Types of R.C.C. Column Failure

            Columns are the most important parts of a structure. They transfer loads of the structure to the surrounding soil through the foundations. So we need to build strong columns, otherwise, failure will occur.

            Column is a long cylindrical member subjected to axial compression. Column carries self-weight and load coming on it. You know that columns are used to transfer the load vertically to the horizontal beam. Column is categorized based on its height. Every material used in the building experience both compression and tension stress that’s how we designed the modern buildings.

            Columns consist of two major material one is concrete and second one is steel. Before designing the columns, civil engineers should calculate total stress due to live and dead load of the building. When the applied stress exceeds the permissible stress (calculated) the structure will fail.

             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.

In this article, different types of column failures are discussed.

Types of Column Failure :-

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).

1) Compression Failure

2) Buckling Failure

3) Shear Failure

1. COMPRESSION FAILURE:-

             This type of failure is also called as “Column Failure due to Pure Compression”. When columns are axially loaded, the concrete and steel will experience some stresses. When the loads are greater in amount compared to the cross-sectional area of the column, the concrete and steel will reach the yield stress and failure will be starting without any later deformation.

            The concrete column is crushed and collapse of the column is due to the material failure. In this type of failure, the material fails itself, not the whole column. This type of failure mostly occurs in shorter and wider columns. 

             To avoid this, the column should be made with sufficient cross-sectional area compared to the allowable stress. 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.

2. BUCKLING FAILURE:-

             This type of failure is also called as “Column Failure due to Elastic Instability”. Buckling failure generally occurs in long columns. Because they are very slender and their least lateral dimension is greater than 12. In such condition, the load carrying capacity of the column decreases very much for given c/s area and % of 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. Building Code prevents usage of such long columns for slenderness ratio greater than 30 (for unbraced columns) for the use in concrete structures.

3- SHEAR FAILURE:-

            This type of failure is also called “Column Failure due to Combined Compression and Failure”. In engineering, shear strength is the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force.

            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.