Column Footing:
In a building, the load path flows from
Safe Bearing Capacity of Soil:
General Guidelines:
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:
- Given Data
- 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
- Find Area Required, A = Total Axial Load / SBC
- Find Side of Column Footing, B = SQRT of Area
- Find Projections: l = (B - b)/2 (where b = column width and B = Footing width)
- Find Pa = Factored Axial Load / Area
- Find B.M. = Pa2 /2
- Find effective depth, d using Equation: MR = C x Z
- Find Overall Depth = d + Clear Cover + Half of Dia. of bar
- Find Ast using Equation: MR = T x Z
- Find One-way Shear Force: Vu = (Pa x l ) (l-d)
- Check for Shear: same as Singly Reinforced Beam
- Check for Two-way Shear: τbd < τpermissible
- Find τbd = Pa [ L x b – (b+d)(l+d)] / 2 [(b+d)+(l+d)]
d
- τpermissible = Ks τc
τc = SQRT of fck
Ks = 0.5 + βc
βc = b/l
- 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:
- Given Data
- Find Total Load to be transferred to soil = P1 + P2 + Self. wt of footing at 10% of Total axial load
- Total Axial Load, P = 1.10 (P1 + P2) (if Factored Load is given, get Axial Load, P = Pu/1.5)
- Find Area Required, A = Total Axial Load / SBC
- Assume Width of Footing, B
- Find Length of Footing, L = Area / B
- Find Projections: l = (B - b)/2 (where b = column width and B = Footing width)
- Find Pa = Factored Axial Load / Area
- Find B.M. = W2 /2
- Find effective depth, d using Equation: MR = C x Z
- Find Overall Depth = d + Clear Cover + Half of Dia. of bar
- Find Ast using Equation: MR = T x Z
- Find One-way Shear Force: Vu = (Pa x l ) (l-d)
- Check for Shear: same as Singly Reinforced Beam
- Check for Two-way Shear: τbd < τpermissible
- Find τbd = Pa [ L x b – (b+d)(l+d)] / 2 [(b+d)+(l+d)] d
- τpermissible = Ks τc
τc = SQRT fck
Ks = 0.5 + βc
βc = b/l
- 17. Draw Plan and Cross Section of Combined Column Footing with Steel Reinforcement Details