Question

Arrive at the size of the rectangular RCC footing for a column of size 300 mm × 450 mm supporting an axial service load of 1200 kN. The SBC of soil is 200 kN/m². Take M30 grade concrete, Fe415 steel, the angle of internal friction of soil is 30⁰ and unit weight of soil is 19 kN/m³.

           

Answer 1

Answer) To arrive at the size of the rectangular footing we have

Area of the footing = (Dead load of structure, footing and surcharge + live load on column or wall)/ safe bearing capacity of the soil.

we are given axial Service load on column =1200KN

Dead load of structure, footing and surcharge is not given but they are generally considered as 10%of the load on column = 0.10*1200=120KN

Safe Bearing capacity of the soil is given = 200KN/m2

Now Area of footing is given by= (120+1200)/200

Area of footing =6.6m2

As no restricton is given on length and breadth of the footing we can Assume any size for footing (say) L=1.2B

1.2B2 =6.6

B2= 5.5

B=2.35m

and L= 1.2*2.35

L=2.82m

Approximate depth of foundation is given by Df=

Where qa = Safe Bearing capacity of the soil is given = 200KN/m2

= unit weight of the soil =19KN/m3

= Angle of internal friction of soil = 30

Df=

Df=1.17m

Use footing of 2.82m x 2.35m of footing at a depth of 1.17m.