Introduction

A footing supporting more than a single column or wall is called a combined footing, and when many columns (more than two) are involved, terms such as continuous strip footing (if columns are aligned in one direction only) and raft foundation or mat foundation are used. Multiple column foundations become necessary in soils having very low bearing capacities. However, even in soils having moderate or high ‘safe bearing capacity’ for the use of individual footings, combined footings become necessary sometimes — as when:

• columns are so closely spaced that isolated footings cannot be conveniently provided, as the estimated base areas tend to overlap;
• an exterior column located along the periphery of the building is so close to the property line that an isolated footing cannot be symmetrically placed without extending beyond the property line.

Distribution of Soil Pressure

The prediction of the exact distribution of base pressure under a footing is difficult, as it depends on the rigidity of the footing as well as the properties of the soil. If this is difficult for an isolated footing, indeed, it is more so for a combined footing.
For a very rigid footing supported on an elastic soil base, a straight line pressure distribution is appropriate. Such an assumption is found to lead to satisfactory designs in the case of relatively rigid footings. However, for relatively flexible footings, such an assumption is not realistic; the problem is rather complex and involves consideration of soil-structure interaction.

Geometry of Two-Column Combined Footings

The geometry of the footing base should preferably be so selected as to ensure that the centroid of the footing area coincides with the resultant of the column loads (including consideration of moments if any, at the column bases). This will result in a uniform distribution of soil pressure, which is desirable in order to avoid possible tilting of the footing.
The footing may be rectangular or trapezoidal in shape, depending on the relative magnitudes of loads on the two columns which the footing supports. When the exterior column (which has the space limitation for an independent footing) carries the lighter load a rectangular footing or a trapezoidal footing (with a reduced width under the exterior column) can be provided. On the other hand, when the exterior column carries the heavier load the wider end of the trapezoidal footing should be located under the exterior column.

Design Considerations in Two-Columns Footings

Fixing Plan Dimensions

The plan dimensions of the two-column combined footing may be selected to satisfy the following two requirements

1. Base area of footing A = Total (service) load/q_a.
2. The line of action of the resultant of the column loads must pass through the centroid of the footing.

In the case of a rectangular footing, the second requirement results in a length L of the footing equal to . The edge distance a may be fixed with reference to a property line; otherwise, it may be suitably assumed. Having fixed L, the footing width B is obtained as A/L. In the case of a trapezoidal footing usually the length L is selected first, and the dimensions B₁, B₂ adjusted to satisfy the two requirements cited above.

Load Transfer Mechanism

As in the case of isolated footings, the factored net soil pressure q_u is computed as the resultant factored load divided by the base area provided, and the pressure may be assumed to be uniformly distributed