51³Ô¹Ï

Topic

Investigation of brittle failure in steel-to-CLT self-tapping screw connections

Department of Civil Engineering

Date & location

• Tuesday, July 8, 2025

• 6:00 P.M.

• Virtual Defence

Reviewers

Supervisory Committee

• Dr. Lina Zhou, Department of Civil Engineering, 51³Ô¹Ï (Supervisor)

• Dr. Ying Hei Chui, Department of Civil and Environmental Engineering, University of Alberta (Outside Member) 

External Examiner

• Dr. Thomas Tannert, School of Engineering, University of Northern British Columbia 

Chair of Oral Examination

• Dr. Catherine Stevens, School of Earth and Ocean Sciences, UVic 

Abstract

Self-tapping screws (STSs) are becoming increasingly popular in the mass timber construction industry due to their ease of installation, high load-carrying capacity, and suitability for prefabrication. While the newly introduced STS provisions in CSA O86-24 covers both ductile and brittle failure modes for 90-degree STS connections under lateral load, it has not fully addressed the brittle failure of inclined STS connections due to a lack of research data and the brittle failure modes defined in CSA O86-24 do not fully capture the actual failure mechanism of steel-to-CLT STS connections. To fill this knowledge gap, a series of tests on steel-to-CLT connections with STSs at different insertion angles (45° and 90°), penetration lengths (two layers and three layers of CLT panels) and lateral load directions (parallel and perpendicular to the CLT major strength direction) were conducted. The results reveal that plug-step shear is the dominant brittle failure mode, characterized by a combination of plug shear failure in layers with grain direction parallel to the load and step shear in layers with grain direction perpendicular to the load. Different from the plug shear defined in CSA O86-24 where the side shear planes are continuous across the effective thickness of a CLT panel regardless of the grain direction, the side shear planes observed in this study only developed in layers with grain direction parallel to the load. Based on the observed failure mechanisms, a mechanics-based model for plug step shear was proposed. The calculated effective depth in CSA O86-24 was found to align well with the observed actual failure depth, and models with calculated effective depth demonstrated better predictive performance compared to the ones using penetration depth. The proposed plug-step shear equations and the design equations in CSA O86-24 were evaluated with the test results. The plug-step shear model demonstrated a strong predictive performance. A modification factor of 0.65 is recommended for STS connections loaded parallel to the CLT major strength direction while no modification is recommended for connections loaded perpendicular to the CLT major strength direction. Although CSA O86-24 brittle failure model does not reflect the actual failure mechanism of steel-to-CLT STS connections, it provides reasonably accurate predictions.