Bond–slip behaviour and constitutive modelling of screwed-in threaded rods in glulam joints

Steel screwed-in threaded rods (STRs) can form high strength connections in glued laminated timber (glulam) structures. However, the bond–slip behaviour of STRs embedded in glulam is not well understood and detailed constitutive models to describe this behaviour are lacking. This study addresses that knowledge gap through an extensive experimental programme comprising 280 pull-out tests on 16 and 20 mm diameter STRs under monotonic loading with STR-to-grain angles ranging from 0°–90°. Key bond–slip properties, including withdrawal strength, bond stiffness, slip at peak stress, maximum slip, and fracture energy, are quantified in terms of both the mean and 5th-percentile values. The results show that withdrawal strength increases with STR-to-grain angle up to 30° and then remains approximately constant, while bond stiffness decreases and both slip at peak stress and maximum slip increase with angle. A larger rod diameter generally increases withdrawal strength, maximum slip, and fracture energy, and reduces scatter, indicating reduced sensitivity to local defects. Mechanics-informed regression models are developed for the mean and characteristic bond–slip properties as functions of glulam density, rod diameter, and inclination angle. A new constitutive model is proposed that captures the full nonlinear bond–slip response of STRs in glulam. The model shows strong agreement with experimental results and provides length-independent parameters suitable for analytical design methods and interface-based numerical simulations.