Birch plywood has superior mechanical properties compared with plywood made from most softwood species, which makes it suitable for structural application. Plywood is also more environmentally friendly, cost-efficient, and less prefabrication demanding than steel plates. For a proper design of birch plywood as joint plates in timber-timber connections, the embedment behavior of mechanical connectors into plywood needs to be properly investigated. In this study, the authors performed embedment tests of dowel connectors into birch plywood specimens under five different angles from parallel to perpendicular to the face grain with a step size of 22.5°. Dowel connectors with three different diameters were utilized to study the influence of dowel diameter on embedment strength and stiffness. Besides, test series using self-drilling screws and acetylated birch plywood specimens were conducted to study the effect of both fastener type and acetylation. Moreover, full-hole and half-hole test results were compared. The embedment strengths were calculated analytically according to different term definitions. The test results were then compared with the analytical results based on formulas reported in the literature, e.g., Eurocode 5 and other design standards.

This study presents the feasibility of constructing moment joints using birch plywood plates and glulam beams. Beam-beam joints consisting of birch plywood gussets and glulam beams connected by self-tapping screws were built and tested in the four-point bending regime. Analytical calculations based on the fastener group's torsional moment resistance, the proposed elastic and post-elastic fastener group's load-bearing criteria, and the design formulas given in Eurocode 5, are performed to predict the connection capacity in both elastic and post-elastic stages. Under the four-point bending, all specimens with birch plywood plates with the face-grain orientation of either 0 degrees or 22.5 degrees to the beam length direction, showed brittle failure either in the glulam beam or birch plywood plate. Numerical models are constructed to capture the global load-deflection behavior of all beam specimens. The foundation zone modeling scheme is adopted to consider the influence of the fasteners' embedment on the macroscopic load-deflection curves. As a result of comparison, the analytical calculations give reasonable pre-dictions on both the elastic and post-elastic ultimate beam capacity, and the numerical models give satisfactory predictions on the bending stiffness for all tested beam specimens.

Birch is a short-lived hardwood species widespread in the Northern Hemisphere. Plywood made from birch has superior mechanical properties compared with that made from most softwoods, which makes it suitable for structural application. In this study, the feasibility of using birch plywood as gusset plates in timber-timber connections is presented. Test frames consisting of birch plywood gussets and glulam beams connected by nails were built and tested. A 2D analytical model based on truss theory and a 3D finite element model were proposed and constructed. Both models showed satisfactory agreements with the test results in terms of stiffness and strength. Tensile failure on the birch plywood gussets along the outermost row of nail holes was observed in the experiment. The observed failure modes and the stress distributions in the 3D numerical model suggest that the spreading angle (Whitmore effective width) theory should be considered in the design phase of birch plywood gusset plates. Besides, a modified spreading angle theory is proposed to both approximate the stress distribution and predict the load-bearing capacity.

This study reveals timber-timber composite joints consisting of glulam pieces and birch plywood plates with three different load-to-face grain angles. Utilizing a similar number of fasteners and arranging the fastener array from narrow to wide, uniaxial tension specimens were manufactured with four different fastener patterns. The thickness of birch plywood was intentionally under-designed so that the failure modes for all connections were net tension failure of birch plywood plates. Thereafter, the influence of the fastener pattern and face grain orientation on the load-bearing capacity and stiffness of the investigated composite joints was studied. The load capacity and nominal strength generally increased when the nail patterns varied from narrow to wide. This observation was associated with the Whitmore effective width theory (load spread angle) in steel gusset plate design. Moreover, to derive valid analytical methods to predict the net-tension capacity of birch plywood plates, the classic spread angle model that assumes rectangular stress blocks and the modified spread angle model that considers the summation of stresses from each fastener row were discussed. Both models were adopted to predict the net tension capacity of investigated specimens at 0°. In addition, the stiffness of joints was measured and compared with slip modulus formulas in Eurocode 5. The measured local stiffness values were found to be independent of the fastener pattern and load-face grain angles. The analytical slip modulus assuming the case without predrilling exhibited a satisfactory prediction, while formulas assuming the case with predrilling tend to give overestimations.

This study reveals the load spread model in the design of birch plywood gusset plates when subjected to uniaxial tension. Plywood plates with elevating widths at three different load-face grain angles weredestructively tested until reaching force plateaus. Two load spread models, namely, the classic and modified load spread models, adopted from the Whitmore effective width theory, were investigated to account for the observed phenomenon. The classic load spread model considers a rigid fastener array and an evenly distributed stress block. A closer-to-reality modified model considers the summation of stress blocks contributed from each fastener line. For both models mentioned, the magnitudes of corresponding spread angles were calibrated utilizing a fitting scheme considering maximized R-square values. The validity of both models was later examined and validated versus the previous experimental data reported in the literature. The classic model gave over-estimations on the load-bearing capacities of several connection patterns despite some close predictions. The modified model was found to be conservative for almost all investigated fastener patterns. Accordingly, a hybrid adoption of load spread models was suggested.

This study investigates frame corner joints built of birch plywood plates and glulam elements connected via self-tapping screws. Analytical calculations based on the fastener group’s torsional moment resistance, the proposed fastener group’s elastic and post-elastic load-bearing criteria, and the design formulas in Eurocode 5 were performed to predict the connection capacity in both elastic and post-elastic stages. A combined action check formula was adopted to predict the capacity of birch plywood plates and glulam elements. Frame corner specimens constructed with three different plywood thicknesses were planned to study the influence on global behavior and rotational stiffness. The specimens were intentionally designed so that failure occurred either in plywood or in glulam, in order to examine the robustness and validity of analytical calculation models. Another supplementary test group with 21 mm plywood and fewer fasteners was also designed and tested, in which the plastic yield of fasteners was expected. The test results of this supplementary group served to calibrate the analytical model that predicts the elastic and post-elastic capacity of the connection group. As a result of the comparison, the analytical calculations gave reasonable predictions on the failure of plywood, glulam, and the capacity of the fastener group. Only when the exposed moment exceeded the post-elastic limit of the fastener group did the plastic yielding of fasteners become observable. Moreover, numerical finite element models adopting the foundation zone-modeling scheme were constructed, which were proven to capture all test configurations' linear loading stiffness satisfactorily.