Timber Connections Research Articles

Shape-Changing Wood Joints in Crafts and Industry and Their Potential for Building Construction and Wood Culture

Abstract Timber has regained popularity in construction in recent years due to its ecological benefits. The connection methods used in this study play a vital role in the sustainability of structures and materials. Monomaterial timber connections are sustainable alternatives to metal fasteners and adhesives commonly used in construction. Wood is an anisotropic material with dimensional changes resulting from changes in atmospheric conditions. Understanding and accounting for this property are crucial for the longevity and functionality of wooden structures. The cumulative knowledge of wood´s material characteristics and its use in design, construction, and human culture can be defined as wood culture developed through artists’ and craftsmen’s experiences, science, and industry. The development of various techniques by artisans to leverage the dimensional change in wood to join timber elements is a major contribution to wood culture. In contrast, until now, the timber industry has mainly focused on limiting or controlling these changes in standardized production and has neglected their use for joining timber elements. However, technological advances have changed dramatically. The digital manufacturing and analysis of wood structures have the potential to guide machine tools and may allow the integration of dimensional changes, especially in the design and construction of timber joints. This study explores the state-of-the-art utilization of dimensional changes in timber to join elements in craft, material science, and industrial production. The potential of techniques utilizing this behavior for innovation in modern design and construction and their implications for wood culture were examined. Research gaps and avenues for further research are identified.

Numerical modelling of the structural response of a novel hybrid densified wood filled-aluminium tube dowel for structural timber connections

This paper deals with the finite element modelling of the structural response of timber connections assembled using a novel hybrid dowel, made of densified wood filled aluminium tube, for the first time. Predictive and comprehensive finite element models, using the LS-Dyna Software, are developed to thoroughly investigate the non-linear 3D mechanical behaviour and optimize the design of the aluminium-to-timber connections. The materials parameters of the models are, first, identified based on experimental data from three-point bending tests. Then the models are validated by comparison to experimental data from slotted-in aluminium plate timber connections assembled either using steel dowel or hybrid densified wood filled aluminium tube dowel. The results are compared in terms of load-slip curves as well as in terms of failure modes. In addition, a parametrical study is conducted using the validated finite element model to investigate the influence of some material and geometrical parameters. The results showed that the hybrid densified wood filled aluminium tube dowels can be a potential substitute for conventional steel dowels. The study highlights the efficiency of the proposed finite element model in terms of quality of results and efficiency of use upstream the design process of such structures.

Proposing new adhesive-free timber edge connections for cross-laminated timber panels: A step toward sustainable construction

The use of timber as a building material is becoming increasingly popular thanks to its superior environmental performance compared with concrete and steel. However, timber structures rely on solid connections to improve their weak expansibility. Steel connections can be prone to corrosion over time, leading to the decreased structural integrity. Additionally, steel connections require more material and energy to manufacture and install compared with timber connections. This article focuses on the flexural performance of cross-laminated timber (CLT) panels with adhesive-free edge connections under four-point bending tests. First, numerical models of experimentally tested CLT panels were constructed using the finite element (FE) software ABAQUS. Then, these FE models were validated with the comparisons of their results with those of the experimental tests. Afterward, four new adhesive-free edge connections using timber for the CLT panels were developed in this study, helping sustainable construction. Utilizing the designed edge connections of the current study, forty-one parametric studies were numerically conducted on the connected CLT panels to investigate their ultimate loads, strains, displacements, moment capacities, failure modes, and effective stiffness. The factors affecting the edge connections’ load-bearing capacity were also examined and discussed. The study provides helpful insights into the development of CLT as a sustainable construction material with improved adhesive-free edge connections.

Impact of Acetylation on the Behaviour of Single-Dowel Timber Connections

This paper presents an experimental study where the mechanical behaviour of single-dowel timber connections made of acetylated Scots pine is compared with the behaviour of connections made from untreated Scots pine. The main aim was to evaluate the influence of the acetylation on the connection brittleness and also to compare the experimental results to the design provisions of the current European structural timber code, Eurocode 5 (EC5). The experiments included embedment tests and tests with connections loaded parallel and perpendicular to the grain, and, for the latter tests, applying different end and edge distances. The acetylated wood showed a 2% increase in density and a 31% increase in embedment strength compared to the untreated wood. For tests on connections loaded parallel to the grain, all specimens made from acetylated wood failed in a brittle manner, while the connections made from untreated wood and complying with minimum end distance of the EC5 design provisions failed due to embedment failure followed by splitting involving cracking along the grain. The connections made of acetylated wood showed a 13–15% higher capacity than the corresponding specimens made from untreated wood. Thus, to fully utilize the potential of the increased embedment strength parallel to the grain, it is concluded that reinforcement of the joint, e.g., by self-tapping screws or externally applied sheet reinforcement would be necessary if the minimum end distances of EC5 are applied. The current design provisions for loading perpendicular to the grain overestimated the capacities severely with predicted characteristic values being 20–50% higher than mean values from tests for the recommended minimum edge distances. Finally, it was found that the splitting capacity in loading perpendicular to the grain was 10–18% lower for the specimens made from acetylated wood compared to the untreated wood.

Evaluating the load-carrying performance of single-bolted connections of inorganic-bonded bamboo composite

ABSTRACT In the field of civil engineering, the utilization of bio-based composites is a viable solution for promoting environmentally friendly practices. A novel bio-based inorganic-bonded bamboo composite (InorgBam) prepared with roller-pressing impregnation, cold pressing, and molding technology was first introduced, and 72 double-shear specimens were tested to investigate the load-carrying performance of InorgBam single-bolted connections. The failure characteristics of the InorgBam single-bolted connections were evaluated using the European yield model (EYM). By incorporating the existing bilinear and Attiogbe models, the load-displacement relationship of the InorgBam single-bolted connections was foretasted with precision. The effect of bolt diameter and component thickness on load-bearing capacity was compared, and relevant design suggestions were proposed. Prediction equations of load-bearing capacity for timber connections in existing standards were briefly reviewed and compared with the test results to illustrate their applicability for InorgBam connections. Comparison results indicated that the equations for timbers exhibited an adequate safety redundancy for the InorgBam. Finally, a simplified equation was proposed based on nonlinear fitting that aims to provide a convenient approach to design activities for InorgBam bolted connections and promote the structural application of InorgBam material.

Beam-on-Foundation modelling of dowel-type single fastener connections in cross laminated timber

In this study, a numerical model that aimed to predict the strength and stiffness, of a single laterally loaded mechanical fastener in steel-to-cross laminated timber (CLT) connections using a Beam-on-Foundation (BoF) model, is presented. The BoF model was used to predict the ductile failure modes of steel-to-CLT connections and considered the layered structure of CLT. When compared to the European Yield Model (EYM), the BoF model was found to be advantageous as it not only predicted the strength but also the stiffness of the connections. A comparison of the slip modulus and strength from BoF model simulations of experimental tests carried out on steel-to-CLT connections showed good agreement with the experimental results. Using the BoF model, the influence of; (i) density, (ii) deck layer orientation, (iii) embedment behaviour, (iv) dowel diameter, (v) steel plate thickness, and (vi) embedment length, on steel-to-CLT connections was investigated in a parameter study. The results presented herein highlight the benefit of the BoF model that considers CLT layer specific embedment behaviour in determining the shear capacity and stiffness of steel-to-CLT connections.

Glulam beams adhesively bonded by birch plywood plates in moment-resisting beam-to-beam connections

The design of timber connections is of vital importance in timber structures. Bonded connections exhibit the advantages of lower cost, higher load-bearing capacity, and higher stiffness compared to conventional mechanical connections. However, the potential of the bonded connections has yet to be fully exploited, not only due to their sensitivity to the adhesive types and process-related parameters but also due to the lack of studies regarding the structural performance of the bonded connection in various loading conditions. In this paper, birch plywood plates were utilized to adhesively connect two glulam beam halves to create a longer span. Plywood made of birch was chosen because birch is highly resourced on the Eurasian continent, with its mechanical properties better than most softwoods. Specifically, glulam beams were connected by birch plywood plates at mid-span and then loaded in four-point bending. Four test series with two different bonding areas and birch plywood face grain orientations were carried out. The bonded region was designed as the weakest part to investigate the failure modes, moment capacity, bending stiffness, and moment-rotation angle relationships. Furthermore, numerical models were developed to predict the structural behaviors in the linear elastic stage, while analytical models were proposed and subsequently modified to predict the moment-carrying capacities. Both numerical and analytical models displayed satisfactory agreement with the test results.

Enhance resilience in timber connections with shape memory alloy and densified veneer wood

Dowel-type timber connections are commonly found in historic buildings and require reinforcement and repair to ensure that their resilience against natural disasters is maintained. This study aims to enhance the resilience of dowel-type connections by equipping them with shape memory alloy (SMA) dowels and densified veneer wood (DVW) reinforcement. The research findings reveal that the performance of SMA bars under cyclic bending is superior to that of steel, in terms of self-centring effect and ductility. Dynamic testing of the SMA-dowelled timber connections shows that connections using SMA exhibit lower residual deformation and better self-centring than those using steel. Additionally, the DVW reinforcement improves not only the self-centring but also the resistance of the SMA-dowelled connection. The comparison between solid bars and tubes shows that the solid bars perform better in terms of damping, resistance and self-centring. These results demonstrate the potential of developing resilient timber connections and improving the performance of SMA-dowelled timber connections.

Feasibility of using Uruguayan Eucalyptus for sustainable timber connections

Uruguayan fast-growing Eucalyptus grandis has emerged as a promising sustainable material for structural building components. However, traditional methods for joining timber components, such as beams and columns, typically rely on metallic fasteners, and mass timber products like glued laminated timber often use adhesives. In response to growing environmental concerns over the past five years, researchers have explored the substitution of petroleum-based adhesives and metallic fasteners with wooden pegs or dowels. To create effective connections, these dowels must be fashioned from denser wood than the lamellae. The embedment strength of both woods with varying densities is a critical factor in the design of timber connections using fasteners. This paper presents the results of a structural performance analysis of fully eucalyptus dowelled connections, constructed by combining Eucalyptus grandis (? = 473.2 kg|m3, COV = 13.2 %) for the lamellae and Eucalyptus tereticornis (? = 1042.4 kg|m^3, COV = 3.2 %) for the dowel. The bending strength of dowels was determined for specimens with 15- and 18-mm diameters, yielding 173.0 MPa (COV = 10.7 %) and 184.0 MPa (COV = 8.3 %), respectively. Four series of connections were tested, varying the fiber orientation of the lamellae (longitudinal L, transversal T) and dowel diameter (15, 18), following the half-hole method described in ASTM D 5764 standard. The embedment strength values obtained for the connections are as follows: L-15 = 31.3 MPa (COV = 9.3 %), T-15 = 11.2 MPa (COV = 28.7 %), L-18 = 29.6 MPa (COV = 10.1 %), T-18 = 16.7 MPa (COV = 25.7 %). These results reveal that the longitudinal application of the load, aligned with the fiber orientation, exhibits approximately twice the strength of the transversal orientation, with less variability in the outcomes. Furthermore, a strong correlation with the lamellae density was observed, while no significant dependency on the dowel diameter was found. These findings represent a crucial initial step in characterizing and designing glue-free wood-wood connections. The utilization of different eucalyptus species presents a promising path for the development of connections that promote more sustainable timber construction systems.

Bio‐based laminated truss structures with bolted steel connections: Experiment, modeling, and model‐updating

AbstractBio‐based laminated structures offer a timely solution to meet the pressing demand for resource‐efficient and environmentally responsible construction practices in civil engineering. Bolted connections are commonly used in truss joints and significantly impact their global mechanical behaviors. However, existing studies on the complex behavior of joints for integrated bio‐based structures are lacking. Based on previous work by the authors about joint behavior, this study seeks to experimentally investigate the axial behavior of bolted steel to laminated timber and bamboo (glubam) connections with slotted‐in steel plates in planar truss structures. First, cyclic tests were conducted on Bio‐based laminated planar truss structures to evaluate their global and local hysteretic behavior. The test matrix encompasses eight distinct connection configurations, carefully selected to assess the connection behavior of Laminated Veneer Lumber (LVL) and Glued Laminated Bamboo (glubam). Subsequently, through the incorporation of the hysteretic model for truss joints, an advanced hybrid truss model was developed within the OpenSees framework and successfully validated using experimental data. Finally, this study proposes and demonstrates the effectiveness of an efficient model‐updating framework, which combines the power of the genetic algorithm (GA) and neural network, resulting in a significantly enhanced accuracy of the model.

Cyclic pull-out behavior of screwed-in threaded rods embedded in glulam elements: Experimental and numerical studies

In this study, to support the application of screwed-in threaded rods (STRs) in timber connections, STRs with varying anchorage lengths were tested under cyclic pull-out loading. A new type of hysteresis model was developed in OpenSees to describe the cyclic shear-slip behavior of the timber layer in STRs by modifying the classical SAWS hysteresis model, and the parameters of the model were identified through the genetic algorithm implemented in MATLAB. Based on the numerical model, a cohesive layer possessing the modified SAWS shear-slip hysteresis rule was developed in an ABAQUS user subroutine and adopted into finite element (FE) models to simulate the pull-out behavior of STRs. The test results demonstrated that the withdrawal capacity of the STRs increases with increasing anchorage length, while the anchorage length has a limited influence on the withdrawal stiffness at the serviceability state. The hysteretic curves of the STRs with an anchorage length of 200 mm or 300 mm show a significant pinching effect due to the shear failure of the timber layer, while those of the STRs with an anchorage length of 400 mm exhibit plump hysteresis loops owing to the fully utilized plastic behavior of the steel rod. The numerical analyses indicated that the cyclic shear-slip behavior of the timber layer in the STRs can be well captured by the modified SAWS model. Benefiting from the specially developed cohesive layer and the elastic–plastic damage material constitutive model in the FE analysis, the FE numerical results show good agreement with the test results in terms of the failure mode, withdrawal properties, cumulative energy, and load–displacement curves.

Characterizing properties of fungal-decayed cross laminated timber (CLT) connection assemblies

The effects of brown rot decay fungi on mechanical properties of CLT connection assemblies were investigated. CLT connections evaluated were assembled with code approved angle bracket connectors and modelled as floor-to-wall systems in mass timber buildings. Physical changes, mass loss and quasi-static cyclic tests were used to assess the performance of connection assemblies up to 40 weeks after fungal inoculation. Peak load, stiffness, energy dissipation and ductility of connections were characterized based on force–displacement data generated from the destructive test of connections. Assemblies experienced up to 57 % loss in load carrying capacity and 90 % loss in energy dissipating capacity of the connections after 40 weeks of fungal exposure. Connection stiffness was only slightly impacted over this period but wetting and redrying caused significant degradation of connection ductility.

Numerical modelling of moment-transmitting timber connections

In recent years, the use of timber in construction substantially increased due to the material’s renewable nature, lower climate impact and increased economic competitiveness. Another driving factor is great improvements in modelling techniques for the design of timber structures. Suitable prediction of the connection behaviour, as a fundamental part of the structural behaviour of timber structures, is crucial for a more economic and reliable design. However, the more realistic and complete connection models, the more complex and difficult to handle they are, which might hinder their practical application. A good trade-off between complexity and computational efficiency can be achieved with the so-called Beam-On-Foundation (BOF) method, which is applied herein in a 2-step hierarchical model to analyse and predict the rotational stiffness, ductile capacity and load distribution among fasteners of four different configurations of moment transmitting beam-to-column timber-to-timber connections. The connection model is validated with experiments on the global response of the connection as well as with a 3-D solid FEM model. The herein proposed connection model well predicted the overall connection response and provided insight into the local fastener behaviour. As compared to the 3-D solid model, which additionally gives access to more realistic local stresses in the timber, the 2-step model is however much more efficient with a great reduction of computation time. This makes the approach suitable for parametric studies and the analysis and engineering design of timber structures.

Experimental investigation on axially-loaded threaded rods inserted perpendicular to grain into cross laminated timber

This paper presents an experimental work on axially-loaded threaded rods inserted perpendicular to grain into the narrow face of cross laminated timber (CLT). Penetration length, loading type (tension, compression, fully reversed), and loading configuration (pull-pull and pull–push) were varied. The use of self-tapping screws as reinforcement was also explored. Stiffness under cyclic and monotonic loading, damping ratio, withdrawal capacity, failure mode, reinforcement effect, and influence of loading history were investigated. The experimental results highlight the high withdrawal stiffness and capacity of threaded rods embedded into CLT elements, and hence their effectiveness as fasteners for stiff timber connections.

Design and analysis of timber-concrete-based civil structures and its applications: A brief review

Abstract Builders, designers, and the research community are becoming interested in incorporating timber and concrete based composite structures because they effectively integrate the structural qualities of timber and concrete. The structure’s stiffness, ductility, and load capacity are all affected by the quality of the timber connections used in construction. However, timber-concrete-based structures are limited due to a lack of design knowledge and the brittle failure behaviour of timber under shear or tensile loading. Experimental, numerical, and analytical methods have been proposed in the literature, and the key parameters influencing the performance of timber-concrete structures have been discussed. This study addresses the current challenges in designing timber-concrete connections and their failure modes and suggests simple performance-based analytical models that determine the failure mode. It looks at some of the best numerical design methods used in the past and tries to determine the best way to use timber as a possible way to use safe design principles for timber–concrete composite structures in the future.

Combined effects of axial and lateral loading for Glued-In steel rods in laminated veneer lumber

Glued-in rod timber connections are used to achieve structural continuity in timber structural members and as moment connections due to their high efficiency, stiffness and axial load transition. Existing studies have focused on axial withdrawal load capacity but there is limited research on laterally loaded glued-in rods and combined effects of lateral and axial loading which is common loading condition for all connections. This study investigates the relationship between axial and lateral loading in the ductile failure modes of glued-in steel rods in hardwood LVL. In all specimens an unbonded length was provided based on the latest design recommendations to mitigate splitting failure. The experimental findings suggest that the introduction of lateral load had no effect on the axial load capacities of the rods when the governing failure mode was rod yielding. This is not currently addressed in Eurocode 5. The lateral force capacity exceeded the design code prediction by 82%, and design recommendations are discussed for more accurate estimation of the lateral capacity. The use of the component method can be adopted to estimate accurately the rotational stiffness of moment connections with glued-in rods in LVL.

Moment response of post-tensioned timber beam-column connections in fire – Experimental behaviour of a simple connection

Post-Tensioned Timber (PTT) frames combine high-strength tendons with heavy timber elements to create seismic low-damage or longer span timber structures. Although many aspects of structural performance, including seismic and long-term performance, have been investigated the behaviour of beam-column connections in fire has not been studied. Since it is the ductile rocking beam-column connections that give this system its advantageous response, the behaviour of these connections is key to evaluating the full-structural performance in fire. This paper presents the results of an experiment of a simple PTT beam-column subassembly under an applied load and exposed to an ISO 834 Standard Fire. The connection decompressed after 3 min and the beam rotation increased exponentially until it the applied moment was unable to be resisted at 20 min. Temperatures in the timber connection away from and at the connection are presented. Although the tendon force decreased from the effects of heating, the primary reduction in tendon force was due to crushing of timber under the anchorage.

ASPID: An asymmetric pinching damaged hysteresis model for timber structures

This paper presents a new high-fidelity phenomenological-based hysteretic model called ASPID that includes asymmetry, pinching, and strength/stiffness degradation for timber members. To capture asymmetry, one force–displacement envelope and ten physical-based hysteretic parameters are used for each loading direction. Pinching is added with an unloading–reloading monotone smooth piecewise function composed of two quadratic Bézier polynomials and one linear segment. Strength degradation at target displacement is taken using a novel fatigue law based on the maximum displacement and hysteretic dissipated energy. Exponential stiffness degradation of pinching and reloading phases is adopted in terms of their maximum displacement, whereas irreversible damage is captured using two threshold displacements independent of each loading direction. The update force algorithm is included for their computational implementation, and a Python version can be downloaded for free. The model is validated using six experimental benchmark tests of mass and lightweight timber connections and assemblies with symmetric/asymmetric behavior, where an optimized parameter identification process is considered. Moreover, the model test responses are compared with three well-known hysteretic models: SAWS, Pinching4, and DowelType. The ASPID model’s test results show an error less than 7.6% for the capacity and 2.9% for the cumulative dissipated energy as well as fits with high precision the force and dissipated energy history, getting a Normalized Root Mean Square (NRMS) error less than 4.3% and 2.8%, a Normalized Mean Absolute (NMA) error less than 2.7% and 1.5%, and a coefficient of determination R2 over 94.88% and 99.09%, respectively. Finally, comparing the four hysteretic models, the ASPID model gives the highest R2 values in all tests and has the smallest NRMS and NMA errors regarding force history and the smallest NMA errors for the dissipated energy history.

Strain Rate Effect on the Embedment Mechanical Properties and Fracture Behavior of Softwood Laminated Veneer Lumbers: Implications to Timber Connections

Strain Rate Effect on the Embedment Mechanical Properties and Fracture Behavior of Softwood Laminated Veneer Lumbers: Implications to Timber Connections

Seismic Behavior of Balloon Frame Cross-Laminated Timber Connections

Seismic Behavior of Balloon Frame Cross-Laminated Timber Connections