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.

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

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).

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

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

The designing of strap or cantilever footing is based on trial and error method, it begins with a trial value of e so that the length L1 is known. The calculations are repeated with another value of e till values of the reactions R1 and R2 calculated by both the procedure, coincide.

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.

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.

If the resultant of the soil pressure coincides with the resultant of the loads and the center of gravity of the footing, the soil pressure is assumed to be uniformly distributed.