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RC Columns

A Column may  be defined as element used primarily to support axial compression loads and with a height of at least three times its least lateral dimension.
When the ratio of effective length of column to its least lateral dimension doesn't exceed 12, then it is termed as short column.

When the ratio of effective length of column to its least lateral dimension exceeds 12, then the column is considered as long column or slender column.
Pedestal: It is a short column of effective length not greater than 3 times of least lateral dimension.

  • In case of pedestal in which the longitudinal reinforcement is not taken in account in strength calculations, nominal longitudinal reinforcement not less than 0.15 percent of the cross-sectional area shall be provided.

Points to Remember

  • The columns are reinforced with longitudinal, transverse or helical reinforcement. The longitudinal reinforcement is also termed as main steel. The transverse reinforcement is in the form of links around the main steel and is termed as lateral ties, links or polygonal links.

Pitch of lateral ties or lateral reinforcement

The Pitch of transverse reinforcement shall not be more than the minimum value of the following:
  • least lateral dimension of column
  • Sixteen times the diameter of longitudinal reinforcing bar nearest to the compression face of the member
  • 300mm

Diameter

The diameter of polygonal links or lateral ties should not be less than one-fourth of the diameter of the largest longitudinal bar. In no case, it should be less than 6mm.

Helical reinforcement

In a column, with helical reinforcement, the permissible load is based on the core area. The least lateral dimension of such a column should be taken as equal to the diameter of core.  The pitch shall not be less than 75mm and not more than one-sixth of the core diameter of the column or less than 25mm or less than three times the diameter of steel bar forming helix.

Specifications of columns As per IS 456:2000

1. Minimum percentage of steel = 0.8%
2. Maximum percentage of steel
  • 4% if bars are lapped
  • 6% if bars are not lapped
3. Minimum diameter of longitudinal bars = 12mm
4. Minimum number of bars 
  • For circular = 6
  • For rectangular = 4
5. Maximum spacing of longitudinal bars = 300mm
6. Load carrying capacity of column is increased by 5% when helical reinforcement is provided as a transverse reinforcement.
7. Minimum compressive strain in concrete in axial compression is taken to be 0.002

Design of columns

1. Short Column 

Load carrying capacity: load carrying capacity of steel, P(steel)+ load carrying capacity of concrete   P(concrete)

2. Long Column


3. Circular Column

  • with separate ring used as a stirrup
  • with helical reinforcement

Minimum eccentricity


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