The effects of timber properties and screw geometry on the withdrawal strength in Tasmanian plantation hardwood

With increasing demand for timber-rich structures, the use of Eucalyptus nitens ( E. nitens), a fast-growing hardwood species cultivated extensively in Tasmanian plantation forests, has expanded beyond its traditional use for fibre production to structural applications. Despite its rapid growth and abundant supply, there is limited information on timber connection performance made from this species. This study investigates the screw withdrawal strength of E. nitens sourced from fibre-managed plantations to assess its suitability in structural timber connections. The experiments used five partially threaded screws: G14 (6.46 mm), G8 (4.36 mm), and G10 (4.96 mm) with a 60° thread angle, and SP5 (5.10 mm) and SP6 (6.10 mm) with a 35° thread angle. A total of 540 screws, with screw thread bearing area ranging from 11 mm 2 to 19 mm 2 , were tested to evaluate the influence of key parameters on withdrawal strength, including effective screw thread bearing area, screw thread angle, effective penetration length, screw-grain angle and timber density. The study identified four failure modes based on the interaction between screw material yielding capacity and screw withdrawal strength in the timber. The results indicate that longer screw penetration lengths and larger thread bearing areas negatively impact withdrawal strength, confirming the critical role of screw geometry in connection design. Additionally, timber density significantly influences screw withdrawal performance, with denser samples demonstrating higher withdrawal resistance. These findings provide valuable insights into optimising screw geometry for timber connection design using E. nitens, supporting its use in sustainable timber construction and contributing to the development of more efficient, low-carbon building solutions. • Eucalyptus nitens, a fast-growing hardwood mainly used for fibre, is now being explored for structural applications. • Supports the growing demand for sustainable materials in construction. • Effects of screw geometry, such as thread bearing area, angle, and penetration depth, on withdrawal strength were analysed. • Density at 12 % MC and screw-grain angle were evaluated for their impact on withdrawal strength.