Withdrawal resistance and tightening-induced axial forces of threaded inserts in cross-laminated timber

Threaded inserts are of interest in demountable connections as they can enable disassembly and reassembly. In service, such fasteners may be subjected to axial forces resulting from an external load or the tightening of connected elements. In this study, the effect of embedment depth (60, 80, and 100 mm) on the withdrawal resistance of threaded inserts in five-layer cross-laminated timber (CLT) was investigated. The relationship between tightening torque and the resulting axial force was also assessed to determine the torque coefficient (K) of the 100 mm inserts with an M8 threaded rod screwed into the insert and a nut tightened at the opposite end. Average withdrawal resistance increased significantly with embedment depth (p < 0.05), by 31% from 60 to 80 mm and 44% from 80 to 100 mm. However, withdrawal resistance per unit embedment depth was relatively similar for the 60 and 80 mm embedment depths. The axial force generated during tightening increased linearly with applied torque within the tested range, with a torque coefficient of K = 0.293. Based on the obtained K, a tightening torque of 27.4 kN.mm (95% CI: 25.3 - 29.5 kN.mm) was estimated to create an axial force corresponding to 40% of the mean withdrawal resistance of the 100 mm inserts. Tightening torque should therefore be considered when designing connections using such fasteners as the resulting axial forces are substantial relative to their withdrawal resistance.