Nonlinear Rotation of Capacity-Protected Foundations: The 2015 Canadian Building Code

When foundations are capacity-protected, inelastic deformations will occur primarily in the seismic force-resisting system. Soil flexibility can be ignored when determining seismic loads, but footings will rotate when subjected to the maximum overturning moment, and this may increase building drifts, particularly in lower stories where gravity-load columns are less flexible. A “hand calculation” method is presented for estimating rotation of a footing from the uniform bearing stress distribution required to resist the applied overturning moment. The method, which has been adopted in the 2015 Canadian building code, accounts for initial linear rotation of footings and additional nonlinear rotation due to footing uplift and nonlinearity of soil. A quick, safe estimate can be made using approximate equations, or a more accurate estimate can be made by determining two parameters from figures. Design examples demonstrate how the method can be used to design foundations for improved performance at a small additional cost.