ECCENTRICALLY LOADED FOOTINGS.

The footings are so designed and proportioned that the C.G. of the superimposed load coincides with the C.G. of the base area, so that the footing is subjected to concentric loading, resulting in uniform bearing pressure. However, in some cases, it may not be possible to do so. For example, if the wall (or column) under construction is near some other property, it will not be possible to spread the footing to both the sides of the wall or column. Such a situation is shown in Fig. 3.7.

FIG. 3.7 ECCENTRIC LOADING.

 

Let this resultant load have an eccentricity e with respect to the centre of base width B. This eccentric weight
is equivalent to (i) a centrally placed load W and (ii) bending moment M = W . e.

Due to these two, a trapezoidal soil pressure diagram, having pressure intensities q1 and q2 will result.

 The magnitude of q1 should not exceed the safe bearing pressure for the soil. Also in order that the footing may remain in contact with soil, q2 should be positive (i.e. no tension should be developed). In the extreme case, q2 = 0, when e = B/6. This gives the maximum value of eccentricity. In that case

average pressure on the foundation. If e is greater than B/6 tension will be developed, in which case, the end B of the footing will have loose contact with the soil.