Timber Engineering Research Articles

Evaluation of [formula omitted]-curves and cohesive law in mode I of European beech

This work addresses the determination of the cohesive laws in Mode I and tangential–longitudinal (TL) crack propagation system of Fagus Sylvatica L. This species is one of the ever-growing and most widely used hardwood species in Europe for engineered timber products. Double Cantilever Beam (DCB) tests are performed. The strain energy release rate (GI) is derived from the R-curves by applying the Compliance-Based Beam Method (CBBM), which has the advantage of not requiring the measurement of the crack length during propagation, but only the global load–displacement curves. The cohesive law of the material is determined from the relationship between GI, and the crack tip opening displacement (CTOD) monitored for each specimen using Digital Image Correlation (DIC). Numerical finite element models are developed by implementing the average cohesive law through Cohesive Zone Models (CZM). An average GI value of 0.46 kJ/m2 is obtained for this species. The numerical load–displacement curves are consistent with the experimental results, which demonstrates the suitability of the method for the identification of the cohesive laws in beech. The fracture properties obtained are essential in the development of advanced and reliable numerical models in timber engineering design using this species.

The Italian instructions for the design, execution and control of timber constructions (CNR-DT 206 R1/2018)

In 2007 the Italian National Research Council published the technical document CNR-DT 206/2007 “Instructions for the design, execution and control of timber structures”, with the purpose to provide a technical support to the operators of the sector, in line with the most advanced knowledge at that time. Since then, the world of timber engineering has largely used such document, even though the instructions are not mandatory standard rules, so that they became the most common tool in Italy for the structural use of timber, opening the markets and favouring competition and new applications. In latest years new studies, researches and innovative proposals have promoted the development and the growth of timber constructions also in the civil residential field. The framework of standard rules for constructions and products, both Europeans and Italians, has evolved too. For these reasons, CNR has considered as opportune to proceed to the editing of a new version, updated and widened, namely with the acronym DT 206-R1. The document comes from the spontaneous cooperation of an open group of specialists and operators of the sector, based on a wide discussion on the common scientific and technical expertise and knowledge. The current version has already taken into account the results of the public inquiry phase (concluded by now).The paper presents the main contents of the document, evidencing the innovations.

Laboratory of Timber Engineering, Research Group of Forest Science, Division of Fundamental Agriscience Research, Research Faculty of Agriculture, Hokkaido University

Laboratory of Timber Engineering, Research Group of Forest Science, Division of Fundamental Agriscience Research, Research Faculty of Agriculture, Hokkaido University

Energy‐efficient and sustainable composite facade constructions

Society currently undergoes a transformation process in understanding the environmental impact of its resource exploitation. The growing importance of resource‐efficiency and the reduction of the environmental impacts are being widely discussed and investigated. Therefore, the European Union (EU) countries have implemented energy efficiency regulations in all sectors, including the building sector, which is accountable for 40 % of energy consumption and 36 % of CO2 emissions within the EU. In modern facade constructions, the use of large‐scaled glass elements is inevitable, which has led to the investigation of potential optimization of structural performance, energy efficiency and ecological characteristics of the structural building components. The Department of Structural Design and Timber Engineering (ITI) at the Vienna University of Technology (TU Wien) developed several timber‐based composite systems, combining timber and wood concrete on the one hand, as well as timber and structural glass components on the other hand. Through various combination of the individual composite systems, certain advantages as well as the synergies resulting from the different combinations could be shown. Different types of combined timber, wood concrete and glass composite structures for facades are developed, assessed and compared to conventional facade structures. The comparison is carried out regarding the materialization on component level, as well as the overall construction level, whereby a scale‐independent assessment of the examined construction types could be reached. This assessment shall further be enabled by a broad‐spectrum computation, ranging from static and dynamic thermal simulations for annual heating and overheating periods up to various thereof resulting ecological impact calculations. In terms of ecological building design, the conceptual implementation of the building envelope plays an important role in the thermal comfort of the inner rooms, as well as the glare and reflection by the use of daylight. One advantage of large‐scale glass facades is the possibility to implement a high portion of solar entries into the total energy concept. The extremely versatile application possibilities allow different sectors of power generation, such as photovoltaic cells in the facade, thermal collectors for heat generation or algae panels, which generate biogas by photosynthesis process. The gained energy can further operate heating and cooling systems but also hot water supply.

Thermal properties of wood and wood composites made from wood waste

1. Adl-Zarrabi B., 2004. Determination of thermal properties of wood and wood based products by using TPS. Proc. 8th World Conference on Timber Engineering, Lahti, Finland. Google Scholar

Self-camber of timber beams by swelling hardwood inlays for timber–concrete composite elements

Timber–concrete composites (TCC) are smart solutions for slabs in residential and office buildings regarding the sustainable and performance-optimized use of materials. However, a non-negligible disadvantage is the deflection of the timber elements caused by in-situ concrete casting during construction. This paper presents an approach to camber timber elements without external forces by using the innate swelling capacity of wood. Oven-dried hardwood inlays can be inserted transversally into cuts on the top side of a timber element. After an increase of the moisture content in the inlays, the swelling pressure will result in a self-camber of the timber element. In this study, a procedure for prediction of the self-camber is derived and the model is validated using an experimental test series. The results demonstrate that the self-camber of spruce elements using beech inlays is both feasible and predictable. On this basis, practical application scenarios for TCC elements in timber engineering are shown and discussed.

Glued-in multiple steel rod connections in cross-laminated timber

ABSTRACT The increasing popularity of cross-laminated timber (CLT) provides opportunities to extend the use of wood beyond traditional low-rise residential construction. For the high-performance material CLT to achieve its full utilisation, adequate joining techniques are required. With that in mind, while glued-in rods (GiR) are a powerful technical solution for numerous timber engineering applications, with proofed performance in solid wood, only few studies are available on the performance of GiR connections in CLT. Critical research gaps exist regarding the performance of multiple GiR, which this paper suggest to fill. Herein, experimental investigations on the performance of multiple GiR in CLT are presented. Steel rods of diameter d = 12.7 mm were glued-into CLT panels with two anchorage lengths (10d and 18d), three numbers of rods (1, 2 and 3), and two spacings between rods (s = 4d and 6d). In total, 10 test series with 5 replicates, thus 50 specimens, were manufactured and subsequently tested under uni-axial quasi-static monotonic tension. The results, assessed in terms of load-carrying capacity, demonstrated that GiR in CLT offer an alternative high-performance timber connection, and allows for more insights relating load capacity to the number, the anchorage length, and the spacing between rods.

Accelerated curing of glued-in rods: Influence of manufacturing defects

Adhesive bonding is considered a special process, for which very high quality standards exist with regard to manufacturing. These include, amongst other, controlled environmental conditions and avoidance of any contamination of adherend’s surfaces, for which reasons it is performed in dedicated shops by staff specially trained for these tasks. Additionally, because bonding is performed in dedicated environments, a large palette of techniques to accelerate curing can effectively be applied. Totally different is the situation in the construction and building industry, where—excluding prefabrication—significant work has to be performed on site, with all imaginable environmental and contaminational issues critical for adhesive bonding. This paper indicates, for glued-in rods (GiR), a typical joint type of wide use in timber engineering, that it is in principle possible to perform structurally loaded bonded connections on-site, considering all aforedescribed adverse issues. For that, 192 GiR, resulting of the combination of four adhesives, two accelerated curing techniques, and four imperfection types considered relevant, were considered, and their effect on joint capacity evaluated. The results showed that, depending upon the adhesive, a relatively high level of robustness with regard to the adverse conditions prevailing on construction and building sites can be achieved.

Glued-in CFRP and GFRP rods in block laminated timber subjected to monotonic and cyclic loading

Current advances in timber engineering with the use of timber at longer spans necessitate the use of high performance joints. Glued-in rods exhibit high axial load capacity and stiffness and are suitable for such applications. FRP rods can increase their durability and fire performance but ways to produce progressive failure mechanisms are necessary to enhance their resilience. In this study CFRP and GFRP rods glued in block laminated timber with epoxy are investigated under both monotonic and cyclic loading. Joints with constant glue-line thickness and variable bond-line geometry are studied to induce progressive resin failure mechanisms. A stepped wedge shaped geometry with varying glue-line thickness and a mirrored version were investigated. The effect of elastic modulus (CFRP vs GFRP) and glue-line thickness on the bond strength, stiffness and material viscous damping are explored. CFRP rods exhibit 11% higher axial load capacity under monotonic loading but GFRP rods exhibit higher bond stiffness at the serviceability stage. The cyclic loading does not discount the bond performance and results in higher stiffness due to viscoelastic creep deformation. An increase in glue-line thickness results in higher axial withdrawal capacity and viscous material damping. Specimens with a wedge shaped bond-line exhibit superior mechanical performance. An FEA model is presented for a variable bond-line geometry and the numerical results agree well with the experimental findings.

Application and potential of shape memory alloys for dowel-type connections in timber structures

ABSTRACT The present study aims at identifying and analyzing challenges and prospects related to the use of pseudoelastic shape memory alloys for well-established double shear connections in timber engineering. The unique behavior of shape memory alloys will allow to bear large reversible deformations without loss of cohesion. Thereby, the main objective of the present work is to study the behavior of a connection that is able to stay completely reversible even upon very large deformations. In this context, the stiffness of the connection as well as functional and structural integrity and damping behavior have been improved by stepwise adaption of the detailing. Proof of concept is provided, and current limitations are discussed, based on the application of bolts made from a commercially available shape memory alloy.

Reliability analysis of the improved four-level assessment standardization for bearing capacity of timber components

In practical timber engineering, various problems widely exist in the current reliability assessment for the bearing capacity of components. Improving the current assessment standards of timber components has important guiding significance for their design and repairing optimization. Considering the long-term load effect and the latest code GB50005-2017 of timber structures with a new resistance coefficient, a superior limit state function (LSF) of timber components with bearing capacity was established. Using this realistic LSF, the reliability index values β were discussed and analyzed by two versions of Chinese codes (GB50068-2001 and GB50068-2018) with different load partial coefficients γL and three design reference periods (DRPs) with different statistical parameters. Furthermore, the four-level reliability assessment standardization with bearing capacity was improved based on superior LSF, different load partial coefficients and three DRPs. The results showed that the load partial coefficients γL and DRPs significantly impact the reliability index values β. By contrast, the impact load partial coefficients and DRPs on the limits of four-level assessment (k) is less significant than that on the reliability index values β. The practical four-level assessment standardization is improved based on the target reliability index [β] and superior level principles. The reasonableness and superiority of the four-level reliability assessment standardization were verified by calculating the 28 different load cases, 3 different DRPs and 26 kinds of ratios k of resistance to load effect. To meet the requirements of the minimum reliability index values β, this study suggests that the bearing capacity design values of R/γ0S are set to 1.0, 0.93, and 0.87 as the limits of au/bu, bu/cu, and cu/du for main timber components, while values of R/γ0S are set to 1.0, 0.87, and 0.82 as the limits for general timber components.

The role of the hold-down in the capacity model of LTF and CLT shear walls based on the experimental lateral response

Cross Laminated Timber (CLT) and Light Timber Frame (LTF) shear walls are widespread constructive technologies in timber engineering. Despite the intrinsic differences, the lateral response of the two structural systems may be quite similar under specific connection layouts, boundary constraints, and size of the shear walls. This paper compares the experimental cyclic responses of CLT and LTF shear walls characterized by the same size 250×250cm, and loaded according to the EN 12512 protocol. The rigid-body rotation of the shear walls prevails over the deformation and rigid-body translation in the post-elastic displacement range. As a consequence, a capacity model of the two systems based on the sole hold-down response accurately seizes the observed cyclic response, despite ignoring the other resisting contributions. The authors examine the differences exhibited by the CLT and LTF shear walls and the related error corresponding to a capacity model based on the sole hold down restraints. Additionally, it is assessed the overstrength of the CLT panel and LTF sheathing to the shear walls collapse due to the hold-down failure. The estimated overstrength factor is the most meaningful difference between the two structural systems in the considered experimental layouts.

Effect of thermal aging on the mechanical performance of timber-timber single-lap adhesive joints

Abstract Timber is a natural material traditionally used in the building industry. In the last decades, developments in scientific research have made timber a high-tech building material. In the field of timber engineering, recent developments concerned advances in the study of adhesive joints and their applications in the field of structures. The growing interest in adhesive joints in timber structures is due to the advantages offered by adhesive technology over traditional mechanical joining techniques. In this paper, the effect of exposure of adhesive joints between timber adherends to high values of temperature and relative humidity is investigated. In particular, two different two-component (2K) epoxy adhesives (EPX1, EPX2) and one polyurethane (PU) in single-lap adhesive joints between mahogany and pine adherends have been used and compared. This research work investigates the possibility of realising new joints between timber adherends in innovative building components (e.g. windows and curtain walls). The results obtained from the experimental campaign showed that PU adhesive has lower mechanical performance than epoxy adhesives, regardless of the aging condition. On the other hand, the epoxy adhesives tested showed high performance and different mechanical behavior after artificial aging.

Accelerated curing of G-FRP rods glued into timber by means of inductive heating – Influences of curing kinetics

ABSTRACT Glued-in Rods (GiR) are nowadays widely established as an adhesively bonded connection type in timber engineering. The assemblies consist of rods, e.g. threaded rods or rebars, made of steel, or Fibre-Reinforced Polymers (FRP), which are bonded into wooden construction elements. In contrast to mechanical fastening, adhesive bonding often results in a better load transfer between the adherends, and consequently a reduction of local stress peaks in comparison to mechanical fasteners. Bonding of GiR is typically performed with 2 K polyurethanes (2 K-PUR) or 2 K epoxies (2 K-EPX), for which full curing can last up to several days. Consequently, before the connection can be stressed, or moved for further processing, components have to be maintained in fixed position. This process can be very time, and cost, consuming for contractors. In order to create a speedier, thus more efficient, bonding process, the present study investigated accelerated curing of large-scale GiR specimens with the help of inductively heated Curie particles (CP). The CP are added to the adhesives, and exposed to a high frequency (HF) electromagnetic field (EMF). The EMF heats the mix up until the Curie temperature (T C) is reached, at which point the heating automatically stops. Due to the generated heat, polymerisation is significantly accelerated in a process independent of external monitoring techniques that prevents adhesive overheating. In order to design this practitioner-friendly and controllable manufacturing process, kinetic models were developed, allowing the prediction of the degree of cure, α, in dependency of CP content. The kinetic models were applied to experimentally determined temperature profiles during inductive heating of large-scale GiR specimens, considering three commercially available 2 K-EPX, and one 2 K-PUR, all of which are widely used. The kinetic modelling allowed for a much better interpretation of the heating behaviour, highlighting, in particular, the contribution of enthalpy for adhesive curing, and by extension of the whole induction process.

Wood City: Timberizing the City’s Building Blocks

“Wood City” applies mass timber engineering to the 19 building categories that essentially shape our built environments, otherwise financialized as real estate products by Wall Street. Suburban offices, fast-food restaurants, metal warehouses, big-box grocers, garden apartments, single-family houses, hotels, self-storage facilities, assisted living facilities, strip-shopping centers, etc. constitute 75% of the built environment. Given that the US will double its built environment in just one generation coupled with the urgency to develop low-carbon futures, what if cities were built from the only construction system sequestering carbon and engineered to be “energy positive” – wood? Real estate value chains are undergoing transformations in sectors like fuel retail, fast food, grocery, and logistics, while new interest from venture-capital is hybridizing housing, hospitality, healthcare, and the senior services markets. Innovations in timber-engineered buildings to date have been associated with signature projects involving tall buildings and cultural institutions. Alternatively, Wood City outlines a design research agenda for mass timber through the pattern languages of ordinary building sectors already undergoing novel mixings of space, services, technologies, and experiences.

Experimental study and analytical model of bolted connections under monotonic loading

Dowel-type connections are essential components of safe, economical timber buildings. This study analyzes the mechanical properties of different configurations of bolted connections with slotted-in steel plates. The test results indicate that the parallel-to-the-grain connections exhibited three typical failure modes, whereas wood cracking was the predominant failure mode of the connections perpendicular-to-the-grain. The mechanical properties, such as initial stiffness, yielding load, maximum load capacity and ductility, are obtained from load-displacement curves and compared with a design code. The relationship among the mechanical properties, bolt diameter and the thickness of glulam timber is investigated. Based on the experimental load-displacement curves, a two-parameter analytical model related to the initial stiffness and load-carrying capacity is proposed for the prediction of nonlinear load-displacement behavior, and the curves generated using the analytical model are consistent with the test curves. The two physical parameters can also be calculated using the design formulas, rendering the analytical model more useful in practice. This work is beneficial by contributing mechanical knowledge and providing a database for improved connection designs in timber engineering.

Resistive curing of glued-in rods

Adhesive bonding is a joining technique that enjoys increasing popularity in industry, including timber engineering. However, two major shortcomings still delay their widespread use, in particular on-site: long process times, and temperature requirements for curing. Accelerated curing is seen as one way to overcome both. In this paper, resistive heating is investigated as a method to both substantially accelerate curing, and to make it possible even under low temperature. The technique is illustrated on glued-in rods (GiR), a typical adhesively bonded structural joint widely used in timber engineering. Resistive heating was achieved by wrapping a thin Constantan wire within the threads of a rod, which was then subjected to an electrical current. In a series of tests, the performance of three two-component epoxies (2 K-EPX) and one two-component polyurethane (2 K-PUR), all commercially available, were investigated in terms of load capacity after resistive heating under room temperature (RT), +5 °C, and –10 °C. The results indicate that GiR bonded with two of the 2 K-EPX exhibited no significant differences in terms of joint capacity, regardless of the temperature under which they were cured, but that the 2 K-PUR proved very sensitive from departures from the recommendations of the technical data sheet. The results give, in particular practitioners, valuable indications on how to implement resistive heating in real-life applications.

Accelerated curing of glued-in threaded rods by means of inductive heating — part IV: curing under low temperatures

ABSTRACT Glued-in rods (GiR) are a successful class of structura bonded joints for timber engineering applications. However, they suffer from two limitations: relatively slow curing processes, and requirements related to minimum temperatures for the bonding process. Both, to some extent, limit their wider application, in particular on site. Inductive heating was investigated as a method to overcome both drawbacks by speeding up the process, and enabling it under low temperatures. To verify that, a series of four commercially available adhesives was used to manufacture GiR with beech LVL and Ø16 threaded rods embedded by 100 mm, which were then cured under a variety of conditions. Firstly, cold-curing according to the prescriptions of the technical data sheets and inductively, both at +23°C; secondly, inductively at +5°C; and thirdly, at – 10°C. Numerical modelling complementing these studies allowed for insights related to temperature distributions and curing progress otherwise inaccessible through purely experimental means. The results of this study clearly show that, if selecting appropriate adhesives, curing of glued-in rods is not only accelerated from several days down to a few minutes, but that the process can also be carried at much lower temperatures than those prescribed by the technical datasheets. Based thereupon, a completely new range of practical applications, in which gluing of rods can potentially be made independent of ambient temperatures on-site, has been unleashed.

Accelerated curing of G-FRP rods glued into timber by means of inductive heating using Curie-particles – large-scale experiments at room temperature

ABSTRACT Glued-in Rods (GiR) connections are widely used in timber engineering as a connector between individual load-bearing members. Typically, GiR consist of a rod, e.g. rebars or threaded rods, glued into timber-based elements. However, rods made of new materials, as Glass-Fibre Reinforced Polymers (G-FRP) are increasingly being used due to their advantageous material properties like high tensile strength and good resistance against corrosion or chemical media. The cure of usually used and commercially available adhesives for GiR applications, mostly cold-curing 2 K-epoxies (2 K-EPX) and 2 K-polyurethanes (2 K-PUR), is time consuming. After adhesive application, the components must be fixed and maintained in position from several hours to days before the connection can be stressed. This process, depending on the connection type, can be significantly time, and cost, consuming for contractors, which makes the process difficult on-site. In order to overcome these restrictions, the present study investigated accelerated curing with the help of Curie-Particles (CP) exposed to a high-frequency (HF) alternating electromagnetic field (EMF). The CP are added to the 2 K adhesives, the resulting mix is then exposed to the EMF, generating thermal energy, which in turn accelerates the chemical reaction and associated adhesive curing. Due to the very nature of the CP, heating proceeds until their Curie temperature is reached, at which point the heating automatically stops. As a result, accelerated curing is achieved independently of external monitoring. In the present study, G-FRP rods were glued into blocks of spruce Glued Laminated Timber (GLT), and inductively cured through the addition of 33.3 w/w-% of CP. In total, four commercially available adhesives (three 2 K-EPX, and one 2 K-PUR) were investigated. The authors demonstrated on a total of 67 large-scale GiR specimens that inductively heated specimens achieved almost identical failure loads, and fracture patterns (wood failure), compared to cold-cured references accompanied by a reduction of curing time from as long as 1–10 days down to 10 min.

Design curves for stiffened engineered timber wall systems: A verified analytical approach

This manuscript introduces the mathematical modelling of efficient stiffened engineered timber wall systems and presents a collection of corresponding design curves. An analytical approach via the exact finite strip method through the Wittrick-Williams algorithm was used in this study to understand and predict the behaviour of such systems. Appropriate orthotropic material models and strength limits have been incorporated into the analytical method for the engineered timber panel and sawn cut timber stud stiffeners respectively. This is the first time that this approach has been used in this field of timber engineering and hence was verified through experimental testing and detailed Finite Element Analysis (FEA). Compared to FEA there is over a thousand-fold decrease in computational cost which allowed many parameters to be varied incrementally including: the thickness of the panel, number of stiffeners, height of the wall and applied load. The results of which has enabled a set of versatile and simple validated design curves to be developed and presented. With these design curves, for a desired load capacity, the optimal system configurations are given. Likewise, for a chosen configuration the allowable axial load is given.