Numerical and analytical investigations of steel-to-wood connections with fully threaded self-tapping screws under wetting

Self-tapping screws (STSs) are widely used in mass timber connections due to their high strength and ease of installation. A new provision of STS connections has been implemented to the latest edition of Canadian timber design standard, CSA O86–24. However, the performance of STS connections under wetting condition has not been fully addressed and remains a critical concern since STS breakage was reported in several mass timber projects exposed to wetting during construction. This study investigates the failure mechanisms of steel-to-wood connections with fully threaded STSs under wetting condition through numerical and analytical approaches. Cohesive zone modeling method was employed to simulate the wood-screw interaction, validated with experimental data, while analytical solutions were developed to predict STS stress distributions. The investigated parameters included screw diameter, penetration length, moisture content change, and installation-induced loads. Results revealed two primary failure modes: (1) STS yielding close to the screw head due to the combined stress from wetting and installation loads, and (2) localized withdrawal failure at the screw tip caused by excessive wood swelling, particularly for the connections with long penetration length (>25d₀, where d₀ is the nominal diameter of STS) or large moisture content increase (e.g., 12 % to fiber saturation point). The proposed closed-form analytical model, whose key input parameters can be determined based on STS withdrawal load-displacement curves or empirical equations, showed good agreement with the verified numerical modeling results, offering a practical tool for designers to predict the wetting-induced STS stress in the design phase. This work bridges gaps in the current Canadian timber design standard by providing quantitative methods to evaluate moisture-dependent STS performance in steel-to-wood connections, therefore enhancing the safety and reliability of mass timber construction.