Determination of Canadian Seismic Force Modification Factors for Post-Tensioned Cross-Laminated Timber Rocking Walls

Post-tensioned cross-laminated timber (PT-CLT) rocking walls with energy dissipation devices (EDDs) have proved to be a low-damage seismic force–resisting system (SFRS) due to their self-centering capability and stable energy dissipation. Global efforts have been made to explore the applicability of PT-CLT walls as primary SFRS in mass-timber buildings. Studies have demonstrated satisfactory performance of such systems in high seismic risk zones in Canada. Nonetheless, challenges remain for this system to be practically adopted in Canada due to the absence of SFRS force modification factors (overstrength-related factor, Ro, and ductility-related factor, Rd) in the latest editions of the National Building Code of Canada (NBC). This study evaluates Rd and Ro factors for PT-CLT rocking walls with EDDs following the performance-based unified (PBU) procedure and FEMA P695. The PBU procedure is used to assess system performance against life safety and collapse prevention performance levels, whereas global collapse analysis is conducted following FEMA P695. An Ro of 1.5 and three trial Rd factors, Rd=3, 4, and 5, were initially considered. To reflect potential variability in building geometry, system configuration, and seismic hazards, 75 archetype buildings were designed using each Rd factor, leading to 225 archetype buildings in this study. A robust multispring numerical modeling strategy in OpenSeesPy was first validated with full-scale quasi-static cyclic and shaking table tests and then used to model all archetype buildings. Nonlinear static analysis was first conducted to validate the Ro factor, followed by nonlinear response history analyses and incremental dynamic analyses to validate trial Rd factors by examining building performance and collapse margin ratios. The investigation indicated that an Ro of 1.5 and an Rd of 4 could make all designed archetype buildings satisfy the acceptance criteria of both the PBU procedure and FEMA P695.