Skip to main content

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


Comments

Popular posts from this blog

Correction due to curvature and refraction AND combined correction in surveying

In geodetic surveying, error due to curvature and refraction is taken into action when the area is greater than 256 km²   Correction due to curvature (Cc)             Error due to curvature is taken into action because during leveling with theodolite    or Autolevel the horizontal line and level line do not coincide. Level line is curved line parallel to the earth surface and horizontal line is straight line.               this correction is given by Cc =0.07849 d² OR 0.0785d²                                                                 here, d is the linear distance (in km)   Correction due to refraction (Cr)             Error due to refraction is taken into action due to the changing of medium of light either from a denser medium to lightier medium or viceversa.                    it is 1/7 of correction due to curvature                                               1/7x(0.785 d²)                                                   it is equals to 0.112d²          

Prestressed concrete

The basic concept of prestresssing the concrete consists in introducing the artificially the compressive stresses in a structure before it is loaded. The tensile stresses in the prestressed concrete structure may be reduced to a great extent or even entirely eliniminated depending upon the magnitude of prestressing. In a prestressed concrete structure, the cost of supporting structure and foundation is reduced, dead load of structure is reduced and cracking of concrete is avoided. The high strength concrete and high tensile steel should be used in a prestressed concrete member. According to Indian standards, the cube strength of the concrete used should bot be less  than 35N/mm 2 . The ultimate strength of high tensile steel wires used in prestressing varies from 1500N/mm 2 for 8mm diameter bars to 2350N/mm 2 for 1.5 mm diameter bars. The various methods adopted in prestresssing are as follows: Pre-tensioning The method of providing desired amount of compressive stress in

Estimation of maximum flood discharge

The following empirical formulae may be adopted to compute the maximum flood discharge: Dicken's formula Ryve's formula Inglis formula Nawab Jang Bahadur formula Fanning's formula Fuller's formula  1) Dicken's formula According to Dicken's formula, the flood discharge (Q) in cumecs is given by The value of C taken as 11.4 for Northern India, 13.9 to 19.5 to Central India and 22.5 to 25 for Western Ghats 2) Ryve's formula According to Ryve's formula, for Madras catchments, the flood discharge (Q) in cumecs is given by The value of C is taken as 6.75 for areas within 24 km from the coast, 8.45 for areas within 24 km to 161 km form the coast and 10.1 for limited areas near hills 3) Inglis formula According to Inglis formula, the flood discharge (Q) in cumecs is given by This formula is used for estimating flood discharge for the catchments of former Bombay presidency. 4) Nawab Jang Bahadur formula According to Nawab Ja