This study presents an analytical, experimental, and numerical investigation of full-scale glulam trusses connected through birch plywood gusset plates. Three trusses were joined through mechanical connectors, while three were adhesively bonded. Failure was designed to occur in the plywood plates subjected to a multi-axial stress state due to the convergence of three glulam elements in the node. The thickness of these plates varied between 9, 12 and 21 mm. This research aimed to evaluate the structural behaviour and failure mechanisms of birch plywood plates in timber connections. Experimental results showed that glued trusses generally exhibited larger ultimate loads and greater global stiffness compared to mechanically connected trusses. Nevertheless, the global stiffness was independent of the thickness of the central plywood plates. Analytical estimations for specimens that failed in the plywood generally showed good agreement with the experimental capacity despite a slight overestimation, which was attributed to the size effect of the plywood's mechanical properties. Glued trusses with thicker plywood plates (12 and 21 mm) exhibited failure in the glue line between the central plywood plate and the glulam member. Despite the size of the glued area being the same for these two trusses, the experimental capacity differed significantly due to the varying values of wood failure percentage in the glued connections that failed. Furthermore, 2D planar numerical models demonstrated good prediction of the global experimental stiffness for both glued and dowelled trusses.

The proper design of timber connections requires precise characterization of the embedment strength and stiffness values. Valid embedment data are usually sourced from experiments following different testing standards. However, these values have been known to be influenced by discrepancies among different test standards, making it challenging to compare available embedment test data from various literature sources and use these values in predicting the strength and stiffness of connections. Therefore, it is essential to investigate and understand the differences among embedment test standards, of which EN 383 and ASTM D5764 are the two most representative ones. In this study, embedment tests on nearly 200 specimens were conducted following specifications in EN 383. First, the loading procedures, specimen configurations, and orientations slightly varied among different test series, so the significance of certain subtle parameters within EN 383 was examined. Second, the measured values were compared with the literature data tested following ASTM D5764. Thereby, by considering the difference in veneer layups, the reduction factor for plywood was analytically derived based on the formula for LVL as recommended in Eurocode 5. A combined formula was thereby derived to predict the in-plane anisotropic embedment strength values and compared with the experimental embedment data.

The accumulation of self-mass and the resultant permanent deformation are critical in the design of high-rise timber buildings, especially in the case of compression perpendicular to the grain (CPG). Metal fasteners, e.g., self-tapping screws, are conventional and standardized reinforcements against CPG in timber members. As a comparison, wooden rods provide a more environmentally friendly and cost-efficient solution. This study utilizes glued-in wooden rods and laminated densified wooden (LDW) rods as CPG reinforcements. Test series are first planned to characterize the single-fastener behavior by applying direct loading on the single rod. Thereafter, the global behavior of reinforced glulam specimens, following the loading of Case A and Case B as defined by prEC5, is investigated. All possible associated failure modes are observed and classified. The capacity and stiffness data are utilized to validate the proposed analytical loading mechanism of Case A. The number of glued-in wooden rods varied with the specimen geometry to investigate whether the effective numbers apply. Digital image correlation measurements were also performed on featured Case B specimens to visualize the stress dispersion and the reinforcing effect of the rods. Stiffness-related parameters are also discussed for all investigated specimens.

In timber connections, adhesively bonded connections are, in general, cheaper, stiffer, and stronger compared to typically adopted mechanical connections. However, bond line quality is sensitive to various process-related parameters; thus, it is generally preferable to do the gluing in the factory with these parameters controlled during assembly and then transport the entire structure to the construction site. This leads to a limitation on the size of the timber structure owing to the limited transportation capacities. Another concern regarding fully glued timber-to-timber connections falls in their non-ductile behavior prior to the ultimate failure, which is crucial, especially in seismic regions. This paper focuses on the design of two innovative connection systems aimed at overcoming these two key limitations. Both systems offer the advantages of indoor adhesive application and on-site adhesive-free assembly. The first connection system involves a hybrid solution connecting prefabricated elements by means of steel rods and special ‘‘wheel-geared’’ notches of birch plywood, while the second connection system employs pure plywood notched connections during the on-site assembly. These two novel connection systems have potential for use in both moment- and force-resisting applications. In this study, they were introduced and designed in the format of a frame corner, where bending moment, axial force, and shear force are present. Analytical models predicting the capacity for each possible failure mode were developed and then validated by the test results. It can be found that the first connection system exhibits moderate ductile behavior, and its load-bearing capacity is considered to be satisfactory. The capacity can be further improved to be as strong as the fully glued connection if thicker plywood plates are utilized. The second connection system possesses lower strength and stiffness. However, it could still be applied in non-critical connection regions where no substantial external load exists.

This study reveals the magnitude of stress spread angles in the design of plywood gusset plates when subjected to uniaxial tension, with a specific focus on mechanical connections. Plywood plates with elevating widths at three different load-face grain angles were destructively tested. The test series continued with consecutively increased plate widths until the measured forces reached plateaus. Two models, namely, the classic and modified stress spread models, adopted from the Whitmore effective width theory, were investigated to account for the observed phenomenon. The classic stress 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. It was found that the classic model, despite some close predictions, gave over-estimations on the load-bearing capacities of several connection patterns. The modified model was found to be conservative for almost all investigated fastener patterns. Accordingly, a hybrid adoption of stress spread models was suggested.

This study investigates the possibility of utilizing steel rebars as alternative mechanical fasteners in timber structures. Adopting rebars rather than conventional smooth dowels as fasteners in timber-timber connections might have positive aspects in terms of environmental impact and structural capacity. As to the environmental aspect, it should be mentioned that rebars are typically from two primary sources, namely: a) reused rebar offcuts or b) recycled steel scraps. Regardless of different origins, rebars have reduced environmental impact, especially since rebar offcuts are the primary waste source in concrete construction. As to the strength aspect, which is the main focus of this paper, the authors aimed to check whether enhanced load capacity of dowel-type connections could be achieved due to the higher withdrawal capacity of rebars as compared to smooth dowels. To verify this, tension-shear specimens with either two or four shear planes, adopting smooth dowels or steel rebars as the mechanical fasteners, were experimentally investigated. Plywood with thicknesses of 15 mm or 21 mm was adopted to further verify the contribution from the friction, assuming that thicker plywood generates a higher withdrawal capacity. Analytical models in Eurocode 5 were also adopted to predict the shear plane capacity and the connection stiffness during loading phases. Thereafter, a numerical model was constructed to verify the structural benefits of rebars by adopting different friction models. The model aims to explain experimental observations both quantitively and qualitatively.

Previous studies have demonstrated the potential of birch plywood as a substitute for steel plates in multi-plane shear connections of timber structures due to, among other things, its low environmental impact, better workability, and relatively low cost. However, models in modern building codes, such as Eurocode EN1995–1 (EC5), can be used to determine the load-carrying capacity of timber connections with up to two shear planes. Furthermore, some studies have shown that EC5 design model tend to underestimate the actual load-carrying capacity of timber connections. Therefore, there are some uncertainties concerning suitable design models to assess the load-carrying capacity of such connections. This study was preliminary conducted to shed light on the reasons for such discrepancies, conducting several experiments on both doweled and screwed birch plywood-to-timber connections, with either two or four shear planes. The analytical load-carrying capacities estimated by EC5 showed underestimation of the experimental results, with greater underestimation when fully threaded screws are adopted as fasteners, rather than smooth dowels. Furthermore, regardless of the type of fastener, a substantial discrepancy between EC5’s prediction and experimental results was observed when the number of shear planes was increased from two to four. The results of the investigations indicate that the main cause of the discrepancies might be associated with the so-called “rope effect” which is taken into account by EC5’s design approach in an over-conservative manner.