Timber Engineering Research Articles

Split wooden rods for novel wood-based boards in the construction sector

Wood has been utilized as a building material for thousands of years. Nowadays, its renewable nature and carbon-storing capacity can become important factors in climate change mitigation efforts. This has led to a resurgence of timber engineering in recent years, with impressive multi-story timber buildings worldwide. However, it should not be overlooked that the wood sector will face several challenges in the coming years and decades to pave the way to a leading role of wood in the desired transition toward bioeconomy. Based on the assumption that an increasing demand for wood will make it a more precious resource, a couple of strains will emerge across the entire value chain of wood processing. This calls for innovations to address issues arising from predicted changes in resource provision, to increase material yields, and to promote reuse after the end of life. Our conceptual article proposes a new wood separation and processing method. This approach is inspired by the well-known production of wood shingles and is currently being developed for the implementation of new wood-based products.

Project-based learning in timber engineering education: A recent example

Project-based learning in timber engineering education: A recent example

End-grain bonding of spruce timber at low curing temperatures – effects on tensile strength and how to improve

ABSTRACT The end-grain bonding of timber components using Timber Structures 3.0 technology (TS3) represents an emerging construction method in the field of timber engineering. For onsite applications, research is being conducted to determine how low temperatures during the curing process affect the bonding and to explore potential methods of mitigating any adverse impacts. The current research results reveal that low curing temperatures detrimentally influence the mechanical properties of the bond. Conversely, investigations into bonding with heated casting resin as a means to counteract the effects of low curing temperatures demonstrate highly beneficial outcomes. Overall, this study provides design-relevant tensile strength values, which, when coupled with suitable application strategies, enable effective bonding under low ambient temperatures. Future investigations will delve deeper into the failure mechanism (adhesion – cohesion) as it relates to curing temperature variations.

Assessment of timber engineering needs and practices in Bhutan: A comprehensive study

A survey was conducted to explore the intensity of timber usage and evaluate the knowledge and experiences amongst working professional timber handlers in Bhutan. Subsequently, a need assessment survey was carried out to understand the need and demand for certification courses in timber engineering in the country. Despite having an abundant timber resource and massive usage, Bhutan still needs to gain expertise in timber engineering. This study involved 90 respondents from timber processing, construction and manufacturing sectors across 12 districts in the country. Twenty-one respondents are timber technocrats who have significantly contributed to timber associate infrastructure, while 61 are from the Wood-based Industries, specifically from the timber manufacturing sectors. The key findings from this survey observed several barriers to timber use that include limited knowledge amongst designers, clients and construction companies, alongside concerns about durability and fire resistance. As per the findings, these challenges could be addressed through a dedicated timber engineering course, an idea supported by participants, with 80% of manufacturing professionals and 55–60% of others. The respondents also highlighted the demand for timber testing facilities in the country. This research underscores the need for timber engineering expertise in Bhutan's construction industry.

Low-temperature adhesive curing in timber engineering: About the relationship between curing kinetics and mechanical properties

Adhesive bonding plays a pivotal role in timber engineering, enhancing structural integrity, sustainability, and aesthetic appeal, while also addressing environmental concerns. The assessment of strength in adhesively bonded timber joints involves cohesive strength, adhesive strength, and substrate failure, all of which are crucial considerations for designing dependable timber structures. A comprehensive investigation was carried out with the aim of improving adhesive bonding for construction by revealing the relationship between curing progress and mechanical adhesive properties. For that, dynamic DSC measurements, kinetic modelling, tensile tests to determine the cohesive and adhesive strength, as well as tests for the evaluation of stiffness and hardness were performed using a two-component polyurethane adhesive. The investigation yielded valuable insights, particularly regarding the time- and temperature-dependent development of the curing degree and the aforementioned material properties. Additionally, the correlation between the curing degree and the respective material properties could be determined, showing that cohesion and stiffness built up occurs quite similar while the build-up of adhesive strength correlates well with hardness. It was thus concluded that Shore D hardness might represent a practical indicator for monitoring the progress of curing at low temperatures, which is a suitable means of improving adhesive applications in the construction industry.

Enhancement of mechanical characteristics and novel damage assessment for timber post and beam structures with K-braces

Glued-laminated Timber Post and Beam Structural System (GTPBSS) gains widely application in timber engineering because of direct force transmission and simple construction. The present work investigates the influence of introducing K-bracing on the lateral performance of the GTPBSS. To this end, four full-scale GTPBSS specimens with and without braces under horizontal loading conditions were carried out. The obtained test results highlight that the GTPBSS without braces exhibited large deformation. The failure modes predominantly centered on the occurrence of wood cracking at both the column-base joint and beam-column joint. In contrast, the GTPBSS specimens equipped with K-bracing elements exhibited failure modes marked by the instability of the brace components. The inclusion of brace system within the GTPBSS structures yielded positive results, effectively reducing lateral deformations while concurrently augmenting both the elastic stiffness and the later maximum load-carrying capacity. However, it is essential to exercise caution with regard to potential out-of-plane instability. A dual-parameter damage assessment model was developed to evaluate structural damage, incorporating factors related to energy dissipation and deformation. The evaluation results derived from this model were consistent with the test results, affirming the rationality and validity of the proposed approach. This model is expected to provide a scientific foundation for formulating strategies for repair and maintenance.

The Current State of Gender-based Diversity within the Field of Canadian Professional Structural Timber Engineering

There is currently minimal data that may be used for retention and recruitment strategies for timber professional engineering in Canada. A comprehensive survey performed in 2022 to address this lack of data is discussed. The survey was generated and analyzed by York University (Canada) and distributed through the Canadian Wood Council’s membership directory. This survey explored participants’ motivations for working on timber-based structural projects and the existing state of EDI training within the workspace. Findings from the context of analysis illustrated that retention in the work force appears not to be affected by gender, with participants overwhelmingly indicating their plan is to continue to work in the timber structural engineering sector. Gender specific findings suggest that most men who practice timber structural design have had more recorded work experience, whereas women were more likely to obtain a PhD when pursuing graduate school.

Monotonic testing of single shear-plane CLT-to-CLT joint with hardwood dowels

Reducing embodied carbon emissions in buildings and striving for carbon neutrality through sustainable design has become a primary goal in the construction industry. The industry is expanding its use of wood-engineered products, such as cross-laminated timber (CLT) to achieve those sustainability goals. Furthermore, replacing steel fasteners with lower embodied carbon alternatives is gaining increased attention in timber engineering. This paper aims to quantify the mechanical behavior of single shear-plane CLT-to-CLT joints using hardwood dowels. The study includes 154 single shear-plane experimental tests with four different CLT species, two hardwood dowel species, and two dowel diameters. The test specimens were subjected to cyclic and monotonic loading until failure, and mechanical properties such as yielding and ultimate strength, serviceability and yielding stiffness, and ductility were quantified. The hardwood dowel CLT-to-CLT joints demonstrated high strength with up to 10.3 kN (2.3 kips) capacity per dowel and lateral stiffness of up to 5.2 kN/mm (30.8 k/in) per dowel. This strength and stiffness are roughly equivalent to or exceed values for mass timber wood screws, making hardwood dowels a good alternative. In addition, the observed mean ductility of joints ranged from 2.4 to 4.5. Finally, analytical equations for the joints' yielding strength, lateral serviceability stiffness, and yielding stiffness were derived with non-linear regression analysis and validated using the experimental test results.

Multi-step creep tests of notched beams of silver fir (Abies alba) at different initial moisture contents

Silver fir is an important species for French timber engineering industry but its application faces drying issues. This paper presents the results of creep and ramp-loading bending tests on full-scale silver fir notched beams at different initial moisture contents. Initial moisture content showed no discernible impact on instantaneous maximum force, while dynamic compliance appeared to be proportional to this force, possibly due to the dowelling effect. Beam-theory equations are solved using finite differences with crack propagation assimilated to a height reduction, providing an estimation of the compliance and that total deflection primarily results from creep rather than crack. No correlation is observed between the maximum relative compliance and the initial moisture content. Besides this maximum value never reaches the deformation factor for serviceability design in Eurocode 5, suggesting oversizing for green wood. The discrepancy between experimental and theoretically derived crack openings highlights limitations in traditional beam theory, especially concerning fracture mechanics analysis and non-linear effects. For all tests upward crack trajectories towards compression zone are observed and physically discussed. These findings contribute to a better understanding of the mechanical behaviour of wood, particularly regarding creep and crack propagation, with implications for structural design and material characterisation.

Reinforcement for earthquake-damaged glued-laminated timber knee-braced frames with self-tapping screws and CFRP fabric

Glued-laminated Timber Post and Beam Structure (GTPBS) is favored in timber engineering, primarily due to its commendable seismic resistance. Nonetheless, under severe earthquakes, these structures are susceptible to damage. This study aims to understand the impact of diverse reinforcement techniques on the lateral resistance of earthquake-damaged GTPBS. For this purpose, two full-scale frames were reinforced using self-tapping screws and Carbon Fiber Reinforced Polymer (CFRP) fabric before being subjected to horizontal low-cycle reversed loading tests. An in-depth comparison was made considering failure modes, hysteresis curves, and other pertinent parameters of the specimens both before and after reinforcement. Evidently, the employment of both self-tapping screws and CFRP fabric reinforcements curtailed the initiation and progression of joints cracks, averting premature structural failures. In addition, the better ductility of knee brace makes it more suitable for reinforcing earthquake-damaged GTPBS. These reinforcement techniques managed to recover over 90% of the bearing capacity of earthquake-damaged GTPBS, with a notable boost in their stiffness and ductility. The study culminates with an energy analysis that delves into the transformation and relative contributions of hysteresis energy and elastic strain energy during loading. From this analysis, it emerged that reinforcements using self-tapping screws and CFRP fabric not only ensure structural robustness but also maintain consistent energy dissipation.

On-Site Application of End-Grain Bonded Timber Under Low Curing Temperatures

The end-grain bonding of timber components using the Timber Structures 3.0 technology (TS3) is an advancing construction method in timber engineering. This technology allows the realisation of any plate size by bonding plates on-site where low temperatures influence the performance. Therefore, investigations are underway to evaluate the influence of the low temperatures on the curing process. Additionally, research is in progress to discover techniques to reduce any adverse effects of low curing temperatures on the mechanical properties of the bond. The implementation of particular measures, such as the inclusion of milled heating wires into the joint and the pre-heating of the joint with a hot air blower, has been identified as advancing the potential for grouting even in the face of low external temperatures, as recent research has indicated.

Advancements and Applications of One-Component Polyurethane Adhesive in Timber Engineering: A Mini Review

Advancements and Applications of One-Component Polyurethane Adhesive in Timber Engineering: A Mini Review

Uncertainty quantification in timber-like beams using sparse grids: Theory and examples with off-the-shelf software utilization

When dealing with timber structures, the characteristic strength and stiffness of the material are made highly variable and uncertain by the unavoidable, yet hardly predictable, presence of knots and other defects. In this work, we apply the sparse grids stochastic collocation method to perform uncertainty quantification for structural engineering in the scenario described above. Sparse grids have been developed by the mathematical community in the last decades, and their theoretical background has been rigorously and extensively studied. The document proposes a brief practice-oriented introduction with minimal theoretical background, provides detailed instructions for the use of the off-the-shelf Sparse Grid Matlab kit (freely available online and straightforward to use) and discusses two preliminary examples inspired from timber engineering problems that highlight how sparse grids exhibit superior performances compared to the plain Monte Carlo method.

Numerical modeling in timber engineering – moisture transport and quasi-brittle failure

Numerical modeling in timber engineering – moisture transport and quasi-brittle failure

Physical and Mechanical Properties Performance between Untreated and Treated with CCA Treatment at Different Age Groups of Fast-Growing Acacia Hybrid of Sarawak

An effort was carried out to fully utilise fast-growing Acacia hybrid usage in the timber engineering field; however, the research data are still lacking. This paper aims to evaluate the physical and mechanical properties performance between untreated (control) and treated with 10% copper chrome arsenic of Acacia hybrid collected from Daikin Plantation Sdn. Bhd. Bintulu, Sarawak at air-dry condition at different age groups using the small clear method. Mechanical properties test refers to shear parallel to grain (tangential and radial directions), cleavage (tangential and radial directions), and tension parallel to grain test. Meanwhile, the physical properties test refers to moisture content (MC) and density test. After treatment, mechanical properties increase with an average of 13.67%; meanwhile, moisture content decreased with an average of 0.58% or 0.09% MC, and density slightly increased with an average of 0.44% or 0.002 g/cm3. Results indicate that 10-year-old Acacia hybrid exhibits the highest strength values in shear parallel to the grain, tension parallel to the grain, and cleavage, followed by 13-year-old and 7-year-old. Treated samples in the tangential direction performed better with consistent mean results than that of the untreated samples, while radial direction gave a high average strength increment when treated.

Densified delignified wood as bio-based fiber reinforcement for stiffness increase of timber structures

Carbon fiber reinforcements are applied in timber engineering thanks to their high strength, but their fossil origin compromises sustainability. Furthermore, applications for which an increase of stiffness is beneficial, such as in slab-type elements, still remain unexploited. Here, we reinforced beech wood with an emerging bio-based fiber type, densified delignified wood (DDW). We comparatively adhered DDW, flax, and carbon fibers to wood using a polyurethane adhesive. The specimens’ stiffness was tested before and after reinforcement using bending tests, and tensile tests of the isolated fiber composites were performed for modeling purposes. Whereas no beneficial effect was reported for flax fibers, DDW and carbon fibers were able to provide a stiffness increase of up to 10.5% and 16.7%, respectively, demonstrating the potential of DDW. On this basis, a design case study conducted for a timber–concrete composite slab showed that a DDW stiffness reinforcement can be a material saving alternative.

Residual stresses in adhesively bonded wood determined by a bilayer flexion reporter system

The application of adhesives in modern timber engineering often introduces moisture into the wood, leading to permanent residual stresses after hardening. This paper proposes a novel approach to assess these residual stresses by using wooden bilayers as a reporter system. For thin bilayers, moisture-induced stresses lead to pronounced visible flexion that can be used to identify the stress-driving parameters of the adhesive’s gelation process. These parameters depend solely on the wood/adhesive combination and are inversely determined by fitting a finite element method model on the experimentally obtained flexion state. In a subsequent step, the determined parameters are used to calculate the residual stresses in the adhesive bondline of cross-laminated timber plates, emphasizing this approach’s scale independence and general applicability to larger scale structures. All combinations of European beech and Norway spruce with the adhesives Melamine–Urea–Formaldehyde (MUF), Phenol–Resorcinol–Formaldehyde (PRF), and Polyurethane (PUR) were investigated.

A serviceability investigation of dowel-type timber connections featuring single softwood dowels

Dowel-type connections are common in timber engineering, but current design codes are largely based on empiricism and are oversimplified. This inhibits the optimised use of connections, which is essential for the design of economically efficient timber structures. This study investigates the use of 3D computational modelling to predict the slip modulus (a key measure of stiffness) of single and double shear, dowel-type connections featuring a single softwood dowel. Initial modelling was conducted on parallel- and perpendicular-to-grain connections to establish a suitable mesh size and to validate the model against experimental work. The slip modulus in angled orientations was then investigated, a relationship given no consideration in design codes. The results show that the current design codes greatly overestimate the slip modulus in both single and double shear connections involving timber dowels. In comparison, the models in this study are more than twice as accurate at predicting slip modulus. Furthermore, slip modulus was shown to vary sinusoidally as the grain-to-grain angle changes between the parallel and perpendicular orientations. Differences between model and experimental values can be attributed to uncertainty in the mechanical properties of the timber in the experiments, the assumption of uniform properties for timber in each principal direction in the models, and the inherent variability of timber which affects experimental results.

Low-temperature curing of adhesives – Large-scale experiments

ABSTRACT In contrast to mechanical joining techniques, which can be manufactured under any ambient conditions, adhesively bonded joints always require some minimum temperatures – typically above 10 °C – in order to cure safely. Due to this limitation, either many technical applications exclude adhesives right from the start or practitioners at high costs are forced to adapt their production processes to the specific environmental requirements of the polymers to be cured. As a response to these manufacturing challenges, a new process for accelerated curing – induction heating with Curie particles (CP) – was studied to cure three different structural adhesives (two epoxy resins and one polyurethane) at low temperatures. The CP are added to the adhesives as a typical filler material and can only be heated by an externally applied electromagnetic field up to their material-specific Curie temperature T c. Thus, a self-regulating process is created, which effectively prevents the adhesives from overheating while simultaneously eliminating the need to control curing temperatures by external measures like IR cameras. Recently, the authors published a series of papers in which CP were used to cure at ambient temperatures (+23 °C); the results of which are now to be extended towards low-temperature curing. For that, an experimental campaign was carried out using the exemplary connection type of Glued-in Rods (GiR), which are frequently used in various timber engineering applications. In detail, the heating behaviour of inductively cured GiR conditioned at –10 °C and –30 °C was compared to GiR inductively cured starting from +23 °C. In addition, reference sets with and without added CP were produced, whereby all GiR were subjected to tensile tests in the following. Afterwards, the series were compared on the level of load–displacement curves, failure loads, as well as observed fracture patterns. The results proved that CP-curing is capable of curing adhesives in temperature ranges currently not intended for structural adhesives by technical regulations. When cured under low-temperature conditions, the GiR involving epoxy adhesives showed a fracture behaviour that proved to be equal to that of the references. The failure loads of the 2 K-PUR dropped to 50–75% of the reference values, which, however, is strongly assumed to be related to moisture on the cold adherends and not to a general unsuitability of polyurethane adhesives for the process – an assumption that will be validated by low-temperature experiments using single lap shear joints presented in a subsequent study.

Threaded rods grouted in beech laminated veneer lumber

AbstractGlued‐in rods are a class of adhesively bonded joints for timber engineering applications resulting in high‐strength and stiffness connections. However, the use of polymeric adhesives may lead to issues related if the temperatures exceed their glass transition temperature, restricting their performance under quasi‐static, or more critically, sustained loads. To overcome these, the substitution of polymeric adhesives by mineral high‐performance grout was investigated. It was found that primers have neither a significant effect on strength nor on the failure mode; threated wood surfaces, however, resulted in a significant improvement of the latter. Based thereupon, grouted‐in rods were manufactured. The best performance was achieved with a threaded wood surface, which achieved roughly 50% of the strength comparable adhesively bonded glued‐in rod's strength. While the obtained strength may seem quite low, it is important to remind that the latter will largely remain unaffected by temperature; accordingly, made at room temperature, the comparison between grouted and glued rods is in favor of adhesive bonding, it may well be different at elevated ones.