Reinforced concrete column is a compression member and transfers the loads from structure to the ground through foundations. There are three types of concrete columns based on its height and lateral dimension. Long columns are those whose ratio of height to least lateral dimension is more than 12. When the height to least lateral dimension is less than 3, it is called a pedestal and if it is between 3 and 12, it is called as a short column.
The load carrying capacity and modes of failure of a reinforced concrete column is based on the slenderness ratio. Slenderness ratio is the ratio of the effective length Le and least lateral dimension of the column as per Indian and British Standards. But as per American Concrete Institute Code of Practice, the slenderness ratio is defined as the ratio of effective length of column to its radius of gyration, which is same as used for structural steel design as per IS Code. Effective length of a column depends on its support conditions at ends.
Based on the slenderness ratio of the column, there are three modes of failure of reinforced concrete columns. The columns are assumed to be centrally loaded (no eccentric loads).
Mode – 1: Column Failure due to Pure Compression:
When reinforced concrete columns are axially loaded, the reinforcement steel and concrete experiences stresses. When the loads are high compared to cross-sectional area of the column, the steel and concrete reach the yield stress and column fails without undergoing any lateral deformation. The concrete column is crushed and collapse of the column is due to the material failure. To overcome this, the concrete column should have sufficient cross-sectional area, so that the stress is under the specified limit. This type of failure is generally seen in case of pedestals whose height to least lateral dimension is less than 3 and does not experience bending due to axial loads.
Mode – 2: Column Failure due to Combined Compression and Failure:
Short columns are commonly subjected to axial loads, lateral loads and moments. Short columns under the action of lateral loads and moments undergo lateral deflection and bending. Long columns undergo lateral deflection and bending even when they are only axially loaded. Under such circumstances when the stresses in steel and concrete reach their yield stress, material failure happens and RCC column fails. This type of failure is called combined compression and bending failure.
Mode – 3: Column Failure due to Elastic Instability:
Long columns are very slender, i.e. its effective length to least lateral dimension is more than 12. Under such condition, the load carrying capacity of reinforced concrete columns reduces drastically for given cross-sectional area and percentage of reinforcement steel. When such type of concrete columns are subjected to even small loads, they tend to become unstable and buckle to any side. So, the reinforcement steel and concrete in such cases reach their yield stress even for small loads and fail due to lateral elastic buckling. This type of failure is unacceptable in practical concrete constructions. Code prevents usage of such long columns for slenderness ratio greater than 30 (for unbraced columns) for the use in concrete structures.
Fig: Failure Modes of Concrete Columns For Different Slenderness Ratio