Withdrawal strength and stiffness of screw fastenings in cross-laminated timber

Introduction – Cross-laminated timber (CLT) is a product that is becoming increasingly common in the construction industry. CLT is categorized as homogeneous or combined, where homogeneous has the same strength classes throughout the entire cross-section and combined has varying strength classes across the cross-section. The strength and stiffness are compensated in the combined material by placing the highest strength class furthest out in the cross-section. When it comes to stiffness in timber structures, however, joints often represent the weakest link. This is particularly relevant in the construction of taller buildings where stiffness becomes a crucial variable for stability. To broaden the knowledge of screw connections in CLT this study aims to investigate the comparison of homogeneous and combined CLT with regards to strength and stiffness in axially loaded screw connections in CLT. Method – A literature review was conducted to design a test setup for measuring the strength and stiffness of axially loaded screw connections in CLT. Relevant studies and standards were examined to develop a suitable setup for the study. Tensile tests were conducted to determine the difference in strength and stiffness between homogenous and combined CLT as well as between strength class C24 and C35, providing numerical values for comparison. Result – The literature study resulted in empirical data presenting various test setups for measuring axially loaded screws connections in CLT. The experiment, conducted in the form of tensile tests, resulted in data in the form of measured force in relation to displacement, which could be used to calculate strength and stiffness. Analysis – The test setup which was designed to measure the strength and stiffness of axially loaded screw connections in CLT was based on guidelines from SS-EN 1382 (SIS, 2016) and ISO 6891 (ISO, 1983), as well as inspiration from previous studies with similar test setups. The tensile tests showed that no significant difference in strength and stiffness could be observed between the combined and homogeneous CLT elements that were tested. Discussion – The results show no significant percentage differences in relation to the coefficient of variation between combined CLT and homogeneous CLT. However, the results do show a significant difference when only strength class C35 and C24 are tested separately. Additionally, the experiments show that displacement measurements should be made locally due to deformations in the tensile testing machine and equipment. Due to deformations in the screw, it also turns out to matter where in relation to the test specimen and screw the local measurements are made