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

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 raft is subdivided into a series of continuous beams (strips) centered on the appropriate column rows to establish shear failure and moment diagram.

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 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.

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

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.

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.