In this study , torque loading tests on small shear blocks were performed to evaluate the rolling shear strength of cross-laminated timber (CLT). The CLT plates in the tests were manufactured with Mountain Pine Beetle-afflicted lumber boards and glued with polyurethane adhesive; two types of layups (five-layer and three-layer) with a clamping pressure 0.4 MPa were studied. The small block specimens were sampled from full-size CLT plates and the cross layers were processed to have an annular cross section. These specimens were tested under torque loading until brittle shear failure occurred in the middle cross layers. Based on the test results, the brittle shear failure in the specimens was evaluated by detailed finite element models to confirm the observed failure mode was rolling shear. Furthermore, a Monte Carlo simulation procedure was performed to investigate the occurrence probability of different shear failure modes in the tests considering the randomness of the rolling shear strength and longitudinal shear strength properties in the wood material. The result also suggested the probability of rolling shear failure is very high, which gives more confident proof that the specimens failed dominantly in rolling shear. It was also found that the torque loading test method yielded different rolling shear strength values compared to the previous research from short-span beam bending tests; such a difference may mainly be due to the different stressed volumes of material under different testing methods, which can be further investigated using the size effect theory in the future.
This paper presents a study on evaluating rolling shear (RS) strength properties of cross laminated timber (CLT) using torsional shear tests and bending tests. The CLT plates were manufactured with Spruce-Pine-Fir boards and glued with polyurethane adhesive. Two types of layups (3-layer and 5-layer) and two clamping pressures (0.1 MPa and 0.4 MPa) were studied. For the torsional shear tests, small shear block specimens were sampled from the CLT plates and the cross layers were processed to have an annular cross section. Strip specimens were simply sampled from the CLT plates for the bending tests. Based on the failure loads, RS strength properties were evaluated by torsional shear formula, composite beam formulae as well as detailed finite element models, respectively. It was found that the two different test methods yielded different average RS strength value for the same type of CLT specimens. The test results showed that the CLT specimens pressed with the higher clamping pressure had slightly higher average RS strength. The specimens with thinner cross layers also had higher RS strength than the specimens with thicker cross layers.
In this study, the duration-of-load effect on the rolling shear strength of cross laminated timber (CLT), with different cross-sectional layups (five-layer and three-layer), was evaluated. A stress-based damage accumulation model is chosen to evaluate the duration-of-load strength adjustment factor of the rolling shear strength of CLT. This model incorporates the established short-term rolling shear strength of material and predicts the time to failure under arbitrary loading history. The model has been calibrated and verified based on the test data from low cycle trapezoidal fatigue tests (damage accumulation tests) in the previous study. The long-term rolling shear behaviour of CLT can then be evaluated from this verified model. As the developed damage accumulation model is a probabilistic model, it can be incorporated into a time based reliability assessment of the CLT products, considering short-term, snow, and dead load only loading cases. The reliability analysis results and factors reflecting the duration-of-load effect on the rolling shear strength of CLT are compared and discussed. The characteristic of this modeling theory lies in that the verified model is also able to predict the duration-of-load behaviour of CLT products under arbitrary loading history, such as long-term dead load case; then, these predictions of time to failure from the damage accumulation model can elucidate duration of load by the stress ratio evaluation approach. The results suggest that the duration-of-load rolling shear strength adjustment factor for CLT is more severe than the general duration-of-load adjustment factor for lumber; this difference should be considered in the introduction of CLT into the building codes for engineered wood design.
This paper presents a study on rolling shear damage accumulation and duration of load of cross-laminated timber (CLT) with low cycle fatigue tests. The study of the duration-of-load (DOL) effect on strength properties of wood products is typically challenging; it may be more challenging for non-edge-glued CLT considering crosswise layups of wood boards, existing gaps, and non-uniform stress distributions in cross layers. In experimental studies, short-term ramp loading tests and low cycle trapezoidal fatigue loading tests were used to study the DOL behaviour of the CLT rolling shear. The ramp tests were performed to establish the short-term CLT rolling shear strength properties. The low cycle trapezoidal fatigue tests were performed to evaluate the damage accumulation process for the matched specimens under controlled rolling shear stress levels. A stress-based damage accumulation model was further used to investigate the rolling shear DOL effect with model parameters treated as random variables calibrated against one set of the test data. The calibrated model predicted well comparing with the other set of the test data. This verified model provides a robust tool to quantify the DOL effect on rolling shear strength in the core layers of CLT that can be used in future studies of DOL behaviour in CLT under arbitrary loading histories.
In this study, the duration-of-load (DOL) effect on the rolling shear strength of cross laminated timber (CLT) was evaluated. A stress-based damage accumulation model is chosen to evaluate the DOL effect on the rolling shear strength of CLT. This model incorporates the established short-term rolling shear strength of material and predicts the time to failure under arbitrary loading history. The model was calibrated and verified based on the test data from low cycle trapezoidal fatigue tests (the damage accumulation tests). The long-term rolling shear behaviour of CLT can then be evaluated from this verified model. As the developed damage accumulation model is a probabilistic model, it can be incorporated into a time-reliability study. Therefore, a reliability assessment of the CLT products was performed considering short-term and snow loading cases. The reliability analysis results and factors reflecting the DOL effect on the rolling shear strength of CLT are compared and discussed. The results suggest that the DOL rolling shear strength adjustment factor for CLT is more severe than the general DOL adjustment factor for lumber; and, this difference should be considered in the introduction of CLT into the building codes for engineered wood design.
Cross laminated timber (CLT) shear walls typically consist of solid engineered timber panels connected by metal hardware such as hold-downs, angle-brackets and others. Under seismic loads, the panel elements deform mainly in a rocking mode coupled with a sliding mode and small amount of in-plane bending/shear deformations. The connection system normally governs the lateral behaviour of CLT shear walls. This paper presents a finite element wall model CLTWALL2D to study CLT shear wall behaviour. The model consists of elastic orthotropic plate elements for the panels and nonlinear spring elements for the connections. Contact elements are also used for the panel-to-panel interactions. The nonlinear spring properties are represented by a subroutine called HYST that is able to model the strength and stiffness degradation and the pinching effect commonly observed in timber connections. The HYST parameters are calibrated by experimental data of CLT connections and embedded to the CLTWALL2D model. The wall model is validated against experimental data of a CLT shear wall test database. Parametric studies are then carried out to study the influence of gravity loads and vertical connection densities on the wall behaviour in terms of strength, stiffness, ductility, and energy dissipation.
Currently the massive timber shear walls are mainly made from Cross Laminated Timber (CLT), which possesses a high in-plane shear strength and rigidity. But only part of its elements (mainly the vertically aligned laminae) are engaged in carrying the vertical load and that could be a limitation when designing taller timber structures or wherever higher vertical load is present. This project studied alternative solutions to massive timber shear wall system, based on Nailed Laminated Timber (NLT) and post laminated LVL (Laminated Veneer Lumber).
The test was conducted on three levels: shear test on glue/nail line, bending-shear test on a small element, and full size wall test under lateral loading. The former two tests investigated the properties of basic elements in NLT and post laminated LVL. The results were used to design and predict the performance of full size shear walls.
The NLT walls were tested under two conditions: without sheathing and with plywood sheathing. The wall without sheathing had the lowest load-carrying capacity and lowest stiffness. Adding plywood sheathing significantly increased its strength and stiffness. The failure in the wall with sheathing was at the sheathing connections, in the forms of nail withdrawal, nail head pull through, and nail breakage. The NLT wall with sheathing had a peak load up to 60% higher than the comparable light wood frame wall, also with a higher stiffness and better ductility. NLT shear walls have an internal energy dissipating capacity which CLT and post laminated LVL walls lack. The post laminated LVL walls behaved as a rigid plate under lateral loading, with little internal deformation. The failure occurred at the holdowns not within the wall. The size effect of its shear strength was studied and an equation was developed to calculate the shear strength of a large size wall plate.
Both products have efficient vertical load bearing mechanism by arranging all elements in the vertical direction. They may serve as alternative to light wood frame walls or CLT walls. Some guidelines for the application and design of NLT shear walls and post laminated LVL shear walls were proposed.
This paper presents results of an experimental study of commonly used angle bracket and hold-down connections in Cross Laminated Timber (CLT) wall systems under bi-directional loading. Monotonic and cyclic tests of the connections were carried out in one direction, while different levels of constant force were simultaneously applied in a perpendicular direction. The experiment aims to consider the combined and coupling effect of loads for connections in a rocking CLT shear wall system. Key mechanical characteristics of those connections were calculated, evaluated and discussed. The results show that shear and tension actions for hold-downs are quite independent but strongly coupled for angle brackets. The study gives a better understanding of hysteretic behaviour of CLT connections, and provides reliable data for future numerical analysis of CLT structures.
This study aims to develop an improved understanding of the interfacial bond behavior of softwood glulam joints with bonded-in threaded steel rod. A total of 39 glulam joints with bonded-in single-threaded steel rods were tested to failure in the pull-pull configuration. The test results were presented in term of failure modes, load-relative movement response, pullout strength, and the corresponding slip. The distributions of bonded-in rod axial strain, interfacial bond stress, and relative movement were also analyzed to evaluate the local bond stress– relative movement response in the bond line. The results confirmed that the bond-relative movement response is dependent on the locations along the anchorage length, and the bond-relative movement responses located near both the loaded end and the anchorage end were observed to be stiffer than those at other locations. Finally, the predictions for the load capacity of the glulam joints with bonded-in threaded steel rod were carried out based on several existing empirical formulas.
In order to study the lateral resistance of reinforced glued-laminated timber post and beam structures, nine cyclic tests on full-scale one-storey, one-bay timber post and beam construction specimens were carried out. Two reinforcement methods (wrapping fiber reinforced polymer (FRP) and implanting self-tapping screws) and two structural systems (simple frame and knee-braced frame) were considered in the experimental tests. Based on the experimental phenomena and test results, feasibility of the reinforcement was discussed, contribution between different methods was evaluated, and the seismic performances of the specimens were studied. Results showed that both the two reinforcement methods could restrain the development of crack, and recover the strength, stiffness and energy dissipation capacity. It also showed that the lateral resistance could be improved significantly when the failed simple frame retrofitted by reinforcing the joint and adding knee-brace, and this approach can be very practical in engineering.