Numerical Investigation of the Axial Stress Distribution of Self-Tapping Screws in Mass Timber Products during Wetting or Drying

Self-tapping screws (STS) are used in wood-to-wood and wood-to-steel connections in timber structures. Premature failure of STS during the construction phase has been reported by structural engineers and contractors in relation to a few North American mass-timber projects. This STS failure type is suspected of having been precipitated by additional axial stress exerted on STS from swelling of the wood resulting from prolonged wetting. This study investigates the axial stress distribution of STS installed in two mass timber products, cross-laminated timber (CLT) and glulam, under axial loading and changing moisture conditions in the linear elastic regime. The focus is on modelling the stress distribution of STS during wood-wetting. Properties of self-tapping screws under axial loads, such as tensile and withdrawal properties, along with swelling properties of CLT and glulam, have been investigated. A numerical method to predict the axial stress distribution of the screw from these material property tests has been developed. The numerical model has been calibrated with test results. The method developed in this research helps in understanding the premature failure of self-tapping screw connections under the moisture content variation of wood. The next step will be developing an analytical model to predict the axial stress distribution of STS.

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