Investigation of brittle failure in steel-to-CLT self-tapping screw connections
Date
2025
Authors
Wang, Huiqi
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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 new STS provision 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 connections with inclined STSs. 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, which consists of plug shear failure in layers with grain parallel to the load and step shear failure in layers with grain perpendicular to the load, is the dominant brittle failure mode. 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. Models with calculated effective depth demonstrated better predictive performance compared to the ones using penetration depth. 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.
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Keywords
Steel-to-CLT connection, Brittle failure, Lateral load, Inclined self-tapping screw