An isolated column footing transfers the loads from a single column to the supporting soil. the size of the footing is determined by the allowable soil bearing pressure. The footing is designed for flexure, punching or two-way shear, and flexural or one way shear. The depth of the footing is generally governed by punching shear.
The ACI code, 1 in Sections 15.4 and 11.12, specifies the critical sections in the footing for flexure and shear. For a reinforced concrete column the reaction plane is defined in ACI Section 15.4.2 as being located at the face of the column. The location of the critical sections for flexure and shear are specified with respect to this plane and are illustrated in Fig. 1(a-d). The length of the critical perimeter for punching shear is given by
where
| c 1 | = short side of column |
| c 2 | = long side of column |
| d | = effective depth of footing reinforcement |
The reaction plane for a footing supporting a column with a steel base plate is specified as halfway between the face of the column and the edge of the base plate. The maximum moment is computed at this plane, with the flexural shear at a distance d from this plane and the punching shear at a distance of d/2 from this plane, as shown in Fig. 2.
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