Cross-laminated Timber Elements Research Articles

Bending behaviour of cross-laminated timber stressed-skin panels manufactured with mountain pine (Pinus uncinata Ramond ex DC.)

Cross-laminated timber (CLT) are wood-based structural products in worldwide expansion. Its application as a bending element implies the consumption of a significant amount of wood as a raw material in comparison to alternative solutions. In this research, an innovative construction system based on the stressed-skin panels concept has been designed using thin CLT skins and finger-jointed wood ribs of mountain pine (Pinus uncinata Ramond ex DC.) with the aim of adding value to underused wood and to increase the efficiency in the use of wood. Additionally, they are combined with cork insulation materials to obtain high natural and renewable building products to improve energy efficiency in buildings. To fulfil these objectives, two different types of stressed-skin panels, for roof and floor uses, were designed, manufactured and tested in bending, obtaining key mechanical performance information. A variation of the Gamma method was used to model the stiffness and compared with the experimental results. The efficiency in the use of wood for these innovative panels was compared with CLT elements of similar stiffness. The results will allow valorizing and wisely using this alternative species, contributing to the development of local economies in rural regions in Southwestern Europe through sustainable wood-based construction.

A Timber–Concrete–Composite Edge Connection for Two-Way Spanning Cross-Laminated Timber Slabs—Experimental Investigations and Analytical Approach

This paper introduces a method to create a moment-resisting edge connection between two cross-laminated timber (CLT) panels. With this connection, two-way spanning cross-laminated timber slabs can be realised, where the span exceeds the manufacturing and transport-related width of the individual CLT panels. Until now, mostly on-site gluing solutions have been suggested for such connections. In this study, a solution using a timber–concrete–composite (TCC) system is proposed. For this, self-tapping screws are inserted along the edge face of the CLT component, enabling the formation of a lap splice between two adjacent CLT elements. This lap splice is reinforced by additional rebars and filled with concrete. This means that only familiar, easy-to-handle materials are used on-site, and there is no need for adhesives, which can be difficult to apply. To evaluate the load-bearing capacity of the connection, it was subjected to a pure bending load in four-point bending tests, where load-bearing capacities of up to 70% of the characteristic load-bearing capacity of the solid CLT elements were achieved. An analytical approach for a simplified engineering calculation model is introduced to determine the load acting upon the screws. Based on the experimental results, it is shown that the analytical approach is able to adequately represent the load-bearing capacity of the connection. The analytically determined forces on the screws may then be used to carry out further verifications on this connection method.

Vibration reduction indices for junctions between cross-laminated timber floors and lightweight timber framed walls

Cross laminated timber elements are often used as the floors and the external walls in mass timber buildings in Canada, but the internal walls of these buildings are typically lightweight timber framed walls. The calculation of the flanking transmission loss and the apparent sound transmission class for the mix of lightweight and mass timber constructions requires the laboratory measured vibration reduction indices or flanking level differences of the different flanking paths. Junctions constructed in the new four room flanking facility at the National Research Council Canada have been used to evaluate both the vibration reduction indices and the flanking level differences of a cross laminated timber floor connected to lightweight timber walls.

An Experimental Comparison of Airborne Sound Insulation between Dovetail Massive Wooden Board and Cross-Laminated Timber Elements

Adhesives and metallic fasteners play a pivotal role in the domain of engineered wood products (EWPs). Nevertheless, owing to their origins in petroleum, adhesives can pose environmental hazards, whereas metal fasteners can complicate end-of-life disposal and reusability. Nonetheless, a resolution emerges in the form of dovetail massive wooden board elements (DMWBEs), characterized by their pure wood composition and absence of adhesive metal connections. The existing literature pertaining to DMWBEs has predominantly focused on inadequate structural analysis and model testing of connection specifics rather than appraising the efficacy of a structural member, such as a floor slab. This article presents a comparative analysis between a DMWBE and a correspondingly sized cross-laminated timber (CLT) panel, focusing on their respective airborne sound insulation capabilities. Experimental samples of model scale with dimensions of 200 mm thickness, 1160 mm width, and 1190 mm length were employed for both CLT and DMWBE. The evaluation of airborne sound insulation performance was conducted in accordance with ISO 10140-2 standards. The findings underscored the superior performance of DMWBE (Rw = 43 dB) in contrast to CLT (Rw = 40 dB) concerning airborne sound insulation efficacy. Additionally, the damping of the panel increased due to the different composition of the DMWBE, as evidenced by a higher measured total loss factor (TLF) compared with CLT.

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.

Dynamic evaluation of a nine-storey timber-concrete hybrid building during construction

Timber-concrete hybrid buildings are an innovative solution to increase the amount of timber materials used in modern buildings. This study presents a dynamic evaluation of a nine-storey timber-concrete hybrid residential building during construction. The building has an irregular shape and consists of three to seven storeys of cross-laminated timber (CLT) on top of two storeys of concrete. In order to study the influence of non-structural elements and a heavily increased slab load in construction, ambient vibration tests were conducted seven times during the 13-month construction period. The results show a clear decrease in the natural frequencies of the building as the building gets higher and more elements are installed. However, a slight increase in the natural frequency was observed following the installation of the non-structural walls in the final construction stage. A corresponding finite element analysis is presented for each test, providing additional insights into the parameters typically used in the structural design process. The study demonstrates the importance of properly selecting reduction factors for CLT elements in a dynamic finite element analysis. It also shows the importance of considering non-structural walls, both regarding weight and stiffness, even in buildings where the number of non-structural walls is relatively small compared to structural walls.

Relationships between stiffness of material, lamellas and CLT elements with respect to out of plane bending and rolling shear

The use of cross laminated timber (CLT) for construction has increased greatly in recent years and the large volumes of wood used for CLT means that it is important to optimize the use of the material. This requires relevant grading of lamellas and knowledge of relationships between lamella and CLT properties. In the present study, the relationship between dynamic axial modulus of elasticity (MoE) of lamellas and the quasi-static out of plane bending stiffness of CLT is investigated. By means of four-point bending test, it is shown that the effective quasi-static MoE of lamellas in CLT is only 2–6% lower than the average axial dynamic MoE of the individual lamellas. With this knowledge, producers of CLT can easily predict and control the important out of plane bending stiffness of the produced CLT. Moreover, it is shown that effective rolling shear stiffness of layers in CLT can be accurately determined by means of digital image correlation performed in connection to four-point bending of CLT, even for long test spans. For layers of lamellas of Scots pine of size 40 × 190 mm2 the average apparent or effective rolling shear modulus was determined to be 159 MPa. The average rolling shear modulus of the same material was determined to be 56 MPa.

Effect of Bearing Direction and Mounting Techniques on Cross-Laminated Timber Elements in the Field

Vibration reduction index (Kij) measurements in the field have some challenges compared to laboratory measurements. Firstly, the measurement requires access to a construction site during the short time span when the cross-laminated timber (CLT) elements are apparent. Secondly, building contractors are often on a tight time schedule. Therefore, it is important to find a solution that minimizes the measurement time on site. Moreover, Kij measurements in the field include several types of junctions with different bearing directions which may be of importance. This paper aims to evaluate two different mounting techniques with accelerometers on CLT elements and to discuss how the bearing direction could affect the vibration level difference of junctions. Measurement data indicates few deviations between mounting techniques with bee wax or double-sided adhesive tape when accelerometers are attached on CLT elements. Furthermore, field measurements indicate that the vibration level will decrease with an increased number of lamellas over the same CLT element. Double-sided adhesive tape is an adequate substitute for bee wax in the field for mounting accelerometers on CLT elements, with some limitations for high frequencies. Measurement data concludes that the bearing direction of CLT elements can influence the vibration reduction index of a junction.

An efficient analytical method to obtain the homogenised frequency-independent elastic material properties of cross-laminated timber elements

Cross-Laminated Timber (CLT) is a renewable, sustainable, and cost-efficient building element that has been growing in popularity in recent years. To improve one of CLT’s weaknesses, its suboptimal noise and vibration isolation performance, computationally efficient, accessible, and extensible CLT vibro-acoustic models are required. An effective approach to achieve such models is the homogenisation of layered materials. This paper presents a validated homogenisation method based on First-order Shear Deformation Theory (FSDT) that obtains the frequency-independent elastic material properties of CLT. This method is applicable to arbitrary linear elastic material properties, orientations, and stacking sequences of the layers. The homogenised material properties are utilised with FSDT Equivalent Single-Layer (ESL) models that are readily implemented with many finite element method codes to calculate the vibro-acoustic behaviour of CLT elements, even including thickness resonance effects when applied with an appropriate model. The presented homogenisation method for CLT is validated in a numerical study by comparing the mechanical mobilities of ESL models against layerwise dynamic models. The numerical study is conducted based on an experimentally validated 5-ply model for 2- to 7-ply CLT plates with proportionally increasing thicknesses and three idealised boundary conditions. The frequency-independent material properties also allow for the graphical exploration of the anisotropic nature of CLT and the calculation of the universal anisotropic index. The flexibility of the homogenisation method, combined with its ready implementation in already widely implemented FSDT models, can have an application and impact beyond the vibro-acoustic considerations of CLT into the general mechanical modelling of CLT as it is implemented in ever more advanced applications.

A unified design proposal for shear stress prediction in crossing areas for cross laminated timber at in-plane shear and beam loading conditions

Environmental and urbanization challenges during the last few decades encouraged steady growth of mass timber construction where attention is drawn to cross laminated timber (CLT) as one of the most interesting products in terms of mechanical properties, versatility, efficient prefabrication and sustainability. Standardisation and codification regarding testing and design of CLT elements are hence pointed out as one of the main issues within the ongoing revision procedure of Eurocode 5. A consistent and unified design approach for CLT at pure in-plane shear loading conditions (shear walls) and at in-plane beam loading conditions is however still missing. This paper deals with analytical models for the determination of stress components related to predictions of load bearing capacity of CLT with respect to shear failure mode III – shear failure in the crossing areas constituted by the flatwise bonded areas between laminations of adjacent layers. This failure mode is relevant for both pure in-plane shear loading and in-plane beam loading conditions. The paper presents a review of previously proposed models for the prediction of shear stresses in crossing areas of CLT, for both loading conditions. Comparisons between FE-results and model predictions are reviewed indicating significant differences between them concerning the predicted influence of the CLT element lay-up and values of maximum shear stresses. Based on simplifications of models previously presented, a unified design proposal that is based on a rational and consistent mechanical background for both loading situations and that shows overall good agreement with FE-results is presented.

Experimental Stress Analysis of Cross-Laminated Timber Elements under Cyclic Moisture

Cross-laminated timber (CLT) panels have been adopted as main structural elements in the mass timber constructions that have recently caught the attention from different stakeholders. The enhanced mechanical performances allow the design of impressive structures like tall buildings. However, not so much is known about the performance of CLT panels under service life condition. In this context, the strain distribution taken from an experimental campaign carried out using Digital Image Correlation (DIC) is considered as a basis to investigate the stress distribution presented by CLT panels under cyclic moisture variations. Four CLT configurations promoting different moisture flow were conditioned in climatic chambers for 324 days at a constant temperature of 20°C, and with a humidity cycle varying every 21 days between 30% and 90%, getting constant at 65% after the first 142 days. Strains calculated through DIC, along with moduli of elasticity defined by ultrasound tests, were used to estimate strained and released stresses, respectively, before and after cutting specimens. Results show that sealing CLT panels play an insignificant role on distribution of stress induced by cyclic moisture, but peak stresses, both tension and compression resulted from wetting and drying cycles, respectively, were reduced in many specimens, especially when longitudinal faces were sealed. Moreover, as general recommendations to take in the design of CLT panels regarding moisture changes, it is possible to identify the need to limit the longitudinal length of panels, to use more thin layers rather than less thick ones, and to protect panels in the longitudinal direction, owing to high stress concentrated along this direction, particularly in the borders.

Experimental Investigation of Cross Laminated Timber Elements with Holes or Notches at In-Plane Beam Loading Conditions

Environmental and urbanisation challenges have encouraged steady growth of mass timber structures where cross laminated timber (CLT) stands out in applications as full-size wall, floor, or beam elements. Beam elements are used mainly in situations where cross layers have a reinforcing effect on the tensile stress perpendicular to the beam axis, such as when introducing holes or notches, which is common practice in beams, due to engineering, installation, or architectural requirements. This paper presents experimental investigations of CLT beams with holes or notches for comparison and validation of an analytical model provided in the literature. Different sizes of holes and notches as well as different placements of the holes were considered in the experiments. All relevant failure modes were analysed and discussed in detail. Two predominant failure modes were indicated, i.e., bending failure and shear failure in crossing areas (mode III). Results further indicate that reduced lamination widths near the hole, notch, or element edges have a relatively small influence on the beam strength. Parametric studies indicate net shear failure (mode II) and tensile failure perpendicular to the beam axis as the critical failure modes in most of the considered cases, indicating their strong underestimation in design verifications according to the analytical model.

Glued timber concrete composite walls using ultra-high-performance concrete

This paper gives a short introduction of the development of cross laminated timber (CLT) concrete composite walls with a glued connection. In the composite walls, prefabricated ultra-high-performance concrete (UHPC) lamellas replace timber lamellas at the core layer of the CLT elements. To check the feasibility of gluing timber to UHPC small-scale shear, delamination and bonding tests were performed and showed promising results. The load bearing behaviour was analysed with centrically and eccentrically loaded tests on wall segments. Analytic modelling of the wall experiments using an effective bending stiffness, based on shear analogy method, showed a good correlation to the experimental results.

Pull-out Behaviour of Axially Loaded Screwed-in Threaded Rods Embedded in CLT Elements: Experimental Study

Experimental investigation on the pull-out behaviour, including the withdrawal capacity, slip stiffness and interfacial shear stress, of axially loaded screwed-in threaded rod embedded in cross laminated timber (CLT) was conducted. Specimens with varying embedment length and different number of threaded rods were tested in this study. To prevent premature splitting failure of timber, some specimens were reinforced in the direction perpendicular to the timber grain with self-tapping screws. Test results showed that the screwed-in threaded rod connections exhibited good pull-out behaviour with high withdrawal capacity and slip stiffness. Within a certain range, the withdrawal capacity increases considerably with the increase of embedment length, while the average interfacial shear stress shows the opposite tendency. The self-tapping screws played an important role on reducing the splitting of the timber and improving the withdrawal capacity and slip stiffness of the screwed-in threaded rod. Additionally, for the specimens with self-tapping screw reinforcements, the improvement of withdrawal capacity and stiffness when increasing the number of threaded rods are much more obvious than that of the specimens without the self-tapping screws, due to the change of failure modes.

Analysis and Calculation of Stability Coefficients of Cross-Laminated Timber Axial Compression Member.

Cross-laminated timber (CLT) elements are becoming increasingly popular in multi-storey timber-based structures, which have long been built in many different countries. Various challenges are connected with constructions of this type. One such challenge is that of stabilizing the structure against vertical loads. However, the calculations of the stability bearing capacity of the CLT members in axial compression in the structural design remains unsolved in China. This study aims to determine the stability bearing capacity of the CLT members in axial compression and to propose the calculation method of the stability coefficient. First, the stability coefficient calculation theories in different national standards were analyzed, and then the stability bearing capacity of CLT elements with four slenderness ratios was investigated. Finally, based on the stability coefficient calculation formulae in the GB 50005-2017 standard and the regression method, the calculation method of the stability coefficient for CLT elements was proposed, and the values of the material parameters were determined. The result shows that the average deviation between fitting curve and calculated results of European and American standard is 5.43% and 3.73%, respectively, and the average deviation between the fitting curve and the actual test results was 8.15%. The stability coefficients calculation formulae could be used to predict the stability coefficients of CLT specimens with different slenderness ratios well.

Built-in moisture in cross-laminated timber roofs – a field study

Cross-Laminated Timber (CLT) elements have had a growing popularity in recent years due to i.e. low carbon footprint, low weight and efficient construction time. However, the elements are sensitive to moisture and prone to organic growth if not treated properly or if used incorrectly. Roof slabs are particularly exposed, as they have a large area of exposure and the horizontal orientation doesn’t allow rainwater run-off. The efforts made to protect CLT-roofing elements by Norwegian contractors vary widely, as there are few guidelines and little long-term experience. A field study of CLT-roofs on existing buildings was conducted to investigate the conditions after some years in service. The study includes inspection and moisture measurements of CLT elements from the exterior side in 10 building projects 1-9 years old from two regions of Norway. The contractor of each project was interviewed in order to assess the extent of climate exposure and protection measures during construction. The results indicate a correlation between water content, building age and exposure level during construction. There is a clear indication that the drying time for built-in moisture in CLT roof constructions are slow. Keeping built-in moisture to a minimum is therefore paramount.

A Numerical Study of the Stiffness and Strength of Cross-Laminated Timber Wall-to-Floor Connections under Compression Perpendicular to the Grain

The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.

Determination of the dispersion relation in cross-laminated timber plates: Benchmarking of time- and frequency-domain methods

The availability of a reliable estimate of the dispersion relation of plates is of great importance for acoustic modelling. Several experimental methods for the extraction of wavenumber and wave velocity information are discussed in the literature. They differ, among other factors, by the sensitivity to noise and multiple reflections, frequency range of application and implementation of the processing algorithm. While for homogeneous thick or thin plates well-established analytical solutions are available, non homogeneous materials would benefit from experimental characterisation. In this framework, our attention is focused on cross-laminated timber (CLT) elements, load-bearing orthotropic layered wood plates that revolutionised the sector of timber construction. The anisotropic and non homogeneous nature of wood suggested testing methods that, though converging for homogeneous materials, could bring different results in their application to CLT elements. The aim of this paper is therefore to benchmark methods for the extraction of wavenumber information on CLT plates, from which the effective elastic properties to model CLT structures as an equivalent homogeneous plate can be derived. This is achieved through a numerical and experimental benchmark of well-established methods for the evaluation of the real component of the propagating wavenumber in CLT elements. Five relevant methods were selected through a literature review, implemented and analyzed: three in the time domain, namely maximum peak, cross-correlation and kurtosis, and two in the frequency domain: phase difference and wave correlation. First, the five methods were benchmarked by numerical simulation on a homogeneous material. In this ideal case, results showed to be consistent for all methods. Then, wavenumber measurements were performed on a cross-laminated timber plate. The results show that the maximum peak, cross-correlation and the wave correlation method provide the lowest dispersion of the data. Considering the time required by the installation of the setup, the wave correlation method seems to be the best alternative among the proposed ones.

Bond strength of GiR in cross-laminated timber: A preliminary study

The popularity of cross-laminated timber (CLT) has increased significantly over recent years, with numerous low- to mid-rise buildings being constructed with CLT panels as the load-bearing structural system. Glued-in rods (GiR) have been used in the construction and retrofitting of timber structures and recently found its use in CLT elements. Embedded into the timber with structural grade adhesives, GiR enable stiff connections between timber and other structural members. Due to the complex force mechanisms occurring within these joints, there is yet to be consensus on a suitable method to predict their pull-out strength or failure mechanism. There is lack of experimental research on performance of GiR embedded into CLT. This paper examines previous research on glued-in rod connections as typically applied to solid timber and other engineered timber products. It aims to identify the factors contributing to the strength and failure modes of this connection, as well as the current models that are used for the analysis and design of glued-in rods. A series of 30 experiments were undertaken to investigate the load carrying capacity of single GiR bonded into CLT panels to identify the influence of (i) the bonded length of rod; and, (ii) the direction of the middle ply of the CLT panel on these results. The experimental results indicate that the pull-out strength is largely dictated by the embedment length.

Strain variation analysis of cross-laminated timber elements under cyclic moisture

Despite the growing use of cross-laminated timber (CLT) panels in the construction industry, their behaviours under weather conditions, especially cyclic moisture which generated additional strains, requires further investigation. To address this gap, an extensive experimental campaign was carried out to evaluate CLT elements, in which strains are measured, under cyclic moisture changes, using Digital Image Correlation (DIC). In this way, the effects of cyclic humidity changes on amplitude and distribution of strains on four different configurations of CLT panels, divided based on the free flow direction of moisture, are explained. Regarding RH cycle tests, CLT specimens were conditioned in climatic chambers for 324 days at constant temperature 20 (°C) with a humidity cycle varying every 21 days between 30% and 90%, getting constant at 65% after the first 142 days. Results show that, in the main face, drying periods present tensile strain while wetting periods present compressive strains in the longitudinal direction. But, in the tangential direction, restrained strains tend to reduce significantly from first to last drying cycles for all test configurations. Moreover, in the lateral face, drying periods present tensile strains while wetting periods present compressive strains in the tangential direction. Experimental results confirmed that the ratio between tangential and longitudinal directions tends to decrease with the number of RH cycles, and successive RH cycles do not cause cumulative strains on CLT panels.