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.

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

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.

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

For a rectangular combined footing, xÌ… = L/2 and for a trapezoidal combined footing solutions lies between the limits L/3 < xÌ… < L/2.

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

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.

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.

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.