Axial Performance of Self-tapping Screws in Mass Timber Products Under Moisture Content Variation

• Author / Creator

  Khan, Mehsam T.

• Self-tapping screws (STS) are widely used in wood-to-wood and wood-to-steel connections in timber structures. The premature failure of STS during the construction phase has been recently reported by structural engineers and contractors of several mass timber projects in North America due to the increase of the wood's surrounding moisture content. This type of STS failure is suspected of having been precipitated by additional axial stress exerted on the STS from moisture swelling of the wood. This research investigates the axial performance of STS installed in two mass timber products, CLT and glulam, under axial loading conditions and moisture content change of the wood to understand this failure mechanism. This research emphasizes the case of moisture content increase (wood wetting) of CLT and glulam. A moisture transport model for predicting the moisture content variation distribution in CLT and glulam during moisture adsorption has been presented here. Then, a method to determine the swelling of the CLT and glulam from the moisture content variation distribution has been proposed. The properties of self-tapping screws under axial loading conditions, such as tensile and withdrawal properties, have been investigated. Subsequently, an analytical model to predict the stress distribution along the length of the screw has been developed. The analytical model has been verified with a numerical (finite element) model with the input properties from material properties tests conducted on the self-tapping screws, CLT and glulam. The numerical model results and the analytical model predictions of the screw stress distributions showed good agreement for the wood wetting cases. Finally, the stress distribution model of the screw has been linked to the stress distribution in self-tapping screws in wood-to-wood and wood-to-steel connections. The database and the methods developed in this thesis lays the foundation for developing failure criteria and guidelines to prevent the premature failure of self-tapping screw connections under moisture content variation.

• Subjects / Keywords

  • Self-tapping screw
  • Mass timber product
  • Moisture content variation

• Graduation date

  Fall 2022

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-h8ad-eg08

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.