Criterion for Applying Two-Step Analysis Procedure to Seismic Design of Wood-Frame Buildings on Concrete Podium

Abstract

Neither the 2015 National Building Code of Canada (NBCC) nor the 2014 Canadian wood design standard, Engineering design in wood, explicitly provides seismic design guidelines for wood-frame buildings on a concrete podium. The NBCC commentary and ASCE standard, Minimum design loads and associated criteria for buildings and other structures, provide different criteria for designing podium buildings using a two-step analysis procedure. Of specific interest in this paper is which criterion is more suitable for wood-frame podium buildings. Wood-frame podium buildings with different stiffness ratios, mass ratios, and period ratios were designed with two-step analysis procedure in accordance with the NBCC and Canadian wood design standard. Nonlinear time history dynamic analysis was conducted using the selected earthquake ground motions based on the guidelines proposed for NBCC. Pure wood-frame buildings were also analyzed as benchmark cases for evaluating the seismic performance of midrise podium buildings designed with a two-step analysis procedure. The analysis results show that neither the criterion in the NBCC commentary nor that in the ASCE standard can guarantee that podium buildings designed using the two-step analysis procedure meet the performance-based seismic design criterion. An empirical criterion based on the normalized stiffness ratio and normalized mass ratio was developed. It is concluded that when the normalized stiffness ratio is equal to or greater than 10 times the normalized mass ratio, the buildings designed with the two-step analysis procedure are able to meet the performance-based seismic design criterion.