Development of provisions for simplified design of rocking foundations

A simplified design procedure has been developed for potential inclusion within NZS1170.5 to allow rocking shallow foundations for low- to mid-rise buildings without special study. Rocking foundations allow a nonlinear uplift and soil yielding mechanism to form at the soil-foundation interface, and can significantly reduce the required size of foundations (or avoid requiring deep foundations) and reduce seismic demands on a building. The procedure has been used to design a series of buildings and the performance of these buildings has been evaluated using a displacement-based assessment procedure that accounts for soil-foundation-structure interaction. Given that the equivalent static procedure in NZS1170.5 contains several conservative assumptions for multiple storey buildings, and the displacement-based approach is a first mode approximation, the majority of designs and assessments were based on a single degree-of-freedom (SDOF) system. Variations in the design and soil property assumptions were considered, as well as different thresholds for proposed limitations on the applicability of the simplified procedure. Seven performance measures were used to evaluate and demonstrate that the limitations in the proposal result in adequate behaviour for ultimate limit state and serviceability limit state actions. A displacement correction, to conventional NZS1170.5 displacement procedures, is proposed as a concentrated rotation at the underside of the foundation. This correction accounts for foundation rotation prior to reaching the moment capacity.