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

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

The strap used does not remain in contact with soil, and thus does not transfer any pressure to the soil.

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

If the Center of gravity of the load coincides with the centroid of the raft, the upward load is regarded as a uniform pressure, which will be equal to the downward load divided by the area of the raft.

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.

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.

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

When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one-half of the area and it may prove more economical to use mat or raft foundation.