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

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

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

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

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

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.

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

A raft may undergo large settlement without causing harmful differential settlement. For this reason, almost double settlement of that permitted for footing is acceptable for raft. Therefore if a maximum settlement of 50 mm (2 in) is permitted for a raft, the differential settlement is not likely to exceed 20 mm (0.75 in).

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.