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

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.

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.

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).

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.

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

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

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

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