Withdrawal behaviour of screw groups in Tasmanian eucalyptus nitens GLT: Effects of installation configuration and glue-line position

This study investigates the withdrawal performance of screw groups in Tasmanian E. nitens GLT. A total of 126 specimens were tested across six screw installation configurations, considering different screw types and positions relative to the glue line. The results show that timber density, screw geometry and screw arrangement significantly influence withdrawal capacity. Higher timber density and larger overall effective thread bearing area increased withdrawal resistance, while multiple screws reduced the per-screw capacity due to group interaction effects. Screws installed directly along the glue line generally showed improved performance compared with those embedded within the timber, whereas screws positioned close to the glue line showed only a minor reduction in capacity. Among the tested screw types, G14 screws achieved the highest and most consistent capacities, while SP5 and SP6 screws tended to fail by shank fracture, particularly in higher-density timber. Existing code-based models were found to underestimate withdrawal capacity or show inconsistent trends. Two regression-based models were therefore developed to account for screw group effects, with the linear model providing the most consistent and reliable predictions.