Search

Dowel bearing strength of three cross-laminated timber (CLT) species (Douglas-fir, Norway spruce, spruce pine fir) were periodically assessed through 40 weeks of exposure to two brown rot fungi species (Gloeophyllum trabeum and Rhodonia placenta). Time-dependent regression models were developed to describe the relationship between level of fungal exposure and dowel bearing strength of CLT. Obtained dowel bearing strength values were used to predict the capacity of floor-to-wall CLT connection systems. Predicted strength values were compared with capacity of actual connection systems which were subjected to similar biodeterioration treatments. Significant reductions in the dowel bearing strength of CLT were observed 30 weeks after fungal inoculation. Similar patterns of degradation were observed for both fungi in all three CLT species, but R. placenta was slightly more aggressive than G. trabeum, causing a 16.2 MPa reduction in dowel bearing strength of Douglas-fir after 40 weeks of exposure. Connection capacities estimated using National Design Specification yield models for dowel type fasteners and the dowel bearing strength of fungal-damaged CLT were consistent with observed properties and were within 85% of actual capacity of connections tested, especially in early stages of decay. [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]

Limited research exists on the deformation compatibility behavior of mass timber beam-to-column connections. Therefore, to investigate this behavior, large-scale experimental monotonic testing was performed on five different glue-laminated (glulam) beam-to-column connections. The purpose of this research was to create a better fundamental understanding of the deformation compatibility behavior of mass timber beam-to-column connections. Three connections were custom-designed connections, and two were pre-engineered connections. Specimens included the presence of cross-laminated timber (CLT) decking on the beam, as well as combined lateral and gravity loading. Monotonic tests pushed specimens to a target story drift of 7.2% at a rate of 0.51 mm/s. The data were analyzed, and the yield force, yield displacement, initial stiffness, secant stiffness, and connection classification (categorized as either rigid, semirigid, or flexible) were calculated for each connection. Per the connection classification methodology used, all connections were classified as flexible (simple) connections. However, the pre-engineered connections use substantially more screws and plates thereby increasing the cost and effort of installation. The results demonstrate that in some cases simple custom connections can have a larger yield force and initial stiffness than pre-engineered connections, and due to the reduction in number of screws, there was less damage within the connection at the completion of the test. The forces required to push connections to 2% drift all exceeded the lateral force capacity of the connections; however, all connections maintained load-carrying capacity throughout testing. [ABSTRACT FROM AUTHOR]

Mass ply panel (MPP) is a new mass timber product that was recently introduced for structural applications in buildings. Although there have been several studies on characterizing the mechanical properties and structural performance of MPP, there is a lack of understanding regarding in-plane shear properties of the panels. More intensive studies should be done for MPP to provide a comprehensive understanding of its behavior. This paper presents an experimental study to characterize the effective in-plane shear modulus and shear strength of MPP in the major strength direction of the panel. Two experiments, including multiple span-length center-point bending tests and a novel large-scale shear test, were conducted. The test results were presented and compared with previous studies and the manufacturer's product report. The new test proposed for shear strength characterization has the potential to be an industry standard for mass timber panels. [ABSTRACT FROM AUTHOR]

Crossarms are heavy timbers used to support electrical lines on utility poles. The requirements for crossarm material are fairly strict especially with respect to limitations on the presence of knots, which represent a gap or hole in the timber and also result in grain deviations that can affect properties. The strict grading rules can sometimes result in shortages of acceptable material. The potential for accepting arms with minor, but currently unacceptable defects was explored on a population of 200 rejected Douglas-fir crossarms and 50 arms that met current standards. Knot diameter and location on the arms were mapped and the arms were tested to failure in a specially designed apparatus that simulated an actual line load. The resulting data were used to calculate Modulus of Rupture (MOR) and Modulus of Elasticity (MOE). Nearly all arms failed above the minimum American National Standards assumed MOR value of 53.78 MPa and many of the rejected arms had higher MOR values than acceptable arms. While there was more variability in properties for the rejected arm population, correlations between knot diameter or knot location and MOR were all exceeding poor (r2 ≤ 0.25). The results suggest the need for further evaluation of the emphasis on knots in grading rules. [ABSTRACT FROM AUTHOR]

The performance of cross-laminated timber (CLT) panels in building applications have been investigated extensively. However, limited information is available regarding the performance of CLT panels for bridge applications, as well as the torsional performance of CLT panels under asymmetric loading, geometry, and boundary conditions. The objective of this study was to investigate the torsional performance of skewed CLT pedestrian interior bridge deck panels through large-scale structural testing. The panels were designed as a pedestrian bridge crossing over an atrium of The Canyons building in Portland, OR. The test panels were subjected to line and uniformly distributed loads to replicate the reactions from the adjacent panels and design loads directly applied to the panel, respectively. Two different loading configurations based on the 2014 Oregon Structural Specialty Code (OSSC) were applied with up to three different loading intensity levels. An additional loading configuration, which was beyond the scope of the OSSC, was also applied in the second series of tests to determine if panel failure could be induced. The test results demonstrated that the test panels were able to recover their original shape without suffering major damages on the panel. The measured residual deformation after the removal of the applied load was very small. The findings of this study broaden the current state of knowledge of the structural performance of a CLT bridge deck panel under asymmetric loading, and the test results can be used for future design problems. [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]

Mass timber represents a range of composite materials, including glued- or nail-laminated timber, cross-laminated timber, laminated veneer lumber, and mass plywood panels that are increasingly used to construct multi-story structures. The potential of these wood materials has energized the architectural and engineering communities as they seek less energy-intensive building materials. However, performance optimization of mass timber buildings will require careful consideration of the potential effects of moisture intrusion on performance. This paper reviews the roles of moisture in building performance, discusses the potential effects of moisture on mass timber, outlines steps to limit moisture intrusion and, if this cannot be prevented, discusses possible steps for wood protection. The paper concludes with a call for further collaborative research to better understand the role of moisture in mass timber structural performance. [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]

This 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 connection assemblies were toe-screwed (TS) at 45° and cyclically tested in tension and in-plane shear to evaluate the failure modes and applicability as seismic connections in CLT shear walls. The connection assemblies were representative of full-scale CLT shear walls connected to CLT floors as often seen in platform construction. Toe-screwed CLT connections exhibited screw fracture, head pull-through, and pinched hysteresis loops. Mechanical connection properties and backbone curves were extracted for comparison with design estimates. A head pull-through design method was developed, which predicted peak strength within 5% of test results. Connection parameters for ASCE/SEI 41 idealized-component curves were presented for use in performance-based design nonlinear-static pushover analysis. Combining the presented ASCE/SEI 41 parameters with the proposed design method and test results provides a simple and reasonably accurate nonlinear toe-screwed connection response curve. Partially threaded washer-headed screws were found to be superior to fully threaded screws in seismic applications due to their 15 times greater ultimate deformation capacity, substantially greater energy dissipation, similar peak strength, and similar elastic stiffness. However, the yield strength of partially threaded STS was found to be 48% of the yield strength of fully threaded STS. [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]

• Cross-laminated timber rocking walls dissipate energy through connection damage. • Slip-friction connections were originally developed for steel structures. • The SFC system was designed as a hold-down in a self-centering CLT rocking wall. • Quasi-static, cyclic tests with SFC connections showed elastic-plastic behavior. • Passive damping was close to typical idealized friction hysteresis models. Cross-laminated timber (CLT) rocking walls limit internal building forces and dissipate seismic energy mostly through severe connection damage and related wood crushing. While collapse-prevention and life-safety performance can be achieved, the connections and CLT walls are permanently damaged and may be unreliable for resisting aftershocks. Slip-friction connections (SFCs), also known as slotted-bolted connections, are highly resilient passive dampers originally developed for steel structures. As part of a thorough research program to apply SFCs to CLT rocking walls, connection tests were conducted to better understand a novel application of multiple modern timber technologies. The research objectives were to design and test a slip-friction connection connected to cross-laminated timber with inclined self-tapping screws (STS) to determine the stiffness and passive damping properties of the assembly. The SFC system was designed for use as a slip-friction hold-down in a self-centering CLT rocking wall system. A series of quasi-static, cyclic axial tests with slip-friction connections was shown to impart ductile and elastic–plastic characteristics to an otherwise non-ductile CLT connection. Because the timber connection forces were limited below the STS yield strength by the slip-friction connectors, no wood damage was observed during SFC sliding. Compared to other sources, passive damping was found to be excellent and close to typical idealized friction hysteresis models. The slip-friction connections are a unique design using an efficient CLT to SFC connection, with no slack, that performs consistently after consecutive tests. Additionally, cursory equivalent-viscous-damping ratios were extracted for the elastic range of the STS to steel connection for small amplitude excitations below the slip force threshold. [ABSTRACT FROM AUTHOR]

• Evaluated effects of two brown rot fungi (Gloeophyllum trabeum and Rhodonia placenta) on performance of laterally loaded connections. • T-shaped connection assemblies modelling floor-to-wall connections in buildings were inoculated with fungi for up to 40 weeks. • Evaluated capacity, stiffness, energy dissipation, and ductility of connection assemblies through quasi-static cyclic connection tests. • Load carrying capacity and energy dissipating capacity were the most adversely affected of all the parameters evaluated. 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. [ABSTRACT FROM AUTHOR]

• Cross-laminated timber requires connection development for seismic resiliency. • Slip-friction connections tested in CLT shear walls with static, cyclic loadings. • Idealized hysteretic models accurately represent CLT rocking walls. • Repeatable performance and structural protection beyond allowable loads. • CLT walls were observed to consistently return to center. Construction of tall-wood buildings is increasing in North America. Cross-laminated timber (CLT) is one of the dominant structural mass-timber products and is a high strength, lightweight, value-added, wood-composite panel suited for shear walls. The advent of tall-wood buildings necessitates further connection development for increased seismic resiliency to meet life-safety and performance-based design objectives. A novel slip-friction connection (SFC) was tested on six 1.52 m by 3.04 m CLT walls subject to static and pseudo-static cyclic loading. The SFC used Belleville springs and brass shims to provide stable slip forces and equivalent-viscous damping ratios between 0.28 and 0.63. Half of the wall tests included a short post-tensioned restoring rod with Belleville washers. Inclined self-tapping screws (STS) were used to connect the SFCs to the CLT walls, and the SFCs were designed to reliably dissipate energy while protecting the rocking walls from damage. The wall's response followed the idealized parallelogram, or flag shaped, hysteretic models observed in other slip friction systems. Results indicate repeatable performance and structural protection at least 2.5 times STS design loads. Self-centering and partially-centering wall conditions were tested by changing the SFC slip forces, while maintaining an initial 23 kN restoring force. Based on these experiments designers will be able to apply SFCs to CLT rocking walls. [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]