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

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.

It is assumed that combined footings are rigid and rests on a homogeneous soil, so as to give rise to a linear stress distribution on bottom of footing.

The soil in a simplified elastic foundation is assumed to be continuous elastic medium obeying Hooke?s law.

According to Skempton and Bjerrum, total settlement of a footing in clay may be considered to consist of three components
S = Si + Sc + Ss.

The earth pressure varies from point to point of a raft, and hence resultant pressure at any point is considered to be w + mw, where w is constant and m varies throughout raft.

In spread footing, load is transmitted through an isolated column or wall to subsoil; hence this is most common type of foundation.

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, differential settlement is not likely to exceed 20 mm (0.75 in).

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

Iw = influence factor = 1.06 for rigid rectangular footing, with L/B = 1.5 = 1.70 for rigid rectangular footing, with L/B = 5.