Seismic design and behaviour of coupled balloon-type cross-laminated timber shear walls with high-performance screwed connections

Although encapsulated mass timber construction is currently permitted up to twelve storeys in Canada, the National Building Code of Canada lacks the design rules and seismic force modification factors required for an equivalent static design of balloon-type cross-laminated timber (CLT) shear walls. This dissertation presents a comprehensive investigation into the seismic design and performance of these systems. The study began with a survey of design professionals, which indicated strong industry interest in balloon-type systems. The research then followed a multi-stage methodology to address the identified knowledge gaps: experimental characterization of key connections, static system modelling, and dynamic performance assessment. Experimental results for hold-downs with mixed-angle self-tapping screws showed significant improvements in strength and stiffness compared to traditional light gauge steel brackets. Vertical panel-to-panel connection testing of half-lap and tongue-and-groove joints further characterized the seismic performance of these connections, which contribute significantly to the strength and ductility of CLT shear walls. Experimental results were used to calibrate non-linear hysteretic connection models. Static cyclic analysis of wall systems up to twelve storeys indicated that while single walls concentrate deformation in the hold-downs, coupled walls engage the vertical joints, providing higher strength, additional energy dissipation, and displacement ductility. Consequently, a ductility factor of 2.0 was proposed for these systems. A mechanics-based hand calculation design methodology was developed in consultation with practicing structural engineers. The design method was validated using numerical models, achieving a system overstrength of 1.7 to 2.0, depending on the wall height. Finally, incremental dynamic analysis was used to validate the proposed seismic force modification factors. Using guidance from FEMA P-695 and the performance-based unified assessment procedure, median-adjusted collapse margin ratios ranging from 2.4 to 5.2 were found, depending on the design parameters. This confirms that a ductility factor, Rd, of 2.0 and a system overstrength factor, Ro, of 1.5 are appropriate for equivalent static design. These findings provide a technical foundation for codifying balloon-type CLT shear walls and support a more prescriptive design pathway for tall mass timber buildings.