A column is a very important component in a structure. It is like the legs on which a structure stands. It is designed to resist axial and lateral forces and transfer them safely to the footings in the ground.
Columns support floors in a structure. Slabs and beams transfer the stresses to the columns. So, it is important to design strong columns.
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.
The axial load carrying capacity of a column is deduced from the formula
Please see the link for formulas to calculate axial loads in columns. I would recommend using advanced structural design software like ETabs or Staad Pro for design of structures. Column design does not depend only on axial loads, but also on many other factors. There are bending moments and tortional forces induced due to beam spans, wind loads, seismic loads, point loads and many other factors.
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
2. Based on slenderness ratio
The ratio of the effective length of a column to the least radius of gyration of its cross section is called the slenderness ratio.
- Short RCC column, =< 10
- Long RCC column, > 10
- Short Steel column, =<50
- Intermediate Steel column >50 & <200
- Long Steel column >200
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 RCC columns
- Spiral RCC columns
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
- 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.
- 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)
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);
- 1/6th of the core diameter of the column
Pitch should not be less than,
- 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)
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