Design of Spread Footing

Axial Load: 1200 kN
Column Size: 450 mm x 450 mm
Steel: 4-250 bars of Fe415
Concrete Grade: M25

Step 1: Calculation of Column Load

The column load is the axial load that the footing must carry. In this case, it is 1200 kN.

Step 2: Calculation of Area of Footing

To calculate the area of the footing, the bearing capacity of the soil and the column load must be considered. The footing should be designed in such a way that the pressure on the soil does not exceed its bearing capacity.

Allowable pressure on soil = 105 kN/m²
Area of footing = Column load / allowable pressure on soil
Area of footing = 1200 kN / 105 kN/m²
Area of footing = 11.43 m²

Assuming a square footing, the side of the footing = √11.43 m² = 3.38 m

Therefore, the area of the footing = (3.38 m)² = 11.43 m²

Step 3: Calculation of Thickness of Footing

The thickness of the footing can be calculated using the formula:

Thickness of footing = (Column load x eccentricity) / (Bearing capacity of soil x width of footing)

Assuming an eccentricity of 0.25 m, the thickness of the footing is:

Thickness of footing = (1200 kN x 0.25 m) / (105 kN/m² x 3.38 m)
Thickness of footing = 0.84 m

Step 4: Calculation of Steel Reinforcement

The steel reinforcement required for the footing can be calculated using the formula:

Steel reinforcement = (0.5% x width of footing x thickness of footing) / area of one bar

Assuming 4-250 bars of Fe415, the area of one bar is 4.91 cm².

Therefore, steel reinforcement = (0.5% x 3.38 m x 0.84 m) / (4 x 4.91 cm²)
Steel reinforcement = 357.2 cm²

The number of bars required = Steel reinforcement / area of one bar
Number of bars required = 357.2 cm² / 4.91 cm²
Number of bars required = 72.7

Therefore, 73 bars of 250 mm diameter are required.

Conclusion

The spread footing to carry an axial load of 1200 kN through a column of size 450 mm x 450 mm having 4-250 bars can be designed with a square footing of 3.38 m x 3.38 m and a thickness of 0.84 m. 73 bars of 250 mm diameter are required for the steel reinforcement.