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

For a rectangular combined footing, xÌ… = L/2 and for a trapezoidal combined footing solutions lies between the limits L/3 < xÌ… < L/2.

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.

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.

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

In the pressure intensity equation q2 = a. qs, if a= 0 we have a triangle with xÌ… = L/3.

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.

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

In strap footing, two independent columns are supported by as trap or beam at the bottom.