STRIP FOOTINGS: Depth of concrete bed block.

The depth of concrete bed block depends upon the type of concrete, the projection of the block and the soil bearing pressure. It is found on the basis of the bending moment imposed on it, and on the basis of safe modulus of rupture.


For a footing shown in Fig. 3.3 let

d = depth of concrete block in cm.
a = projection beyond the masonry face in cm.
m = safe modulus of rupture of concrete mix, in kg/cm^2.
q = net soil bearing pressure, in kg/m^2.

FIG. 3.3

The projected concrete block will be subjected to bending moment due to upward soil presure q. The maximum B.M. will  be about plane A-A. Consider 1 cm length of the footing ( or wall ).

In the above expression, q and m are in the same units (KN/m^2) while d and a area also in the same units ( i.e. either in mtres or in cm or in mm).

Equation 3.4 (a) is also valid if both q and m are expressed in t/m^2 units.

The value of modulus of rupture m for various types of concrete are given in Table 3.2

TABLE 3.2 SAFE MODULUS OF RUPTURE OF CONCRETE.