Mechanical connections using birch plywood as gusset plates in timber structures

QC240425

Abstract

The use of steel gusset plates in connections for timber structures increases the environmental impact of the structure. Wood-based connections could, in this case, be ideal substitutes providing lower environmental impact as well as both better workability and mountability. Birch (Betula spp.) is a hardwood species widespread in the Northern Hemisphere, and plywood made of birch possesses superior strength and stiffness performance compared with plywood made of softwood, making the former promising for structural use.

One idea, in this context, is to use birch plywood in different timber connections, such as truss nodes, or moment-resisting connections, for example, spliced beam joints, beam-column joints, and portal frame corner joints. Such new types of connections could result in significant advantages in terms of a low so-called carbon footprint as well as ease of prefabrication and on-site assembly. However, the feasibility of utilizing birch plywood in structural design needs to be investigated in-depth. In particular, there is a lack of knowledge regarding the mechanical behavior and failure mechanism of birch plywood for different load cases.

The aim of this thesis is to produce new knowledge necessary for designing mechanical connections using birch plywood as gusset plates in timber structures. The specific objectives were, first, to characterize the embedment behavior of single steel fasteners in birch plywood by mechanical tests and analytical models, and second, with further mechanical tests and modeling, to investigate the failure behavior and mechanism in birch plywood gusset plates with multiple fasteners for different load cases.

Embedment tests with steel dowels demonstrated a load-to-face grain angle dependence of the strength and stiffness values, although less pronounced compared with other in-plane mechanical properties of the plywood. A new embedment strength formula for birch plywood that considers both the load-to-face grain angle and the dowel diameter is proposed.

The failure mechanism of birch plywood in spliced beam joints under pure bending moment was examined both experimentally and theoretically. As a result, new analytical models based on Eurocode 5 formulas and the fastener group’s polar moment of inertia resistance are proposed. Based on experiments on glulam frames, the failure mechanism of birch plywood in connections under uniaxial tension loads was also studied. In this case, results verified that the load spread (Whitmore effective width) phenomenon exists in birch plywood.

The design of birch plywood connections loaded in uniaxial tension was further addressed regarding the influence of fastener patterns and face-grain orientations. The magnitude of spread angles in the classic and modified load spread models was determined by conducting tests on plywood with increasing widths. Their validity was examined by predicting the capacities of the aforementioned uniaxial tension joints. Furthermore, frame corner joints consisting of glulam and birch plywood were studied, where the analytical formulas for predicting the load-bearing capacities of the fastener group, the glulam member, and the plywood plates were validated.

For future work, it is suggested that full-scale experiments be conducted on both trusses and portal frames to further validate the design models and optimize the design of connections using birch plywood and mechanical fasteners.

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