An Analytical Model for Predicting the Axial Stress Distribution of Self-Tapping Screws Due to Axial Load and Moisture Swelling of Mass Timber Products

Self-tapping screws are becoming increasingly popular for use in modern timber structures. The axial stress distribution of self-tapping screws due to a mechanical load has been previously studied. However, the stress distribution of self-tapping screws due to moisture swelling-induced load from wood has not been explored so far. This research presents an analytical model to predict the axial stress distribution in self-tapping screws embedded in mass timber products under the combined effects of axial mechanical loading and wood moisture-induced swelling. The analytical model has been validated with numerical simulation. The input properties of the analytical model can be determined from withdrawal tests of self-tapping screws and the manufacturer’s guide of screw and mass timber products. A simple program has been developed to predict the stress distribution and maximum axial stress in self-tapping screws for a range of effective penetration lengths under a pre-load and moisture content change. Correctly predicting the maximum axial stress in self-tapping screws under the simultaneous action of a pre-load and wood moisture swelling-induced load can help design safer timber structures. This research provides a practical method for practicing engineers to predict the maximum axial stress in self-tapping screws due to pre-load and wood moisture swelling.