According to IS 456: 2000, the minimum and maximum percentage of longitudinal reinforcement in a column (expressed as a percentage of the gross cross-sectional area of the column) are respectively.
IS: 456-2000 specified the requirements of longitudinal (main) steel bars to be used for columns and compression members regarding the percentage of reinforcement, number of steel bars, the diameter of the bars, the spacing of the bars and the concrete cover for the steel bars as under :
Percentage of the longitudinal reinforcement
(A) The minimum percentage of the cross-sectional area of the longitudinal reinforcement shall be not less than 0.8 percent of the gross cross-sectional area of the column.
(B) In any column that has a large cross-sectional area than that required to support the load, the minimum percentage of steel shall be based upon the area of concrete required to resist the direct stress and not upon the actual area.
(C)) The maximum percentage of the cross-sectional area of the longitudinal reinforcement shall be not more than 6 percent of the gross cross-sectional area of the column.
It is to note that the use of 6 percent reinforcement may involve practical difficulties in placing and compacting the concrete; hence lower percentage is recommended. Where the steel bars from the column below have to be lapped with those in the column under consideration. The percentage of steel shall usually not exceed 4 percent.