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The main objective of this work is to characterize the stress–strain relationship of cross-laminated timber (CLT) wall panels when loaded in compression and how different confinement mechanisms provide compression reinforcement for the CLT panels. CLT crushing tests were performed to define the behavior of CLT under compression up to the failure point, defined here as the strain at which the gross stress falls by 20% from the peak value. Testing included monotonic uniform compression experiments of five-ply CLT with specimens that were 0.457 m wide by 1.52 m long. The motion of the specimens perpendicular to loading direction was restrained at midlength to prevent buckling. Twelve specimens were tested, including six bare CLT specimens and six reinforced specimens of CLT—three with self-tapping screws while the other three with self-tapping screws and an additional U-shape steel plate at the bottom of the specimens. Various damage points were observed during the tests, including (1) point at the onset of damage (through visual observation and instrumental observation) (2) point of initiation of strength degradation, (3) point at which buckling occurs, (4) point at which 25.4 mm delamination was observed, and (5) failure point. Three axial compression stress–strain models were adopted, calibrated, and compared. Results indicate that a stress–strain model combining linear and cubic functions for the elastic and softening responses, respectively, captures well the CLT inelastic behavior under compression. [ABSTRACT FROM AUTHOR]

While the use of cross-laminated timber (CLT) panels for building construction has increased over the last several decades, current standards and existing literature provide limited information regarding the design of CLT diaphragms or the prediction of their deflections when subjected to wind and strong earthquake motions. This paper presents the design and assessment of a CLT diaphragm that was part of a full-scale two-story structure subjected to shake-table testing. An analytical model is proposed for diaphragm deflection accounting for in-plane shear and bending stiffness, as well as the stiffness of various connections. Moreover, a refined numerical modeling strategy is proposed in order to consider phenomena such as panel-to-panel gap closure. Results indicate that the analytical model yields conservative results both in terms of deflections and forces when compared to the numerical model that simulates similar sources of strength and stiffness. The analytical model is suitable for the design of symmetric diaphragms with regular shapes, whereas the numerical model can also be used to model asymmetric diaphragms with irregular shapes. [ABSTRACT FROM AUTHOR]

Mass timber construction has seen a growing interest in North America and worldwide. In the US, code provisions allow mass timber buildings to be up to 82 m tall, but with floor slabs requiring a noncombustible, usually cementitious, topping over mass timber panels (MTPs). Designing the MTP and concrete topping to act compositely can increase the strength and stiffness compared to noncomposite MTP floors. Several connections are available to create composite action between an MTP and a concrete topping, with self-tapping screws being the most widely considered practice. A new test fixture and testing method were developed to characterize connection properties and facilitate the selection of fasteners most suitable for developing composite action. Three conventional self-tapping screws were experimentally investigated at the single-fastener scale. The screws were subjected to withdrawal at 90° and 45° orientations to the interface plane between the MTP and concrete. Withdrawal from both the MTP substrate and concrete topping was investigated. Three MTP materials were tested: cross-laminated timber (CLT) consisting of two different wood species and a mass ply panel (MPP). Results demonstrated that the pullout failure of the screwhead from the concrete was the limiting failure mode. Comparisons were made between the experimental results and analytical models for screw withdrawal to assess the applicability of available models to the different MTPs. These models underestimated the capacity of the tested fasteners for all samples except for the fully threaded screws. [ABSTRACT FROM AUTHOR]

Cross-laminated timber (CLT) is an engineered wood product offering several advantages for wood utilization in midrise and tall buildings. However, moisture intrusion poses significant risks since it can affect the durability of CLT structural members and their connections. There has been much research on connection performance to understand their seismic response. However, the impact of moisture on the performance of CLT connectors is less understood. Cyclic tests were conducted on a series of CLT shear wall-to-diaphragm L-bracket (angle bracket) connections after subjecting them to a wetting and redrying cycle. The study includes three exposure durations and four wood species (Douglas fir, Southern yellow pine, Norway spruce, and spruce-pine-fir) to investigate the effect of moisture exposure on the connection performance. The performance was characterized in terms of strength, stiffness, and energy dissipation. In addition, two existing engineering force-displacement models were fitted to the experimental data. Results indicate that the changes due to wetting and redrying cycling were not statistically significant for the load-carrying capacity and stiffness. In contrast, a statistically significant decrease in the total energy dissipation capacity was observed. [ABSTRACT FROM AUTHOR]

A full-scale 2-story mass timber building was tested on the University of California San Diego Natural Hazards Engineering Research Infrastructure (UCSD-NHERI) uniaxial shake table during the period from June 2017 to September 2017. The main objective of the experimental program was to test the performance of mass timber building designs with different seismic lateral force–resisting systems. The focus of this study is on a building configuration designed using self-centering post-tensioned cross-laminated timber (CLT) rocking walls with U-shaped steel flexural plate energy dissipators. The shake-table tests were designed to subject the building to a series of earthquake ground motions of increasing intensity, ranging from a service-level earthquake to 1.20 times the maximum considered earthquake intensity. Between each ground motion, low-amplitude white-noise excitations were applied to the building, which responded as a quasilinear system. In this paper, two output-only operational modal analysis methods are used to estimate the modal parameters (frequency, damping, and mode shapes) based on acceleration data collected during the white-noise shake-table tests. The correlations of observed damage and repairs performed during the experimental program with changes in estimated modal features are reported. The modal parameters estimated from the testing program are also compared with a linear finite-element model that is used to validate the modal identification results and study the performance of the two system identification methods for CLT rocking structures. [ABSTRACT FROM AUTHOR]

This paper presents the behavior of floor diaphragms of a shake-table experiment of a full-scale 2-story mass-timber building structure. The structure consists of glued-laminated timber beams and columns, and floors and walls were designed and built making use of cross-laminated timber panels. Two different floor systems were designed, where the roof consists of a topped cross-laminated timber (CLT)-concrete composite system, and the floor level consists of untopped CLT panels connected with plywood single-surface splines. The CLT floor systems were designed to remain essentially elastic over the whole series of shake-table tests, which included testing of three lateral force–resisting systems tested at three different seismic intensity levels (service level, design basis, and maximum considered earthquake) for a total of 34 shake-table earthquake tests. Results from the testing indicate that CLT diaphragms designed to remain essentially elastic based on basic principles of structural mechanics and existing test data can achieve desired seismic performance objectives. In addition, sources of overstrength in certain elements of the diaphragm need to be explicitly considered for a holistic diaphragm design. [ABSTRACT FROM AUTHOR]

Timber-concrete composite (TCC) floors are becoming more commonly used in mass timber construction due to their increased stiffness and strength relative to nonconcrete composite timber floors. Current building codes do not recognize the contribution of the composite action of TCC floors to their combined strength and fire performance. Extensive structural testing is required for structural engineers to design fire protection in timber buildings that use TCC floors using either alternate means and methods or structural fire engineering. This paper presents two experimental studies conducted to examine (1) properties of shear connection systems used in TCC floors through ambient temperature direct shear tests, and (2) the behavior of TCC floors through large-scale fire tests performed at the National Research Council (NRC) in Canada. The main objective of this research is to collect data on the fundamental behavior of TCC floors in a fire. The results indicate that TCC floors have improved fire performance when compared with mass timber floors. In addition, the research quantifies the force-slip behavior of the shear connectors used and benchmarks the flexural behavior of TCC floors at ambient and elevated temperatures to existing analytical models. [ABSTRACT FROM AUTHOR]

Connections in mass timber structural systems transfer lateral forces from mass timber elements such as shear walls to floor diaphragms. Cross-laminated timber (CLT) is a prominent engineered mass timber material used to manufacture walls and floor assemblies. Fire performance research of CLT walls and floors has been abundant in recent years in an effort to develop an understanding of the performance of structural members under fire. Some fire-protected structural elements, including connections, may not be directly exposed to flames in a fire event but will experience elevated temperatures. There is limited research on elevated temperature performance of CLT connections, and consequently a lack of understanding of their fire performance in CLT structures. To aid in bridging this knowledge gap, in this study a series of cyclic shear tests were conducted on a CLT wall-to-floor bracket connection assembly to characterize thermal degradation according to a matrix of 28 exposure duration–temperature combinations. Two different methods were employed to develop force-displacement backbone models from the experimental hysteresis data. Models from both methods indicate an overall degradation of the connection performance in terms of load carrying capacity, elastic stiffness, ductility, and energy dissipation with increasing exposure to elevated temperatures. [ABSTRACT FROM AUTHOR]

Connections in mass timber structural systems are critical for transferring lateral forces from mass timber elements such as shear walls and diaphragms. Cross-laminated timber (CLT) is a prominent mass timber material used to manufacture these wall and floor assemblies. Although research exists that investigated the fire performance of CLT walls and floors, very little investigation has been devoted to the thermal performance of the connection systems themselves. This void in the data and knowledge surrounding CLT connections is an impediment for modeling the elevated temperature performance of CLT structures. Therefore, a series of shear tests were conducted on a CLT L-bracket connection assembly to characterize the thermal degradation of peak loads and initial stiffness as a function of exposure duration at a given temperature. A total of 116 specimens, including four control specimens, were tested according to a matrix of 28 exposure duration-temperature combinations. Two analytical models are developed to explain the thermal degradation—one assuming a mechanism based on first-order kinetics and the second using a statistical regression. The results of this work indicate that the degradation of peak load and initial stiffness with respect to exposure duration occurred at a linear rate and depended on temperature, according to the Arrhenius activation energy theory. This research is a step toward a holistic evaluation of elevated temperature modeling of CLT structures. [ABSTRACT FROM AUTHOR]

• Twenty in-plane shear tests on CLT surface spline connections were performed. • Splines were constructed using nails and three different screw spacings. • Nail spline connections were more ductile than the screw connections. • An engineering performance model for the CLT splines is presented. Cross-laminated timber (CLT) panels have been used as structural diaphragm elements that form the primary lateral force-resisting system in mass-timber buildings. To better understand the overall performance of CLT diaphragms, an experimental program including monotonic and cyclic in-plane shear panel-to-panel spline connections tests was conducted. The spline connections were constructed using nails and screws with different spacings. Experimental results from the testing program allow for characterization of the performance of the connection systems in terms of elastic stiffness, strength, and ductility. Based on the results of this specific testing program, nailed spline connections displayed similar performance in terms of stiffness and strength and were more 58.7% more ductile than the screw connections, which is partly explained by the larger overstrength factors observed for the screw connections. A simplified performance model is presented to aid designers in determining the elastic, inelastic, and post-peak force parameters of the various spline connections tested. [ABSTRACT FROM AUTHOR]