Important concepts related to the design of columns
Columns play an important role in a building structure. Their main function is to carry the loads from the superstructure and finally transmit the load to the foundation. In this article I’m going to discuss some important concepts related to the design of RCC columns in which I will discuss the definition of a column as per the code, what are long columns and short columns, the Detailing of reinforcement for columns, and finally I will discuss the terms Nominal Cover, Clear Cover, and Effective cover.
Definition of column:
As per IS 456 2000, Column is a compression member, the effective length of which exceeds three times the least lateral dimension. By the term “Least lateral dimension” we mean the smaller side of the column cross-section.
For illustration, if we have a column of size 230mm x 450mm its least lateral dimension would be 230mm. Similarly for a column of size 200mm x 300mm, the least lateral dimension would be 200mm. As per the definition of a column, the minimum effective length of a 230mm x 450mm column should be 3 x 230 i.e. 690mm and for the column of size 200 x 300 mm, the effective length shall be at least 3 x 200 i.e., 600mm. However, if the effective length is less than the least lateral dimension, such a member is called a “Pedestal”
What are long columns and short columns?
To define such columns, we must know the concept of slenderness ratio. Slenderness ratio is the geometrical parameter defined for a compression member (column), to determine whether it’s a long column or a short column. It is the ratio of the effective length of a column to its least lateral dimension (Lateral dimension perpendicular to the axis of bending).
If the slenderness ratio of a column is greater than 12, such a column is termed as ‘Long Column’, and if this ratio is less than 12, then it would be called a ‘’Short Column’’.
Detailing of reinforcement for a column:
Columns are provided with 2 types of reinforcement bars i.e., Longitudinal reinforcement and Transverse reinforcement. Longitudinal reinforcement is the vertical main bars and the transverse reinforcement is nothing but the lateral ties which are also known by the term stirrups when it comes to beams.
As per IS 456 2000, the area of cross-section of longitudinal reinforcement shall not be less than 0.8% of the gross cross-sectional area of the column. Also, the maximum area of longitudinal reinforcement should not exceed 6% of the gross cross-sectional area of the column. However, from the practical point of view, the maximum percentage of steel shall be limited to 3% to avoid the congestion of bars while concreting and especially when there is a need to overlap the bars.
For illustration, if we have a column of size 230mm x 300mm provided with 6 No 12 mm steel bars, the area of steel for this column will be equal to 6 times the area of the cross-section of each bar. or simply 6(π/4)D2
Where D is the diameter of the longitudinal bars. The Gross cross-sectional area is the total area of this section including the area of steel.
Area of steel = 6 x 3.14/4 x 122 = 678mm2
Gross cross-sectional Area = 230 x 300 = 69000mm2
Let’s check if the calculated area of steel satisfies the criteria.
Percentage of steel = Area of steel/Gross Cross-Sectional Area x 100 = 678/69000 x 100 = 0.98% > 0.8% (OK)
It should be noted that the minimum diameter of the bars inside a column shall be 12mm. Also, a minimum of 4 No of bars shall be provided in rectangular cross-sections and 6 No of bars to be provided in circular columns.
IS 456 2000 also suggests that the spacing of longitudinal bars shall be limited to 300 mm.
Talking about the lateral ties. Is 456 2000 suggest that the diameter of the lateral ties shall not be less than 1/4th of the diameter of the largest longitudinal bar and in no case less than 6mm whichever is greater?
For illustration, if we have a column provided with 8 No of longitudinal bars out of which 4 No of 16mm bars are provided at the corners and 4 No of 12mm bars at the intermediate positions. The diameter of the largest longitudinal bar would be 16mm.
Diameter of ties = ¼ x 16 = 4mm < 6mm
Therefore, we can provide 6mm ties in such a column.
However, it’s practically recommended to provide a minimum of 8mm ties.
Now let’s discuss what should be the maximum and minimum spacing of lateral ties
Spacing/Pitch of lateral ties
The spacing of the lateral ties shall not be more than the least value from the following 3 criteria;
- Least lateral dimension of the member
- 16 times the diameter of the smallest longitudinal bar
For illustration, if we assume a column of size 230mm x 450mm with 4 No of 16mm bars and two No of 12mm bars. The diameter of the smallest longitudinal bar would be 12 mm.
- Least lateral dimension = 230mm
- 16 x d = 16 x 12 = 192mm
If we compare all the 3 criteria, 192mm is the least among all. Hence, we can provide a spacing of 192 mm.
Nominal Cover and Effective cover:
While doing the reinforcement detailing, we come across the terms, Nominal cover, Clear Cover, and effective cover. The steel in RCC members is provided with sufficient concrete cover to protect it from the surrounding atmosphere. To understand all the 3 terms let’s consider the cross-section of a column.
Clear Cover/Nominal Cover:
The clear cover is the distance between the exposed concrete surface to the surface of the nearest reinforcing bar. It should be noted that the concrete surface doesn’t include the finishing layer like plaster or any other finishes. However, as per IS 456 2000, the term Clear Cover is replaced by the term Nominal Cover.
For columns provided with a minimum diameter of 12mm bars. The minimum nominal cover provided is 40mm.
The term Effective Cover is usually used in design calculations. Effective cover means the distance between the exposed concrete surface to the centroid of the main reinforcement. To understand this, let’s consider the cross-section of a member. Let D be the diameter of the main bar and d be the diameter of the stirrup.
Centroid of main bar = D/2
Effective Cover = Nominal cover + Diameter of stirrup (d) + Centroid of main bar (D/2)