It is assumed that the combined footings are rigid and rests on a homogeneous soil, so as to give rise to a linear stress distribution on the bottom of the footing.

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 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.

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.

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

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.

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

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.

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