Combined Footing

The purpose of a footing is to transfer the loads from the structure to the soil underneath as well as support the foundation and resist settling.

The selection for perfect type of footing is based on the depth at which the bearing strata rests, the condition of soil and the type of superstructure.

Combined footing: When two or more columns in a straight line are provided on a single spread footing, it is known as a combined footing.

The combined footings are arranged on the basis of the following conditions :-

When two columns are adjacent to each other resulting overlap of adjoining isolated footings.

Where the bearing strength of soil is low, resulting overlap of adjoining isolated footings.

Closely linked building line or subsisting building or sewer which are closed to a building column.

The combined footing appears as rectangular, trapezoidal or tee-shaped in plan.

The geometric proportions and shape are set in such a manner that the centeroid of the footing area overlaps with the resultant of the column loads. It leads to produce identical pressure underneath the entire area of footing.

Trapezoidal combined footing is arranged when one column load remains greater than the other. Consequently, the both projections of footing beyond the faces of the columns will be restrained.

Also Read: How to calculate footings in excel following reference drawing

Rectangular combined footing is useful when one of the projections of the footing is restrained or the width of the footing is restrained.

Longitudinally, the footing functions as an upward loaded beam extending among columns and cantilevering beyond. With statics, the shear force and bending moment diagrams in the longitudinal direction are drawn. Moment is verified at the faces of the column. Shear force is critical at distance ‘d’ from the faces of columns or at the point of contra flexure. Two-way shear is verified under the heavier column.

The footing is susceptible to transverse bending and this bending is expanded over a transverse strip adjacent to the column.