及譯文文獻、資料題目:Simple Beam Layout文獻、資料來源:網(wǎng)絡文獻、資料發(fā)表（出版）日期:2012228 院（部）:管理工程學院業(yè)：信息管理與信息系統(tǒng) 級：信管082 名：聞龍 號:2008021326指導教師：郭兵姜衛(wèi)杰翻譯日期：2012.5.25山東建筑大學畢業(yè)設計外文文獻及翻譯外文文獻:Simple Beam LayoutThe layout of a sim pie p restressed-c on crete beam is con trolled by two critical secti ons: the maxi

2、mum mome nt and the end sect ions. After these secti ons are desig ned, in termediate ones can ofte n be determ ined by insp ecti on but should be sep arately inv estigated whe n n ecessary. The maximum mome nt sect ion is con trolled by two load ing stages, the in itial stage at tran sfer with mini

3、mum mome nt MG act ing on the beam and the work in g-load stage with maximum desig n mome nt MT. The end secti ons arecontrolled by area required for share resistance, bearing plates, anchorage spacings, and jacking clearances. All intermediate sections are designed by one or more of the above requi

4、rements, depending on their respective distances from the above controlling sections. A common arrangement for posttensioned members is to employ some shape, such as I or T, for the maximum moment section and to round it out into a simple rectangular shape near the ends. This is commonly referred to

5、 as the end block for posttensioned members. For pretensioned members, produced on a long line process, a uniform I, double-T, or cored section is employed throughout, in order to facilitate production. The design for individual sections having been explained in Chapters 5, 6, and 7,the general cabl

6、e layout of simple beams will now be discussed.The layout of a beam can be adjusted by varying both the concrete and the steel. The section of concrete can be varied as to its height, width, shape, and the curvature of its soffit or extrados. The steel can be varied occasionally in its area but most

7、ly in its position relative to the centroidal axis of concrete. By adjusting these variables, many combinations of layout are possible to suit different loading conditions.This is quite different from the design of reinforced-concrete beams, where the usual layout is either a uniform rectangular sec

8、tion or a uniform T-section and the position of steel is always as near the bottom fibers as is possible.Consider first the pretensioned beams, Fig. 8-7.Here straight cables are preferred, since they can be more easily tensioned between two abutments. Let us start with a straight cable in a straight

9、 beam of uniform section, (a).This is simple as far as form and workmanship are concened,山東建筑大學畢業(yè)設計 外文文獻及翻譯But such a section cannot often be economically designed, because of the conflicting requirements of the midspan and end sections. At the maximum moment section generally occurring at midspan,

10、it is best to place the cable as near the bottom as possible in order to provide the maximum lever arm for the internal resisting moment. When the MG at midspan is appreciable, it is possible to place the c. g. s. much below the kern without producing tension in the top fibers at transfer. The end s

11、ection, however, presents an entirely different set of requirements.Since there is no external moment at the end, it is best to arrange the tendons so that the c. g. s. will coincide with the c. g. c. at the end section, so as to obtain a uniform stress distribution. In any case, it is necessary to

12、place the c. g. s. within the kern if tensile stresses are not permitted at the ends, and not too far outside the kern to avoid tension stress in excess of allowable values.It is not possible to meet the conflicting requirements of both the midspan and the end sections by a layout such as ( a ). For

13、 example, if the c. g. s. is located all along the lower kern point, which is the lowest point permitted by the end section, a satisfactory lever arm is not yet attained for the internal resisting moment at midspan. If the c.g. s. is located below the kern, a bigger lever arm is obta ined for resist

14、i ng the mome nt at mids pan, but stress distributi on will be more unfav orable at the en ds. Besides, too much camber may result from such a layout, since the en tire len gth of the beam is subjected to n egative bending due to p restress. In sp ite of these objecti ons, this simplearra ngeme nt i

15、s ofte n used, esp ecially for short spans.山東建筑大學畢業(yè)設計外文文獻及翻譯noFig 8-7. Layouts for p rete nsioned beams.fM IFor a uniform con crete secti on and a straight cable, it is p ossible to get a more desirable layout tha n ( a ) by simple vary ing the soffit of the beam, as in Fig. 8-7( b ) and ( c ); ( b

16、) has a bent soffit, while ( c ) has a curved one. For both layouts, the c. g. s. at mids pan can be dep ressed as low as desired, while that at the ends can be kept n ear the c. g. c. If the soffit can be varied at will, it is p ossible to obtain a curvature that will best fit the given loading con

17、dition; for example, a parabolic soffit will suit a uniform loading. While these two layouts are efficient in resisting moment and favorable in stress distribution, they possess three disadvantages. First, the formwork is more complicated than in ( a ). Second, the curved or bent soffit is often imp

18、ractical in a structure, for architectural or functional reasons. Third, they cannot be easily produced on a long-line pretensioning bed.山東建筑大學畢業(yè)設計 外文文獻及翻譯When it is possible to vary the extrados of concrete, a layout likeFig. 8-7( d ) or ( e ) can be advantageously employed. These will give a favor

19、able height at midspan, where it is most needed, and yet yield a concentric or nearly concentric prestress at end section. Since the depth is reduced for the end sections, they must be checked for share resistance. For ( d ), it should also be noted that the critical section may not be at midspan bu

20、t rather at some point away from it where the depth has decreasd appreciably while the external moment is still near the maximum. Beam ( d ), however, is simple in formwork than ( e ), which has a curved extrados.Most pretensioning plants in the United States have buried anchors along the stressing

21、beds so that the tendons for a pretensioned beam can be bent, Fig. 8-7( f ) and ( g ). It may be economical to do so ,if the beam has to be of straight and uniform section, and if the MG is heavyenough to warrant such additional expense of bending. Means must be provided to reduce the frictional los

22、s of prestress produced by the bending of the tendons. For example, the tendons may be tensioned first from the ends and then bent at the harping points.It is evident from the above discussion that many different layouts are possible. Only some basic forms are described here, the variations and comb

23、inations being left to the discretion of the designer. The correct layout for each structure will depend upon the local conditions and the practical requirements as well as upon theoretical considerations.Most of the layouts for pretensioned beams can be used for posttensioned ones as well. But, for

24、 posttensioned beams, Fig. 8-8, it is not necessary to keep the tendons straight, since slightly bent or curved tendons can be as easily tensioned as straight ones. Thus, for a beam of straight and uniform section, the tendons are very often curved as in Fig. 8-8( a ). Curving the tendons will permi

25、t favorable positions of c. g. s. to be obtained at both the end and midspan sections, and other points as well.山東建筑大學畢業(yè)設計 外文文獻及翻譯4A comb in ati on of curved or bent tendons with curved or bent soffits is freque ntly used, Fig. 8-8( b ), whe n straight soffits are not required. This will p ermit a s

26、maller curvature in the tendons, thus reduc ing the fricti on. Curved or bent cables are also comb ined with beams of variable dep th, as in ( c ). Comb in atio ns of straight and curved tendons are sometimes found convenient, as in ( d ).Variable steel area along the len gth of a beam is occasi on

27、ally p referred. This calls for sp ecial desig n of the beam and invo Ives details which may offset its economy in weight of steel. I n Fig. 8- 8( e ), some cables are bent up ward and an chored at top flan ges. In (f ), some cables are stopped part way in the bottom flan ge. These arran geme nts wi

28、ll save some steel but may not be justified uni ess the sav ing is con siderable as for very long spans carry ing heavy loads.8-3 Cable P rofilesWe stated in the previous section that the layout of simple beams is controlled by the maximum moment and end sections so that, after these two sections ar

29、e designed, other sections山東建筑大學畢業(yè)設計 外文文獻及翻譯can often be determined by inspection. It sometimes happens, however, that intermediate points along the beam may also be critical, and in many instances it would be desirable to determine the permissible and desirable profile for the tendons. To do this,

30、a limiting zone for the location of c. g. s. is first obtained, then the tendons are arranged so that their centroid will lie within the zone.The method described here is intended for simple beams, but it also serves as an introduction to the solution of more complicated layouts, such as cantilever

31、and continuous spans, where cable location cannot be easily determined by inspection. The method is a graphical one; giving the limiting zone within which the c. g. s. must pass in order that no tensile stresses will be produced. Compressive stresses in concrete are not checked by this method. It is

32、 assumed that the layout of the concrete sections and the area of prestressing steel have already been determined. Only the profile of the c. g. s. is to be located.Referring to Fig . 8-9, having determined the layout of concrete sections, we proceed to compute their kern points, thus yielding two k

33、ern lines, one top and one bottom, ( c ) . Note that for variable sections, these kern lines would be curved, although for conveniencethey are show n straight in the figure rep rese nti ng a beam with uniform cross secti on.For a beam loaded as show n in ( a ), the minimum and maximum mome nt diagra

34、ms for the girder load and for the total worki ng load res pectively are marked as MG and MT in ( b ).1 n order that, un der the work ing load, the cen ter of p ressure, the C-l ine, will not fall above the top kern lin e, it is evide nt that the c. g. s. must be located below the top kern at least

35、a dista neea仁MT/F (8-1)山東建筑大學畢業(yè)設計外文文獻及翻譯fa) Bmm EkvAlion%Girdr Moment andMonunt 01口皿Upper lirtwlGJ limiting Zone fur 亡事工Fig 8-9. Locati on of limit ing zone for c. g. s.If the c. g. s. falls above that upper limit at any point, then theC-line corresponding to moment MT and prestress F will fall abov

36、e thetop kern, resulting in tension in the bottom fiber.Similarly, in order that the C-line will not fall below the bottomkern line, the c. g. s. line must not be positioned below the bottomkern by a distance greater than which gives the lower limit for thelocation of c. g. s. If the c. g. s. is pos

37、itioned above that lowerlimit, it is seen that the C-line will be above the bottom kern andthere will be no tension in the top fiber under the girder load andinitial prestress F0.山東建筑大學畢業(yè)設計 外文文獻及翻譯Thus, it becomes clear that the limiting zone for c. g. s. is givenby the shaded area in Fig. 8-9( c ),

38、 in order that no tension will existboth under the girder load and under the working load. The individualtendons, however, may be placed in any position so long as the c. g. s.of all the cables remains within the limiting zone.The position and width of the limiting zone are often an indicationof the

39、 adequacy and economy of design, Fig. 8-10. If some portion of theupper limit falls outside or too near the bottom fiber, in ( a ), eitherthe prestress F or the depth of beam at that portion should be increased.On the other hand, if it falls too far above the bottom fiber, in ( b ),either the prestr

40、ess or the beam depth can be reduced. If the lowerlimit crosses the upper limit, in ( C ), it means that no zone is available for the locati on of c. g. s. , and either the p restress F or the beam depth must be in creased or the girder mome nt must be in creased to dep ress the lower limit if that

41、can be done. On the other hand, as will be discussed later, the case show n in Fig. 8.10( c ) may be very satisfactory whe n are allowi ng ten sile stress in con crete.山東建筑大學畢業(yè)設計外文文獻及翻譯9Upper limitILowet fhmttfflj Upf*r Uroit 仏 轉(zhuǎn)*Lomt kmriW Upper Limit Tod P*r alxrr Bottomfcj UppCT aad Laww liouti C

42、roaaFig 8-10. Un desirable p ositi ons for c. g. s. zone limits.山東建筑大學畢業(yè)設計外文文獻及翻譯中文譯文:簡支梁布局一個簡單的預應力混凝土梁由兩個危險截面控制 :最大彎矩截面和端截面。這兩部分設計好之后，中間截面一定要單獨檢查，必要時其他部位也要單獨調(diào)查。最大彎矩截面在以下兩種荷載階段為控制情況，即傳遞時梁受最小彎矩MG勺初始階段和最大設計彎矩MT時的工作荷載階段。而端截面則由抗剪強度、支承墊板、錨 頭間距和千斤頂凈空所需要的面積來決定。所有的中間截面是由一個或多個上述要 求，根它們與上述兩種危險截面的距離來控制。對于后張構(gòu)件的

43、一種常見的布置方 式是在最大彎矩截面采用諸如I形或T形的截面，而在接近梁端處逐漸過渡到簡單 的矩形截面。這就是人們通常所說的后張構(gòu)件的端塊。對于用長線法生產(chǎn)的先張構(gòu) 件，為了便于生產(chǎn)，全部只用一種等截面，其截面形狀則可以為 I形、雙T形或空心的。在第 5 、 6 和 7 章節(jié)中已經(jīng)闡明了個別截面的設計，下面論述簡支梁鋼索 的總布置。梁的布置可以用變化混凝土和鋼筋的辦法來調(diào)整?；炷恋慕孛嬖诟叨?、寬 度、形狀和梁底面或者頂面的曲率方面都可以有變化。而鋼筋只在面積方面有所變 化，不過在相對于混凝土重心軸線的位置方面卻多半可以有變化。通過調(diào)整這些變 化因素，布置方案可能有許多組合，以適應不同的荷載情

44、況。這一點是與鋼筋混凝 土梁是完全不同的，在鋼筋混凝土梁的通常布置中，不是一個統(tǒng)一的矩形截面便是 一個統(tǒng)一的T形，而鋼筋的位置總是布置得盡量靠底面纖維。首先考慮先張梁，如圖 8-7 ，這里最好采用直線鋼索，因為它們在兩個臺座之 間加力比較容易。我們先從圖 (a) 的等截面直梁的直線鋼索開始討論。這樣的布置 都很簡單，但這樣一來，就不是很經(jīng)濟的設計了，因為跨中和梁端的要求會產(chǎn)生沖 突。通常發(fā)生在跨度中央的最大彎矩截面中的鋼索，最好盡量放低，以便盡可能提供最大力臂而提供最大的內(nèi)部抵制力矩。當跨度中央的梁自重彎矩MG相當大時,就可以把 c.g.s 布置在截面核心范圍以下很遠的地方，而不致在傳遞時在頂

45、部纖維中引起拉應力。然而對于梁端截面卻有一套完全不同的要求。由于在梁端沒有外力矩，因為在最后的時刻，安排鋼索要以c.g.s與c.g.c在結(jié)束區(qū)段一致，如此同樣地獲得克服壓力分配的方法。無論如何，如果張應力在最后不能承受，放置c.g.s.是必需緊排的，而且緊排的不能太遠，避免張拉應力超過應力允許值。10山東建筑大學畢業(yè)設計外文文獻及翻譯圖8-7布局預應力梁同時滿足跨中和梁端兩種截面的布局需求這是不可能的，舉例來說，如(a )，如果c.g.s.全都放在核心下界處，那么這對梁端截面來說，已經(jīng)是容許的最低點，面對跨中截面來說，則還沒有達到足夠大的力矩臂來提供令人滿意的內(nèi)部 抵抗力矩。如果c.g.s.緊

46、排在下面位置，在中跨處的抵抗力就可以達到要求了, 但是最后壓力分配將不太容易，此外，過大的反撓度也可能導致這樣的布局，由于 預應力在整個光纖內(nèi)受到負面彎曲。盡管有這些不對的地方，但這往往是最簡單的 布局，特別是一些短跨。對于直線鋼索等截面的混凝土梁，有可能獲得比 (a) 更理想的布置，只要變化一下梁的底面形狀，如在圖 8-7 里的( b ) 和( c ) ; (b)中的底面是折線的，而( c ) 中則是弧線的。對于這兩種布置，對 c.g.s. 在跨中可以盡量放在低的位置， 而在兩端可以保持 c.g.s 不變，如果梁的底面可以任意改動，這樣就有可能獲得最 適合于荷載情況的曲線。舉例來說，一個拋物

47、線底面最適合于勻布荷載。雖然這兩 個布置有效地抵抗應力分布，但是有三個缺點，首先，在 (a) 處模板要更加復雜 ; 第二，由于建筑或功能的原因，弧形或折線形的底面往往不切合實用; 第三，它們在 長線法預應力臺座上都很難生產(chǎn)出來。11山東建筑大學畢業(yè)設計 外文文獻及翻譯只要有可能變化混凝土梁的頂面，那么就可以有利地采用圖 8-7( d ) ，( e )那樣的布置方案。這樣在最需要高度的跨中具有良好的高度，而且在梁端截面可以 得到一個共軸的或者近乎共軸的預加應力。因為高度在梁端截面減少，所以一定要 經(jīng)常檢查。例如 ( d ), 也應該注意危險截面可能不在跨中，寧可布置在一些遠離它 的點，在最大值附

48、近高度略微有點降低。梁 ( d ) 在模板方面要比 ( e ) 項中具有弧 線形頂面的梁簡單。美國的大多數(shù)先張預制工廠沿張拉臺座埋設有錨頭，以便于先張法梁的力筋也 可以折如圖8-7的、(g)。倘若梁必須是等截面的直梁，而且倘若梁自重彎矩MG的確大曲，得有必要作這種額外花費的彎曲的話，那么這樣做也可能是經(jīng)濟的。不過必須 設法減少力筋的彎曲所引起的預應力的摩擦損失。例如，在末端就先張拉，然后再 受拉彎曲。顯然，從上述討論中，許多布置都是可能的。只有一些基本的形式在這方面介 紹了，變化的組合需要自行設計。正確的布置結(jié)構(gòu)將取決于當?shù)氐臈l件和實際需求但是，對于適筋梁，像圖8-8，沒有必要保持彎矩包絡圖是直線，因為稍微彎 曲或弧線形的力筋同直線力筋一樣可以輕松張拉。因此，在等截面直梁中，力筋往 往彎曲，例如在圖8-8.(a)處。把力筋彎曲將會允許c.g.s.在梁兩端和跨中以及 其他各點的截面中都獲得有利的位置。只要不要求用直線的底面，那么就常?？梢圆捎萌鐖D8-8( b )所示的把弧線形或折曲的力筋配合弧線或折線底面一同使用。這樣可以使力筋彎曲得小些，從而 降低摩擦力。12山東建筑大學