The bearing capacity
(a,) calculated using the foregoing
equations is that of the soil at the depth of the footing.
The load applied at the depth of the footing includes the
structural ioad (a,), the weight of the footing itselt
the weight of any soii immediately above the footing
Since the unit weight of a typical concrete footing is not too
different from soil, the weight terms are often grouped
together approximately as follows:
= (WI + W
where 'A = fonting area
-unfactored surcftarge at Ole fOOting oepth (0,) for a S9il of u it
The net load capacity (a,,,) then is the ultimate capacity less the weight load.
The aliowable load capacity (a,) is the net capacity divided by a factor of safety: