How To Design The Footing For A House

Footing foundation construction is done to dispenses the load, that is generated from the column, into the soil. The size of footing is based on the type of soil on which the building stands.

For all types of framed buildings with columns mainly provided on hard soil, the general practice is to build up separate independent footings for every columns. Such a footing is known as isolated footing. The shape of the isolated footings varies from square, rectangular, or circular in plan. Rectangular footings are constructed for rectangular columns. Square footings are ideal for square or circular columns. Circular footings are provided for circular columns.

SELECTION OF ISOLATED FOOTINGS:

Isolated spread footing is intended for framed structure i.e. (GF + 3) ultimate storey, with strong soil in nature i.e. SBC of soil beyond 150 KN/m2 based on the cost and execution convenience liable to other design consideration.

When the soil has less strength, with SBC < 150 KN /m2, the spreading area should cover 60% of the soil area and the spacing will be closer among the footings will occur.

For other conditions, isolated footing should not be recommended. In this situation, strip footing or raft foundation are suitable.

(i) Bearing strength of the soil
(ii) Type of Loads
(iii) Type of Structure
(iv) Economy
(v) Allowable differential settlement

TYPES OF ISOLATED FOOTING:

The footing may be a Pad footing or a sloped footing or stepped footing. It may be axially loaded or eccentrically loaded.

Also Read: Some useful guidelines on the construction of RCC foundation

At the time of casting a footing, a straight width of 50 to 75 mm on all four sides of the column is retained to arrange the seating of formwork for column. In sloped footing a Pedestal is sometimes provided to reduce the footing design cost. The Pedestal provides the following benefits:

(i) It minimizes effective cantilever of footing and consequently the bending moment and shear are also minimized.
(ii) It provides greater width to withstand the bending moment.
(iii) It provides greater perimeter while checking two way shear.

The plans for RCC staircase should be accurate and proper. But the most important factor is to properly read these plans. So, there should proper knowledge regarding the main symbols and signs in the plans.

Pedestal is casted once the footing is casted. Normally, M20 grade concrete mix is applied in footing. However the column may include higher grade of concrete. The concrete mix of Pedestal should be same as column. To arrange the casting of the Pedestal, 75 mm straight length at the top of footing should be casted. At the time of providing Pedestal, width of resisting bending moment is considered as the width of Pedestal + 150 mm.

If you want to learn how to make the design of any rectangular sloped footing, go through the following video tutorial, presented by the renowned engineer Parag Pal.

Video Source: Parag Pal