簡支梁布局外文資料翻譯（word13頁）

1、 南 京 理 工 大 學 紫 金 學 院畢業(yè)設(shè)計(論文)外文資料翻譯系： 機械工程系 專 業(yè)： 土木工程 姓 名： 袁洲 學 號： 050105140 (用外文寫)外文出處： Design of prestressed concrete structures 附 件： 1.外文資料翻譯譯文；2.外文原文。 指導教師評語：袁洲同學完成的外文翻譯內(nèi)容基本完整，語句較通順、 表達較清晰、格式較規(guī)范，符合畢業(yè)設(shè)計的要求。 簽名： 年 月 日注：請將該封面與附件裝訂成冊。附件1：外文資料翻譯譯文8-2 簡支梁布局一個簡單的預應(yīng)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(shè)計好之后，中間截

2、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的初始階段和最大設(shè)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

3、面的設(shè)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。混凝土的截面在高度、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(yīng)不同的荷載情況。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

4、，但這樣一來，就不是很經(jīng)濟的設(shè)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部纖維中引起拉應(yīng)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(yīng)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(yīng)力超過應(yīng)力允許值。圖8-7 布局預應(yīng)

5、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(yīng)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 ） ; （

6、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，一個拋物線底面最適合于勻布荷載。雖然這兩個布置有效地抵抗應(yīng)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(yīng)力臺座上都很難生產(chǎn)出來。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

7、軸的或者近乎共軸的預加應(yīng)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(yīng)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(shè)有錨頭，以便于先張法梁的力筋也可以折曲，如圖8-7的（f）、（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(shè)法減少力筋的彎曲所引起的預應(yīng)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

8、本的形式在這方面介紹了，變化的組合需要自行設(shè)計。正確的布置結(jié)構(gòu)將取決于當?shù)氐臈l件和實際需求以及理論上的思考。圖8-8 使鋼筋后張的梁的布局但是，對于適筋梁，像圖8-8，沒有必要保持彎矩包絡(luò)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D 8-8( 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。弧線的或折曲的鋼索也可以配合變高度梁使用。

9、如在（ c ）處。有時發(fā)現(xiàn)同時使用直線的和弧線的力筋頗為有利，如圖（ d ）所示。沿長度方向改變鋼筋面積的布置方案偶爾也是可取的。這樣的梁必須經(jīng)過專門設(shè)計，而它所必須用到的細節(jié)構(gòu)造卻可能抵消掉所節(jié)省的鋼材。在圖8-8（e）中，一些鋼索被向上彎曲而且布置在最高的邊緣。在(f) 處，一些鋼索在底部的邊緣中被省略。這些布置方案雖然可以節(jié)省一些鋼材，不過除了像用在承受重荷載的很長跨度的梁上那樣能節(jié)約大量鋼材的情況之外，可能不值得的采用。8-3 鋼索的縱斷面我們在上一節(jié)已經(jīng)討論了，簡支梁的布置是受到最大彎矩和梁端兩種截面控制，因而在這兩種截面設(shè)計哈之后，介于其間的其他截面就往往可以通過觀察來確定。然而，

10、有時沿梁長度方向的中間點上也可能出現(xiàn)危險截面，乃至在許多情況中宜于為鋼索確定容許的并且理想的縱斷面。要做到這一點，c.g.s.在限制區(qū)的位置是首先需要確定的，然后再布置鋼索，使其重心保持在限定區(qū)之內(nèi)。描述的方法在這里是為簡支梁，但它也可作為解決更為復雜布局的方法，如懸臂梁和連續(xù)跨越梁，檢查電纜的位置是不容易確定的。方法是圖解式的；c.g.s.在給定的限制地域里面，生產(chǎn)時一定要通過井然有序且沒有張應(yīng)力的過程。壓應(yīng)力混凝土中沒有檢查這個的方法。據(jù)推測，布局的具體方法和地區(qū)的預應(yīng)力鋼已經(jīng)確定時只有形象的c.g.s.的位置。在談到圖8-9時，在確定具體的布局部分時，我們開始計算他們克恩點，從而產(chǎn)生兩個

11、克恩線，一個頂部和底部的一個，如（ c ）處。請注意，對于變截面，這些克恩線將被彎曲，但為方便起見，他們將表現(xiàn)出連續(xù)的數(shù)字以代表梁截面。因為光纜裝載顯示在（a）處, 在（ b ）處最低和最高的時刻梁負荷載和總的工作負荷分別被標記為MG和MT。為了根據(jù)工作負荷，壓力中心的C線，將不屬于上述頂端克恩線，很明顯，c.g.s.必須位于下方頂端克恩處。a1=MT/F (8-1)圖8-9 c. g. s.的限制區(qū)域如果c.g.s.屬于上述上限在任何地點，然后在C線相應(yīng)的MT和預應(yīng)力F載上述頂端克恩線處，底部光纜將造成嚴重受壓。同樣，為了使C線不低于底部克恩線，c.g.s.線不得低于定位底部克恩線的位置。如

12、果c.g.s.定位高于下限，這里看到的C線將高于底部克恩線，這樣就不會產(chǎn)生頂端光纖梁下的負荷和初始預應(yīng)力。因此，它可以清楚地看到限制區(qū)c.g.s.給出了陰影面積圖, 如圖8-9（c），為了將根據(jù)梁負荷下的工作負荷不存在。然而，個別的腱可能被放在任何的位置，如此就當做 c.g.s. 保持在所有的電纜中的限制地域里面。位置和寬度的限制區(qū)往往說明是否是適當和經(jīng)濟的設(shè)計,如圖8-10。如果上限的一些部分外面或者在底部的光纖附近落下，在（a）處, 預應(yīng)力F或光纜的深度在那一部分應(yīng)該被增加。另一方面，如果它屬于上述底部纖維，在（ b ）中，預應(yīng)力梁高度是可以降低的。如果穿越下限，在（ C ）中，這意味著，

13、如果是可以做到?jīng)]有c.g.s.提供的位置，然后在F或預應(yīng)力梁深入時必須增加，以降低下限。另一方面，將討論后，該例題中顯示圖8-10（c）可能是非常令人滿意的是，允許布局在拉應(yīng)力混凝土。圖8-10 限制c.g.s.的不利位置附件2：外文原文8-2, Simple Beam LayoutThe layout of a simple prestressed-concrete beam is controlled by two critical sections: the maximum moment and the end sections. After these sections are des

14、igned, intermediate ones can often be determined by inspection but should be separately investigated when necessary. The maximum moment section is controlled by two loading stages, the initial stage at transfer with minimum moment MG acting on the beam and the working-load stage with maximum design

15、moment MT. The end sections are controlled 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 requirements, depending on their respective distances from the above controlling sections.

16、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 as the end block for posttensioned members. For pretensioned members, produced on a l

17、ong 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 cable layout of simple beams will now be discussed.The layout of a beam can be adjusted by

18、 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 mostly in its position relative to the centroidal axis of concrete. By adjusting these var

19、iables, 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 section or a uniform T-section and the position of steel is always as near the bottom fi

20、bers 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 beam of uniform section, (a).This is simple as far as form and workmanship are conce

21、ned, 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, it is best to place the cable as near the bottom as possible in order to provide the maximum lever arm

22、 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 section, however, presents an entirely different set of requirements. Since there is no external moment

23、 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 place the c. g. s. within the kern if tensile stresses are not permitted at the ends, and not too far

24、 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 example, if the c. g. s. is located all along the lower kern point, which is the lowest point permit

25、ted 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 obtained for resisting the moment at midspan, but stress distribution will be more unfavorable at the ends. Besides, too

26、 much camber may result from such a layout, since the entire length of the beam is subjected to negative bending due to prestress. In spite of these objections, this simple arrangement is often used, especially for short spans.Fig 8-7. Layouts for pretensioned beams.For a uniform concrete section an

27、d a straight cable, it is possible to get a more desirable layout than ( a ) by simple varying the soffit of the beam, as in Fig. 8-7( b ) and ( c ); ( b ) has a bent soffit, while ( c ) has a curved one. For both layouts, the c. g. s. at midspan can be depressed as low as desired, while that at the

28、 ends can be kept near the c. g. c. If the soffit can be varied at will, it is possible to obtain a curvature that will best fit the given loading condition; for example, a parabolic soffit will suit a uniform loading. While these two layouts are efficient in resisting moment and favorable in stress

29、 distribution, they possess three disadvantages. First, the formwork is more complicated than in ( a ). Second, the curved or bent soffit is often impractical in a structure, for architectural or functional reasons. Third, they cannot be easily produced on a long-line pretensioning bed.When it is po

30、ssible to vary the extrados of concrete, a layout like Fig. 8-7( d ) or ( e ) can be advantageously employed. These will give a favorable 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 secti

31、ons, they must be checked for share resistance. For ( d ), it should also be noted that the critical section may not be at midspan but 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 for

32、mwork than ( e ), which has a curved extrados.Most pretensioning plants in the United States have buried anchors along the stressing 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 sec

33、tion, and if the MG is heavy enough to warrant such additional expense of bending. Means must be provided to reduce the frictional loss 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

34、 from the above discussion that many different layouts are possible. Only some basic forms are described here, the variations and combinations 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 w

35、ell as upon theoretical considerations.Most of the layouts for pretensioned beams can be used for posttensioned ones as well. But, for 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. T

36、hus, for a beam of straight and uniform section, the tendons are very often curved as in Fig. 8-8( a ). Curving the tendons will permit favorable positions of c. g. s. to be obtained at both the end and midspan sections, and other points as well. Fig 8-8. Layouts for posttensioned beams.A combinatio

37、n of curved or bent tendons with curved or bent soffits is frequently used, Fig. 8-8( b ), when straight soffits are not required. This will permit a smaller curvature in the tendons, thus reducing the friction. Curved or bent cables are also combined with beams of variable depth, as in ( c ). Combi

38、nations of straight and curved tendons are sometimes found convenient, as in ( d ).Variable steel area along the length of a beam is occasionally preferred. This calls for special design of the beam and involves details which may offset its economy in weight of steel. In Fig. 8-8( e ), some cables a

39、re bent upward and anchored at top flanges. In ( f ), some cables are stopped part way in the bottom flange. These arrangements will save some steel but may not be justified unless the saving is considerable as for very long spans carrying heavy loads.8-3 Cable ProfilesWe stated in the previous sect

40、ion that the layout of simple beams is controlled by the maximum moment and end sections so that, after these two sections are designed, other sections can often be determined by inspection. It sometimes happens, however, that intermediate points along the beam may also be critical, and in many inst

41、ances it would be desirable to determine the permissible and desirable profile for the tendons. To do this, 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 b

42、eams, but it also serves as an introduction to the solution of more complicated layouts, such as cantilever 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

43、no tensile stresses will be produced. Compressive stresses in concrete are not checked by this method. It is 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, hav

44、ing determined the layout of concrete sections, we proceed to compute their kern points, thus yielding two kern lines, one top and one bottom, ( c ) . Note that for variable sections, these kern lines would be curved, although for convenience they are shown straight in the figure representing a beam

45、 with uniform cross section.For a beam loaded as shown in ( a ), the minimum and maximum moment diagrams for the girder load and for the total working load respectively are marked as MG and MT in ( b ). In order that, under the working load, the center of pressure, the C-line, will not fall above th

46、e top kern line, it is evident that the c. g. s. must be located below the top kern at least a distancea1=MT/F (8-1)Fig 8-9. Location of limiting zone for c. g. s.If the c. g. s. falls above that upper limit at any point, then the C-line corresponding to moment MT and prestress F will fall above the top kern, resulting in tension in the bottom fiber.Similarly, in order that the C-line will not fall below the bottom kern line, the c. g. s. line must not b