A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete.
RCC Column
A column forms a very important component of a structure. Columns support beams which in turn support walls and slabs. It should be realized that the failure of a column results in the collapse of the structure. The design of a column should therefore receive importance.
Supporting the slabs is the main function of the columns… Such slabs are called Simply Supported Slabs. Simply supported slabs could be either one way slab or a two-way slab. It depends on the dimensions of the slab.
In this article, we are going to discuss in detail the basis of classification of columns and different types of reinforcement required for a certain type of column.
A column may be classified based on different criteria such as:
1. Based on shape
- Rectangle
- Square
- Circular
- Polygon
2. Based on slenderness ratio
- Short column, ? ? 12
- Long column, ? > 12
3. Based on type of loading
- Axially loaded column
- A column subjected to axial load and unaxial bending
- A column subjected to axial load and biaxial bending
4. Based on pattern of lateral reinforcement
- Tied columns
- Spiral columns
Minimum eccentricity
Emin > l/500 + D/30 >20
Where, l = unsupported length of column in ‘mm’
D = lateral dimensions of column
Types of Reinforcements for columns and their requirements
Longitudinal Reinforcement
- Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.
- Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross cross-section area of the column.
- The bars should not be less than 12mm in diameter.
- Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.
- Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.
Transverse reinforcement
- It maybe in the form of lateral ties or spirals.
- The diameter of the lateral ties should not be less than 1/4th of the diameter of the largest longitudinal bar and in no case less than 6mm.
The pitch of lateral ties should not exceed
- Least lateral dimension
- 16 x diameter of longitudinal bars (small)
- 300mm
Helical Reinforcement
The diameter of helical bars should not be less than 1/4th the diameter of largest longitudinal and not less than 6mm.
The pitch should not exceed (if helical reinforcement is allowed);
- 75mm
- 1/6th of the core diameter of the column
Pitch should not be less than,
- 25mm
- 3 x diameter of helical bar
Pitch should not exceed (if helical reinforcement is not allowed)
Least lateral dimension
- 16 x diameter of longitudinal bar (smaller)
- 300mm
Guidelines to be followed for making a column layout
Today, we will discuss something very general. Inspite of knowing these general thumb rules, Civil Engineers still end up making disastrous mistakes which would not only cost them but also cost the people living in the building designed by these engineers.
Earlier, I wrote an article describing one of my projects where structural designing was executed on site (which was extremely pathetic) even before Architectural design was done.
In this article, we will go through the essential thumb rules to be followed for giving a column layout. Ofcourse RCC columns have to be designed in accordance to the total load on the columns but apart from that it is essential for every Civil engineer and Architect to remember a few thumb rules so that they are prevented from making mistakes.
Three thumb rules to be followed are as follows:
- Size of the Columns
- Distance between Columns
- Alignment of columns
Thumb rule no.1
Size of the columns
The size of the columns depends on the total load on the columns.
Minimum size of the column should not be less than 9”x9”.
9”x9” columns are to be used for a single storey structure with M15 grade of concrete.
In case, 9”x9” column size is to be used for 1 and half storey structure, then it is advised to use M20 grade concrete.
A safe and structurally sound column size for a 1 and half storey structure should not be less than 12”x9” using M15 grade concrete. This should be in your most preferred and practical options list.
Thumb rule no.2
Distance between the columns
Try to maintain equal distance between the centres of two columns. Always plan a column layout on a grid.
The distance between two columns of size 9”x9” should not be more than 4m centre to centre of column.
If larger barrier free distances are required then going for larger column size is to be used.
The size of the columns increase because of two factors:
- Increase in the distance between two columns (This increases the dimensions of the columns as well the depth of the beam.)
- Height of the building (Increase in the number of floors is directly proportional to the dimensions of the columns.
Thumb rule no.3
Alignment of Columns
A rectangular grid is to be made for placing the columns. This helps in avoiding mistakes and placing in columns can be done in the right way.
The columns can preferably be arranged in two different fashions:
- In a straight line with the help of a grid
- In a circular fashion for circular buildings.
Zigzag arrangement of columns is an absolutely wrong way of working out Structural design. It should be remembered that when columns are erected, beams are laid connecting the columns.
The Zigzag column placement causes three major issues:
- Unbalanced load transfer
- Problems in wall construction
- Problems in laying beams
If these three thumb rules are followed by Civil Engineering and Architecture students, implementation of wrong Structural design can be prevented.
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