Reinforced concrete column

A reinforced concrete column is a structural member designed to carry compressive loads, composed of concrete with an embedded steel frame to provide reinforcement. For design purposes, the columns are separated into two categories: short columns and slender columns.

The strength of short columns is controlled by the strength of the material and the geometry of the cross section. Reinforcing rebar is placed axially in the column to provide additional axial stiffness. Accounting for the additional stiffness of the steel, the nominal loading capacity P_n for the column in terms of the maximum compressive stress of the concrete f_c', the yield stress of the steel f_y, the gross cross section area of the column A_g, and the total cross section area of the steel rebar A_st

where the first term represents the load carried by the concrete and the second term represents the load carried by the steel. Because the yield strength of steel is an order of magnitude larger than that of concrete, a small addition of steel will greatly increase the strength of the column.

To give a conservative estimate and build redundancies into the final structural system, the ACI Building Code Requirements give a maximum reduced design load of ${\displaystyle \mathrm {{\phi }P_{\mathrm {n} }} \,\!}$ where ${\displaystyle \mathrm {\phi } \,\!}$ is the strength reduction factor for the type of column used. For spiral columns

where ${\displaystyle \mathrm {\phi } =0.75\,\!}$. For tied columns

where ${\displaystyle \mathrm {\phi } =0.65\,\!}$. The additional reduction past the strength reduction factor is to account for any eccentricities in the loading of column. Distributing a load toward one end of the column will produce a moment in the column and prevent the entire cross section from carrying the load, thus producing high stress concentrations towards that end of the column.

Spiral columns are cylindrical columns with a continuous helical bar wrapping around the column. The spiral acts to provide support in the transverse direction and prevent the column from barreling. The amount of reinforcement is required to provide additional load-carrying capacity greater than or equal to that attributed from the shell as to compensate for the strength lost when the shell spalls off. With further thickening of the spiral rebar, the axially loaded concrete becomes the weakest link in the system and the strength contribution from the additional rebar does not take effect until the column has failed axially. At that point, the additional strength from spiral reinforcement engages and prevents catastrophic failure, instead giving rise to a much slower ductile failure.

The ACI Building Code Requirements put the following restrictions on amount of spiral reinforcement.