There are various types of RCC Column based on its shape, length and forces. Function and construction methods are discussed here for these types of column.
Column is a vertical member which takes complete load of the beam, slabs and the entire structure and the floor and other area of the building is adjusted as per the requirement of the client or owner.
The size of the columns, quantity of cement sand and aggregate to be mixed, the number of steel bars to be placed, spacing between the stirrups is all mentioned in the structural drawing which is designed by structural designer as per the actual load on the column and considering the factor of safety.
A column is a vertical member which effectively takes load by compression. Basically column is a compression member as load acts along its longitudinal axis. Bending moment may occur due to wind earthquake or accidental loads.
Column transfers the load of the structure of slabs beams above to below, and finally load is transferred to the soil. Position of the columns should be so that there are no tensile stresses developed at the cross section of the columns. Columns location should be such that it hides in the walls partially or fully.
Types of RCC Column and its Functions and Construction Methods
Types of RCC Column
Types of RCC Column based on Shapes
- Circular for exposed outside for good architecture view
- Square or rectangular traditional for any structure
Types of RCC Column based on length
- Short column – if L/B<=12
- Long column – if L/B > 12
Where L is the height of the column, B is width
Generally, floor height is approximately 3 m or 10 feet, L/B ratio will be less than 12, so in maximum cases short column is placed. In case where height of floor is more than 3 m or 10 feet, we need to check L/B ratio so result may be long or short column. Generally, on long column there are more forces generated so should be designed carefully.
Types of RCC Column based on moments
- Biaxial column: Designed for axial load and moment in 2 directions
- Uniaxial columns: Designed for axial load and moment in 1 direction
Generally, in a building corner most columns are biaxial columns and side column is uniaxial column and internal columns can be any of these.
Construction Methods of RCC Columns
Construction of RCC columns involve following four steps:
- Column layouts
- Column reinforcement work
- Column formwork
- Pouring of concrete
Layout of Columns
Column layouts are done by laying rope in the grid lines and mark the location of columns
Column reinforcement works needs following checklist on site:
- Check the numbers and diameter of vertical bars
- Spacing between vertical bars
- Check Development length which depends on diameter of bar
- Lapping in alternate bars should come at same height.
- Lapping should not come inside beam or slab.
- Lapping should be at l/3 or 2l/3 of column as per structural notes
- Spacing between stirrups is as per the drawing
- Hook should be bend properly at right angles.
- Check the stirrups corner. Vertical bar should come at right angle edge of stirrup properly bound with binding wire.
Column Reinforcement and Layout
Columns carry the load of structure and hence they are critical. It is important to align them for verticality so that load is transmitted properly. The column shuttering should be strong enough to take the pressure of fresh concrete and remain in position during concreting.
Some Guidelines on column shuttering
- To hold the concrete shuttering firmly in place and for proper alignment a concrete pad called starter is cast before fixing the shuttering. The thickness is about 45 mm to 60 mm and dimensions are precisely the same as the dimensions of proposed column. The starter should be cured for a day or 2 so that it is hard enough to fix the shuttering around it.
- Column box or shuttering for columns is made of plywood sheets or steel sheets fabricated with adequate stiffeners.
- A thin films of oil or grease should be applied to inner surface of the shuttering to enable easy removal of the column after the concrete hardens.
- Shuttering should be properly aligned to its verticality and diagonals to be checked to ensure accuracy in dimensions.
- Formwork has to be thoroughly supported with props size before pouring the concrete so that it does not moves horizontally or vertically during concreting.
- The gaps near the shuttered joints should be sealed with plaster or a piece of wood to prevent any leakage of slurry.
- Appropriate space is to be provided in the inner face of the shutter and reinforcement by fixing cover blocks of about 40mm.
- It is preferable to remove shutters after 24 hrs. of casting and if they need to be removed earlier, it should not be removed within sixteen hours.
- The removal of shutters has to be done gently without jerks so that edges of columns are not damaged.
- Care should be taken regarding fixing and supports of column shuttering to prevent it from movement during concreting.
- Diagonals of the shuttering to be checked to ensure dimensional accuracy.
Concreting of RCC Column
Concreting of an RCC column can be done
- With the help of machine or pump
Manual Concreting of RCC Column
Concreting of RCC Column using Pump
Following points to be followed during and after concreting:
- For less quantity machine mix concrete is done and for larger quantity ready mix concrete (RMC) is ordered.
- Approval of placing concrete whether by pump or manually has to be taken from client.
- Concrete should be poured up to slab bottom; the remaining column gets concreted during pouring of slab & beam.
- Mechanical vibrator to be compulsorily used in the column but excess vibration can cause segregation. Each layer should be thoroughly compacted.
- Target slump to be 160 mm.
- Height of pouring concrete should not exceed 1.5 m.
- Construction joint should be avoided in the column.
- Proper cover as per structural drawings to be maintained.
- Temperature should be below 30 degree while pouring concrete.
- After the pouring of concrete and vibrating it with the help of a vibrator the horizontality and verticality of column to be checked