Timber Connections Research Articles

Behaviour of small-scale glulam timber connections with slotted-in steel plates and dowel-type fasteners reinforced with self-tapping screws

Braced frames are a common lateral force-resisting system in mass timber structures. In timber braced frames, timber-steel dowel connections are relied upon to provide ductility and energy dissipation capacity under earthquake loads. However, the ductility and energy dissipation capacity of these connections can be limited by the onset of a brittle failure mechanism (e.g., row-shear or group tear-out) prior to significant dowel yielding. This paper presents an experimental campaign investigating the structural performance of timber-steel dowelled connections reinforced with self-tapping screws. Twelve connections were tested; with 8 tested under monotonic loading and 4 tested under a cyclic loading regime. The tested connections included two different fastener diameters of 11 mm and 16 mm, with one or two internal steel plates, both with and without reinforcing screws designed to prevent brittle shear failure. The results demonstrate that the reinforcing screws are most effective for improving the ductility and energy dissipation for connections with similar design values of dowel yield and row shear capacities. In addition, the increases in ductility and energy dissipation with reinforcement were greatest for connections with high slenderness (defined as the ratio of dowel diameter to timber thickness) dowels. There was little improvement in the ductility of connections whose design capacity was dominated by row shear, although there was improvement in the energy dissipation of the connections. Overall, results of this study demonstrate the potential for using self-tapping screws to reinforce steel dowel connections and improve the seismic performance of mass timber braced frames.

Characterizing the Performance of Adhesive Used in Glued-In Rod Timber Connections at Elevated Temperatures

A glued-in rod, which combines timber, a metal rod, and structural adhesive, is a type of connection whose fire performance is complicated to define since the rod depends on the adhesive’s thermomechanical properties. The objective of this study was to characterize the fire performance of adhesives used in glued-in rods. A total of five structural adhesives were tested: three epoxies and two polyurethanes. The tests performed consisted of applying an axial load to the glued-in rods after heating the connection to a target temperature. Load-slip curves were generated for each sample to determine their behavior at different temperatures. The results show that a reduction of up to 70% can occur in the initial stiffness of the glued-in rod when the temperature at the bond line exceeds the glass transition temperature of the adhesive determined by a dynamic mechanical analysis. This research helps to determine the guidelines for the fire design of glued-in rods and related testing and ultimately leading to a design method for providing fire-resistance to connections made of glued-in rods.

Study of screwed bamboo connection loaded parallel to fibre

A method for connecting unfilled natural bamboo was proposed, comprising screws and metal plates. The method is commonly used in timber connections, yet in bamboo connections screws are rarely seen, mostly due to the perceived risk of splitting. This study aimed to assess the applicability of screws in bamboo through extensive testing of short-term laterally loaded connection parallel to fibre, including variation of several parameters. An existing timber model was found to be appropriate to estimate the connection yield capacity. In terms of the environmental benefits, connection capacity, stiffness and ductility, the method appears to be an attractive alternative to the common mortar-infilled bolted bamboo connection. The risk of splitting was found to be low provided appropriate spacing parallel and across fibre between screws is provided. The main outcome of the study is the design guidance for the proposed connection, including spacing rules and prediction of the connection capacity.

Effects of Re-drying on properties of cross laminated timber (CLT) connections

As mass timber buildings continue to gain widespread acceptance in North America, there is the need to address concerns, which might impact the performance of these novel buildings. Moisture intrusion poses significant levels of risks to mass timber elements and has been associated with durability concerns and possible reduction in performance of structural elements and connection systems. While it is recommended to employ design approaches that eliminate moisture accumulation in buildings, the possibility of elements getting wet during, or post construction remains reasonably high. Hence, it is important to re-dry elements once wet to avoid these issues. Additionally, connections constitute an important part of the lateral force resisting system and must be well studied for optimum performance under all conditions. This study therefore characterized the wetting and redrying performance of Cross Laminated Timber (CLT) angle bracket connections made from four CLT species. The effects of three different drying methods on the load carrying capacity, stiffness, and energy dissipating capacity of the connections were studied. Load carrying capacity of the connections was barely affected, but aggressive drying induced brittleness in some connections, and consequently impacted energy dissipating capacity. Physical observation of the connection assemblies also showed varying degrees of damage, which can be attributed to accumulation of drying stresses depending on the drying method employed.

Experimental testing on timber connections considering the influence of gap size and intumescent sealants

AbstractThis paper investigates the influence of gap size (openings/spaces between two structural members) and the presence of intumescent sealants on the development of temperature when timber connections are exposed to fire conditions. The experimental results of 21 samples representing a concealed steel‐to‐timber connection configuration that was exposed with a 0, 3, 6, and 10 mm gap to the ISO834 standard fire for 120 min are presented. Half of the samples in each sample group were protected with an intumescent fire protection sealant. The temperatures in the timber were measured at various locations around the gap and directly next to the steel component. The experimental results show that the presence of a gap increases the temperature of the timber more than predicted by the thermal penetration models available, with current code guidelines (e.g., Eurocode 5) possibly being non‐conservative. The use of an intumescent fire protection sealant in the gap is shown to be an effective protection method, but that the application configuration is important to ensure effective protection, especially as the gap becomes smaller. The results also show that the use of an intumescent fire protection sealant in a gap increases the predictability of the temperatures in the timber in longer duration fires. Unprotected samples and 0 mm gap samples exhibited large variability in the thermal development outcomes over time.

Distributed fiber optic strain sensing in axially loaded glued-in steel rods in glued-laminated timber

The rise in popularity of mass timber structures globally has increased the demand for high-performance timber connections. Glued-in rod connections which are advantageous compared to other dowel-type connections because of their high strength and stiffness, have been the subject of extensive research in the literature; However, the lack of an in-depth understanding of their behavior and a uniform set of guidelines for their design continues to limit the use of glued-in rods in practice. To develop an improved understanding of the behavior of glued-in rod connections this paper examines, for the first time, the use of distributed fiber optic sensors to measure the complete strain distribution along the length of a glued-in rod connection. Axial pullout tests were conducted on twenty-five glued-in rod connections in glued-laminated timber with varying embedment lengths and glue-line thicknesses. Results of the study demonstrate the ability of fiber optic sensors to capture the full strain distribution along the length of the steel rod and along the epoxy-adhesive interface, enabling the study of localized behavior that would be difficult to measure using conventional strain gauges. In addition, results demonstrate the ability of distributed fiber optic strain sensors to infer axial stress and force distributions along the length of a glued-in rod connection, which are compared with theoretical distributions from the Volkersen model, as well as measure bond-slip behavior.

Experimental Investigation of Ultimate Shear Force Capacity of Glulam Reinforced with Glued-in Threaded Rods

Background: Glued-in threaded (GIT) rods have been considered an effective timber connection method having several advantages, such as high load-carrying capacity, great stiffness, and good fire resistance. This type of connection has widely been utilized in newly built modern timber structures and for reinforcing the existing ones. Objective: The purpose of this experimental research is to investigate how the ultimate shear force capacity (USFC) of glulam is affected if it is reinforced with GIT rods and to study the interaction between the glulam and GIT rods. The results of this research can help researchers and engineers to select a suitable method of reinforcing the glulam with GIT rods at a superior inclination angle (IA) to the fiber direction in order to achieve higher USFCs. Methods and Experimental Tests: A total of forty glulam samples were tested for failure. The experimental tests consisted of five different series of samples with eight samples in each series. Sample series 1 were unreinforced glulam. Sample series 2 and 4 were glulam reinforced with the GIT rods at two different IAs of 45° and 90° to the fiber direction, respectively. Sample series 3 and 5 were glulam reinforced in the same way as sample series 2 and 4, but they were split in the middle. The samples were tested for failure using a standard testing machine. The machine was connected to a computer that recorded all the values obtained from the tests. Results: The results of the study were obtained as force-displacement graphs. The stiffness of the samples and interaction between the glulam and GIT rods were also calculated. The results achieved from the experimental tests of different series were compared and discussed. Conclusion: The experimental tests resulted in a 12.5% increase in the average USFC of sample series 2. However, the average USFC of sample series 4 was not enhanced significantly. The interaction between the glulam and GIT rods was obtained as 31.8% for the samples with reinforcing at the IA of 45° to the fiber direction and 3.8% for the samples with reinforcing at the IA of 90° to the fiber direction. Reinforcing the glulam with the GIT rods demonstrated the potential of enhancing its USFC. However, reinforcing the glulam with the GIT rods at the IA of 45° to the fiber direction was found to be superior to reinforcing the glulam with the GIT rods at the IA of 90° to the fiber direction.

Review of timber connection design in Australia

ABSTRACT This paper reviews previous research and current practice in timber connection design in Australia with reference to methods of deriving design information from test data for nails and screws. The two key documents for timber connection design in Australia are AS 1720.1 design of timber structures and AS 1649 test methods and evaluation of timber connections. These documents have a long history of development, with major changes in design methodologies – from deterministic working stress design to probabilistic limit state design. The supply chain for timber connections has changed with a reduction in the utilisation of timber species and an increase in new fasteners developed for specific purposes. A new approach to the design of timber connections has been proposed to cope with the changing situation. The review is of historic interest but also to ensure that the proposed new approach is consistent with past developments.

The Bolted Timber Connections; Bearing Shear Failure with the European Yield Model (EYM)

This study is done to evaluate the structural performances of bolted timber connections in local hardwoods. In Sarawak, Belian hardwood is typically used to construct the floor and wall diaphragm for unreinforced masonry (URM) buildings. Unfortunately, there is a lack of experimental data on bolted connection of Belian hardwoods. This gap in experimental data might lead to difficulties in designing bolted timber connections for strengthening purposes. Therefore, the study was carried out to evaluate the existing equations in predicting the bolted connection in Belian hardwood. A calibration factor for Belian hardwood will be provided to optimise the use of the current design equation in predicting the row shear failure mode. The European Yield Model (EYM) and Malaysia Standard (MS) will also be compared to the experimental results to verify the suitable equation to predict the strength of the ductile mode of failure. From the comparison between experimental versus prediction values, EYM and Row Shear equation were acceptable to predict the bearing and row shear failure while the MS was found too conservative with a range from 0.16 to 0.25 compared to the 5th percentile experimental results.

Experimental parametric study of perforated steel plate fuses for mass timber seismic force resisting systems

The lack of practical connections with reliable desired energy dissipation capacity as the main components of a seismic force resisting system (SFRS) is a major obstacle in using mass timber in tall buildings located in seismically active regions. A common method to dissipate seismic energy in timber structures is through the yielding of steel material found in the mechanical connections. Under the induced seismic motions, these dissipative connections are intended to yield while the rest of the structure remains intact with minimum damage. In this research, the focus is on the use of perforated steel plate as a dissipating energy device and a seismic fuse. Previous tests have demonstrated the potential use of perforated steel plates as seismic fuses in mass timber SFRS and confirmed their scalability and predictability. Despite promising results under monotonic loading, their behavior under cyclic loading can be improved. This paper presents an experimental parametric study investigating the effects of the number of rows, shape, and size of perforations, the steel plate thickness, and the steel link size on the mechanical performance of the perforated plates. The obtained hysteresis responses and envelope curves are compared. It is found that the ellipse and stagger patterns possess the largest ultimate displacement and stiffness, respectively. In addition, no significant enhancement in the ultimate deformation was observed in the cyclic response of the fuse, beyond three rows of perforations. Within the range of the configurations considered, the reduced ultimate deformation under cyclic loading remains a technical challenge that needs to be addressed if this type of seismic fuse is to be installed in SFRS. The results will assist with the design of future test programs on full-size mass timber connections consisting of such fuses.

FEM Analysis of 3D Timber Connections Subjected to Fire: The Effect of Using Different Densities of Wood Combined with Steel

This work aims to present a study approach for double-shear connections of wood under fire with dowel pins and plates in steel material, using different types of glulam. The simplified Eurocode equations for ambient temperature were used to determine the dimensions and the number of dowel pins that each studied connection needs in order to resist an applied tensile load. Following this methodology, the finite element method was used to assess the thermal analysis of the studied connections under fire. The study aims to increase the information on these connections, where the wood material represents a complicated behavior in fire circumstances, with the addition of the steel material. The heat conducted by the dowel pin inside the connection, and the steel plate and its effect on the wood were analyzed. According to the results, it can be assumed that the temperature evolution is due to the geometry of the connection, the dowel pin or plate position, and the glulam density. Inside the wood element, the temperature remains lower, and externally a charred depth is developed when the target temperature of 300 °C is reached, and, in the vicinity of the dowel pin or the steel plate, a burned wood depth is indirectly formed. The rate of the charred layer is not constant throughout the entire fire exposure. Steel-to-timber connections with an internal steel plate with high glulam density have greater fire resistance due to the lower temperatures obtained.

Influence Factors on the Load-Carrying Capacity of Bolted Timber Connections with Slotted-In Steel Plates

Influence Factors on the Load-Carrying Capacity of Bolted Timber Connections with Slotted-In Steel Plates

Numerical modeling investigation of cross-laminated timber connections consisting of multiple glued-in rods

Glued-in rods are stiff and high-capacity connections that are used in timber structures. Regardless of the many advantages of glued-in rods, research on the glued-in rods in CLT is limited, especially on numerical modeling. This paper provides the numerical modeling principles, validation details, and parametric studies of glued-in rods embedded on the edge of CLT. The numerical modeling was performed on a 3D finite element basis, and the cohesive surface method was used to define the bond lines along the rods. To validate the numerical models, 24 replica experiments were performed and investigated the effect of the rod diameter and of the embedment length on the pull-out strength of rods embedded perpendicular, parallel, and on the boundary of two cross-wised layers. The numerical models were validated according to the maximum load, stiffness, as well as the failure mode of the glued-in rods. The numerical models showed high reliability in simulating the experiments and predicted the strength of the glued-in rods with a maximum of 4% difference. After validation of the numerical models, the parametric study covered the pull-out strength investigation of rods with 20, 24, and 30 mm diameter and anchorage lengths of 250, 350, and 450 mm embedded in the perpendicular, parallel, and on the boundary of cross-wised layers on the edge of 5 and 7 layer CLT panels. The findings showed that increasing the embedment length and rod diameter enhanced the pull-out strength of the glued-in rods, regardless of where the rod was positioned on the CLT's edge. Rods embedded perpendicular to the grain showed the highest pull-out strength, and two rods with 30 mm diameter and 450 mm embedment lengths reached 961 kN pull-out strength. Numerical models showed that for the same diameter and embedment length of two rods embedded on the parallel and on the boundary of two cross-wised layers reached 836 and 738 kN pull-out strength, respectively. Finally, A design equation is provided to determine the pull-out strength of the glued-in rods embedded in CLT based on the shear strength of the bondline.

Timber connection behaviour characterisation based on similitude laws

Testing a full size structure requires very expensive means. Therefore, an alternative approach consists of testing a representative scaled down model, whose design is based on similitude laws. The aim of this article is to establish similitude factors in order to predict the full scale behaviour of timber connection from 1/2 and 1/3 reduced scales behaviour. Ultimately, similitude factors are applied to timber construction which depend on joint behaviour. These similitude factors were evaluated from experimental tests on connections and then compared to the theoretical ones. Due to distortion similarities, disparities have been observed between experimental and theoretical similitude factors.

Adhesively bonded joints between spruce glulam and birch plywood for structural applications: experimental studies by using different adhesives and pressing methods

ABSTRACT Due to the increasing interest in reducing the usage of metals in the construction sector, birch plywood has shown good potential in structural engineering and timber connections as a substitute for the more conventional slotted-in metal plate solutions. However, a proper way of bonding birch plywood with other timber parts that results in adequate connection strength has not been fully investigated. In this paper, experimental studies were carried out to investigate the proper workflow when manufacturing bonded connections made of birch plywood and spruce glulam beams. Tensile shear mechanical tests were then conducted on produced bonded joints to evaluate the bond strength. Three adhesives, i.e. melamine-urea-formaldehyde (MUF), phenol–resorcinol–formaldehyde (PRF), and two-component polyurethane (2C PUR) were evaluated in dry and moist conditions. The influence of three pressing methods, i.e. (a) screw-gluing, (b) clamping by means of clamps, and (c) clamping by application of weight loads, on the bonding strength was investigated. The bonding strength was thereafter compared with the shear strength of spruce glulam and the wood failure percentage was also examined in this study.

Optimizing composite floors for sustainability and efficiency: Cross laminated timber, concrete types, and ductile notch connectors with enhanced shape

Timber-concrete composite (TCC) structures allow designing efficient building floors in terms of lightness, slenderness, and acoustic insulation. The late development of ductile notch connectors has further advanced TCC structures. This work aims to further optimize the shape of ductile notch connections and to optimally design TCC floors with minimum embodied carbon, floor thickness, self-weight and cost.Firstly, an experimental campaign was carried out to characterize the shear behavior of ductile notch connectors with different shapes, which varied from rectangular to trapezoidal to avoid the use of screws, and concrete types, which varied from normal concrete to ultra-high performance fiber reinforced concrete. For the sake of comparison, a glued connection was also considered. Then, a multi-criteria optimization method was performed to optimally design TCC floor with the developed notch connectors and concrete types. The latest design recommendations for TCC floors developed in Canada were considered. A Non-Linear Finite Element Analysis (NLFEA) was carried out for validation. The presented results push the current design limits in terms of embodied carbon and volume of construction materials towards climate-positive floors.

Bolted steel to laminated timber and glubam connections: Axial behavior and finite-element modeling

This study aims to characterize the axial monotonic and cyclic tensile behavior of bolted steel connections to laminated timber and bamboo, using slotted-in steel plates, and to provide a high-fidelity simulation using finite-element modeling. Forty-eight experimental tests were conducted, including eight different configurations, varying in material (Laminated Veneer Lumber (LVL) and Glued Laminated Bamboo (glubam)), number of bolts, and diameter of bolts, under tension, both monotonic and cyclic (three repetitions). The force and displacement were recorded, and used to define the initial stiffness, yielding strength, ultimate strength, ductility, viscous damping, and energy dissipation. The failure modes of the connections were monitored using Digital Image Correlation (DIC) techniques. A novel high-fidelity finite-element model integrating Hill’s yielding and element removal criteria was proposed to predict the mechanical performance and fracture behavior of the examined connections. The numerical simulation was validated by comparing the experimental load–displacement curves and the strain field measurements obtained with DIC. Finally, theoretical formulas for predicting the stiffness of bolted connections were proposed based on the FEM simulations.

Embedment strength of dowel-type fasteners for the case of cross-laminated timber plane side insertion

The strength property of dowel-type connections in cross-laminated timber (CLT) is dependent on the embedment strength of the adopted dowels. In this study, eight groups with 164 CLT cubic specimens were tested based on the half-hole embedment test configuration to address the effects of the potential factors on the embedment strength. In the embedment tests, the axial direction of the smooth dowels was perpendicular to the plane face of the CLT cubic specimens (i.e., in the case of CLT plane side insertion). The considered influencing factors included the loading angle θ, the ratio between the thickness of the transverse layers to the thickness of CLT (RTrLay), and the gap between the longitudinal laminas within the embedment area. The experimental results were further utilized to verify the current empirical equations that could predict the embedment strength. Results show that the embedment strength of the dowels in the case of CLT plane side insertion is enhanced by 19.96–68.40 % when changing the θ from 90 degree to 0 degree. When the gap between the longitudinal laminas exists in the embedment area, the distinction between the embedment strength measured through the θ of 0 degree and that measured through the θ of 90 degree can be weakened. When the θ is 90 degree, the gap existing in the embedment area has almost no effect on the CLT embedment strength. In contrast, when the adopted θ is switched to 0 degree, the gap existing in the embedment area can weaken the CLT embedment strength by up to 37.11 %. When no gap exists in the embedment area, the embedment strength declines with an increase of the RTrLay; whereas when a gap between the longitudinal laminas exists in the embedment area, a growing trend of the embedment strength is illustrated with the increasing RTrLay.

Thermal response of timber connections in the cooling phase

AbstractStructures are conventionally designed to maintain load‐bearing capacity during the heating phase of a fire. However, in structures with moderate or high thermal inertia, the thermal field which results in the lowest structural resistance is likely to occur after the heating phase. This is of particular interest for timber connections because the strength and elastic modulus of timber reduces until the formation of char while steel plates and fasteners, which transfer forces between elements, conduct heat through the connection. It is unclear how thermal fields develop in timber connections during the cooling phase of fires and what influence different cooling rates have. Experiments on identical timber beam‐column subassemblies exposed to the same heating duration but two different cooling phases are presented. The results show that exposed steel components conduct heat into the connection, which propagates a thermal wave through the elements. Although the thermal waves had similar speeds, the specimen absorbed more thermal energy during the longer cooling phase, resulting in higher temperatures. Since the strength and elastic modulus of timber decrease at temperatures below 100°C, these results provide evidence that the structural resistance of a timber connection decreases in the cooling or post‐cooling phases and that a longer cooling phase is more severe than a shorter one. Further investigation into thermal exposure during the cooling phase of realistic compartment fires and the response of a wide variety of timber connections is required to quantify the reduced performance and support the development of appropriate design methods.

Experimental Investigation on Self-Centering Steel-Timber Hybrid Beam-Column Connections

Self-centering heavy timber frames rely on the self-centering beam-column timber connections to limit damage and provide the recentering capability. However, low compressive strength and stiffness perpendicular to the grain of the timber columns have long been a design challenge, which yields low initial post-tensioning forces and possibly a significant loss of post-tensioning forces over the building service life. To address this issue, this paper proposes a new design solution for the self-centering steel-timber hybrid beam-column connections, in which the timber column is replaced by a steel-timber composite column. Cyclic tests were conducted on four beam-column connection specimens with post-tensioning. During the gap-opening, the composite column provided a stiff foundation to the timber beam. The hysteretic curves of all four connections were in a flag shape. Compared with self-centering beam-column timber connections that had the same geometry and material properties, the proposed connection had a larger connection stiffness and an improved efficiency of dissipating energy. After the cyclic loading, the loss of post-tensioning force of the proposed connection was also lower than that of the counterpart timber connections.