1、Axial Compression 轴向受压轴向受压承受压力的构件被称作柱子承受压力的构件被称作柱子 Columns are defined as members that carry loads chiefly in compression.For this purpose,longitudinal and transverse steel is wired together to form cages,which are then moved into the forms and properly positioned before placing the concrete.因此，将纵向钢
2、筋和箍筋绑扎在一起，因此，将纵向钢筋和箍筋绑扎在一起，形成钢筋骨架，然后将其移入模板中，形成钢筋骨架，然后将其移入模板中，在浇筑混凝土前正确摆放。在浇筑混凝土前正确摆放。For another,transverse reinforcement is needed to prevent the highly stressed,slender longitudinal bars from buckling outward by bursting the thin concrete cover.另外，箍筋能够防止应力较大的、细长另外，箍筋能够防止应力较大的、细长纵筋向外弯曲，崩裂单薄的混凝土保护纵筋
3、向外弯曲，崩裂单薄的混凝土保护层。层。Behavior of Axially Loaded Short Column轴心受压短柱的特性轴心受压短柱的特性 When an axial load is applied to a reinforced concrete short column,the concrete can be considered to behave elastically up to a low stress of about(1/3)3/1(cf.cf当荷载作用在钢筋混凝土短柱形心时，当荷载作用在钢筋混凝土短柱形心时，应力低于应力低于)3/1(cf,混凝土被认为具有弹性特
4、性。混凝土被认为具有弹性特性。At this load the concrete fails by crushing or shearing outward along inclined planes,and the longitudinal steel bars fail by buckling outward between ties,as shown in Figure 4.26(a).普通箍筋柱在荷载普通箍筋柱在荷载Na作用下破坏。在作用下破坏。在此荷载下，由于混此荷载下，由于混凝土压碎或向外沿凝土压碎或向外沿斜面受剪破坏，纵斜面受剪破坏，纵筋由于在箍筋之间筋由于在箍筋之间向外弯曲而破
5、坏，向外弯曲而破坏，如图（如图（4.26a）所）所示。示。It is at this stage that the confining action of the spiral has a significant effect,and if sizable spiral steel is provided,the load will ultimately fail the column by causing the spiral steel to yield.正是在这个阶段，螺旋箍发挥着重要正是在这个阶段，螺旋箍发挥着重要的约束作用。如果配置大量的螺旋箍，的约束作用。如果配置大量的螺旋箍，当荷
6、载使螺旋筋屈服时，柱子发生破当荷载使螺旋筋屈服时，柱子发生破坏。坏。Both tied and spirally reinforced columns behave similarlyTied column fails suddenlyYield point(Spirally reinforced concrete shell spalls off)Spirally reinforced column shows ability to deform prior to failureDeformationNaload(a)Unsupported bars buckle (b)Concrete s
7、hell spalls offFailure of tied and spirally reinforced concrete columnShort Columns under Combined Axial Force and Moment轴力及弯矩共同作用下的短柱轴力及弯矩共同作用下的短柱 When a member is subjected to combined axial compression N and moment M,such as in Figure 4.28(a),it is usually convenient to replace the axial load and
8、 moment with an equal load N applied at eccentricity NMe/,as in Figure 4.28(b).当柱子承受轴力当柱子承受轴力N以及弯距以及弯距M的共同作用的共同作用时，如图时，如图4.28a，通常用轴力，通常用轴力N作用在距作用在距离轴心离轴心e=M/N处取代构件承受的轴力和处取代构件承受的轴力和弯矩，如图弯矩，如图4.28b所示，是一种很简便的所示，是一种很简便的方法。方法。NNeMBalanced failure界限破坏界限破坏Under this mode of failure,yielding of the tensile
9、steel on the far side of the loading point occurs simultaneously with the attainment of limit compressive strain of 0.0033 in concrete.在这种破坏方式下，远离轴向力作用在这种破坏方式下，远离轴向力作用点一侧的受拉钢筋屈服，同时受压区点一侧的受拉钢筋屈服，同时受压区混凝土达到混凝土达到0.0033的极限压应变。的极限压应变。00/he00/heCompression failure of the column occurs when(a)the relative
10、eccentricity is small,or(b)although the relative eccentricity is large,but the tension reinforcement ratio is large.当相对偏心距当相对偏心距e0/h0较小时，或尽管相对较小时，或尽管相对偏心距偏心距e0/h0较大，但受拉钢筋配置较多较大，但受拉钢筋配置较多时，柱子会发生受压破坏。时，柱子会发生受压破坏。Columns of this type fail by crushing of the concrete and compressive yielding of the stee
11、l on the side near the load,while the reinforcement on the side farthest from the load does not yield.这种破坏表现为离这种破坏表现为离荷载作用点近的一荷载作用点近的一侧混凝土压碎钢筋侧混凝土压碎钢筋受压屈服，而另一受压屈服，而另一侧钢筋没有屈服，侧钢筋没有屈服，如图如图4.29b所示。所示。Capacity of short columns under combined axial force and moment 短柱抵抗弯压共同作用的能力短柱抵抗弯压共同作用的能力The strength
12、interaction diagram defines the failure load and failure moment for a given column for the full ranges of eccentricity from zero to infinity.强度相关图给出了某一柱子偏心距从零到强度相关图给出了某一柱子偏心距从零到无穷大时的破坏荷载与弯距。无穷大时的破坏荷载与弯距。0.00330.00330.0033e=0e0 smalle0 largeThe extreme concrete fiber strain reaches ultimate strain(0.
13、003)simultaneously with yielding of the extreme layer of steel on the opposite side(fy/Es=0.002).混凝土的最大应变达到极限混凝土的最大应变达到极限应变（应变（0.0033），另一侧），另一侧的最外层钢筋同时达到屈服的最外层钢筋同时达到屈服应变（应变（fy/Es=0.002）。）。The interaction diagram further reveals that as the axial force Nu becomes larger the section can carry smaller
14、Mu before failing in the compression zone.相关曲线进一步显示，在受压破相关曲线进一步显示，在受压破坏区，构件破坏前，随着轴力增坏区，构件破坏前，随着轴力增大，截面所承受的弯距越来越小。大，截面所承受的弯距越来越小。The large axial force produces high compressive strain of concrete keeping smaller margin available for additional compressive strain due to bending.轴力大，混凝土产生压应变就大，从轴力大，混凝土
15、产生压应变就大，从而无法承受更多的由弯曲引起的附加而无法承受更多的由弯曲引起的附加压应变。压应变。This tensile yield stress reduces with the additional compressive stress due to additional axial load.随着附加轴力引起附加的压应力随着附加轴力引起附加的压应力增大，这种受拉钢筋拉应力降低。增大，这种受拉钢筋拉应力降低。Just as for simple bending,the actual concrete compressive stress distribution is replaced
16、by an equivalent rectangular distribution having depth x.仅对单纯的受弯而言仅对单纯的受弯而言,采用受压区截面高采用受压区截面高度为度为x的等效矩形应力分布图替代实际混的等效矩形应力分布图替代实际混凝土压应力分布。凝土压应力分布。e0e0Actual and equivalent rectangular stress distribution of eccentrically loaded column at ultimate strength另外，混凝土压应力、钢筋作用力以另外，混凝土压应力、钢筋作用力以及外力及外力N关于受拉钢筋合力中
17、心线的关于受拉钢筋合力中心线的弯距和必须等于弯距和必须等于0。In addition,sum of the moments of the concrete compressive stress,reinforcement force and the external force N about the centerline of the resultant force in the tension reinforcement must equal zeroAdditional eccentricity附加偏心距附加偏心距Slender Columns长柱长柱On the other hand,
18、the slender or long columns may fail at a much lower value of the load when sudden lateral displacement of the member takes place between the ends.另一方面，当构件在两支承端之间另一方面，当构件在两支承端之间突然发生横向侧移，长柱会在非常突然发生横向侧移，长柱会在非常低的荷载作用下破坏。低的荷载作用下破坏。Capacity of Axially Loaded Slender Column轴心受压长轴心受压长柱承载力柱承载力stability redu
19、ction coefficient稳定系数稳定系数The lower ratio limit is necessary to ensure resistance to bending moment not accounted in the analysis and to reduce the effects of creep and shrinkage of the concrete under sustained compression.为了确保抵抗分析中没有考虑的弯矩以及为了确保抵抗分析中没有考虑的弯矩以及减少混凝土在持续荷载压力作用下混凝土减少混凝土在持续荷载压力作用下混凝土徐变和收缩的
20、影响，必须设置配筋下限。徐变和收缩的影响，必须设置配筋下限。Spacing between longitudinal bars measured along the centroid should not exceed 300mm.纵筋净间距不超过纵筋净间距不超过300mm。When the depth of the eccentrically loaded column is greater than 600mm,additional longitudinal bars with a diameter of 1016mm should be placed on the lateral sid
21、es.当偏心受压柱的截面高度大于当偏心受压柱的截面高度大于600mm时，沿两侧向应该各增加一根直径时，沿两侧向应该各增加一根直径10-16的附加纵筋。的附加纵筋。The ties shall be so arranged that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie having an included angle of not more than,and no longitudinal bar shall be farthe
22、r than 300mm on either side from such a laterally supported bar.135箍筋应该如此布置，柱的每个角以及每隔箍筋应该如此布置，柱的每个角以及每隔一根纵筋都应该有由箍筋的角提供的侧向一根纵筋都应该有由箍筋的角提供的侧向支撑，箍筋夹角不超过支撑，箍筋夹角不超过135o，纵向钢筋的纵向钢筋的任何一侧离具有这样的侧向支撑钢筋的净任何一侧离具有这样的侧向支撑钢筋的净间距不应超过间距不应超过300mm。Cross-sections in buildings In addition,a minimum ratio of spiral steel
23、is imposed such that the structural performance of the column is significantly improved,with respect to both ultimate load and the type of failure,compared with an otherwise identical tied column.另外，采用配筋比较小的螺旋箍筋有利于另外，采用配筋比较小的螺旋箍筋有利于改善柱子的性能。与同样配箍率的箍筋柱改善柱子的性能。与同样配箍率的箍筋柱相比，其极限荷载有了极大的提高，其破相比，其极限荷载有了极大的提
24、高，其破坏型式有所改善。坏型式有所改善。While spirals are not required to run through the column-to-floor connection zones,ties should be inserted in those zones to maintain proper confinement,especially if horizontal beams do not frame into these zones.对于不需要贯穿柱对于不需要贯穿柱-板联结区的螺旋箍而板联结区的螺旋箍而言，尤其是水平梁不与柱固结时，应将言，尤其是水平梁不与柱固结时，应将箍筋插入这些区域，保持对纵筋的适当箍筋插入这些区域，保持对纵筋的适当约束。约束。45 结束语结束语
