Bonding strength between spruce glulam and birch plywood at different load-to-plywood face grain angles

There is growing interest recently in reducing the usage of metals in timber structures. Birch plywood possesses satisfactory mechanical properties compared to other wood-based panels and is promising to be utilized in timber connections as a substitute for the more conventional slotted-in metal plate. There are essentially two possibilities to connect plywood plates and other timber elements by means of either mechanical connections or adhesively bonded connections. Despite the more commonly adopted mechanical connections in current timber structures, the adhesively bonded connections hold the distinct advantages of being more cost-effective, stiffer, and with a lower risk of moisture penetration in the timber elements. When employing birch plywood in timber structure applications such as trusses and frame corners, stresses from different directions need to be transmitted by the plywood gusset plate. However, it is still uncertain how the bonding strength is affected by different loading angles to the face grain. This research question, specifically concerning the bonding strength between birch plywood and spruce glulam, has been addressed in this paper. It was found that the bonding strength varies within a relatively small range when the load-to-plywood face grain angle varies from 0 degrees to 90 degrees, which is promising for the development of adhesively bonded joints. Failure mainly occurred in glulam at 0 degrees and 15 degrees; while at other angles, a mixture of cohesive failure in glulam and plywood face veneer was dominant. The weak angle-dependence of the bonding strength can be explained by further checking the shear strength of the weaker wood adherends between glulam and plywood. A strong positive correlation was observed between bonding strength and the wood shear strength.