Branches make the tree resilient to natural hazards by building a robust, fault-tolerant structure. In conifers, branches are formed annually at the internodes, giving rise to the crown. Branches that receive sufficient sunlight contribute to photosynthesis and are maintained, whereas shaded branches under competitive conditions lose their photosynthetic function and are shed. The cellular arrangement at the branch–stem junction reflects the biological principles of growth, function, and survival strategy. The relevant literature, however, has so far tended to describe this junction as an optimised load-bearing structure designed for maximum mechanical performance, with the aim of deriving bio-inspired engineering solutions from this understanding. The authors conclude that in the observed structure, and perhaps more generally in nature, the principle at work is not optimisation but rather resilience to the environment and survival. This hypothesis is supported by the analysis of microtome images of the junction’s morphology, which has enabled an improved representation and biological understanding of the fibre arrangement of Norway spruce. This improved understanding primarily concerns the orientation and organisation of the fibres on the upper side of the branch. Through the formation of a collar and a sacrificial tissue, nature creates a structure that is mechanically below optimal performance. From this we derive that the underlying principle is not mechanical performance, but resilience and survival.

Timber joints are still built almost exclusively with steel fasteners, despite the high carbon footprint of steel production. Wooden dowels made from dense hardwoods offer a circular and renewable alternative, but current design codes do not yet cover their failure mechanisms. This study investigates the bending–shear interaction (Mode V) in rotary-cut birch and profiled beech dowels through a combination of dynamic and quasi-static testing. Ninety specimens, in two diameters and three gap-to-diameter ratios, were tested to evaluate the influence of geometry and wood directionality on strength. Free–free modal tests provided axial, bending, and shear moduli, which proved to have limited correlation with the measured shear strength, highlighting the practical limits of dynamic grading for dowel performance prediction. In contrast, quasi-static double-shear tests revealed two purely geometric parameters, the unsupported span and loading direction (radial versus tangential), that govern the shear strength of hardwood dowels. A minimalist linear model based on these parameters predicted the yield strength of birch and beech dowels with cross-validated errors below 5%. Expressed in normalised stress space, all results follow an almost linear bending–shear failure line, confirming that Mode V is a brittle shear-dominated fracture mechanism, markedly different from the curved interaction surfaces assumed for ductile steel fasteners. The study concludes by proposing a simplified design format for hardwood dowels and outlining the boundaries of applicability for dynamic grading approaches. 

Plywood is a versatile composite material valued for its high strength-to-weight ratio and ease of fabrication, yet its structural use in exterior environments remains limited due to moisture sensitivity and biological degradation. This study investigates the influence of two industrial-scale preservative pressure treatments, waterborne Tanalith E3475 and oilborne Tanasote S40, on birch and spruce plywood composites. Thirteen full-size panels were treated using full-cell (Tanalith) and empty-cell (Tanasote) processes, followed by controlled drying. Retention and penetration were quantified by mass gain and copper mapping with the dithiooxamide reagent, while mechanical and physical performance were assessed by bending (MOR/MOE), in-plane compression, Brinell hardness, and dimensional stability observations. Retention increased systematically with decreasing panel size, reflecting the edge-to-volume ratio. Spruce panels generally achieved higher retention than birch, attributed to their thicker veneers and more permeable adhesive lines. Tanalithtreated panels exhibited notable warping and veneer cracking, whereas Tanasote-treated panels remained dimensionally stable. Relative to untreated controls, Tanalith reduced birch bending strength and stiffness by approximately 25%-30%, while Tanasote caused only minor strength losses (similar to 8%) and enhanced surface hardness (+35%-40%). The results indicate that oilborne treatment more effectively preserves both dimensional integrity and mechanical performance, highlighting its suitability for exterior-grade structural plywood.

In timber structures, the high bearing stresses at the supports in combination with timber’s low strength and stiffness perpendicular to the grain may result in local failure and/or substantial deformation in regions adjacent to the contact area. Common methods involve the use of self-tapping fully threaded screws to reinforce timber in compression perpendicular to the grain. The calculation models have been incorporated in the upcoming edition of Eurocode 5 (prEN 1995–1–1:2023). Although this reinforcement solution with fully threaded screws has been proven to work well, it has some shortcomings, e.g. that there might be a risk that the reinforcing screws would collide with steel parts present in the area, such as slotted-in steel plates and/or other mechanical fasteners. One alternative method to avoid this issue while also reducing the carbon footprint is to reinforce wood with wood. In this study, two birch products, namely, birch plywood and birch rod, are proposed as reinforcement materials. Comprehensive experimental campaigns were conducted to examine the efficiency of various assembly techniques in two different load configurations, one in continuous support with both ends fully supported and the other one in discrete support. Test results were further compared with the predictions based on the analytical models incorporated in prEN 1995–1–1:2023 for screws and steel rods. It was found that the current prediction model that simply sums up the contributions of unreinforced and reinforcement materials without considering their interaction effect, is conservative and results in the same predicted capacity when plywood is screwed or glued to the unreinforced material. It is thus suggested to introduce different interaction factors, α, in the prediction model for different reinforcement techniques.

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.

This paper investigates the load-carrying capacity of trusses consisting of glulam elements joined through birch plywood plates by means of either adhesively gluing or mechanical fasteners. A total of 6 trusses with a span of 7 meters were tested, three trusses for each connection type. Furthermore, the thickness of the plywood plates varied between 9 and 21 millimetres (mm). The main purpose of this research was to gain a better understanding of the failure mode of birch plywood subjected to a multi-axial stress state with loads from multiple directions. Therefore, the truss specimens were designed so that failure occurred in the central plywood plate, which was subjected to a multi-axial stress state. All specimens with mechanical connectors, as well as the bonded specimen with the thinnest plywood plate (9 mm), failed in the plywood plates. In contrast, bonded specimens with 12 and 21 mm thick plywood failed at the bonded interface between plywood and glulam. For specimens that failed in the plywood plates, the experimental results showed an increase in load-carrying capacity with increasing plywood plate thickness. Furthermore, glued specimens generally exhibited higher load-carrying capacity and elastic stiffness than those with mechanical connectors. For both glued and mechanically connected specimens, the thickness of the plywood plate did not show a significant influence on the stiffness. The analytical load-carrying capacity estimations slightly overestimated the experimental results of specimens that showed plywood failure, but they generally showed good agreement. The overestimation was attributed to the size effect of birch plywood's mechanical properties.

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.

The strength of timber when loaded perpendicular to the grain is significantly lower than when being loaded parallel to the grain. For softwoods, the compression strength perpendicular to the grain is approximately one-tenth of the strength parallel to the grain. Typical methods to increase the bearing strength of a timber beam at the supports include enlarging the support length and/or inserting reinforcing screws at the beam's support. In this study, two new methods of reinforcement for stress perpendicular to the grain are investigated through laboratory tests and analytical methods. The proposed methods utilize birch timber products which are adhesively bonded to the beam at the supports, i.e., where high bearing stresses occur. The adopted birch products: plywood with varying thicknesses and rods with a diameter of 19 mm. The laboratory test results show that the bearing capacity of softwood beams can be significantly increased if reinforced either by means of birch plywood plates or glued-in birch rods. The results pave the way for a new eco-friendly reinforcement technique. A simple analytical model to predict the load-bearing capacity for stress perpendicular to the grain of beams reinforced with the proposed technique is also suggested.

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.