Improving Procedures for Seismic Design of Foundation of Steel Braced Frames in Canada

In addition to the option of designing building foundations to fully develop the capacity of the seismic force-resisting system (SFRS), Canadian design provisions (NBCC 2020, CSA A23.3:19) allow for foundations to be sized to develop partial SFRS capacity. This results in two types of foundations: Capacity-Protected (CP) and Not-Capacity-Protected (NCP). NCP foundations can uplift and rotate, thereby limiting the forces transmitted to the superstructure. However, foundation rotations increase superstructure displacements, which must be considered in SFRS design. Examples from Canadian practice demonstrate that foundations of steel frame buildings designed for seismic loads are often large and costly. In this study, 6-storey steel buildings with X-type tension-compression bracing were designed for Vancouver, Canada, to explore different foundation design strategies. Two soil types were considered: very dense soil or soft rock (Class C site) and soft soil (Class E site). In addition to fully compliant design, two alternatives were examined: (i) SFRS deformations due to foundation rotations were neglected, and (ii) capacity-design considerations were not applied, similar to practices in the United States (ASCE/SEI 7). The inelastic response of the structure-foundation-soil system was analysed using STKO/OpenSees software and compared with design predictions. Foundation displacements and soil response were evaluated to assess the consequences of foundation flexibility on global response. The results suggest that while omitting capacity design is not recommended due to potential excessive permanent soil settlements and bearing pressures, some relaxation of design requirements may be acceptable, particularly for competent soils.