DRCS - Quick Reference (Unit 5)

Column Footing:

In a building, the load path flows from 

• Slabs to Beams, 
• Beams to Columns
• Columns to Column Footings
• Column Footings to the Underlying Soil

Types of Foundations:

• Isolated Footing with Uniform depth
• Isolated Footing with Stepped Footing
• Isolated Footing with Sloped Footing 
• Combined Footings 1) Rectangular 2) Trapezoidal
• Strap Footing (Beam connected columns with slab)

Safe Bearing Capacity of Soil:

• Hard Rocks: (Granite) 2500 to 3500KN/Sqm
• Sand stone, Lime Stone: 1000 to 2500 KN/Sqm
• Soft Rock: 450 to 500 KN/Sqm
• Non-Cohesive Soils: 1) Gravelly Sand: 350 to 450 KN/Sqm 2) Find Sand: 150 to 250 KN/Sqm
• Cohesive Soils: 1)Stiff Clay soft shale: 250 to 350 KN/Sqm 2) Soft Clay: 100 KN/Sqm

General Guidelines:

• Minimum Edge Thickness is 150mm
• Clear Cover is 50mm
• Minimum Steel Reinforcement: same as Slab
• Critical Section for BM is at the face of the column
• Critical Section for Shear: 1) at a distance 'd' from face of column for One-way Shear 2) Punching Shear around the column at a distance of 'd/2' from the face of the column

Area of Footing = Total Load on Soil / Safe Bearing Capacity of Soil

Types of Problems:

1. Design of Square Footing for a given Axial Load

(Design for Working Load Only. Don't consider Factored Load)

Procedure:

1. Given Data
2. Find Total Load to be transferred to soil = Axial Load + Self. wt of footing at 10% of axial load. i.e., Total Axial Load, P = 1.10 x  P 

(if Factored Load is given, Axial Load, P = Pu/1.5)

1. Find Area Required, A = Total Axial Load / SBC
2. Find Side of Column Footing, B =  SQRT of Area
3. Find Projections: l = (B - b)/2 (where b = column width and B = Footing width)
4. Find Pa = Factored Axial Load / Area
5. Find B.M. = Pa² /2
6. Find effective depth, d using Equation: MR = C x Z
7. Find Overall Depth = d + Clear Cover + Half of Dia. of bar
8. Find Ast using Equation: MR = T x Z
9. Find One-way Shear Force: Vu = (Pa x l ) (l-d)
10. Check for Shear: same as Singly Reinforced Beam
11. Check for Two-way Shear: τ_{bd <} τ_permissible
12. _Find τ_bd  = Pa [ L x b – (b+d)(l+d)] / 2 [(b+d)+(l+d)] d
13. τ_permissible = K_s τ_c

τ_c = SQRT of fck

K_s = 0.5 + βc

βc = b/l

1. 
   

1. 17. Draw Plan and Cross Section of Column Footing with Steel Reinforcement Details

2. Design of Square Footing for a given Axial Load

Procedure: same as above except the following:

• Find Size of Rectangular Footings 1) In the same ratio as Column dimentions 2) An equal overhang 'x' kept alround the column (Recommanded)
• Equal Overhang Method: (b+2x) (l+2x) = Area (where b = short side of column, l = long side of column)
• Placement of Reinforcement:

Along long span = same as required

Along short span:

1. Ast for Central Band = 2 Ast / (β + 1)

2. Ast for remaining portion = (Ast - Ast for Central Band)/2

3. Design of Combined Footing for 2 columns carrying Axial Loads

Procedure:

1. Given Data
2. Find Total Load to be transferred to soil =  P1 + P2 + Self. wt of footing at 10% of Total axial load
3. Total Axial Load, P = 1.10 (P1 + P2) (if Factored Load is given, get Axial Load, P = Pu/1.5)
4. Find Area Required, A = Total Axial Load / SBC
5. Assume Width of Footing, B
6. Find Length of Footing, L =  Area / B
7. Find Projections: l = (B - b)/2 (where b = column width and B = Footing width)
8. Find Pa = Factored Axial Load / Area
9. Find B.M. = W² /2
10. Find effective depth, d using Equation: MR = C x Z
11. Find Overall Depth = d + Clear Cover + Half of Dia. of bar
12. Find Ast using Equation: MR = T x Z
13. Find One-way Shear Force: Vu = (Pa x l ) (l-d)
14. Check for Shear: same as Singly Reinforced Beam
15. Check for Two-way Shear: τ_{bd <} τ_permissible
16. _Find τ_bd  = Pa [ L x b – (b+d)(l+d)] / 2 [(b+d)+(l+d)] d
17. τ_permissible = K_s τ_c

τ_c = SQRT fck

K_s = 0.5 + βc

βc = b/l 

1. 
   

1. 17. Draw Plan and Cross Section of Combined Column Footing with Steel Reinforcement Details