Since the ratio of internal to perimeter loads is 2 : 1, i.e. the same as in Design Example 2 (section 12.3.4), the centroid of loads will again be 2.0 m from grid line B. A 9.0 m long base, as in Design Example 2, would therefore again be required to achieve a balanced rectangular foundation.
This relatively long base would however be associated with comparatively large bending moments and reinforcement areas. A more economic foundation is likely to be achieved using a shorter trapezoidal balanced foundation.
|Fig. 12.11 Trapezoidal balanced foundation design|
Condition for a balanced trapezoidal foundation
Again the condition for a balanced foundation is for the centre of gravity of the base to coincide with the centroid of the applied loads.
about x–x, the location of the centre of gravity of the base of area A is given by
Area of base
The values of B1 and B2 would normally be chosen to minimize the size of the base. This would result in a bearing pressure equal to the allowable bearing pressure, na, giving a base area
Dimensions of base
The end of the base furthest from the site boundary will, in this instance, be chosen to extend beyond grid line B by the same amount as a standard 3.65 m × 3.65 m internal pad foundation (see section 12.3.4), i.e. extending by 3.65/2 = 1.825 m.
Thus, from Fig. 12.11,
Ultimate design pressure
The combined dead and imposed partial load factor is γP = 1.51, as in the previous examples. The ultimate design pressure for reinforcement design, pu, is given by