DRCS - Quick Reference (Unit 6)

General Design Approach:  Top to Bottom

• Design of Slab (Unit 4)
• Design of Roof Beam (Unit 2 & 3)
• Design of Columns (Unit 6)
• Design of Column Footings (Unit 5)

Columns:

Compression members are called as 'Strut' or 'Pillar' or 'Columns'

Types of Columns: (based on slenderness ration)

• Short Columns - Slenderness Ratio <= 12
• Long Columns  - Slenderness Ratio > 12 

Slenderness Ratio = eff. length of Column / least lateral dimension

Columns are subjected to 

1. Axial Load
2. Axial Load + Uni-axial Bending 
3. Axial Load + Bi-axial Bending 

Load carrying capacity of Column: Pu = 0.4 fck Ac + 0.67 fy Asc 

(for columns subjected to minimum eccentricity of 0.05 times of least lateral dimension of column)

Ac = Cross Section Area of Concrete

Asc = Area of Compression Steel Reinforcement

Design Guidelines:

• Minimum Steel Reinforcement is 0.8%
• Maximum Steel Reinforcement is 6%
• No. of bars: 1) 4 for Square Column 2) 6 for Circular Columns
• Minimum dia. of steel bar is 12mm
• Minimum Clear cover is 40mm
• Spacing of longitudinal bars around the circumference of the column shall be not less than 300mm

Type of Problems:

1. Design Column of given size subjected to Axial Load

Procedure:

1. Given Data
2. Assume % of Steel Reinforcement (Minimum is 0.8)
3. Find Steel Required by equating, Pu = 0.4 fck Ac + 0.67 fy Asc 
4. Assume dia. of longitudinal steel bar
5. Provide No. of bars (minimum 4 for Square Column)
6. Find Dia. of Later Tie: greater of 1) 1/4 of dia. of main bar 2) 6mm
7. Find Pitch or Spacing of Lateral Tie: least of 1) Least Lateral Dimension of Column 2) 16 times of smallest longitudinal bar 3) 300mm
8. Draw Longitudinal and Cross Section of Column with Steel Reinforcement Details

2. Design Column of given size subjected to Axial Load + Uni-axial Bending

Procedure:

1. Given Data
2. Find d'/D
3. Find Pu / fck b D
4. Find Mu / fck b D²
5. Referring Charts 27 to 62 based on Grade of Steel, Shape of Column, d'/D, Reinforcement Placement (Opposite Faces or Equally All faces): Find p/fck value 
6. Find % of Steel, p = fck x value obtained for chart
7. Check for Minimum and Maximum Reinforcement
8. Provide No. of bars
9. Find Dia. of Later Tie: greater of 1) 1/4 of dia. of main bar 2) 6mm
10. Find Pitch or Spacing of Lateral Tie: least of 1) Least Lateral Dimension of Column 2) 16 times of smallest longitudinal bar 3) 300mm
11. Draw Longitudinal and Cross Section of Column with Steel Reinforcement Details

3. Design Column of given size subjected to Axial Load + Bi-axial Bending

Procedure:

1. Given Data
2. Find d'/D\
3. 1st Trail: Assume % of Steel
4. Find Load Carrying Capacity, Puz
5. Find Pu/Puz where Pu = Load coming on the column
6. Find α_{n (based Pu/Puz values)}
7. Find Pu / fck b D
8. Find p/fck = % of steel / fck
9. Find Mux1 / fck b D² from chart
10. Similary Find Muy1 / fck b D² from chart
11. Apply Check: (M_ux/M_ux1) ^αn   + (M_uy/M_uy1) ^αn   <= 1.0
12. If value is too less than 1, Assume less % of steel than before and do steps 3 to 11 again
13. Once Check is OK, find Longitudinal Steel Reinforcement
14. Provide No. of bars
15. Find Dia. of Later Tie: greater of 1) 1/4 of dia. of main bar 2) 6mm
16. Find Pitch or Spacing of Lateral Tie: least of 1) Least Lateral Dimension of Column 2) 16 times of smallest longitudinal bar 3) 300mm
17. Draw Longitudinal and Cross Section of Column with Steel Reinforcement Details

Note: (from IS Code book)

_{for Pu/Puz <= 0.2,} α_{n is 1.0}

_{for Pu/Puz >= 0.8,} α_{n is 2.0}