Seismic Force Demands on the Foundations of Concentrically Braced Frame Systems

Concentrically braced frames (CBFs) are widely used as lateral force-resisting systems in North America. Canadian seismic design requirements for building foundations distinguish between two foundation types: capacity protected and non-capacity protected (commonly known as “rocking” foundation). For steel bracing systems such as CBFs, there are major ambiguities in how to calculate the foundation design forces, as the governing force for both foundation types is influenced by the capacity of the braces, which have an uncertain overstrength, and by the timing of braces reaching their peak force, which may or may not be simultaneous. In addition, the superstructure, foundation, and underlying soil interact in response to seismic loads, which further influences design force estimates. Therefore, there is a need to better understand the actual force demands on CBF foundations.This study considers example 2-storey and 5-storey steel frame buildings with tension-compression X-bracing, designed for Vancouver, Canada, in accordance with the 2015 National Building Code and the Canadian steel and concrete design standards. The increased drifts of the braced bay caused by foundation rotations are assessed using simplified procedures and included in the design. The buildings are analyzed using advanced numerical models in OpenSees, including brace buckling and P-Delta effects. The results obtained for fixed-base conditions are compared to those including foundation flexibility. For the latter, the effects of soil-structure interaction are represented using nonlinear spring models that can capture foundation rocking and sliding as well as the soil settlement. The results of this study confirm that the Canadian concrete standard estimates reasonably well the demands on the foundations regardless of foundation flexibility but suggest that further study is needed to verify this more reliably.