THE EFFECTS OF SOIL-STRUCTURE INTERACTION ON THE SEISMIC DESIGN OF SHALLOW FOUNDATIONS FOR EBFs

Past research has demonstrated that foundation rocking, can reduce demands on the superstructure by dissipating a portion of earthquake energy through soil-structure interaction (SSI). Canadian seismic provisions allow two approaches for foundation design: capacity-protected (CP) and non-capacity-protected (NCP). CP foundations are designed to fully develop the resistance of the seismic force-resisting system (SFRS), thereby concentrating inelastic activity within the superstructure. In contrast, NCP foundations are designed to develop only a partial capacity of the SFRS, leading to foundation uplift and rotation, which limit the forces transmitted to the superstructure. In practice, Canadian design often results in large foundations for steel frame buildings, significantly increasing construction costs and potentially favoring alternative structural materials. Previous studies on concentrically braced frames have shown that, on competent soils, such large foundations may not be necessary, and satisfactory performance can be achieved even without considering frame overstrength or capacity design. To further validate the applicability of Canadian seismic provisions for foundation design, this study investigates 3- and 8-storey steel buildings with eccentrically braced frames located in Vancouver, Canada. Two site classes (C and E) were considered. Foundation designs followed Canadian seismic design standards (NBCC 2020 and CSA A23.3-19), with an additional comparison to a design approach without capacity considerations, similar to US practice. Non-linear time history analyses were conducted using the STKO/OpenSees platform, incorporating both inelastic frame behavior and nonlinear soil response. The results indicate that for foundations on competent soil (site class C), even without applying capacity design principles, the inter-storey drift and soil capacity limits were satisfied. This design approach resulted in smaller foundations and reduced ductility demands on the superstructure.