Columns

Columns are vertical compression members of a structural frame intended to support the load-carrying beams. They transmit loads from the upper floors to the lower levels and then to the soil through the foundations. Since columns are compression elements, failure of one column in a critical location can cause the progressive collapse of the adjoining floors and the ultimate total collapse of the entire structure.

Column failure is of major significance in terms of economic as well as human loss. Thus extreme care needs to be taken in column design, with a higher reserve strength than in the case of beams and other horizontal structural elements, particularly since compression failure provides little visual warning.

The amount of reinforcement in the case of beams was controlled so as to have ductile failure behavior. In the case of columns, the axial load will usually dominate: hence compression failure behavior in cases of a large axial load/bending moment ratio cannot be avoided.

在框架结构中，柱是被用来支撑承重梁的竖向受压构件。上层楼板的荷载通过柱传到下层，然后经过基础传到土壤中。作为受压构件，如果某一根位于要害部位的柱发生破坏，将会引起邻近楼板的渐进性破坏，最终将会导致整个结构的破坏。

柱的破坏不仅会引起经济方面的巨大损失，而且还会造成人员的伤亡。因此，要特别认真地进行柱的设计工作，应该使其具有比梁和其他水平构件更高的强度储备，这主要是因为在柱发生受压破坏之前没有明显预兆。

对于梁来说，控制钢筋数量的目的是使其具有延性破坏特征。然而对于柱来说，轴向荷载往往占优势。因而，当轴向荷载与弯矩的比值较大时，柱必然会具有受压破坏特征。

As the load on a column continues to increase, cracking becomes more intense along the height of the column at the transverse tie locations. At the limit state of failure, the concrete cover in tied columns or the shell of concrete outside the spirals of spirally confined columnsspalls and the longitudinal bars become exposed. Additional load leads to failure and local buckling of the individual longitudinal bars at the unsupported length between the ties.

随着施加在柱上的荷载继续增加，沿着柱的高度方向，在箍筋部位会产生更多的裂缝。当柱的强度达到临近破坏的极限状态时，普通箍筋柱或螺旋箍筋柱的混凝土保护层会脱落，露出纵筋。继续增加荷载将使构件破坏，并使箍筋之间无支撑长度范围内的纵筋产生局部压屈。

As in the case of beams, the strength of columns is evaluated on the basis of the following principles:

(1) There is no slippage between the concrete and the steel(i. e., the strain in steel and in the adjoining concrete is the same).

(2) The maximum allowable concrete strain at failure for the purpose of strength calculations is 0. 003.

(3) The tensile resistance of the concrete is negligible and is disregarded in computations.

Columns can be classified on the basis of the form and arrangement of reinforcement, the position of the load on the cross section, and the length of the column in relation to its lateral dimensions.

The form and arrangement of the reinforcement identify two types of columns, as shown in Fig. 32. 1:

(1) Rectangular or square columns reinforced with longitudinal bars and lateral ties (Fig. 32.1a).

(2) Circular columns reinforced with longitudinal reinforcement and spiral reinforcement, or lateral ties (Fig. 32.1b).

与梁的情况一样，柱的强度可以根据下述原则进行计算：

(1）混凝土和钢筋之间不会产生滑移（也就是说，钢筋与其周围的混凝土具有相同的应变）；

(2）在强度计算中，混凝土破坏时的最大允许应变值为0.003；

(3）混凝土抗拉强度很小，在计算中可以忽略不计。

可根据钢筋的形式和布置、截面上荷载的位置，以及柱的高度与横截面尺寸的相互关系对柱进行分类。

按照钢筋的形式和布置，柱可以分为两种类型，如图 32.1所示：

(1）配有纵筋和普通箍筋的矩形或正方形柱（图 32.1a），

(2）配有纵筋和螺旋箍筋或者普通箍筋的圆形柱（图 32.1b）；

Although tied columns are the most commonly used because of lower construction costs, spirally bound circular columns are also used where increased ductility is needed, such as in earthquake zones. The ability of the spiral column to sustain the maximum load at excessive deformations prevents the complete collapse of the structure before total redistribution of moments and stresses is complete.

虽然普通箍筋柱因为其造价便宜而得到广泛应用，但是，在诸如地震区这类需要柱具有较大延性的地方，也会采用圆形的螺旋箍筋柱。当发生过大变形时，螺旋箍筋柱具有承受最大荷载的能力，能够防止整个结构在力矩和应力重新分布之前发生完全倒塌。

Based on the position of the load on the cross section, columns can be classified as concentrically or eccentrically loaded. Concentrically loaded columns carry no moment. Eccentrically loaded columns are subjected to moment in addition to the axial force. In practice, however, all columns have to be designed for some unforeseen or accidental eccentricity due to such causes as imperfections in the vertical alignment of formwork.

按照作用在横截面上荷载的位置，柱可分为轴心受压或偏心受压。轴心受压柱不承受弯矩。偏心受压柱除了承受轴向力之外还要承受弯矩。然而，在实践中，所有的柱都应按某些预想不到的或者偶然的偏心进行设计，这是因为在支模板的过程中进行竖向找正时总会产生偏差。

Most columns are subjected to bending moment in addition to axial force. For this reason and to ensure some ductility, a minimum of 1% reinforcement should be provided in the columns. A reasonable reinforcement ratio is between 1.5% and 3.0%. In high-rise buildings where column loads are very large, 4% reinforcement is not unreasonable. A minimum of four longitudinal bars should be used in the case of tied columns. For spiral columns, at least six longitudinal bars should be used to provide hoop action in the spirals.

除了承受轴向力外，大多数柱还要承受弯矩。因此，要保证柱具有一定的延性，所以柱的最小配筋率应该为1%。合理的配筋率应为1.5%～3.0%。在高层建筑中，柱所承受荷载通常非常大，配筋率也可达到4%。普通箍筋柱中应至少采用4根纵筋。对于螺旋箍筋柱，为了保证螺旋筋起到箍的作用，应至少采用6根纵筋。

Failure of columns could occur as a result of material failure by initial yielding of the steel at the tension face or initial crushing of the concrete at the compression face, or by loss of lateral structural stability (i. e., through buckling).

柱的破坏可能会是由于材料破坏而引起，即由受拉面的钢筋屈服，或者由受压面的混凝土首先被压碎；或者由于侧向失稳而破坏（即由压屈引起的破坏）。