Why the minimum reinforcement ratio in the walls is 0.12% or 0.15% and in special cases 0.25% according to ACI318, while, in column, this ratio is high 1%.
Even their behaviors are similar in most cases.
The design method for column specified in the code incorporate separate terms for the load carried by concrete and by longitudinal reinforcement. Reinforcement is necessary to provide resistance to bending which may exist for any of the reasons mentioned above. Normally, the minimum reinforcement percentage of 1.0 percent takes care of the bending moment in column due to the specified minimum eccentricity. It is known that the effect of creep and shrinkage of concrete under sustained loading tends to transfer load to the reinforcement, with a consequent increase in stress in reinforcement. Unless lower limit is placed on this minimum reinforcement ratio, the stress in reinforcement may increase to yield level under sustained load (refer SEFI) .
Similarly columns have different faces experiencing bending compression and tension depending upon the nature of eccentricity or accidental loads and one need to provide reinforcement accordingly on the four faces for example in square columns.
These may be the reason for higher percentage of minimum reinforcement is Columns.
The value of the minimum ratio in columns is 1% to control the deformations by the plastic flow of concrete (creep), due to the action of long-term compression stresses. The axial load ratio, P / Agf´c, in columns is generally greater than 0.1, while in walls it is less than 0.15 which allows the walls to have a lower reinforcement ratio.
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