9.15.3.8. Footing Thickness
When a #footing supports a foundation, the soil exerts a pressure beneath the footing. Where a footing is wider than the foundation wall (or column) it supports, the portion that projects beyond the wall (or column base) is bent upward. If unreinforced, such footings tend to fail along #shear lines that extend downward at an angle of about 45ยบ from the face of the wall. To be within this shear line, therefore, the footing projection is limited to the footing #thickness, unless the footing projection is reinforced.
If the supported loads require very wide footings, their thickness may become excessive. The cost of additional #excavation and #concrete may justify the use of reinforcing or other appropriate measures to reduce the thickness of the footing.
The concrete footing thickness, T, must be at least equal to the footing #projection, P (unless the footing is reinforced), and be not less than 100 mm (4 in.) (Figure 9.15.-8). Note that as special soil conditions or footing loads increase, so must footing thickness.
Quoted from Illustrated Users' Guide - NBC2015
#footingthickness
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