Cross-laminated Timber Connections Research Articles

Bayesian model selection for the nonlinear hysteretic model of CLT connections

Connections are important components to cross-laminated timber (CLT) structures. Although there are some hysteretic models available for CLT connections, the main challenge is how to select a predictable and robust model. In this paper, three popular hysteretic models, including the Bilinear model, SAWS model, and Pinching4 model, are briefly revisited. These hysteretic models exhibit their advantages and disadvantages, making them difficult to select the most suitable model for the simulation of hysteretic response. Based on the test data, the Bayesian approach is used to determine the most plausible model from five candidate models. The identified results show that the symmetric Pinching4 model has the highest evidence in shear loading, while in the tension loading the SAWS model possesses the highest value. For the evaluation of the identified results, both the idealized backbone curve and Park-Ang damage index are compared, showing a good agreement between the test and model results. These mechanical characteristics are crucial to the design and rehabilitation of timber buildings.

Toe-Screwed Cross-Laminated Timber Connection Design and Nonlinear Modeling

AbstractThis research investigated the mechanical characteristics of cross-laminated timber (CLT) connections fastened with structural wood screws, also known as self-tapping screws (STS). The CLT ...

Modeling the Coupling Effect of CLT Connections under Biaxial Loading

AbstractThis paper presents the modeling of coupling effect of tension and shear loading on cross-laminated timber (CLT) connections using a finite-element–based algorithm called HYST developed in ...

Procedure for parameter identification and mechanical properties assessment of CLT connections

Connections in cross-laminated timber (CLT) structures are crucial components that can affect the behaviour of the whole structure. A novel framework that integrates the unscented Kalman filter (UKF) as an estimation tool with a hysteretic model is developed to identify the model parameters of a nonlinear system. The UKF estimates the mean and covariance of the model parameters using unscented transformation (UT) by a set of deterministically chosen sample points. The proposed framework is applied to identify the unknown model parameters of CLT connections using available experimental data, where the cycle behaviour of CLT connections is simulated using a spring element assigned to the hysteretic model. The comparison of hysteretic curves between the test and model shows that the results identified with UKF are precise. Two different approaches are proposed: an assessment according to the EN 12512 standard, and a damage accumulation assessment. The EN 12512 standard assessment is evaluated from the elastic stiffness, ductility ratio, and energy dissipation, whereas the damage accumulation assessment considers the effects of low amplitude and accumulated damage. Together, these two methods fully evaluate the identified result and the mechanical characteristics of CLT connections. The proposed procedure of parameter identification using the UKF (together with the mechanical properties assessment) can be applied to other connections in timber engineering.

Bayesian parameter identification for empirical model of CLT connections

Cross-laminated timber (CLT) is a popular construction material that is widely used in timber buildings. In this study, the mechanical properties of three types of CLT connections were investigated: (i) an angle bracket joint; (ii) a crossed self-tapping screw joint; and (iii) a simple butt joint with self-tapping screws. Three repetition tests for each type of CLT connection were conducted under two different cyclic-loading protocols. The experimental results showed that pull-out failure in the tension direction was dominant, whereas wood crushing and screw failure were typical failure modes in the shear direction. An empirical model was used to simulate the nonlinear behavior of the CLT connections under cyclic loading, where the pinching, cyclic stiffness, and strength degradation were considered. To estimate the model parameters associated with the uncertainty of these mechanical properties, the Bayesian approach was proposed, and the Cramér–Rao lower bound was derived as the theoretical lower bound. The parameter-identification results were evaluated according to the elastic stiffness, yielding point, and ductility via equivalent energy elastic–plastic analysis. Comparison of these mechanical properties between the test results and model estimates revealed good agreement. Additionally, the bounds of the estimated response were investigated according to the parameter variance. In comparison with the experimental data, the error bars covered two standard deviations, indicating the reliability of the estimated results. The values of the identified parameters associated with the standard deviation provide a good reference for further analysis when such types of CLT connections are incorporated into a finite-element model of a structural system. The proposed method of Bayesian parameter identification is also suitable for assessing other types of empirical models.

The elastic and ductile behaviour of CLT wall-floor connections and the influence of fastener length

An investigation was carried out to examine the influence of the fastener length on the elastic and ductile failure behaviour of typical steel-to-timber bracket connections in Cross Laminated Timber (CLT) manufactured from C16 spruce. The behaviour of single screw-type fasteners within CLT of length 25 mm, 50 mm and 75 mm are examined under tension and shear loading. Multiple screw-type fasteners have been utilised to examine monotonic and cyclic behaviour of steel bracket-fastened CLT connections under shear and tension loading. Characteristic values of the experimental results are compared to analytical models from current timber design codes and the literature. The length of fastener has been shown to influence the failure strength and ductility of the connection. The overstrength factors and strength degradation factors have been shown to be similar, regardless of fastener length, and conservative values are recommended.

DESIGN APPROACHES FOR CLT CONNECTIONS

Various design approaches for establishing the resistance of connections in cross-laminated timber (CLT) structures have been developed and adopted in timber design standards worldwide. Although the fundamental principles are similar, the new design provisions for CLT connections have been aligned in some standards with the existing design philosophy and format adopted for sawn timber and glulam using traditional fasteners such as dowels, nails, and wood screws for consistency and simplicity, in the other standards, alternate approaches have been developed. This article presents a snap shot of the various design approaches for connections in CLT adopted in Europe, Canada, the United States, and New Zealand. The intent is for the reader to have a better knowledge of the underpinning assumptions, principles, and the adopted design rules in each of these standards.

In-Plane Strength and Stiffness of Cross-Laminated Timber Shear Walls

The research presented herein investigated the in-plane performance of cross-laminated timber (CLT) shear walls for platform-type buildings under lateral loading. Finite element models of CLT connections (i.e., brackets, hold-downs and self-tapping screws) were developed in OpenSees and calibrated against experimental tests to represent the connections’ hysteresis behaviour under cyclic tension and shear loading. The results were incorporated into models of CLT single and coupled shear walls. The results in terms of peak displacement, peak load and energy dissipation were in good agreement when compared to full-scale shear wall tests. Subsequently, a parametric study of 56 single and 40 coupled CLT shear walls was conducted with varying numbers and types of connectors (wall-to-floor and wall-to-wall) for evaluating their seismic performance. It was found that the strength, stiffness and energy dissipation of the single and coupled CLT shear walls increased with an increase in the number of connectors. Single shear walls with hold-downs and brackets performed better under seismic loading compared to walls with brackets only. Similarly, coupled shear walls with four hold-downs performed better compared to walls with two hold-downs. Finally, ductility of coupled shear walls was found to be 31% higher compared to that of single shear walls. The findings from this research are useful for engineers to efficiently design CLT shear walls in platform-type construction.

Experimental and numerical performance of shear connections in CLT–concrete composite floor

The hybrid cross-laminated timber (CLT) and concrete system has drawn attention as an attractive application for residential and commercial buildings. However, it is crucial to fully understand the mechanical characteristics of the CLT–concrete composite connections before such a hybrid structure becomes widely applicable. This paper assesses the structural performance (force–slip response, slip modulus, and failure mode) of a CLT–concrete composite by conducting fifteen pushout test specimens, which were designed by using the γ-method (Eurocode 5). In these composite specimens, the CLT floor panels were connected to concrete slabs using common mechanical connectors, and the effect of different connection types, connection angles, the penetration depth of the connector, and concrete topping thickness were investigated. The results show that all of the specimens had good trend in the failure mechanisms with a little crush occurred in the CLT and concrete. The finding revealed that the inclination connectors were appeared as an attractive connector layout for CLT–concrete composite components. Non-linear 3D finite element analysis was also developed to simulate the load-slip behavior of the CLT-concrete specimens under shear load. In general, a reliable non-linear finite element model using ANSYS is a great tool for capturing shear capacity and the overall behavior of the hybrid push-out specimens.

Mode Cross-Over and Ductility of Dowelled LVL and CLT Connections under Monotonic and Cyclic Loading

AbstractThis paper presents an experimental study on dowelled connections in cross-laminated timber (CLT) and laminated veneer lumber (LVL) using ϕ20-mm mild steel dowels and internal steel plates....

A novel method for non-linear design of CLT wall systems

In this paper, a non-linear procedure for the seismic design of metal connections in cross-laminated timber (CLT) walls subjected to bending and axial force is presented. Timber is conservatively modelled as an elasto-brittle material, whereas metal connections (hold-downs and angle brackets) are modelled with an elasto-plastic behavior. The reaction force in each connection is iteratively calculated by varying the position of the neutral axis at the base of the wall using a simple algorithm that was implemented first in a purposely developed spreadsheet, and then into a purposely developed software. This method is based on the evaluation of five different failure mechanisms at ultimate limit state, starting from the fully tensioned wall to the fully compressed one, similarly to reinforced concrete (RC) section design. By setting the mechanical properties of timber and metal connections and the geometry of the CLT panel, the algorithm calculates, for every axial load value, the ultimate resisting moment of the entire wall and the position of the neutral axis. The procedure mainly applies to platform-type structures with hold-downs and angle brackets connections at the base of the wall and rocking mechanism as the prevalent way of dissipation. This method allows the designer to have information on the rocking capacity of the system and on the failure mechanism for a given distribution of external loads. The proposed method was validated on the results of FE analyses using SAP2000 and ABAQUS showing acceptable accuracy.

Overstrength of dowelled CLT connections under monotonic and cyclic loading

This paper presents an evaluation of overstrength based on an experimental study on dowelled connections in Cross Laminated Timber (CLT). Connection overstrength needs to be well understood in order to ensure that ductile system behaviour and energy dissipation can be achieved under seismic loading. Overstrength is defined as the difference between the code-based strength, using characteristic material strengths, and the 95th percentile of the true strength distribution. Many aspects contribute to total connection overstrength, which makes its definition challenging. In this study, half-hole embedment tests were performed on CLT to establish embedment strength properties and three point bending tests were performed to determine the fastener yield moment. Different connection layouts, making use of mild steel dowels and an internal steel plate, were tested under monotonic and cyclic loading to evaluate theoretically determined overstrength values and study the influence of cyclic loading on overstrength. Experimental results were compared with strength predictions from code provisions and analytical models for ductile response under monotonic loading. It was found that cyclic loading does not significantly influence overstrength for connections that respond in a mixed-mode ductile way indicating that in future more expedient monotonic test campaigns could be used. This work also provides further experimental data and theoretical considerations necessary for the estimation of a generally applicable overstrength factor for dowelled CLT connections.

Coupled axial-shear numerical model for CLT connections

A new coupled axial-shear numerical model is proposed to simulate the response of typical Cross Laminated Timber (CLT) connections under general loading conditions. The general model is then specialized for hold-down connections. To this aim, a series of experimental tests was first designed and performed at University of Bologna, in order to provide for the necessary data for the characterization of the numerical model. The main parameters required to define the constitutive law of the model are here evaluated by means of linear regression analyses performed on the experimental data. Both monotonic and cyclic test series are taken into account to calibrate and then validate the proposed numerical model, demonstrating its effectiveness in representing the coupled response of hold-down connections under simultaneous application of axial and shear forces. Finally, the response of a whole CLT panel subjected to a cyclic top displacement history is studied, confirming the need of considering axial-shear interaction in modelling of numerical connections.

New design approach for controlling brittle failure modes of small-dowel-type connections in Cross-laminated Timber (CLT)

The introduction of Cross-laminated Timber (CLT) as an engineered timber product has played a significant role in considerable progress of timber construction in recent years. Extensive research has been conducted in Europe and more recently in Canada to evaluate the fastening capacity of different types of fasteners in CLT. While ductile capacities calculated using the yield limit equations are quite reliable for fastener resistance in connections, however, they do not take into account the possible brittle failure mode of the connection which could be the governing failure mode in multi-fastener joints. Therefore, a stiffness-based design approach which has already been developed by the authors and verified in LVL, glulam and lumber has been adapted to determine the block-tear out resistance of connections in CLT by considering the effect of perpendicular layers. The comparison between the test results on riveted connections conducted at the University of Auckland (UoA) and the Karlsruhe Institute of Technology (KIT) and the predictions using the new model and the one developed for uniformly layered timber products show that the proposed model provides higher predictive accuracy and can be used as a design provision to control the brittle failure of wood in CLT connections.