Tuesday, November 6, 2012

Stepped Footing.

When the wall carries heavy loads, or when the safe bearing pressure of the soil is not very high, the base width required from Eq 3.2 wilI be much greater than (T + 2 a). In that case, it is essential to provide masonry offsets, to achieve larger spread, before the load is transferred to concrete base. The height and width of cach offset should be so proportioned that rate of spread does not exceed the permissible value for the masonry Fig. 3.2(b) shows such a stepped footing in which the rate of spread through masonry is n :1 and that through concrete base is n1 :1

The implication of the above recommendations is that  in order to spread the bearing width from original T (width of walI) to B (footing width), the minimum depth required would workout as follows: 

In the case of brick walls, the offset should not be greater than 5 cm; the corresponding height of each step would work out to be 10 cm. As a thumb rule, the width B’ of the bottom brick course should not be less than twice the width of the wall. In the case of stone masonry, the offsets may vary from 7 ½  cm to 10 cm(max.) corresponding to a mim. height of masonry course equal to 15 cm and 20 cm respectively.

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