Abstract:
Kelly (2009) explains how low to medium-rise structures on shallow foundations may not have sufficient weight to prevent foundation rocking, that is cyclic uplifting and reattachment at the ends, during a design earthquake. Such rocking is not generally considered in foundation design but it is known that modest amounts can reduce foundation actions because of period lengthening. The paper presents an approach to shallow foundation design that can incorporate rocking into both forced-based and displacement-based design of structure-foundation systems. The method assesses the small strain elastic rocking stiffness of the foundation and also the moment capacity at a fixed vertical load. These two soil-foundation parameters define the bounds on a nonlinear moment-rotation curve for the foundation; a hyperbolic curve provides a smooth transition between the bounds. A process of iteration, matching capacity with demand, is needed to determine the actions generated on the foundation by a given earthquake. Field data on the rocking response of shallow foundations in Auckland residual clay, supplemented with 3D nonlinear finite element modelling of shallow foundations on cohesive and cohesionless soil, provide supporting evidence for the design approach. The finite element modelling includes nonlinear soil deformation as well as loss of contact, and reattachment, between parts of the foundation and the underlying soil. Insight into the relation between hysteretic damping and foundation rotation is also obtained from the finite element analysis.