Experimental Evaluation of the Durability of Timber–Steel HYBRID Connection Interfaces under Accelerated Ageing

Engineered wood products are increasingly used in structural applications for their favourable strength-to-weight ratio, with uptake further driven by the global shift towards low-carbon construction; however, their structural performance can be limited by a low modulus of elasticity and brittle behaviour. Hybridisation with steel therefore emerges as an effective strategy to improve stiffness, strength, and ductility, provided that continuous connection interfaces along the member length between the timber and steel can maintain strong composite action. The durability of such connections remains uncertain because timber undergoes moisture-induced swelling and shrinkage, while steel exhibits comparatively smaller temperature-driven dimensional changes, which may lead to stress concentrations and connection degradation. This study investigates the durability of continuous laminated veneer lumber (LVL)–steel connection interfaces using two accelerated ageing protocols, namely a modified ASTM D1037 wet–dry cyclic method representing severe exposure and a controlled temperature–humidity variation regime representing more realistic in-service conditions. Four connection configurations are examined, incorporating adhesive bonding, mechanical fastening, or a combination of both. The shear performance of the aged connections is evaluated and compared against unaged reference specimens. Adhesive-bonded connections showed pronounced degradation after ageing, exhibiting complete delamination under the modified ASTM cycling protocol and a 67.2% reduction in shear capacity under the temperature–humidity variation protocol. In contrast, screwed connections retained comparable performance under the temperature–humidity variation protocol, although a 40% reduction was observed under the more severe modified ASTM exposure. Screwed and adhesive-bonded connections experienced capacity reductions of 68.2% and 71.0% under the modified ASTM and temperature–humidity variation protocols, respectively. Overall, the findings demonstrate that cyclic moisture exposure can substantially influence the performance of continuous timber–steel hybrid connections, and while adhesive bonding can provide high initial stiffness and capacity, its durability remains vulnerable under severe wetting and drying conditions, indicating the need for further research into improved adhesive formulations and surface preparation methods.