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

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

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.

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.

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).

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

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.

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

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.

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