Climate-induced shifts in forest composition are altering the availability of wood species for construction, necessitating more adaptable approaches to engineered wood products (EWPs). Structural finger-joints are the foundation for (EWPs) such as Glued-laminated-timber (GLT) and Cross-laminated-timber (CLT) as they create the opportunity to produce boards in the desired length and are standardised in EN 14080; EN 15497. However, these standards specifies that finger joints for EWPs are to be made only from single species of wood. This restriction ensures homogeneity in properties, and quality control. However, combination of species might increase the efficiency of EWPs in better utilization of regional timber resources, and tailored mechanical performance by leveraging the high stiffness and strength of hardwoods alongside the workability and availability of softwoods.
One of the few studies addressing this topic was done by Sjölund and Stenis (2019) who investigated finger-jointed hybrids beams composed of acetylated and untreated birch plywood spruce Glulam by finger jointing. Complementary to this study, Wang et al. (2023) explored the bonding potential of lap jointed birch plywood with spruce glulam. The study reported that the wood failure rate was nearly 100% indicating sufficient bonding was possible between spruce and birch. Further evidence for the feasibility of hardwood-softwood bonding was offered by Musah et al. (2024), who combined nine different north American hardwood and softwood by cross laminating them using two different glues. Shear bonding strength revealed that hybrid combinations involving both hardwoods and softwoods exhibited only 5% lower bonding strength than all hardwood pairings. Subhani and Lui (2024) investigated the shear bonding strength of Eucalyptus nitens and Pinus radiata, and their combination using polyurethane (PUR) adhesives, both with and without primer. The study reported that application of primer on hardwood surfaces was shown to reduce delamination rates and enhancing bond durability. This study addresses a gap in current standards by evaluating the mechanical and bonding performance of hybrid finger joints made from Silver Birch (Betula pendula) and Norway Spruce (Picea abies L.). Unlike prior research focused on lap or cross-laminated joints, this work directly investigates finger joints, which are foundational to EWPs. The findings offer evidence for the viability of hybrid configurations, potentially informing future revisions of EN standards. By enabling more efficient use of regional timber and optimizing mechanical properties through species combination, this research supports sustainable resource utilization and offers the construction industry a pathway to adapt to evolving material availability and performance demands.
Keywords: hybrid finger joints, structural bonding, mechanical properties
Authors
Souvik Ray
Department of Forestry and Wood Technology, Linnaeus University, 35195 Växjö, Sweden
Jonas Faßbender
Wood Biology and Wood Products Department, Faculty of Forest Sciences and Forest Ecology, University of Goettingen, Germany
Holger Militz
Wood Biology and Wood Products Department, Faculty of Forest Sciences and Forest Ecology, University of Goettingen, Germany
Reza Hosseinpourpia
Department of Forestry and Wood Technology, Linnaeus University, 35195 Växjö, Sweden
College of Forest Resources and Environmental Science, Michigan Technological University, Houghton, Michigan 49931, United States
Login to download the PDF