Important Design Considerations For Footing Design.
1. Thickness at the end of the footing in reinforcement and plain concrete footings shall not be less than 150 mm for footing design on soil and nor less than 300 mm above the tops of piles for footing on piles.
2. Total load coming on the footing shall not exceed the safe bearing capacity of the soil. The safe bearing capacity of the soil varies according to their types. The safe bearing capacity of different types of soil is given below.
| Type of Soil | SBC (KN/m²) |
|---|---|
| Hard Strata | 450 |
| Medium Strata | 250 |
| Moist clay | 150 |
| Black cotton soil | 150 |
| Soft strata | 100 |
| Very soft strata | 50 |
3. If plain concrete bed is provided then the minimum cover for reinforcement shall be 50 mm.
4. Depth of Footing :
• Case A : Critical Section For Bending Moment
The critical section for maximum bending moment consideration is taken at the face of the column. For more details on this please refer I.S Code 456:2000, page no 65, clause no 34.2.3.2
Bending Moment in X direction is given as,
Mxx = uplift soil pressure * shaded portion * centroid distance
=
=
Bending Moment in Y direction is given as,
Myy =
=
Greater value from both the bending moment calculations is considered and minimum depth is obtained by treating above moment to limiting moment. Here limiting moment for different grades of steel varies.
- For Fe250 steel = 0.148*fck*bd²
- Fe415 steel = 0.138*fck*bd²
- Fe 500 steel = 0.133*fck*bd²
• Case B : One Way Shear Consideration for footing design
For design shear of footing,
Vu =
The above value of τv should not be less than τc, the critical section for shear stresses in the concrete.
As τv = τc,
Hence the minimum depth required from the consideration of one way shear is given by
The value of τc can be calculated from IS 456-2000, pg no 73, table no 19. For this assume the percentage of steel as per you requirement but generally 0.25 is considered.
• Case C : Two War shear of punching shear consideration :
If Depth of footing is less than column may punch through footing. Therefor for this tendency, Is 456:2000 recommends that the critical section is at a distance of (d/2) from the column face.
Perimeter of punching shear = 4(b+d).
Area of concrete resisting punching shear (A) = 4 (b+d)d ……………….. (equation 1)
Force of punching (S) =
Punching shear stress (τp) = (S/A) …… from (equation 1)
Permissible value of τp = 0.25√fck, In case of footing with uniform thickness, usually one way shear governs the depth and other two cases may be used for checking the selected depth.
5. The minimum reinforcement described for slab is applicable for footing also. i.e, a) 0.12% of c/s area for deformed bars and b) 0.15% of c/s area for mild steel bar.
Design Steps For Footing
1. Find out the required area for footing design. To find out the are of the footing, assume 10 % of the load as self weight of the footing :
where P = Total load in footing.
2. After finding the area of the footing we can easily find the size of the footings.
- For square footing = √Areq.
- Rectangular footing = L*B, for this assume L or B for certain value or L/B ratio = 1.2 to 1.5
- Circular Footing = (Π/4)*D²
3. To find out the net factored upward soil pressure.
4. Depth of the footing By using three considerations which are mentioned above :
- Critical section for bending moment.
- One way shear consideration.
- Two way shear or Punching shear consideration.
5. Area of reinforcement required for footing design :
here B = width of footing.
6. Check for development length is required, since the footing is designed as cantilever with reinforcement subjected to design strength at the column faces. Sufficient bond length should be provided from the face of the column.