Cross-laminated Timber Connections Research Articles

Exploring the relationship between embedment behaviour in cross-laminated timber and its layers: An experimental study

Cross-laminated timber (CLT) has become popular as a construction material and the design of CLT connections is typically based on empirical equations with an average embedment strength of the wood-based product. The experimental study presented herein aims at enhancing the understanding of the relationship between the CLT embedment behaviour and the single layers it is composed of. Non-linear embedment stress–displacement relationships were measured in 244 full-hole and half-hole embedment tests on structural timber boards as well as on CLT produced of parts of the same boards. Three different fasteners, namely a steel dowel with a diameter of 12 mm and the threaded part of screws with a nominal diameter of 6.5 mm and 10 mm, were studied. The design of the experiments allowed to validate a simple mechanical model with parallel springs for the CLT, which showed good agreement with experiments. In addition to a validation of the non-linear spring model, this unique experimental data-set is further exploited in a comparison with empirical equations and for the derivation of a non-linear embedment ratio, between parallel and perpendicular to the grain embedment stresses, over the displacement. Knowledge of the relationship between the non-linear embedment behaviour of the single layers and the CLT element can be further exploited in the engineering design and numerical modelling of CLT connections with dowel-type fasteners.

Reverse-cyclic performance of United States prescriptive code connectors in a novel mass timber structural composite panel

Mass timber has been recently increasing in popularity, especially panel products such as cross-laminated timber (CLT), which has spurred innovation in creating new mass timber materials. One recent mass timber material utilizes structural composite lumber to create a PRG-320 compliant CLT, referred to as a structural composite panel (SCP). To increase the knowledge on the connection performance of the new SCP, reverse-cyclic testing was conducted on standard mass timber panel prescriptive connection systems within U.S. codes with SCP as the substrate. Three different types of tests were conducted: shear tests of a floor-to-wall connector, uplift tests of the same floor-to-wall connector, and shear tests of an inter-panel connector. Two monotonic and six reverse-cyclic tests were done per test configuration. The connections typically failed through a combination of wood failure and either nail bending and withdrawal or nail fracture. Two cyclic tests of the floor-to-wall connector in shear and all tests of the floor-to-wall connector in uplift failed through fracture of the plate connection. Results were compared to previous testing on CLT that was used to validate the connection and design methodology. Higher maximum strengths were observed when comparing the SCP to previously tested CLT, with an average increase of 20 percent. The displacement at maximum force and ultimate displacement capacity were similar between the SCP and previous CLT connections. Connections in SCP exhibited a lower stiffness than comparable CLT specimens for all tests, with an average difference in stiffness of 33 percent. This led to a comparatively lower ductility for the SCP specimens than for the CLT. ASCE 41 trilinear parameters were derived for all cyclic tests to aid in the utilization of the connections with this new material.

P2PE: A finite element formulation for panel-to-panel cross-laminated timber connections

This paper presents a new multi-spring finite element formulation called P2PE that simulates the cyclic behavior of panel-to-panel Cross-Laminated Timber (CLT) connections. The formulation comprises five types of uncoupled linear/nonlinear springs representing the fasteners and contact between panels. For instance, the in-plane fastener behavior is simulated with a co-rotational spring and the Modified Richard–Abbott (MRA) model, which is adapted to account for the asymmetry, pinching, degradation, and low-cycle fatigue of timber connections. The model was implemented into ANSYS through user-element/materials, including all computer implementation steps, and can be freely downloaded. The model's response and sensitivity were studied in three demonstrative CLT diaphragms, and it was validated at the connection and assembly stage with benchmark tests. In the first stage, the fastener model was verified with four cyclic CLT connections, while in the second stage, the model was validated with three medium-to-large scale CLT assemblies. The model accurately predicts the stiffness, strength, deformation, slip, and failure mechanisms of both stages. Finally, a parametric analysis of in-plane bending CLT diaphragms was assessed by varying their panel dimensions. This analysis demonstrated that diaphragms with slender panels have larger capacities, fastener energy dissipation, and shear slips but lower ductilities than shorter ones.

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 assessment of In-plane behavior of multi-panel CLT shear walls for modular structures

Ponderosa pine (Pinus ponderosa) harvested from restoration forests in the Pacific Northwest region of the United States is being considered for use in cross laminated timber (CLT). A prototype modular building was designed and constructed to assess the feasibility of using such CLT panels in emergency housing that can be rapidly assembled, disassembled, and reused. The modular building uses narrow-width CLT panels connected with intra-modular connections (butt joints with inclined screws) that develop the wall modules, which are connected to each other at the corners with point-type inter-modular connections. To accurately predict the behavior of this modular structure under seismic loading, it is necessary to be able to simulate the connections and wall modules under lateral loading. These selected intra- and inter-modular connections in PP CLT have been mechanically characterized in previous experiments. This paper presents the methodology and results from benchmarking the selected in-plane CLT connections against the experimental data in OpenSees. Further, the impacts of the selected connections on the in-plane monotonic and cyclic behavior of multi-panel CLT shear wall modules are also presented. The results show that the Pinching4 material model in OpenSees, when benchmarked using the simulated annealing algorithm, was able to successfully simulate the selected connections. Simulations of multi-panel wall modules indicate that modules with closely spaced screws in the butt joint behave as a single wall panel, while those with widely spaced screws behave as coupled walls. Recommendations on the spacing of intra-modular connections and the applicability of the modular system using the selected connections are provided based on the finite element analysis.

Numerical modeling strategies and parametric analyses of platform-type and balloon-type cross-laminated timber (CLT) shear walls under lateral loads

Despite great contributions having been made on the topic of numerical modeling methods for cross-laminated timber (CLT) shear walls under lateral loads, it is still challenging to simulate the nonlinear wall behavior with both computational efficiency and modeling accuracy. In this paper, a series of novel modeling strategies are developed, aiming to provide a potential solution to this challenge. The modeling strategies involve the revised version of a uniaxial hysteretic model for dowel-type timber joints, physical and phenomenological modeling methods for typical CLT connections (i.e., hold-down and angle bracket connections), and a fiber-section-based modeling method for CLT shear walls. The modeling strategies were validated by existing experimental data, and the modeling results showed that the proposed fiber-section-based model is capable of simulating the lateral performance of both platform-type and balloon-type CLT shear walls with sufficient accuracy while maintaining desirable computational efficiency. The fiber-section-based model was then used to conduct parametric studies, investigating the influence of construction methods, wall aspect ratios, connection stiffness, and vertical load magnitudes on the lateral performance of CLT shear walls. The studies showed that the initial stiffness and ductility ratio of balloon-type CLT shear walls were 39.1–47.1% and 13.7–19.4% larger than the platform-type ones, while the ultimate capacity was less affected by construction methods. Slenderer CLT shear walls had lower ultimate capacities and initial stiffnesses, as well as smaller ductility ratios. Stiffer hold-down connections contributed to larger initial stiffness but resulted in the CLT shear wall failing in a less ductile manner. The vertical load had positive impacts on the ultimate capacity, initial stiffness, and ductility ratio of CLT shear walls, although the effect on the ultimate capacity was quite limited.

Tension resistance properties of hold-down and angle-bracket connections on cross-laminated timber (CLT)

The goal of this study was to propose a practical approach for predicting the tension resistance properties of hold-down (HD) and angle-bracket (AB) connections on cross-laminated timber (CLT) by using modified design equations. The effects of wood density and number of screws on the tension properties of screwed HD and AB connections were experimentally investigated. Pine CLT connections with lower wood density exhibited lower tension resistance (−39%) and yield displacement (−34%) but higher ductility ratio (91%) relative to those of larch CLT connections. The AB connections on larch CLT exhibited lower elastic stiffness (−25 to −53%) and tension resistance (−10 to −22%) than those of the HD connections. The different load distributions of the HD and AB connections led to different failure behaviors associated with wood species. Although the tension resistance of the CLT connections was directly proportional to the actual number of screws, the elastic tension stiffness of the CLT connections was not directly proportional to the actual number of screws. Based on these experimental results, group effects on the tensile properties of the HD and AB connections on CLT were accounted for in the design equations, and the difference between the elastic stiffness, tension resistance, and ductility ratio values predicted using the developed design equations and the corresponding experimentally obtained values were 8, 6, and 7%, respectively.

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

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

Monitoring mechanical behaviors of CLT connections under reciprocating loading based on PZT-enabled active sensing and machine learning algorithms

Monitoring the mechanical behaviors of cross-laminated timber (CLT) connections is of great importance to the condition assessment of timber structures. To date, numerous research works have demonstrated that Lead Zirconate Titanate (PZT)-enabled active sensing approaches can achieve structural healthy state monitoring under monotonic loads, whereas their effectiveness for reciprocating loads still needs to be further studied. Moreover, traditional PZT-enabled active sensing approaches depend on prior knowledge and human judgment, restricting their field applications. Based on the above background, this research proposes an innovative method to monitor the mechanical behaviors of CLT connections under reciprocating loading by integrating PZT-enabled active sensing and eight machine learning (ML) approaches. Meanwhile, a new damage index based on wavelet packet decomposition and multiple signal path fusion is designed to improve the performance of ML methods. Finally, cyclic loading tests on CLT connections are conducted to demonstrate the outstanding capabilities of the proposed method than conventional PZT-enabled active sensing approaches.

An ensemble learning approach to condition assessment of dissipative CLT connections based on piezoceramic sensor data

Novel dissipative connections of cross-laminated timber (CLT) structures, showing great ductility and energy dissipation capability in addition to meeting the requirement of strength and stiffness, have gained increasing attraction in CLT shear walls. As important elements of CLT structures, dissipative connections function as the main part to resist seismic loading. It often displays different mechanical behaviors and suffers stiffness degradation when subjected to external loads. Variations of mechanical behaviors would further affect structural performance during service. The traditional method of analyzing mechanical behaviors of CLT connections relies on destructive tests, which are time-consuming and irreversible. Thus, using the non-destructive method to identify the mechanical behavior of dissipative CLT connections is of great significance. An ensemble learning-aided prediction algorithm combined with piezoceramic-enabled sensing technology is proposed in this study to identify mechanical behaviors of dissipative CLT connections under external loading. The main contribution of this study is to build a bridge between the damage-sensitive feature extracted from non-destructive sensing signals and mechanical behaviors obtained by the destructive loading test through the proposed method, which can avoid the requirement of time-consuming destructive tests and the baseline signal in traditional piezoceramic-enabled sensing method. An energy dissipative hold-down connection model is established in numerical simulations to investigate stress wave propagation mechanisms via different loading conditions. In experimental studies, a reusable piezoceramic-based sensor is firstly designed, and its performance is validated through a preliminary test. Then, a loading test was conducted on a CLT joint with a dissipative hold-down connector to further verify the numerical findings and generate an intact-to-failure piezoceramic-based sensing signal dataset through all loading cases. Consequently, fuzzy entropy is extracted from the signal dataset and fed to an ensemble learning classifier to predict loading behaviors. Prediction results imply the great potential of the proposed method for predicting dissipative CLT joint life-cycle performance in field applications.

Flexural Performance of Splice Connections in Cross-Laminated Timber

This study demonstrates the moment resistance performance of various splice connections of cross-laminated timber (CLT) subjected to flatwise bending. A total of 33 samples in two groups (half-lapped and single-splined) were tested under four-point bending. The influence of fastener types on the half-lapped connections was investigated. Additionally, different lap lengths were considered to understand the influence of lap length on different fastener types. Steel plates with two different thicknesses and plywoods were attached with bolts onto the bottom face only to make the single spline connections. Additionally, plywoods were attached to the CLT members in two ways: (i) with the bolt only and (ii) glue plus bolts. The effect of bolt diameters on the spline connections was also examined, and the connections were tested along both the major and minor axes. To determine the characteristic values of the resistance properties, a statistical analysis was carried out following EN 14358:2016. The results indicate the bolted lap connections experience plastic deformations, whereas the screwed lap connections exhibit relatively linear behaviour until failure. The bolted and screwed lap connection with a lap length of 100 mm showed 39% and 33% higher moment capacity, respectively, than that with a 75 mm lap length. Additionally, the rotational rigidity and ductility of the lap connections increase with the increase in lap length. Irrespective of lap lengths, the bolted lap connections show higher moment capacity, support rotation and ductility, but lower rotational rigidity than screwed lap connections. An increase in bolt diameter increases moment capacity but decreases rotational rigidity. Compared to the plywood spline connections, the steel spline connections showed approximately 24%, 5% and 73% higher moment capacity, rotational rigidity and ductility, respectively. Additionally, the plywood spline connections without glue performed better than glued connections. Overall, compared to the half-lapped connections, the single-spline connections showed better performance.

Brittle failure of laterally loaded self-tapping screw connections for cross-laminated timber structures

The performance of structural timber connections is of utmost importance since they control the global response of the building. A ductile failure mechanism on the global scale is desirable, especially in the design of structures in seismic areas, where dissipative components in which ductile failure modes need to be ensured are considered. Therefore, the knowledge of possible brittle failure modes of connections is crucial. The paper investigates the brittle failures of laterally loaded dowel-type connections in cross-laminated timber subjected to tensile load in a lap joint configuration through experimental investigations and analytical estimations. A set of 13 different test series has been performed with fully threaded self-tapping screws of 8 mm diameter and different lengths (40 to 100 mm) in cross-laminated timber composed of 3 or 5 layers (layer thickness range from 20 to 40 mm), giving rise to the activation of different brittle failure modes at different depths. Plug shear was among the most typically observed failure modes. A previously proposed model for the brittle capacity was applied to the tested connections at the characteristic level. As shown by the performed statistical analysis, the existing model is not reliable and mainly unconservative. A very low performance is observed (CCC = 0.299), but with a good correlation (c = 0.750) for the tests in the parallel direction. Further research work is required to improve the current model predictions and to gain a better understanding of the underlying resisting mechanisms.

Experimental study and finite element analysis on residual carrying capacity of CLT wall-floor angle bracket connections after fire

Fire resistance and residual carrying capacity of cross-laminated timber (CLT) structure are key considerations in multi-story and high-rise buildings. These aspects were studied for CLT wall-floor angle bracket connections using 60 and 80 mm self-tapping screws. Ten connections were tested for static loading at ambient temperature and residual post-fire carrying capacity, giving temperature–time, carbonization and load–displacement values. A finite element model of the connections produces a constitutive relationship for wood transverse-grain compression densification, and nonlinear spring element is used to simulate the failure mode of screw. Findings include that longer screws improve post-fire connection performance but reduce ductility and increase brittleness. This provides a reference for design of fire-resistant CLT connections.

Experimental investigations of glued-in rod connections in CLT

The widespread availability of cross-laminated timber (CLT) provides opportunities to extend the use of wood structures beyond low-rise residential construction. Glued-in rods (GiR) are an interesting technical solution for numerous timber structural applications, amongst them, the not yet exploited potential in conjunction with CLT. This paper summarises experimental investigation on the performance of GiR in CLT. Two 5-ply CLT panel thicknesses (139 and 175 mm), two steel rod diameters (dR = 12.7 and 19.1 mm), and five anchorage lengths (la = 6dR, 10dR, 12dR, 14dR and 18dR) were considered in applications parallel and perpendicular to the CLT major strength axis. A total of 220 uni-axial quasi-static monotonic tension tests were conducted. The results, assessed in terms of load-carrying capacity (ranging from 14 kN to 154 kN) and failure modes (plug-shear failure and lamination tear-out), demonstrated the impact of the connection’s geometric design parameters and that steel rods glued into CLT offer an alternative high capacity connection. The results will serve as the benchmark for subsequent numerical modelling, including the determination of joint capacity.

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.

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.

Withdrawal Properties of Self-Tapping Screws in Japanese larch (Larix kaempferi (Lamb.) Carr.) Cross Laminated Timber

With the increasing popularity of cross-laminated timber (CLT) constructions around the world, there have been attempts to produce CLT using local wood species in different countries, such as Japanese larch (Larix kaempferi (Lamb.) Carr.) in China. Thus, the need to investigate the connection performance also increases to support the design and construction of CLT buildings using local wood species. In this study, the withdrawal properties of three different types of self-tapping screws (STS), with a diameter of 6 mm, 8 mm, and 11 mm, were tested with Japanese larch CLT. The results revealed that the withdrawal strength of STS increased with increasing density and effective length, but decreased with an increasing diameter. With a density increment of 0.05 g/cm3, the withdrawal strength increased by an average of 9.4%. With an effective length increment of 24 mm, the withdrawal strength increased by an average of 1.4%. An empirical regression model was adopted to predict the withdrawal strength of Japanese larch CLT based on the results, which can be used for potential engineering design of CLT connections using STS.

Cyclic behavior of bracket connections for cross-laminated timber (CLT): Assessment and comparison of experimental and numerical models studies

This paper reports the comprehensive test and numerical models assessments for uniaxial cyclic behaviors of standard bracket connections for cross-laminated timber (CLT). Five analysis methods are compared for the determination of yield points of CLT connections. The applicability of the methods to various types of CLT bracket connections is discussed in order to identify the most universal approach. Key mechanical properties of bracket connections such as strength, stiffness, yield point, failure point and ductility are shown and compared. Pinching4 hysteretic model is used and calibrated to match the experimental hysteretic curves. The damage index error based on energy-based accumulation principle is applied to assess the precision of the model on hysteretic details. It is found that the Yasumura&Kawai approach provids a suitable estimation of the yield point based on bilinear approach, avoiding underestimation like the CSIRO and Karacabeyli&Ceccotti methods and improving results from the methods of EN 12512 and ASTM E2126. The damage indicator can precisely indicate the quality of correlation between experimental results and model predictions. The Pinching4 model is proved the ability in modeling the cyclic experimental uniaxial behavior of the CLT bracket connections. It can capture most details of hysteretic load-displacement curves and offer closer predictions to the test results in the advanced stage of loading to damage. But it has limited abilities for modeling high unloading stiffness at the initial stage for CLT connections.

Cyclic response of insulated steel angle brackets used for cross-laminated timber connections

In cross-laminated timber (CLT) buildings, in order to reduce the disturbing transmission of sound over the flanking parts, special insulation layers are used between the CLT walls and slabs, together with insulated angle-bracket connections. However, the influence of such CLT connections and insulation layers on the seismic resistance of CLT structures has not yet been studied. In this paper, experimental investigation on CLT panels installed on insulation bedding and fastened to the CLT floor using an innovative, insulated, steel angle bracket, are presented. The novelty of the investigated angle-bracket connection is, in addition to the sound insulation, its resistance to both shear as well as uplift forces as it is intended to be used instead of traditional angle brackets and hold-down connections to simplify the construction. Therefore, monotonic and cyclic tests on the CLT wall-to-floor connections were performed in shear and tensile/compressive load direction. Specimens with and without insulation under the angle bracket and between the CLT panels were studied and compared. Tests of insulated specimens have proved that the insulation has a marginal influence on the load-bearing capacity; however, it significantly influences the stiffness characteristics. In general, the experiments have shown that the connection could also be used for seismic resistant CLT structures, although some minor improvements should be made.

A Method to Characterize Biological Degradation of Mass Timber Connections

Biological durability issues in cross-laminated timber (CLT) have been majorly ignored in North America because of the European origin of the material and careful construction practices in Europe. However, the risks of fungal and insect attacks are increased by the North American climatic conditions and lack of job-site measures to keep the material dry. The methods to evaluate durability in solid timber are inadequate for use in mass timber (MT) for a number of reasons, such as moisture variation and size being critical issues. This study therefore proposes a method, which is suitable to evaluate the strength of MT assemblies that are exposed to fungal degradation. The objective of the study was to explore a controlled method for assessing the effects of wetting and subsequent fungal attack on the behavior of CLT connections. Two different methods were used to create fungal attack on CLT assemblies. Although they were both successful, one was cumbersome, left room for many errors, and was not as efficient as the other. In addition, a standardized method to evaluate and characterize key performance metric for the connections is presented.