Mechanical properties of hybrid joints in timber structures

Mechanical joints with screws or bolts are widely employed in timber structures and are known to provide high toughness. However, they result in low stiffness, which is a representative limitation of wood structural joints. In this study, we investigated the mechanical properties of hybrid joints comprising mechanical and adhesive joints, to prevent the decrease in stiffness while maintaining high toughness. Wood and steel plates were joined with screws and adhesives, and their stiffness, bearing capacity, and fracture energy were evaluated by lap-shear tests. We inferred that the stiffness and bearing capacity of the hybrid joints change with the type of adhesive used, while the fracture energy primarily depends on the diameter of the screws. Interestingly, when the adhesive strength was too large, a decrease in the fracture energy was observed because the precursory rupture of the adhesive joint triggered the wood failure in the fastened regions and lead to the early pull-out of the screws. To suppress such decrease and early pull-out, an adhesive fillet, i.e., the coating of the adhesive onto a larger area than that of the steel plate, was investigated. Accordingly, we successfully improved the toughness. Our results suggest the importance of optimal designs for the hybrid joint in timber structures.