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

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

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

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 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.

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

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.

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

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.

The maximum bending moment should be adopted as the design value for the reinforced concrete rectangular footing, which should also be checked for maximum shear and bond etc.