土木工程專業(yè)外文翻譯---拱橋的設(shè)計和橋梁裂縫產(chǎn)生的原因分析

1、<p><b>　　本科畢業(yè)設(shè)計</b></p><p><b>　　中英文翻譯</b></p><p><b>　　土木工程學(xué)院</b></p><p><b>　　二○一二年六月一日</b></p><p>　專業(yè)名稱：土木工程</p&

2、gt;<p>　年級班級：XXX</p><p>　學(xué)生姓名：XXX</p><p>　指導(dǎo)教師：XXX</p><p>　　Design of arch bridges and the bridge crack</p><p>　　produced the reason to simply analyse</p>

3、<p>　　This chapter considers the full range of arch bridge types and a range of materials presenting several case studies and describing the design decisions that were made. A general treatment of the analysis of a

4、rches is presented, including the derivation of the basic equations that can be used to undertake hand calculations which may beused to validate computer analysis output. Detailed arch bridge design is outside thescope o

5、f this chapter so only general issues are discussed. Most of the chapter</p><p>　　The origins of the use of arches as a structural form in buildings can be traced back to antiquity (Van Beek, 1987). In tryin

6、g to arrive at a suitable definition for an arch we may look no further than Hooke’s anagram of 1675 which stated ‘Ut pendet continuum flexile, sic stabat continuum rigidum inversum’ – ‘a(chǎn)s hangs the flexible line, so but

7、 inverted will stand the rigid arch’. This suggests that any given loading to a flexible cable if frozen and inverted will provide a purely compressive str</p><p>　　The expansion of the railway and canal sys

8、tems led to an explosion of bridge building. Brickwork arches became increasingly popular. With the construction of the Coalbrookdale Bridge (1780) a new era of arch bridge construction began. By the end of the nineteent

9、h century cast iron, wrought iron and finally steel became increasingly popular; only to be challenged by ferro cement (reinforced concrete) at the turn of the century.</p><p>　　During the nineteenth century

10、 analytical technique developed apace. In particular, Castigliano (1879) developed strain energy theorems which could be applied to arches provided they remained elastic. This condition could be satisfied provided the li

11、ne of thrust lay within the middle third, thus ensuring that no tensile stresses were induced. The requirement to avoid tensile stresses only applied to masonry and cast iron; it did not apply to steel or reinforced conc

12、rete (or timber for that matter</p><p>　　Twentieth century arch bridges have become increasingly sophisticated structures combining modern materials to create exciting functional urban sculptures.</p>

13、<p>　　Types of arch bridge</p><p>　　The relevant terms that are used to describe the various parts of an arch bridge are shown in Figure 1. Arches may be grouped according to the following parameters:&

14、lt;/p><p>　　1. the materials of construction</p><p>　　2. the structural articulation</p><p>　　3. the shape of the arch</p><p>　　Historically, arch bridges are associated w

15、ith stone masonry. This gave way to brickwork in the nineteenth century. Because these were proportioned to minimise the possibility of tensile stress, they tend to be fairly massive structures. By comparison the use of

16、reinforced concrete and modern structural steel gives the opportunity for slender, elegant arches.</p><p>　　Nowadays, timber is restricted to small bridges occasionally in a truss form but more usually as la

17、minated curved arches. Although timber has a high strength to density ratio parallel to the grain, it is anisotropic and strength properties perpendicular to the grain are relatively weak. This requires careful detailing

18、 of connections to ensure economic use of the material.</p><p>　　With regard to structural articulation the arch can be fixed or hinged. In the latter case either one, two or three hinges can be incorporated

19、 into the arch rib. Whilst the fixed arch has three redundancies, the introduction of each hinge reduced the indeterminacy by one until, with three hinges, the arch is statically determinate and hence, theoretically, fre

20、e of the problems of secondary stresses. Figure 2 shows a range of possible arrangements. The articulation of the arch is not only depende</p><p>　　The traditional filled spandrel, where the vehicular loadin

21、g is transferred through the b ackfill material onto the extrados of the arch, represents at first glance the simplest structural condition. As will be seen later this is not the case and has led to much research for the

22、 specific case of the masonry arch bridge in an attempt to improve our understanding of such structures.</p><p>　　The spandrel may be open with columns and/or hinges used to transfer the deck loads to the ar

23、ch. In an attempt to minimise the horizontal thrust on the abutments, the deck may be used to ‘tie’ the arch. Tied arches are particularly appropriate when deck construction depths are limited and large clear spans are n

24、eeded (particularly if ground conditions are also difficult and would require extensive piling to resist the horizontal thrusts).</p><p>　　Returning to Hooke’s anagram, the perfect shape for an arch would be

25、 an inverted catenary – this would only be the case for carrying its own self-weight. Vehicle loading and varying superincumbent dead load both induce bending moments. Consequently the arch has to have sufficient thickne

26、ss to accommodate the ‘wandering’ thrust line.</p><p>　　For ease of setting out and construction simpler shapes are adopted nowadays segmental or parabolic shapes are used. Although in situations where maxim

27、um widths of headroom have to be provided (say over a railway, road or canal) an elliptical shape may be required or its nearest ‘easy’ equivalent the three-centred arch.</p><p>　　It is worth commenting at t

28、his stage regarding the idealization of arch structures. Traditionally arches are perceived as being two-dimensional structures; this, of course is not true – but the extent to which it is not true should be of concern t

29、o the designer/assessor. Even in the case of a three-hinged arch which ostensibly is statically determinate the ‘pins’ are capable of transmitting shear even though they theoretically cannot transmit moments. In the case

30、 of non-uniform transverse distri</p><p>　　From an aesthetic point of view, arches have a universal appeal. In spite of this, care must be taken as the impact of even modest sized bridges is significant. Fil

31、led arches are invariably masonry (or widening of masonry) bridges. Cleanness of line, honesty of conception and the attention to detail are vital ingredients to a successful bridge. Certainly, simple stringcourses and c

32、opings are preferable to elaborate details which would be expensive and inappropriate for most modern bridges. Where</p><p>　　Historically abutments comprised either rock, or else were massive masonry suppor

33、ts relying on their weight to resist the thrust of the arch. In terms of structural honesty this is necessary as it is instinctive to expect such support.</p><p>　　Reinforced concrete and steel arches are al

34、together much lighter structures. ‘The structure consists basically of the arch, the deck and usually some supports from the arch to the deck – in that order of importance. These elements should be expressed in both form

35、 and detail, and with due regard for their hierarchy’ (Highways Agency, 1996).</p><p>　　It is important that the deck, if it rests on the crown of the arch, should not mask it in any way. Any support whether

36、 spandrel columns or hinges (in the case of the tied arch) should not be allowed to dominate. Preferably they should be recessed relative to the parapet and stringcourse.</p><p>　　Concrete arches can be eith

37、er a full width curved slab or a series of ribs. Steel is almost always a series of ribs. Where ribs are used thought should be given (if they are going to be seen from underneath) to the chiaroscuro of the soffit.</p

38、><p>　　The ratio of span to rise should generally be in the range 2:1 to 10:1. The flatter the arch the greater the horizontal thrust; this may affect the structural form selected, i. e. whether or not a tie sh

39、ould be introduced, or the stiffness of the deck relative to the arch.</p><p>　　In recent years, the traffic capital construction of our country gets swift and violent development, all parts have built a lar

40、ge number of concrete bridges. In the course of building and using in the bridge, relevant to influence project quality lead of common occurrence report that bridge collapse even because the crack appears The concrete ca

41、n be said to " often have illness coming on " while fracturing and " frequently-occurring disease ", often perplex bridge engineers and technicians. In f</p><p>　　Concrete bridge crack ki

42、nd, origin cause of formation In fact, the origin cause of formation of the concrete structure crack is complicated and various, even many kinds of factors influence each other, but every crack has its one or several kin

43、ds of main reasons produced. The kind of the concrete bridge crack, on its reason to produce, can roughly divide several kinds as follows :</p><p>　　First, load the crack caused Concrete in routine quiet.<

44、;/p><p>　　Is it load to move and crack that produce claim to load the crack under the times of stress bridge, summing up has direct stress cracks, two kinds stress crack onces mainly. Direct stress crack refer

45、to outside load direct crack that stress produce that cause. The reason why the crack produces is as follows: (1) Design the stage of calculating, does not calculate or leaks and calculates partly while calculating in st

46、ructure; Calculate the model is unreasonable; The structure is supposed and acco</p><p>　　Stress crack once means the stress of secondary caused by loading outside produces the crack. The reason why the crac

47、k produces is as follows, (1)In design outside load function, because actual working state and routine, structure of thing calculate have discrepancy or is it consider to calculate, thus cause stress once to cause the st

48、ructure to fracture in some position. Two is it join bridge arch foot is it is it assign " X " shape reinforcing bar, cut down this place way, section of size desi</p><p>　　In the design, should pa

49、y attention to avoiding structure sudden change (or section sudden change), when it is unable to avoid, should do part deal with, corner for instance, make round horn, sudden change office make into the gradation zone tr

50、ansition, is it is it mix muscle to construct to strengthen at the same time, corner mix again oblique to reinforcing bar, as to large hole in a utensil can set up protecting in the perimeter at the terms of having angle

51、 steel. Load the crack characteristi</p><p>　　Second, crack caused in temperature change.</p><p>　　The concrete has nature of expanding with heat and contract with cold, look on as the external

52、environment condition or the structure temperature changes, concrete take place out of shape, if out of shape to restrain from, produce the stress in the structure, produce the temperature crack promptly when exceeding c

53、oncrete tensile strength in stress. In some being heavy to step foot-path among the bridge, temperature stress can is it go beyond living year stress even to reach. The temperature crac</p><p>　　Third , shri

54、nk the crack caused.</p><p>　　In the actual project, it is the most common because concrete shrinks the crack caused. Shrink kind in concrete, plasticity shrink is it it shrinks (is it contract to do ) to be

55、 the main reason that the volume of concrete out of shape happens to shrink, shrink spontaneously in addition and the char shrink. Plasticity shrink. About 4 hours after it is built that in the course of constructing, co

56、ncrete happens, the cement water response is fierce at this moment, the strand takes shape gradually, s</p><p>　　Studies have shown, influence concrete shrink main factor of crack as follows. (1) Variety of

57、cement, grade and consumption. Slag cement, quick-hardening cement, low-heat cement concrete contractivity are relatively high, ordinary cement, volcanic ash cement, alumina cement concrete contractivity are relatively l

58、ow. Cement grade low in addition, unit volume consumption heavy rubing detailed degree heavy, then the concrete shrinks the more greatly, and shrink time is the longer. For example, in o</p><p>　　Fourth, cra

59、ck that causes out of shape of plinth of the ground.</p><p>　　Because foundation vertical to even to subside or horizontal direction displacement, make the structure produce the additional stress, go beyond

60、resisting the ability of drawing of concrete structure, cause the structure to fracture. The even main reason that subside of the foundation is as follows. (1) Reconnoitres the precision and is not enough for, test the m

61、aterials inaccuratly in geology. Designing, constructing without fully grasping the geological situation, this is the main reason that </p><p>　　拱橋的設(shè)計和橋梁裂縫產(chǎn)生的原因分析</p><p>　　本章涵蓋了許多橋型的拱橋和一系列關(guān)于材料的研

62、究案例，還有介紹了所做的設(shè)計決策是怎么形成的。介紹了分析研究拱橋的一般處理方法，包括推導(dǎo)可用于進行手工計算的基本方程以用來驗證計算機分析的結(jié)果。詳細拱橋的設(shè)計在本章的范圍之外，因此只進行了一般性問題的討論。本章的大部分內(nèi)容介紹的是砌體拱橋。磚石拱橋施工中討論了它的歷史背景和強調(diào)了工程技術(shù)人員采取一種全面的方法來評估和設(shè)計橋梁的重要性。橋梁評估有一個重要原則 ，就是一種在現(xiàn)在和未來都能夠有指導(dǎo)作用的評估方法。這也是建立在以砌體材料的背景資

63、料的基礎(chǔ)之上的。本章最后總結(jié)得出了一種修補的方法和用一張包含了常用的補救措施和加強措施的綜合圖表來說明的維護策略。</p><p>　　拱作為一種建筑結(jié)構(gòu)形式的起源歷史可以追溯到古代(凡比克，1987年)。在試圖為拱尋找一個適當(dāng)?shù)亩x時，我們可以借鑒一下不久前胡克在1675年說的“剛性拱”字謎。這表明任何給定負載量的柔性電纜如果在凍結(jié)并且倒轉(zhuǎn)之后在外部荷載下將是一個平衡的抗壓結(jié)構(gòu)。顯然，加載時任何輕微的變化都會對

64、拱產(chǎn)生誘發(fā)突變。達到合適比例的拱厚度以符合推力線的偏心率的適用范圍是橋梁工程師所面臨的挑戰(zhàn)。即使在中世紀表示贊賞砌體拱橋,但其表現(xiàn)形式基本上是作為幾何和比例決定的審美情趣和穩(wěn)定的重力結(jié)構(gòu)。其壓縮強度可以依賴但是拉伸強度卻不能。根據(jù)經(jīng)驗，大跨度和拱厚度的很多經(jīng)驗關(guān)系融合并成功地應(yīng)用，在整個歐洲產(chǎn)生了很多優(yōu)雅的結(jié)構(gòu)。</p><p>　　鐵路和運河的擴大使用導(dǎo)致了橋梁的爆炸式增長，磚砌拱橋成為越來越受歡迎的形式。隨著

65、施工的Coalbrookdale橋（1780年），標志著拱橋建設(shè)的新時代開始了。到十九世紀，鑄鐵、鍛造鐵和鋼終于越來越受歡迎，只是世紀之交會受到鋼筋混凝土的挑戰(zhàn)。</p><p>　　在十九世紀分析技術(shù)迅速發(fā)展，特別是，卡斯蒂利亞諾(1879)的應(yīng)變能量定理，能適用于砌體拱橋使他們保持彈性。提供推力線屬于中間第三，從而確保沒有拉應(yīng)力誘導(dǎo)，能滿足這一條件。要求避免拉應(yīng)力，因此只適用于砌體和鑄鐵，它并不適用鋼或鋼筋混

66、凝土（或木材），因為這些材料有抗拉伸應(yīng)力的能力。</p><p>　　二十世紀的拱橋已成為越來越多的與現(xiàn)代材料相結(jié)合的復(fù)雜的結(jié)構(gòu)，以創(chuàng)造令人興奮的功能性城市雕塑。</p><p><b>　　拱橋的類型</b></p><p>　　在圖1中顯示用來描述一個拱橋的各部分的有關(guān)術(shù)語。拱橋可以根據(jù)以下參數(shù)進行分組：</p><p&

67、gt;<b>　　1．建筑材料</b></p><p><b>　　2．結(jié)構(gòu)的銜接</b></p><p><b>　　3．拱橋的形狀</b></p><p>　　從歷史上看，拱橋都是與石頭建筑有聯(lián)系的，但是在十九世紀讓位給了磚砌結(jié)構(gòu)。因為他們的比例盡可能的減少了拉伸應(yīng)力的可能性，這使得他們有成為巨大

68、結(jié)構(gòu)的趨勢。相比之下，使用鋼筋混凝土和現(xiàn)代結(jié)構(gòu)鋼為拱橋提供了簡約、典雅的機會。</p><p>　　現(xiàn)在，木材被限制在一些桁架形式的小橋上，但通常更作為復(fù)合曲線的拱橋。雖然木材相比于糧食作物的比例有一個高的強度和密度，但是它的各向異性和橫紋的強度性能是相對比較弱的。這就需要注意的連接細節(jié)，以確保材料的經(jīng)濟使用。</p><p>　　至于銜接拱結(jié)構(gòu)可固接或鉸接。在后一種情況中一個、兩個或三個

69、鉸鏈可以被使用到拱肋。雖然固結(jié)的拱橋存在三個多余約束，但是一個鉸的作用就是減少一個多余約束，而對于三個鉸來說的拱橋是靜定的，因此，從理論上來講，無二次應(yīng)力問題。圖2顯示了一系列可能的布置情況。拱橋的銜接不僅僅取決于鉸的數(shù)量，也從根本上受到橋面的位置和從橋面?zhèn)飨蚬暗暮奢d性質(zhì)的影響。</p><p>　　傳統(tǒng)的填充拱肩是車輛荷載通過其回填材料傳遞到拱背，這是第一眼能夠看到的最簡單的結(jié)構(gòu)狀況。但是稍后我們看到的就不是這

70、種狀況，這就導(dǎo)致了我們在試圖提高對這種結(jié)構(gòu)的認識時，將要對這種磚石拱橋的許多特定案例進行研究。</p><p>　　帶有柱子的拱肩會被做成開放式的，鉸用來傳遞橋面板的荷載到拱。在試圖盡量減少對橋墩的水平推力時，橋面板就會和系桿拱配合使用。當(dāng)橋面的建設(shè)深度受到限制時或者需要大跨度時（特別是如果地面條件也很困難，將需要大量的樁以抵抗水平推力時），系桿拱就會特別的適用。</p><p>　　回到

71、胡克的字謎，拱的完美形狀將是一個倒立的懸鏈線——這是一種抵抗自身重量的情況。車輛荷載和不同的自上而下的恒載是兩種誘導(dǎo)彎曲的情況。因此，拱必須有足夠的厚度以適應(yīng)“蜿蜒”的推力作用線。</p><p>　　為了便于施工放樣，簡單的建筑物形狀現(xiàn)在被分段采用或者采用拋物線形狀。雖然在最大寬度的情況下凈空必須提供（比如在一個鐵路，公路或運河），橢圓形狀可能是其最接近的要求或其最相近的等價物是“簡單”的三個鉸的拱。</

72、p><p>　　這是值得在現(xiàn)階段討論的理想化的拱結(jié)構(gòu)。傳統(tǒng)意義上，拱被認為是二維結(jié)構(gòu)。這當(dāng)然是不真實的——但它在何種程度上是不正確的應(yīng)該是設(shè)計師和評估員所關(guān)心的問題。即使在表面上看是靜定的三鉸拱的情況下，“插腳”都能傳遞剪力，即使他們理論上不能傳遞。非均勻的荷載橫向分布的情況下，鉸鏈會傳遞一個會在拱內(nèi)產(chǎn)生扭矩的不斷變化的剪力。此外，斜拱或非垂直拱肋的情況下，結(jié)構(gòu)具有很高的冗余，因此將需要更多地注意發(fā)布的工程結(jié)構(gòu)方面的

73、詳細信息。</p><p>　　從美學(xué)的角度來看，拱橋有一個普遍的吸引力。盡管這樣，重視規(guī)模不大的橋梁的影響是十分有意義的。實體的拱橋總是磚石（或磚石擴大）橋梁。行清潔、誠信的理念和注重細節(jié)是成功的橋梁的至關(guān)重要的因素。當(dāng)然，簡單的層拱和頂層更適合詳細描述一些昂貴的和與許多現(xiàn)代橋梁不相稱的細節(jié)。使用石頭是因為它材質(zhì)重要的環(huán)境敏感性。應(yīng)采用現(xiàn)代開采技術(shù)（激光切割、金剛石鋸、火焰變形和噴砂)，保留傳統(tǒng)加工保護計劃。如

74、果使用不同的磚砌體可以指定紋理或琉璃磚和灰泥。在這里，層拱可以很有效的掩飾層面方向上的變化。</p><p>　　從歷史上看，橋墩是由巖石或者是大量的厚重磚石結(jié)構(gòu)組成的，依靠自身的重量抵抗拱橋的推力。依靠結(jié)構(gòu)的穩(wěn)定性，這是必要的，因為這是一種本能去期待這種支持。</p><p>　　鋼筋混凝土橋和鋼拱橋有許多輕結(jié)構(gòu)。結(jié)構(gòu)基本上包括拱、橋面板和通常一些拱肋到橋面板的支撐結(jié)構(gòu)——重要的順序。這

75、些元素應(yīng)該表現(xiàn)在細節(jié)和形式上，并充分考慮其層次性（高速公路局，1996年）。重要的是，如果在拱頂處的橋面板，不應(yīng)該用任何方法去掩飾它。無論是層拱還是鉸鏈的任何支撐結(jié)構(gòu)（在系桿拱橋的情況下）都不應(yīng)該被允許占據(jù)主導(dǎo)地位。最好是他們相對于欄桿和層拱能夠隱藏起來。</p><p>　　混凝土拱可以是一個完整的彎曲寬板或者是一系列的肋骨組成，鋼拱橋大多是由一系列的肋骨組成。凡是使用肋骨的時候應(yīng)當(dāng)考慮（如果是從下面看的話）拱

76、腹的明暗變化。對跨度上升的比例應(yīng)控制在10:1到2:1的范圍內(nèi)。拱越平坦水平推力就越大，這可能影響結(jié)構(gòu)形式的選擇等等，不論是系桿是否應(yīng)該被引進，還是橋面板相對于拱的剛度。</p><p>　　近幾年來，我國交通基礎(chǔ)建設(shè)得到迅猛發(fā)展，各地區(qū)修建了大量的混凝土橋梁。在橋梁建造和使用過程中，有關(guān)因出現(xiàn)裂縫而影響工程質(zhì)量甚至導(dǎo)橋梁垮塌的報道屢見不鮮。混凝土開裂可以說是“常發(fā)病”和“多發(fā)病”，經(jīng)常困擾著橋梁工程技術(shù)人員。其

77、實，如果采取一定的設(shè)計和施工措施，很多裂縫是可以克服和控制的。為了進一步加強對混凝土橋梁裂縫的認識，盡量避免工程中出現(xiàn)危害較大的裂縫，本文盡可能對混凝土橋梁裂縫的種類和產(chǎn)生的原因作較全面的分析、總結(jié)，以方便設(shè)計、施工找出控制裂縫的可行辦法，達到防范于未然的作用。</p><p>　　混凝土橋梁裂縫種類、成因?qū)嶋H上，混凝土結(jié)構(gòu)裂縫的成因復(fù)雜而繁多，甚至多種因素相互影響，但每一條裂縫均有其產(chǎn)生的一種或幾種主要原因?；?/p>

78、凝土橋梁裂縫的種類，就其產(chǎn)生的原因，大致可劃分如下幾種：</p><p><b>　　1．荷載引起的裂縫</b></p><p>　　混凝土橋梁在常規(guī)靜、動荷載及次應(yīng)力下產(chǎn)生的裂縫稱荷載裂縫，歸納起來主要有直接應(yīng)力裂縫、次應(yīng)力裂縫兩種。直接應(yīng)力裂縫是指外荷載引起的直接應(yīng)力產(chǎn)生的裂縫。裂縫產(chǎn)生的原因有：（1）設(shè)計計算階段，結(jié)構(gòu)計算時不計算或部分漏算；計算模型不合理；結(jié)構(gòu)

79、受力假設(shè)與實際受力不符；荷載少算或漏算；內(nèi)力與配筋計算錯誤；結(jié)構(gòu)安全系數(shù)不夠。結(jié)構(gòu)設(shè)計時不考慮施工的可能性；設(shè)計斷面不足；鋼筋設(shè)置偏少或布置錯誤；結(jié)構(gòu)剛度不足；構(gòu)造處理不當(dāng)；設(shè)計圖紙交代不清等。（2）施工階段，不加限制地堆放施工機具、材料；不了解預(yù)制結(jié)構(gòu)結(jié)構(gòu)受力特點，隨意翻身、起吊、運輸、安裝；不按設(shè)計圖紙施工，擅自更改結(jié)構(gòu)施工順序，改變結(jié)構(gòu)受力模式；不對結(jié)構(gòu)做機器振動下的疲勞強度驗算等。（3）使用階段，超出設(shè)計載荷的重型車輛過橋；受車

80、輛、船舶的接觸、撞擊；發(fā)生大風(fēng)、大雪、地震、爆炸等。</p><p>　　次應(yīng)力裂縫是指由外荷載引起的次生應(yīng)力產(chǎn)生裂縫。裂縫產(chǎn)生的原因有：（1）在設(shè)計外荷載作用下，由于結(jié)構(gòu)物的實際工作狀態(tài)同常規(guī)計算有出入或計算不考慮，從而在某些部位引起次應(yīng)力導(dǎo)致結(jié)構(gòu)開裂。例如兩鉸拱橋拱腳設(shè)計時常采用布置“X”形鋼筋、同時削減該處斷面尺寸的辦法設(shè)計鉸，理論計算該處不會存在彎矩，但實際該鉸仍然能夠抗彎，以至出現(xiàn)裂縫而導(dǎo)致鋼筋銹蝕。（

81、2）橋梁結(jié)構(gòu)中經(jīng)常需要鑿槽、開洞、設(shè)置牛腿等，在常規(guī)計算中難以用準確的圖式進行模擬計算，一般根據(jù)經(jīng)驗設(shè)置受力鋼筋。研究表明，受力構(gòu)件挖孔后，力流將產(chǎn)生繞射現(xiàn)象，在孔洞附近密集，產(chǎn)生巨大的應(yīng)力集中。在長跨預(yù)應(yīng)力連續(xù)梁中，經(jīng)常在跨內(nèi)根據(jù)截面內(nèi)力需要截斷鋼束，設(shè)置錨頭，而在錨固斷面附近經(jīng)?？梢钥吹搅芽p。因此，若處理不當(dāng)，在這些結(jié)構(gòu)的轉(zhuǎn)角處或構(gòu)件形狀突變處、受力鋼筋截斷處容易出現(xiàn)裂縫。實際工程中，次應(yīng)力裂縫是產(chǎn)生荷載裂縫的最常見原因。次應(yīng)力裂縫

82、多屬張拉、劈裂、剪切性質(zhì)。次應(yīng)力裂縫也是由荷載引起，僅是按常規(guī)一般不計算，但隨著現(xiàn)代計算手段的不斷完善，次應(yīng)力裂縫也是可以做到合理驗算的。例如現(xiàn)在對預(yù)應(yīng)力、徐變等產(chǎn)生的二次應(yīng)力，不少平面桿系有限元程序均可正確計算，但在</p><p>　　在設(shè)計上，應(yīng)注意避免結(jié)構(gòu)突變（或斷面突變），當(dāng)不能回避時，應(yīng)做局部處理，如轉(zhuǎn)角處做圓角，突變處做成漸變過渡，同時加強構(gòu)造配筋，轉(zhuǎn)角處增配斜向鋼筋，對于較大孔洞有條件時可在周邊設(shè)

83、置護邊角鋼。荷載裂縫特征依荷載不同而異呈現(xiàn)不同的特點。這類裂縫多出現(xiàn)在受拉區(qū)、受剪區(qū)或振動嚴重部位。但必須指出，如果受壓區(qū)出現(xiàn)起皮或有沿受壓方向的短裂縫，往往是結(jié)構(gòu)達到承載力極限的標志，是結(jié)構(gòu)破壞的前兆，其原因往往是截面尺寸偏小。根據(jù)結(jié)構(gòu)不同受力方式，產(chǎn)生的裂縫特征如下：（1）中心受拉。裂縫貫穿構(gòu)件橫截面，間距大體相等，且垂直于受力方向。采用螺紋鋼筋時，裂縫之間出現(xiàn)位于鋼筋附近的次裂縫。（2）中心受壓。沿構(gòu)件出現(xiàn)平行于受力方向的短而密的

84、平行裂縫。（3）受彎。彎矩最大截面附近從受拉區(qū)邊沿開始出現(xiàn)與受拉方向垂直的裂縫，并逐漸向中和軸方向發(fā)展。采用螺紋鋼筋時，裂縫間可見較短的次裂縫。當(dāng)結(jié)構(gòu)配筋較少時，裂縫少而寬，結(jié)構(gòu)可能發(fā)生脆性破壞。（4）大偏心受壓。大偏心受壓和受拉區(qū)配筋較少的小偏心受壓構(gòu)件，類似于受彎構(gòu)件。（5）小偏心受壓。小偏心受壓和受拉區(qū)配筋較多的大偏心受壓構(gòu)件，類似于中心受壓構(gòu)件。（6）受剪。當(dāng)箍筋太密時發(fā)生</p><p>　　2．溫度變

85、化引起的裂縫</p><p>　　混凝土具有熱脹冷縮性質(zhì)，當(dāng)外部環(huán)境或結(jié)構(gòu)內(nèi)部溫度發(fā)生變化，混凝土將發(fā)生變形，若變形遭到約束，則在結(jié)構(gòu)內(nèi)將產(chǎn)生應(yīng)力，當(dāng)應(yīng)力超過混凝土抗拉強度時即產(chǎn)生溫度裂縫。在某些大跨徑橋梁中，溫度應(yīng)力可以達到甚至超出活載應(yīng)力。溫度裂縫區(qū)別其它裂縫最主要特征是將隨溫度變化而擴張或合攏。引起溫度變化主要因素有：（1）年溫差。一年中四季溫度不斷變化，但變化相對緩慢，對橋梁結(jié)構(gòu)的影響主要是導(dǎo)致橋梁的縱向

86、位移，一般可通過橋面伸縮縫、支座位移或設(shè)置柔性墩等構(gòu)造措施相協(xié)調(diào)，只有結(jié)構(gòu)的位移受到限制時才會引起溫度裂縫，例如拱橋、剛架橋等。我國年溫差一般以一月和七月月平均溫度的作為變化幅度?？紤]到混凝土的蠕變特性，年溫差內(nèi)力計算時混凝土彈性模量應(yīng)考慮折減。（2）日照。橋面板、主梁或橋墩側(cè)面受太陽曝曬后，溫度明顯高于其它部位，溫度梯度呈非線形分布。由于受到自身約束作用，導(dǎo)致局部拉應(yīng)力較大，出現(xiàn)裂縫。日照和下述驟然降溫是導(dǎo)致結(jié)構(gòu)溫度裂縫的最常見原因。

87、（3）驟然降溫。突降大雨、冷空氣侵襲、日落等可導(dǎo)致結(jié)構(gòu)外表面溫度突然下降，但因內(nèi)部溫度變化相對較慢而產(chǎn)生溫度梯度。日照和驟然降溫內(nèi)力計算時可采用設(shè)計規(guī)范或參考實橋資料進行，混凝土</p><p><b>　　3．收縮引起的裂縫</b></p><p>　　在實際工程中，混凝土因收縮所引起的裂縫是最常見的。在混凝土收縮種類中，塑性收縮和縮水收縮（干縮）是發(fā)生混凝土體積變

88、形的主要原因，另外還有自生收縮和炭化收縮。塑性收縮。發(fā)生在施工過程中、混凝土澆筑后4～5小時左右，此時水泥水化反應(yīng)激烈，分子鏈逐漸形成，出現(xiàn)泌水和水分急劇蒸發(fā)，混凝土失水收縮，同時骨料因自重下沉，因此時混凝土尚未硬化，稱為塑性收縮。塑性收縮所產(chǎn)生量級很大，可達1%左右。在骨料下沉過程中若受到鋼筋阻擋，便形成沿鋼筋方向的裂縫。在構(gòu)件豎向變截面處如T梁、箱梁腹板與頂?shù)装褰唤犹?，因硬化前沉實不均勻?qū)l(fā)生表面的順腹板方向裂縫。為減小混凝土塑性收

89、縮，施工時應(yīng)控制水灰比，避免過長時間的攪拌，下料不宜太快，振搗要密實，豎向變截面處宜分層澆筑?？s水收縮（干縮）。混凝土結(jié)硬以后，隨著表層水分逐步蒸發(fā)，濕度逐步降低，混凝土體積減小，稱為縮水收縮（干縮）。因混凝土表層水分損失快，內(nèi)部損失慢，因此產(chǎn)生表面收縮大、內(nèi)部收縮小的不均勻收縮，表面收縮變形受到內(nèi)部混凝土的約束，致使表面混凝土承受拉力，當(dāng)表面混凝土承受拉力超過其抗拉強度時，便產(chǎn)生收縮裂縫?；炷劣不笫湛s主要就是縮水收縮。如配筋率較大

90、的構(gòu)件</p><p>　　研究表明，影響混凝土收縮裂縫的主要因素有：（1）水泥品種、標號及用量。礦渣水泥、快硬水泥、低熱水泥混凝土收縮性較高，普通水泥、火山灰水泥、礬土水泥混凝土收縮性較低。另外水泥標號越低、單位體積用量越大、磨細度越大，則混凝土收縮越大，且發(fā)生收縮時間越長。例如，為了提高混凝土的強度，施工時經(jīng)常采用強行增加水泥用量的做法，結(jié)果收縮應(yīng)力明顯加大。（2）骨料品種。骨料中石英、石灰?guī)r、白云巖、花崗巖

91、、長石等吸水率較小、收縮性較低；而砂巖、板巖、角閃巖等吸水率較大、收縮性較高。另外骨料粒徑大收縮小，含水量大收縮越大。（3）水灰比。用水量越大，水灰比越高，混凝土收縮越大。（4）外摻劑。外摻劑保水性越好，則混凝土收縮越小。（5）養(yǎng)護方法。良好的養(yǎng)護可加速混凝土的水化反應(yīng)，獲得較高的混凝土強度。養(yǎng)護時保持濕度越高、氣溫越低、養(yǎng)護時間越長，則混凝土收縮越小。蒸汽養(yǎng)護方式比自然養(yǎng)護方式混凝土收縮要小。（6）外界環(huán)境。大氣中濕度小、空氣干燥、溫

92、度高、風(fēng)速大，則混凝土水分蒸發(fā)快，混凝土收縮越快。（7）振搗方式及時間。機械振搗方式比手工搗固方式混凝土收縮性要小。振搗時間應(yīng)根據(jù)機械性能決定，一般以5～15s/次為宜。時</p><p>　　4．地基礎(chǔ)變形引起的裂縫</p><p>　　由于基礎(chǔ)豎向不均勻沉降或水平方向位移，使結(jié)構(gòu)中產(chǎn)生附加應(yīng)力，超出混凝土結(jié)構(gòu)的抗拉能力，導(dǎo)致結(jié)構(gòu)開裂。基礎(chǔ)不均勻沉降的主要原因有：（1）地質(zhì)勘察精度不夠、

93、試驗資料不準。在沒有充分掌握地質(zhì)情況就設(shè)計、施工，這是造成地基不均勻沉降的主要原因。比如丘陵區(qū)或山嶺區(qū)橋梁，勘察時鉆孔間距太遠，而地基巖面起伏又大，勘察報告不能充分反映實際地質(zhì)情況。（2）地基地質(zhì)差異太大。建造在山區(qū)溝谷的橋梁，河溝處的地質(zhì)與山坡處變化較大，河溝中甚至存在軟弱地基，地基土由于不同壓縮性引起不均勻沉降。（3）結(jié)構(gòu)荷載差異太大。在地質(zhì)情況比較一致條件下，各部分基礎(chǔ)荷載差異太大時，有可能引起不均勻沉降，例如高填土箱形涵洞中部比