From Research to Practice: Case Study on Mixed Angle Screws Hold Down Connections for a 6-Storey CLT Shear Wall Building

Abstract

Mass timber structures are a growing topic in the New Zealand design environment. To design mass timber buildings to the moderate or high seismic demands of much of New Zealand, strong stiff, and ductile seismic systems are required. One such system recently developed and tested at the University of Canterbury is the mixed angle screw hold-down connection for Cross Laminated Timber (CLT) Shear walls. Through the innovative use of large European self tapping screws installed at mixed angles with respect to the grain, the strong and stiff performance of screws installed at 45 degrees can be combined with the ductility and displacement capacity of screws installed at 90 degrees to the grain, providing an overall strong, stiff, and ductile connection. Taking research and implementing it into practice isn't always easy. Especially in the frame of timber buildings, where design and guidance is still in development. This paper looks into the application of mixed angle screw hold-down connections in a 6 storey (Concrete podium, 5 storey CLT wall) CLT wall structure designed for a high seismic hazard. Key challenges from the design of this building and their associated learnings are presented. Challenges that had to be overcome included the stiffness prediction, NZS AS 1720.1:2022 requirements for displacement amplification, and the application of capped overstrength for timber structures. These challenges are discussed in detail, and some suggestions for future design are given.