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

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

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

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

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

The penetration resistance N values should be taken at 75 cm intervals for a depth equal to width of the raft, below the base 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.