Behaviour of glue-laminated timber beams subjected to torsion and combined torsion and bending

Abstract

Glued-laminated (glulam) timber is increasingly being used in large-scale structures because of its benefits with respect to embodied carbon when compared to steel and concrete, its high strength-to-weight ratio, and its aesthetically pleasing appearance. However, the behaviour of glulam beams subjected to torsion or combined bending and torsion has seldomly been studied in the literature, even though these loading conditions are likely to occur in practice. Furthermore, current design standards for timber structures in North America (e.g., CSA O-86 in Canada and NDS in the United States) do not provide any guidance on how to consider torsion in design or the combined effects of bending and torsion. The overarching goal of this thesis is to conduct a series of experimental campaigns focused on addressing the knowledge gap related to the behaviour of glulam beams subjected to torsion or combined torsion and bending. The specific objectives are to understand the influence of cross-section size, lamination orientation, and aspect ratio on the behaviour of glulam beams under pure torsion. The study is then extended to consider the behaviour of glulam beams under torsion and bending, with the aim of developing an interaction diagram for this combined loading condition. In all experiments, digital image correlation is used to measure rotation and displacement distributions along the member lengths as well as high-resolution two-dimensional strain fields over the surface of the members. Results of the study demonstrate that under pure torsion, beams fail at rotations less than 5 degrees, characterized by longitudinal shear cracking along the member length, however, beams do exhibit significant post-peak deformability. Under combined loading, there is a transition between shear and flexure failure at a moment-to-torsion ratio of approximately 6.4. A torque–moment interaction diagram is proposed for glulam beams, which could provide a basis for the design of glulam members under combined actions.