In-plane mechanical properties of birch plywood and its performance in adhesively bonded connections

Sammanfattning: Birch (Betula spp.) is a hardwood species with a wide natural distribution on the Eurasian continent, especially in northern Europe. Compared with conventional plywood made from softwood, birch plywood has favorable mechanical properties that could be used in new types of efficient connections for timber structures, and thus enable a substitution of current systems using slotted-in steel plates. Such new connections could result in significant advantages in terms of environmental impact and economy as well as ease of prefabrication and assembly. However, birch plywood has rarely been utilized in connections, and therefore, there is a lack of knowledge necessary to design safe timber structures applying such connection systems. In particular, there is a need for increased knowledge of the mechanical properties of birch plywood and its structural performance under various loading conditions. Current connections in timber structures usually also involve mechanical fasteners, e.g., steel screws and dowels, but there is limited use of adhesively bonded (glued) connections. The aim of this thesis is to gain new knowledge required for the development of adhesively bonded connections using birch plywood as gusset plates in structural applications. Examples of such structural applications are timber trusses and portal frames. In this context it is necessary, first, to characterize the in-plane mechanical properties of birch plywood, and second, to investigate its performance in adhesively bonded connections.The results of the mechanical testing show that birch plywood possesses the highest and lowest tensile, compressive, and bending strength and elastic modulus at 0° (parallel) and 45°, respectively, to the face grain (the fiber direction of the face veneers). The opposite findings were noticed for the shear strength and the shear modulus. All these strength values are similar to or higher than the corresponding strength values of common softwood structural timber in its longitudinal direction. Moreover, a size effect on the in-plane bending strength property was observed at 0° and 90° to the face grain but not at other angles, which is attributed to different failure mechanisms. Based on the experimental work, both analytical and numerical models to predict the in-plane mechanical properties of birch plywood are proposed.Three different adhesives systems were used in the studies: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and a two-component polyurethane (2C PUR). All adhesives used show adequate bonding strength between birch plywood and spruce glulam. However, the use of the adhesive systems should be further investigated in the future. The different manual pressing methods investigated show no significant influence on the bonding strength. Moreover, the bonding strength changes within a relatively small range when the loading direction is varied from 0° to 90°, which is beneficial for the design of birch plywood in adhesively bonded connections. A clear correlation exists between the bonding strength and the shear strength of the weakest wood adherend.In addition, the moment capacity and bending stiffness of adhesively bonded connections using birch plywood were determined experimentally as well as by analytical and numerical models with a satisfactory agreement.In timber connections, especially those that are prevalently loaded in tension and/or compression (e.g., in timber trusses), the contribution of the plywood width on the load-bearing capacity needs to be quantified. The results show that the tensile strength of birch plywood within the bonded area shows very low angle-dependence. This is possibly due to the restricted crack propagation at 22.5° and 45° when the gap between the bonded regions is small. The tensile capacity of birch plywood loaded at 0°, 22.5°, and 45° reaches a plateau at certain widths of the gusset plate, which can be well predicted and explained by the spreading angle theories proposed in this study.In the future, more studies are required for the further development of the adhesively bonded connections with birch plywood. Some preliminary studies serving this purpose have been presented in the thesis as on-going work.