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.
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.