Q. Purpose of setting minimum amount of longitudinal steel areas for columns?
In some design codes it specifies that area of longitudinal steel reinforcement must be not less than a specific percentage of sectional area of column. Initially the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension can be induced in columns during design life of concrete structures. For example tension is induced in columns in case there is uneven settlement of building foundation or upper floors above the column are completely unloaded whereas the floors below column are strictly loaded. Secondly because of the effect of creep and shrinkage, there would be a redistribution of loads between steel and concrete reinforcement. So steel reinforcement can yield easily in case a lower limit of steel area isn't established.
Additionally, test results indicated that columns with too low a steel ratio will render the equation below inapplicable that is used for design of columns:
N=0.67fcuAc+fyAs