PEER-REVIEWED ARTICLE bioresources.com Záborský et al. (2017). “Joint stiffness of wood,” BioResources 12(1), 932-946. 932 Effects of Geometric Parameters of Structural Elements on Joint Stiffness Vladimír Záborský, Vlastimil Borůvka, Daniel Ruman,* and Milan Gaff Joints are one of the most important issues in the design of furniture structures. Joints in furniture structures made from wood and wood materials represent a critical area because furniture most often breaks at the joints of structural elements. This article discusses the analysis of the effect of selected factors: type of loading (compressive, tensile), wood species (Fagus sylvatica L., Picea abies L.), thickness of joint (one-third and half the thickness of the tenon), type of glue (polyvinyl acetate and polyurethane), and the annual ring deflection, on the elastic stiffness of joints. These results indicated significant effects for the wood species, thickness of joint, and type of glue used. The annual ring deflection was on the borderline of statistical significance, while its effect was more significant than the effect of the basic material characteristic, i.e., the wood density. The type of loading was not statistically significant. Keywords: Furniture wood joints; Mortise and tenon; Mechanical loading; Elastic stiffness Contact information: Department of Wood Processing, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 - Suchdol, 16521 Czech Republic; * Corresponding author: dano.ruman@gmail.com INTRODUCTION The topic of joints is one of the most important areas in furniture design. Joints in furniture made of wood and wood-based materials are critical points, as furniture is most frequently damaged where structural elements are joined (Terrie 2009; Brett 2014). Joints can be classified as glued, mechanical, melted plastic, welded, and combined joints (Joščák et al. 2014). Joints can be characterized by their effectiveness, expressed as the ratio of the load capacity of the joint to load capacity of the elements. This effective strength ranges between 10% and 30% (Bašista 1972; Joščák 1999). One of the most dangerous cases of joint stress is stress by bending moment in their angle plane (Eckelman et al. 2004; Erdil et al. 2005; Prekrat and Španic 2009; Uysal et al. 2015). The mortise and tenon joint is one of the most common means of joining structural elements. This joint is usually glued. The mortise and tenon joint must have as little tolerance as possible to ensure joint strength. The greatest joint strength is ensured if adhesive is applied to both the tenon and the inside of the mortise (Forest Products Laboratory 2010). Smardzewski (2002b) examined the distribution of shear stress in joints. Konnerth et al. (2006) determined the behavior and durability of beech and spruce wood joints glued with a PVAc adhesive and compared them with other adhesive types. They determined that the glued joint’s shear strength was 25% higher for beech than it was for spruce. Adhesive type did not have a significant effect on the glued joint’s shear strength. In furniture-making practice, it is common to use thermoplastic PVAc adhesives in gluing products. PVAc adhesives appeared in 1950 and replaced natural adhesives. These