Impact of Moisture Content on the Withdrawal Resistance of Self-Tapping Screws in Timber

Self-tapping screws (STS) have emerged as the state-of-the-art connector for mass-timber structures, often taking advantage of their ability to resist high loads parallel to their axis in withdrawal. This withdrawal resistance is affected by the timber moisture content. Current design standards account for this fact by prescribing service condition reduction factors ( K SF ), which were developed for traditional dowel-type fasteners, creating a need for data specific to STS under realistic moisture exposure scenarios. In this study, the withdrawal resistance of STS was determined through 285 tests on two wood species under five distinct moisture conditioning regimes. The governing failure mode was found to be dependent on the test conditions, with moisture cycling promoting withdrawal failures of long STS, which failed in tension in dry conditions. Experimental K SF values in wet-installed wet-service regime (0.68 and 0.76), indicating that CSA O86 values from lag screws (0.27) are overly conservative, while NDS ( K SF = 0.70 ) and particularly Eurocode 5 ( K SF = 0.89 ) might not be sufficiently conservative.