STRATEGIES FOR SEISMIC DESIGN OF SHALLOW FOUNDATIONS FOR STEEL BUILDING STRUCTURES

Canadian provisions allow two alternatives for the seismic design of foundations: capacity-protected (CP) and not capacity-protected (NCP). CP foundations should develop the full resistance of seismic force resisting system (SFRS) and are favoured whenever possible. With such foundations the inelastic activity occurs predominantly in the superstructure, unexpectedly high seismic demands are better managed and the global system deformations are not increased significantly by foundation rotations. NCP foundations develop a partial capacity of the SFRS. Being weaker than the SFRS, such foundations uplift and rotate thus limiting the forces transmitted to the superstructure. Conversely, the foundation rotations increase displacements of the superstructure which must be considered in design. Canadian design practice shows that foundations of steel frame buildings are often large causing a significant increase in construction cost, which may lead to selection of alternative structural solutions built in different materials. Knowing that the design requirements were developed mainly considering the seismic behaviour of concrete shear walls, characterized by the development of a single plastic hinge at the base, it appears necessary to validate their applicability to steel braced frames that exhibit a distributed yielding mechanism, associated with much larger overstrength and higher capacity design forces on foundations. In this study, 3-storey steel buildings with X-type tension-compression bracing were designed for Vancouver, Canada, to examine different design strategies for foundation design. Two soil types were considered. The foundation design followed Canadian and US seismic design approaches. Non-linear time history analyses were then performed using the OpenSees program. The model included the inelastic frame behaviour and the nonlinear soil response. The forces imposed on foundations obtained from nonlinear time history analysis are compared with design predictions. The foundation displacements and stresses in the soil are also examined to assess the consequences of foundation flexibility on the global structural seismic response.