Basic Rules for Design of Column | Thumb Rules for Column Design:
In this Article today we will talk about the Basic Rules for Design of Column | Thumb Rules for Column Design | Column Design Calculations | RCC Column Design Formulas | Design Steps of RCC Column | Design of RCC Column as per IS 456
What is Column?
The column is a structural member which bear the load of slab, beam and stairs and transfer the total load to the footing and finally to the soil. So, column of any structure must be stronger than another member while designing. Here I have discussed the top 10 quick guides for reinforced concrete column design which will help you to avoid mistakes while designing columns.
The design of the column depends on materials used in construction, types, and intensity of load acting on it, whether it is precast or cast-in-situ, the durability of structure you want to construct, environmental factors, etc.
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.
Basic Rules for Design of Column:
A column may be classified based on different criteria such as:
1. Based on shape
- Polygon Column Design Calculations
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 RCC Column Design Formulas
Emin > l/500 + D/30 >20 Where, l = unsupported length of column in ‘mm’ D = lateral dimensions of column
5. Types of Reinforcements for columns and their requirements
- The cross-sectional area of longitudinal steel in a column shall not be less than 0.8 and not more than 6% of the gross-sectional area of the column.
In places where bars from a column below have to be lapped with those in the column to be designed, the maximum percentage of steel should not exceed 4%.
- The diameter of longitudinal bars should not be less than 12 mm and should not be more than 50 mm.
- Round columns and columns having helical binders should have at least bars.
- The minimum cover of concrete to the outside of longitudinal bars shall be 4 cm or the diameter of the bar whichever is greater. In case where the maximum dimension of a column does not exceed 20 cm and the diameter of the longitudinal bars does not exceed 12 mm, the cover of 2.5 cm may be used.
- Where it is necessary to splice the longitudinal bars, the bars shall overlap for a distance of not less than 24 times the diameter of the smallest bar.
- The spacing of bars measured along the periphery of the column shall not exceed 300 mm. Design Steps of RCC Column
Transverse steel may be provided either in the form of lateral ties or helical bars (spiral).
- The minimum diameter of the ties or helical reinforcement shall not less than 1/4th the diameter of the largest longitudinal bars and in no case less than 5 mm.
- The maximum diameter of the ties or helical steel should preferably be not more than 12 mm and 25 mm respectively.
- The pitch of the ties should not be more than the least of the following
- a) Least lateral dimension of the column.
- b) 16 times the diameter of the smallest longitudinal bar nearest to the compression face of the member.
- c) 48 times the diameter of the tie.
- Pitch of the helical reinforcement should not be more than least of the following:
- a) 1/6th the diameter of the concrete core.
- b) 75mm.
- The least spacing of the lateral ties may be 150 mm and for spirals, the minimum pitch shall be 25 mm or three times the diameter of the helical bars whichever is greater. Design of RCC Column as per IS 456
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