Overturning Moment

The Overturning Safety Factor (OSF) is taken as the ratio of the Resisting Moment /Overturning Moment. The OSF is calculated for each soil load combination and the smallest value for the footing in the each of the major and minor directions is reported with the corresponding load combination. During the optimization process the footings are sized so that a minimum OSF of 1.0 is attained.

The Resisting Moment is the sum of the moments due to each of the following:

Axial loads acting on the supported column
Specified footing surcharge loads
Footing self-weight

The Overturning Moment is taken as the sum of the moments on the column and any shear on the column multiplied by the distance from the base of the column to the base of the footing. If there is uplift on the column then the moment associated with that axial load is also considered in the Overturning Moment.

Note: Footing self-weight can play a significant role in resisting overturning moment. If the user has selected not to consider footing self-weight during the analysis, or has set the footing concrete unit weight to zero, the optimized spread footing size may be significantly larger than otherwise expected due to the absence of footing weight in resisting overturning. This is particularly true for a footing supporting a column which, for a single load combination, has a relatively small axial load and a relatively large base moment.

The OSF is, in some respects, a subjective calculation which may vary between engineering offices and even between engineers. The OSF reported in RAM Foundation is calculated in such a way as to stay consistent with the method used to analyze and optimize the footing.