Timber connections with increased friction in the shear plane: Experimental and analytical investigations into strength and stiffness

Timber connections with increased friction in the shear plane are studied, focusing on experimental and analytical approaches to enhance load-carrying capacity and stiffness. Conventional aluminium connectors, while structurally adequate, pose environmental and fire-resistance limitations. As an alternative, densified veneer wood (DVW) was introduced for system connectors, offering superior sustainability and fire performance. The research explores surface modifications to improve frictional behaviour, thereby optimizing strength and stiffness. Short-term push-out tests examined various surface treatments and screw configurations, revealing that pronounced surface structuring significantly increases ultimate load capacity, while flat, uniform surfaces better maintain stiffness. Long-term duration-of-load tests confirmed that frictional performance remains stable over time, with creep factors for inclined screws significantly lower than those for other dowel-type fasteners. An analytical model was developed to predict load-carrying capacity and stiffness, incorporating friction effects and deformation perpendicular to the grain. The mechanical model proposed in the new generation of EC5 overestimates the stiffness when used for connections with connector plates and inclined screws, because it neglects deformations perpendicular to the grain beneath the connector plate. In contrast, the herein presented spring model incorporates compression behaviour perpendicular to the grain, thus, predicting load-carrying capacity and stiffness accurately for a range of timber components, including CLT and GLT reinforced with fully threaded screws. The results demonstrate that DVW connectors with appropriately modified surfaces can achieve high ultimate loads and reliable long-term performance, thereby supporting their adoption in sustainable timber construction.