Depth of Concrete Bed Block

It depends upon the

(a)        Type of concrete used,

(b)       Projection of the concrete block beyond the bottom most, course of wall footing, and

(c)        Upward pressure of the soil below.

The concrete bed block might fail by shearing, crushing or by bending. The depth of concrete bed block is determined from the consideration of maximum bending moment and the depth thus obtained is found to render the concrete block safe against shear or crushing.

Let      d = depth of concrete block in m.

J  = projection of concrete block on either side of the lower most course of wall footing in metre,

m = safe modulus of rupture of concrete mix used in kN/m², and

p = load on foundation in kN/m².

Let consider the cantilever projection AB of the concrete block. Bending moment, M, about a vertical plane through B,

M = (pj²/2) kN-m

                                                                 Figure 20

A moment of resistance of the concrete block

= (md²/6)kN - m

Equating a maximum bending moment with moment of resistance, we get

pj ²/2 = md²/6 ? d =