Test: Combined Footing - Civil Engineering (CE) MCQ

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.

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.

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

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.

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

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.

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.

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.

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