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 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.

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

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.

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

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

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

When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one-half of the area and it may prove more economical to use mat or raft foundation.

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.

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.