Size effect on the bonding strength between spruce glulam and birch plywood

Wood has recently emerged as a primary alternative to traditional building materials such as concrete and steel, improving the sustainability of modern building construction. In timber engineering, the performance of connections plays an important role. Although adhesive bonding provides excellent load-bearing capacity, its applications is limited by strict environmental requirements. As a result, mechanical connections are more popular than traditional carpentry joints in modern construction. Therefore, investigating the size effect on the bonding strength of adhesively bonded timber joints is necessary, because it can contribute to the progress of adhesive bonding in practical applications. Because birch plywood and spruce glulam both are known for high mechanical strength, so they were used for experiments. The aim of this study was to examine how bonding strength changes with increasing adhesively bonded area. Furthermore, three different face grain angles and adhesive types were tested separately to compare the results and investigate the individual effects of adhesively bonded length and width on bonding strength. In this study, 118 specimens prepared with three different types of adhesives (PRF, Epoxy and 2C-PUR) and tested at three face grain angles (0°, 45°, and 90°), using the modified longitudinal compression shear mechanical equipment to analyse the effect of adhesively bonded area/ length/ width on bonding strength. The results show that bonding strength is influenced by adhesive type and adhesively bonded area. 2C-PUR showed the highest bonding strength, followed by PRF and Epoxy, which closely related with the shear modulus of the adhesives. Additionally, overall bonding strength decreased as adhesively bonded area increased, especially at 45°. Moreover, bonding strength was mainly influenced by adhesively bonded length rather than width, because of uneven shear distribution and defects.