Yield mechanisms of base shear connections for cross-laminated timber shear walls

Abstract

When cross-laminated timber (CLT) shear wall is designed for seismic loads, the Canadian timber design standard recognizes rocking as the sole energy dissipation kinematic mechanism to meet the capacity design requirement. The sliding mechanism, achievable by yielding of the base shear connections of CLT shear walls, is currently excluded. The focus of this research was on the yielding mechanism of base shear connections for mass timber shear walls as potential source for dissipating energy during major seismic events through sliding. Two types of base shear connections were evaluated: one was designed to exhibit ductile behavior through fastener yielding and the other was designed to yield within a perforated zone of steel bracket that was connected to the timber member with over-designed fasteners (i.e., capacity protected). In both cases, self-tapping screws (STS) were used as fasteners. Monotonic and cyclic tests were performed. The strength, stiffness, ductility, over-strength, and failure mechanisms were determined to investigate the use of these connections as reliable energy dissipating mechanisms. It was found that perforated steel plates provide a reliable yield mechanism with predictable yield and ultimate strength and displacement, and that damage in timber elements can be avoided when the fuses are combined with capacity-protected dowel-type fasteners. Both types of base shear connections under consideration behave in a ductile manner with some limitations under cyclic loading. While the perforated plate detail has the potential to qualify as moderately ductile connection, the other connection with fastener yielding mechanism may only exhibit limited ductility. An analytical model for predicting the yield and ultimate strengths, and ultimate deformation of the perforated plate was developed.