If N is less than 5, sand should be compacted by artificial means to rise N above 10, or else piles or piers should be used.

The maximum bending moment should be adopted as the design value for the reinforced concrete rectangular footing, which should also be checked for maximum shear and bond etc.

Ordinarily, rafts are designed as reinforced concrete flat slabs using heavy beams.

The designing of strap or cantilever footing is based on trial and error method, it begins with a trial value of e so that the length L1 is known. The calculations are repeated with another value of e till values of the reactions R1 and R2 calculated by both the procedure, coincide.

The weight of the raft is not considered in the structural design because it is assumed to be carried by directly by the subsoil.

Based on the settlement criterion for raft, the net pressure can be calculated from the following equation having its width greater than 6m.

If the resultant of the soil pressure coincides with the resultant of the loads and the center of gravity of the footing, the soil pressure is assumed to be uniformly distributed.

The design of rigid rectangular footing consists in determining the location of center of gravity of the column loads and using length and width dimension such that centroid of footing and the center of gravity of columns loads coincide.

Mat footing is used where the soil contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.

The net ultimate bearing capacity may be determined from both Skempton and Terzaghi’s equation as given below: