钢筋混凝土梁的剪切和斜拉新.pptx

第第2节节 钢筋混凝土梁的剪切和斜拉钢筋混凝土梁的剪切和斜拉4.2Shear and Diagonal Tension in Reinforced Concrete Beamcatastrophicadj.悲惨的悲惨的,灾难的灾难的希腊颂神歌的一部分，反复的希腊颂神歌的一部分，反复的strophic“下,向下”ampleadj.充足的充足的,丰富的丰富的 For moderate reinforced beam,flexural failure is initiated by gradual yielding of the tension steel,accompanied by obvious cracking of the concrete and large deflection,giving ample warning and providing the opportunity to take corrective measures.对于适筋梁，受拉钢筋逐步屈服引起对于适筋梁，受拉钢筋逐步屈服引起弯曲破坏，随着破坏，混凝土产生明弯曲破坏，随着破坏，混凝土产生明显的裂缝，梁出现巨大的挠曲，发出显的裂缝，梁出现巨大的挠曲，发出各种警示，为采取补救措施提供了时各种警示，为采取补救措施提供了时间。间。Because of these differences in behavior,reinforced concrete beams are generally provided with special shear reinforcement to ensure the flexural failure would occur before shear failure if the member should be severely overloaded.由于这些不同特性，一般在钢筋混凝由于这些不同特性，一般在钢筋混凝土梁中配置专门的抗剪钢筋，确保在土梁中配置专门的抗剪钢筋，确保在构件严重超载时，弯曲破坏先于剪切构件严重超载时，弯曲破坏先于剪切破坏发生。破坏发生。In the case of simply supported beam subjected to two-point loading,the moment and shear distribution is such that the moment is constant in the mid span and in two side spans,shear force is constant.以承受两个集中荷载的简支梁为例，以承受两个集中荷载的简支梁为例，其弯矩在跨中部分是不变的，剪力其弯矩在跨中部分是不变的，剪力在两个边跨范围内不变。在两个边跨范围内不变。At the location of zero shear stress,i.e.,the extreme tension fiber,the principal tensile stress takes the horizontal direction.在剪应力为在剪应力为0处，即最大的拉应力处，即最大的拉应力处，主拉应力沿水平方向。处，主拉应力沿水平方向。At the point of zero normal stress,i.e.,the neutral axis,the principal tensile stress is equal to shear stress,and its direction is 45 degrees with respect to the member axis(Figure.4.12).在正应力为在正应力为0处，即中性轴处，主拉处，即中性轴处，主拉应力等于剪切应力，其方向构件轴线应力等于剪切应力，其方向构件轴线成成45o角。角。Reinforced Concrete without Shear Reinforcement 无腹筋钢筋混凝土梁无腹筋钢筋混凝土梁 For a reinforced concrete without shear reinforcement,flexural crack is formed in regions of large moments,and its direction is parallel to the member axis.对于无腹筋钢筋混凝土构件，在弯矩对于无腹筋钢筋混凝土构件，在弯矩大的区域首先形成弯曲裂缝，平行于大的区域首先形成弯曲裂缝，平行于构件轴线。构件轴线。The longitudinal tension reinforcement has so calculated and placed that it is chiefly effective in resisting longitudinal tension near the tension face.由此计算出纵向受拉钢筋，并将其放置由此计算出纵向受拉钢筋，并将其放置在受拉面附近，主要用来抵抗纵向拉力。在受拉面附近，主要用来抵抗纵向拉力。It does not reinforce the tensionally weak concrete against the diagonal tension stresses that occur elsewhere,caused by shear alone or by the combined effect of shear and flexure.设置纵筋并不是用来提高混凝土的设置纵筋并不是用来提高混凝土的抗拉强度，抵抗任何位置上由剪力抗拉强度，抵抗任何位置上由剪力单独引起的、或由剪力和弯曲综合单独引起的、或由剪力和弯曲综合引起的斜拉应力。引起的斜拉应力。Eventually,these stresses attain magnitudes sufficient to open additional tension cracks in a direction perpendicular to the local tension stress.最后，应力达到一定程度，足以沿垂最后，应力达到一定程度，足以沿垂直于局部拉应力的方向出现另外的拉直于局部拉应力的方向出现另外的拉裂缝。裂缝。The mode of failure is influenced by the shear span to effective depth ratio given bywhere a is the distance between the support and the nearest concentrated load 剪跨比影响梁的破坏方式。剪跨比剪跨比影响梁的破坏方式。剪跨比的计算公式：的计算公式：在这里，在这里，a表示支座与距离最近的表示支座与距离最近的集中力的距离集中力的距离 The diagonal crack,once formed,spread either immediately or at only slightly higher load,traversing the entire beam from the tension reinforcement to the compression face,splitting it into two and failing the beam.一旦形成斜拉裂缝，要么它会立刻延一旦形成斜拉裂缝，要么它会立刻延伸，要么只要稍微增加荷载，它就会伸，要么只要稍微增加荷载，它就会从受拉筋到受压面（从受拉筋到受压面（compression face）贯穿整个梁，将梁劈成两半，）贯穿整个梁，将梁劈成两半，使梁破坏。使梁破坏。The diagonal crack,once formed,spreads toward and partially into the compression zone but stops short of penetrating to the compression face.一旦形成斜裂缝，它向受压区延伸，一旦形成斜裂缝，它向受压区延伸，部分深入到受压区，但是没有穿透受部分深入到受压区，但是没有穿透受压面就停止。压面就停止。For the case of socalled deep beam,i.e.,the shear span to effective depth ratio is very small(a/h0 1.0),the shear resisting mechanism is formed as a compression strut joining the loading and support points.在称为深梁的构件中，即剪跨比很小的在称为深梁的构件中，即剪跨比很小的情况下（情况下（1.0），抗剪体系形成一个），抗剪体系形成一个受压柱连接荷载与支座。受压柱连接荷载与支座。VdTaVczCVextPViAggregate interlockDowel actionCompression zone of concreteThe aggregate interlock forces amounts to one-third and more of the shear force.骨料咬合作用骨料咬合作用Vi，大于等于剪力的，大于等于剪力的1/3。The other internal forces are those in the uncracked portion of the concrete,and those across the longitudinal steel,acting as a dowl,其他的内力，包括未开裂的混凝土产其他的内力，包括未开裂的混凝土产生的生的Vcz，纵向钢筋的销栓作用，纵向钢筋的销栓作用Vd。Dowel n.木钉木钉,销子销子Interlock v.互锁互锁VdTaVczCVextPViReinforced Concrete with Web Reinforcement 配置抗剪钢筋的钢筋混凝土配置抗剪钢筋的钢筋混凝土Economy of design demands,in most cases,that a flexural member be capable of developing its full moment capacity rather than having its strength limited by premature shear failure.大多情况下，设计的经济性要求弯曲大多情况下，设计的经济性要求弯曲构件应该能够充分发挥其抗弯性能，构件应该能够充分发挥其抗弯性能，而不是由过早的剪切破坏限制其具有而不是由过早的剪切破坏限制其具有的强度。的强度。Impend vi.进行威胁进行威胁,即将发生即将发生Stirrups are formed to fit around the main longitudinal bars at the bottom and hooked or bent around longitudinal bars at the top of the member to improve anchorage and providing support during construction.绕着梁底的主纵筋设置箍筋，在梁顶绕着梁底的主纵筋设置箍筋，在梁顶沿梁顶纵筋弯钩或弯曲，加强固定作沿梁顶纵筋弯钩或弯曲，加强固定作用并在施工中提供支承。用并在施工中提供支承。Bent-uplongitudinal bars(e)The requirements for longitudinal flexural reinforcement often conflict with those for diagonal tension reinforcement,and because the saving in steel resulting from use of the capacity of bent bars as shear resistance is small,most designers prefer to provide vertical stirrups for all the shear reinforcement,not counting on the bent part of the longitudinal bars。因为纵筋抵抗弯曲与弯曲纵筋抵抗斜因为纵筋抵抗弯曲与弯曲纵筋抵抗斜拉应力的要求常常发生冲突，并且用拉应力的要求常常发生冲突，并且用弯曲筋抵抗剪力节约的钢筋量很少，弯曲筋抵抗剪力节约的钢筋量很少，所以设计者更愿使用竖向箍筋作为抗所以设计者更愿使用竖向箍筋作为抗剪钢筋，而不愿将纵筋弯起。剪钢筋，而不愿将纵筋弯起。Behavior of web reinforced concrete beams 抗剪配筋混凝土梁的特性抗剪配筋混凝土梁的特性 be free of免于免于Augment v.增加增加,增大增大argumentn.争论争论,辩论辩论,Imminent adj.即将来临的即将来临的,逼近的逼近的This not only exhaust their own resistance but also permits a wider crack opening with consequent reduction of the beneficial restraining effects,point 2 to point 4,above.dowel action（销栓作用）（销栓作用）这不仅耗尽自身的抵抗力而且随着上这不仅耗尽自身的抵抗力而且随着上述第二点到第四点的有益的约束的降述第二点到第四点的有益的约束的降低，裂缝变得更宽。低，裂缝变得更宽。Since web reinforcement is ineffective in the uncracked beam,the magnitude of the shear force or stress that causes cracking to occur（in a beam with web reinforcement)is the same as in a beam without web reinforcement.在未开裂的梁中，抗剪钢筋几乎不在未开裂的梁中，抗剪钢筋几乎不起作用，所以在抗剪钢筋梁与未配起作用，所以在抗剪钢筋梁与未配抗剪钢筋的梁中，引起裂缝的剪切抗剪钢筋的梁中，引起裂缝的剪切力或剪应力的大小是一样的。力或剪应力的大小是一样的。Here Asv is the cross-section area of the stirrup(in the case of the U-shaped stirrup of Figure 1b it is twice the area of one bar)and fyv is the tensile stress in the stirrup.这里，这里，Asv是穿过箍筋的横断面的面是穿过箍筋的横断面的面积（对于图积（对于图1b所示的所示的U形箍筋，是一形箍筋，是一根钢筋面积的两倍），根钢筋面积的两倍），fyv是箍筋的拉是箍筋的拉应力。应力。*The largest tensile stresses are those caused at the outer fiber by bending alone,at the section of maximum bending moment.最大的拉应力处于梁的最大弯矩截面最大的拉应力处于梁的最大弯矩截面外层，是由弯曲单独引起的。外层，是由弯曲单独引起的。*Even though tension cracks form in the concrete,the required flexural tension strength is furnished by the steel,and much higher load can be carried.即使混凝土中出现拉裂缝，钢筋提供即使混凝土中出现拉裂缝，钢筋提供了所需的弯曲抗拉强度，因此梁能够了所需的弯曲抗拉强度，因此梁能够承受更多的荷载。承受更多的荷载。结果，在接近支座处的高剪力区域，结果，在接近支座处的高剪力区域，形成强大的斜拉应力。形成强大的斜拉应力。*In consequence,diagonal tension stresses of significant intensity are created in regions of high shear forces,chiefly close to the supports*The longitudinal tension reinforcement has been so calculated and placed that it is chiefly affective in resisting longitudinal tension near the tension face.由此计算出纵向受拉钢筋，并将其放置由此计算出纵向受拉钢筋，并将其放置在受拉面附近，主要用来抵抗纵向拉力。在受拉面附近，主要用来抵抗纵向拉力。*It does not reinforce the tensionally weak concrete against the diagonal tension stresses that occur elsewhere,caused by shear alone or by the combined effect of shear and flexure.设置纵筋并不是用来提高混凝土的抗设置纵筋并不是用来提高混凝土的抗拉强度，抵抗任何位置上由剪力单独拉强度抵抗任何位置上由剪力单独引起的、或由剪力和弯曲综合引起的引起的、或由剪力和弯曲综合引起的斜拉应力。斜拉应力。in distinction to 与与有差别的有差别的*Flexural cracks are harmless only because adequate longitudinal reinforcement has been provided to resist the flexural tension stresses that the cracked concrete is no longer able to transmit them.因为已经配置了纵向钢筋抵抗弯曲拉因为已经配置了纵向钢筋抵抗弯曲拉应力，开裂的混凝土也不能再传递弯应力，开裂的混凝土也不能再传递弯曲拉应力，所以弯曲裂缝没有什么害曲拉应力，所以弯曲裂缝没有什么害处。处。*In contrast,the beams now being discussed,although furnished with the usual longitudinal reinforcement,are not equipped with any other reinforcement to offset the effects of diagonal cracking，which makes the diagonal cracks much more decisive in subsequent performance and strength of the beam than the flexural cracks.相反，目前被讨论的梁，尽管配置了相反，目前被讨论的梁，尽管配置了常见的纵向钢筋，但还需要其它的钢常见的纵向钢筋，但还需要其它的钢筋抵消斜向开裂的影响。因此，在梁筋抵消斜向开裂的影响。因此，在梁的功能和强度方面，斜拉裂缝比弯曲的功能和强度方面，斜拉裂缝比弯曲裂缝更具有决定性。裂缝更具有决定性。shallow beams 扁梁扁梁span-depth ratio 跨高比跨高比vulnerableadj.易受攻击的易受攻击的,易受易受.的攻击的攻击Penetrate vt.vi.穿透穿透,渗透渗透,弥漫弥漫Stop shortv.突然停止突然停止short of adv.缺乏缺乏,达不到达不到,除除.以外以外,只要没有只要没有