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.

在框架結構中，柱是被用來支撐承重梁的豎向受壓構件。上層樓板的荷載通過柱傳到下層，然后經(jīng)過基礎傳到土壤中。作為受壓構件，如果某一根位于要害部位的柱發(fā)生破壞，將會引起鄰近樓板的漸進性破壞，最終將會導致整個結構的破壞。

柱的破壞不僅會引起經(jīng)濟方面的巨大損失，而且還會造成人員的傷亡。因此，要特別認真地進行柱的設計工作，應該使其具有比梁和其他水平構件更高的強度儲備，這主要是因為在柱發(fā)生受壓破壞之前沒有明顯預兆。

對于梁來說，控制鋼筋數(shù)量的目的是使其具有延性破壞特征。然而對于柱來說，軸向荷載往往占優(yōu)勢。因而，當軸向荷載與彎矩的比值較大時，柱必然會具有受壓破壞特征。

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.

隨著施加在柱上的荷載繼續(xù)增加，沿著柱的高度方向，在箍筋部位會產(chǎn)生更多的裂縫。當柱的強度達到臨近破壞的極限狀態(tài)時，普通箍筋柱或螺旋箍筋柱的混凝土保護層會脫落，露出縱筋。繼續(xù)增加荷載將使構件破壞，并使箍筋之間無支撐長度范圍內(nèi)的縱筋產(chǎn)生局部壓屈。

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).

與梁的情況一樣，柱的強度可以根據(jù)下述原則進行計算：

(1）混凝土和鋼筋之間不會產(chǎn)生滑移（也就是說，鋼筋與其周圍的混凝土具有相同的應變）；

(2）在強度計算中，混凝土破壞時的最大允許應變值為0.003；

(3）混凝土抗拉強度很小，在計算中可以忽略不計。

可根據(jù)鋼筋的形式和布置、截面上荷載的位置，以及柱的高度與橫截面尺寸的相互關系對柱進行分類。

按照鋼筋的形式和布置，柱可以分為兩種類型，如圖 32.1所示：

(1）配有縱筋和普通箍筋的矩形或正方形柱（圖 32.1a），

(2）配有縱筋和螺旋箍筋或者普通箍筋的圓形柱（圖 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.

雖然普通箍筋柱因為其造價便宜而得到廣泛應用，但是，在諸如地震區(qū)這類需要柱具有較大延性的地方，也會采用圓形的螺旋箍筋柱。當發(fā)生過大變形時，螺旋箍筋柱具有承受最大荷載的能力，能夠防止整個結構在力矩和應力重新分布之前發(fā)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.

按照作用在橫截面上荷載的位置，柱可分為軸心受壓或偏心受壓。軸心受壓柱不承受彎矩。偏心受壓柱除了承受軸向力之外還要承受彎矩。然而，在實踐中，所有的柱都應按某些預想不到的或者偶然的偏心進行設計，這是因為在支模板的過程中進行豎向找正時總會產(chǎn)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.

除了承受軸向力外，大多數(shù)柱還要承受彎矩。因此，要保證柱具有一定的延性，所以柱的最小配筋率應該為1%。合理的配筋率應為1.5%～3.0%。在高層建筑中，柱所承受荷載通常非常大，配筋率也可達到4%。普通箍筋柱中應至少采用4根縱筋。對于螺旋箍筋柱，為了保證螺旋筋起到箍的作用，應至少采用6根縱筋。

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).

柱的破壞可能會是由于材料破壞而引起，即由受拉面的鋼筋屈服，或者由受壓面的混凝土首先被壓碎；或者由于側向失穩(wěn)而破壞（即由壓屈引起的破壞）。