Rotational Stability
The factor of safety approach established for sliding and flotation is not appropriate for use in the evaluation of rotational modes of failure. Rotational behavior is evaluated by determining the location of the resultant of all applied forces with respect to the potential failure plane. This location can be determined through static analysis. Limits on the location of the resultant are provided in Table 3-5. The entire base must be in compression for the usual load condition, to maintain full contact between the structure and the foundation, so there is no chance for higher uplift pressures to develop in a crack. This helps ensure linear behavior for common loading conditions. For the unusual load case, higher uplift pressures may develop in a relatively short crack, but this would cause only minor nonlinear behavior. For extreme load conditions on typical civil works projects, a shear or bearing failure will 3-5 EM 1110-2-2100 1 Dec 05 occur before overturning could occur. Therefore, the resultant is permitted to be anywhere within the base, and safety is ensured by the safety factor requirements for sliding and by the limits on allowable bearing stresses.
The analysis for determination of the resultant location in prior guidance has been termed an overturning stability analysis. This is a misnomer since a foundation bearing, crushing of the structure toe, and/or a sliding failure will occur before the structure overturns. This manual replaces the term overturning stability analysis with resultant location.
Best Practices Resources
Stability Analysis of Concrete Structures (EM 1110-2-2100) (U.S. Army Corps of Engineers)
Citations:
Revision ID: 5676
Revision Date: 12/14/2022