(9) Main major modes of Failure in Column

What is a failure in a column?

Failure in column occurring in the vertical plane of the column due to irregularities present in them are called damages. These damages may be due to one specific reason or a combination of many reasons. Damages in RCC columns can be understood by understanding its role and behaviour under many types of different loads. 

The main role of a column in RCC building

Columns are vertical compression members used to transfer the load of the structures, effectively to the foundation. Due to compressive loads, the compressive stresses are developed which control its design.

The transfer of loads to the foundation through columns can come directly from roofs/floor slabs or indirectly from slab to beams and then to columns. Vertical cylindrical/Square members are called columns when they have an effective length more than three times their least lateral dimension (width or diameter).


(a) Role of longitudinal steel in columns seismic

Columns are compression members designed to carry axial loads of the structure. Although the concrete itself is stronger in compression, then why to provide vertical reinforcement in columns? This question generally comes into mind. The reason is that to resist the heavy loads, a concrete column of larger size and shape are required but if we provide longitudinal steel as reinforcement, the size of the column can be reduced and optimum utilization of space can be done.

To resist the heavy loads coming over the columns, concrete is reinforced with longitudinal steel bars so that column can resist tensile stresses caused by some eccentricity of vertical loads. Longitudinal steel should be 0.8% to 6% of the cross-sectional area.

Minimum number of longitudinal bars for square or rectangular section = 4 and, Circular section = 6 Diameter of bars used as longitudinal reinforcement > 12 mm

(b) Role of lateral ties

Lateral ties are nothing but closed loops of steel bars of smaller diameter and placed horizontally at a regular interval (spacing) along the full length of the column.

These lateral ties are also known as “Transverse ties” or “Rings”. Longitudinal steel reinforcement is tied laterally by these ties at suitable spacing. These ties are helpful in avoiding buckling of bars under axial loads. The critical locations (e.g. Near the joint of a column win a beam) the centre to centre spacing of the ties is reduced so that it can be made safe again earthquake movements because joints are most affected during seismic motions. These same is also helpful in reducing transverse shear failures. confinement of concrete core for better strength and preventing longitudinal separation of concrete.

The main types of Damages in columns

Damages in columns can be due to any one of the following factors or combination of these nine types following given below.

1. Damage due to buckling
2. Damage due to shear failure
3. Damage due to floating column concept
4. Damage due to large spacing of lateral ties
5. Damage due to columns of different heights present in the one storey
6. Damage due to inadequate splicing
7. Damages due to inadequate ductility
8. Damages due to lack of “Strong columns weak beam" concept
9. Damages due to casting of top of the column with a beam

1. Damage due to buckling

Buckling is an undesirable phenomenon occurring in longe in which the columns bend out in the least lateral direction due to compressive loads and end conditions acting upon it.
Longer the column, more are the chances of buckling, and hence reduction in the load-carrying capacity. It is always desirable to have short columns which can be achieved by reducing the heights of long columns by bracing them with beams so that they act as a short column.

Note- Column is said to belong when the ratio of effective length (length participating in buckling) to the least lateral dimension is greater than 12.

Condition leff - b > 12 for long column

Due to buckling, axial-flexural (also called combined compression and bending) damages in columns can take place which can ultimately lead to failure of the column and indirectly the structure. This type of failure is generally ductile as vertical reinforcement gives some time before ultimate failure.



2. Damage due to shear failure

Columns which are required to resist earthquake forces must be designed to resist shear failure. Shear failure damages are generally brittle and can be avoided by providing the following given below the points.

• Closely spaced centre to centre lateral ties (rings) at critical junctions.
• By proper analysing the loads and selection of shape and size of the cross-section.
• By proper selection of the grade of steel and concrete to be used.
• By designing the appropriate amount of steel required and their distribution in a particular section. 

In such failures, the column bursts out and there is very less time to escape out of the building.



3. Damage due to floating column concept

This type of damage occurs due to lack of effective load transfer mechanism from the superstructure to the foundation, which is very much essential when seismic forces act upon a building. Complex dynamic behaviour in such type of columns occurs and ultimately leads to failure/damages.



4. Damage due to large spacing of lateral ties

Lateral ties help in carrying shear forces induced by earthquakes and hence help in resisting diagonal cracks. But if the spacing of lateral ties is large enough especially near the ends of columns or near the joint of beam & column, it can lead to diagonal cracks in the column. Further as per IS 13920-1993 it is suggested that the ends of the lateral ties must be bent as 135° hook.



5. Damage due to columns of different heights present in the one storey

The building having both short and long columns in one storey has shown that short column suffers more damages than long columns because for the same displacement induced by the earthquake, short column her than the failure of a long column which means that it will attract more seismic forces. More the stiffness in the column, more seismic forces are required to deform it. Therefore, if not properly designed for such large seismic forces, short column fails. This behaviour of short column is termed as short column effect. 

6. Damage due to inadequate splicing

Splicing means overlapping of vertical bars along the length of the column. Due to the limitations in the available length of bars, there is the number of points where column bars are spliced. This extension in length of the bar can be achieved by properly joining the bars for at least minimum specified length of overlapping called lap length.

This portion of splicing is generally welded or tied properly with binding wire so that it does not slip. Centre to centre spacing of lateral reinforcement is reduced near the portion of splicing to provide confinement. It is further suggested for earthquake resistant structure that splicing should be done at the middle height of the column and not near the ends. Length of splicing in longitudinal bars should be equal to 50 times the diameter of the bar.



7. Damages due to inadequate ductility

Ductility and strength are the main parameters for any structure to be earthquake resistant. Ductility in columns can be achieved by providing a sufficient quantity of steel reinforcement with adequate ductile detailing as per code provisions of IS 13920-1993 whereas adequate strength can be achieved by using rich grades of concrete designed mix.

Lack of ductility means the column will have brittle failure giving very less time for escape and lack of strength will definitely lead to failure of columns due to a large amount of concentration induced due to earthquakes.



8. Damages due to lack of “Strong columns weak beam" concept

For resistant structures, it has been proved that columns should be stronger as compared to

Columns are the vertical members which act as a medium for transferring superstructure to the foundation. Failure of a beam in any part of the building can be less but the failure of columns can lead to the complete collapse of the structure. 

9. Damages due to casting of top of the column with a beam

In framed columns are cast first, for a particular storey, and then the horizontal members are made to rest on columns. For the convenience in the casting of beams and proper joint, some portion (top) of the column is cast with beams monolithically.

This portion of column cast with beam act as a weak plane in columns and are liable to get damaged during seismic movements. This portion of the column is generally denoted as “Topi” (means cap) of the vertical member. This practice is not desirable for the column beam joints subjected to seismic forces.

Also, read - (6) Main major modes of Failure in Beams