Frank Lam

Given the variety of wood species available, understanding of cross-laminated timber (CLT)materials is by no means complete. This dissertation serves to advance the state-of-the-artin understanding the material and structural response of the CLT system and developingengineering tools for modelling and predicting such responses. The investigation consistedof an experimental study, numerical study and reliability analysis. The hypothesis beingtested is that the cross-layers have some contributions towards the CLT’s behaviour underthe axial compression load.In this context, to evaluate the physical and mechanical properties of CLT-lamella(sawnlumber), testing was done on the small-scale (specimens’ length ≤ 250 mm) clear woodand wood contains defects specimens. Then, a medium-scale (495 mm ≤ specimens’ length≤ 1000 mm) 3-, 5-, 7- and 9-layer CLT columns and a full-scale (specimens’ length ≥ 2400mm) 3- and 5-layer CLT elements have been tested. In addition, to characterize the stiffness (modulus of elasticity) of CLT materials, we employed three types of testing, namely,compression test, flexural test, and transverse vibration test.A numerical study is then employed. In order to compute the strength and stiffness ofmedium-scale CLT composite, we developed a nonlinear material model, namely, Subroutine for Orthotropic Materials’ Elasticity & Rate-independent Plasticity (SOME&RIP),and implemented into ANSYS as an UserMat library. In addition, a finite element tool,namely, Analysis of Universal Beam-Columns (AnUBC), considering the material andstructural nonlinearities for the stability analysis of full-scale CLT structures is developedin MATLAB. Finally, reliability analysis is carried out considering the sources of uncertainties that can be resulted from production, construction, material and loading conditions.Results show that characteristic strengths of the medium-scale 3-, 5-, 7-, and 9-layer CLTspecimen groups are 42%, 21%, 64% and 65% higher than the code specified strength,respectively. Moreover, characteristic stiffness is approximately the same as its code’scounterpart. Following the reliability analysis, we conclude that for utilizing CLT capacityefficiently and economically, using the characteristic properties and a performance factor of0.9 instead the current practice value of 0.8 is recommended in the CSA O86 code designequation.

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This dissertation presents a study on the coupling effects of tension and shear force on CLT connections in mid-rise structures. Unlike the current simplified design assumption that CLT connectors only take either pure shear or pure tension force in the structures, this research first considers the significant interactions between tension and shear loads from two orthogonal directions for CLT connections through bi-axial loading experimental tests and numerical analysis. Four sets of experimental tests were conducted: 1) AE 116 angle bracket connection tests under monotonic/cyclic tension loading with four levels of co-existent shear loads (0 kN, 20 kN, 30 kN, and 40 kN), 2) AE 116 angle bracket connection tests under monotonic/cyclic shear loading with four levels of co-existent tension loads (0 kN, 20 kN, 30 kN, and 40 kN), 3) HTT5 hold-down connection tests under monotonic/cyclic tension loading with three levels of co-existent shear loads (0 kN, 10 kN, and 20 kN), and 4) HTT5 hold-down connection tests under monotonic/cyclic shear loading with five levels of co-existent tension loads (0 kN, 20 kN, 30 kN, 40 kN and 60 kN). The specimens exhibited changes in strength and hysteresis behaviour under different configurations. Under bi-axial loading, different interactions between nails and surrounding wood embedment caused the coupling effect. Based on the mechanisms of CLT connections under bi-axial loading, a pseudo-nail model was developed using a modified protocol-independent nail connection algorithm. Two key parameters, a gap size factor β, and an unloading stiffness degradation index γ, were implemented into the original algorithm to capture the unloading stiffness degradation and coupling effect of bi-axial loading. The developed model was able to capture all the characteristics of CLT connections, including strength degradation, reloading/unloading stiffness degradation, the pinching effect, and the coupling effect. Model parameters were calibrated for all 32 configurations from the tests, and compared in terms of load carrying capacity and energy dissipation. Accurate agreements were reached. The results show that the proposed CLT connection model was able to predict the hysteresis behaviour of all nail-based connections.

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This dissertation proposes a strand-based wood composite product to be utilized as the vertical members of post-and-beam (P&B) shear walls. Since the shear wall performance is largely governed by connection systems holding the wall components together, the research focuses on the structural behaviour of two key connection types: nail and hold-down connections. The experimental studies were designed to evaluate the effects of orthogonal properties, such as vertical density profile of the strand-based product, on the connection performance. Static load tests were conducted following ASTM standards and Japanese HOWTEC connection performance guidelines. The test results showed that the connections with fasteners mounted on the face-side of the composite product outperform the ones with fasteners mounted on the product’s edge-side. Subsequently, full-scale shear wall tests were conducted on three P&B wall types to study the effect of the fastener driving direction on the wall performance. The test results confirmed that the shear walls with face-driven nails outperforms the ones with edge-driven nails in terms of load carrying capacity.A detailed mechanics-based finite-element connection model RHYST was also developed to predict the load-displacement relationship of a nail connection. It was developed based on an existing connection model HYST which idealizes a dowel-type connector driven into a wood medium as an elasto-plastic beam embedded in a nonlinear foundation that only acts in compression. RHYST assumes that the lateral response of the wood medium does not decrease at any compressive displacement. The presented model takes into account the contribution of the fastener’s vertical displacements on the response of the foundation. The simulation results of RHYST agreed well with the reversed-cyclic nail connection test results in terms of load carrying capacity and energy dissipation. The model is also able to simulate strength and stiffness degradation between the repeated loading cycles. Moreover, the applicability of RHYST was confirmed by incorporating it as a subroutine in a finite-element shear wall model WALL2D. The simulation results of WALL2D with RHYST showed a good agreement with the wall test results.

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In the beginning of the twenty-first century, the largest mountain pine beetle (MPB) outbreak ever recorded struck western Canada. A huge volume of MPB-attacked lodgepole pine is expected to hit the BC forest industry in the next decade. Technologies to convert MPB-attacked lumber into engineering wood products are urgently required. Cross laminated timber (CLT) is a technology that can produce massive timber members as an engineered wood product for timber structures.In this study, the duration-of-load and size effects on the rolling shear strength of CLT manufactured from MPB-afflicted lumber were evaluated. The study of the duration-of-load effect on the strength properties of wood products is typically challenging; and, additional complexity exists with the duration-of-load effect on the rolling shear strength of CLT, given the necessary consideration of crosswise layups of wood boards, existing gaps and glue bonding between layers.In this research, short-term ramp loading tests and long-term trapezoidal fatigue loading tests (damage accumulation tests) were used to study the duration-of-load behaviour of the rolling shear strength of CLT. In the ramp loading test, three-layer CLT products showed a relatively lower rolling shear load-carrying capacity. Torque loading tests on CLT tubes were also performed. The finite element method was adopted to simulate the structural behaviour of CLT specimens. Evaluation of the rolling shear strength based on test data was discussed. The size effect on the rolling shear strength was investigated.A stress-based damage accumulation theory was used to evaluate the duration-of-load effect on CLT rolling shear. The model was first calibrated against the test data and used to investigate the long-term CLT rolling shear strength. A reliability-based method was then applied to assess the CLT rolling shear performance. Finally, a duration-of-load adjustment factor for CLT rolling shear strength was established. The duration-of-load adjustment factor for the three-layer and five-layer CLT is different from that for lumber. The results suggest that the rolling shear duration-of-load strength adjustment factor for CLT is more severe than the general duration-of-load adjustment factor for lumber, and this difference should be considered in the introduction of CLT into the building codes for engineered wood design.

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This thesis presents theoretical and experimental studies of metal-plate-connected (MPC) wood truss joints under uni-directional tension or out-of-plane bending. A theoretical computer program, SAMPC, was developed based on finite element method (FEM). MPC joint models were constructed using SAMPC, to evaluate the three-dimensional nonlinear performance of the joints.Experimental studies were carried out on MPC truss joints under tension. The joint failure modes were discussed, and the potential reasons for the failure were explored. Data processing techniques were applied to obtain the specific load-displacement relationships, which were in turn used as reference for model calibration and verification. Based on the experimental results, optimized model parameter calibration and model verification were discussed. The program application of MPC joints subjected to out-of-plane bending was investigated. Comparisons of the results from the joint bending test and model verified the applicability of the program for evaluating the out-of-plane rotational stiffness of MPC joints.A reliability analysis was conducted to evaluate the critical buckling load and lateral bracing force of single- and double-braced wood truss web systems. The probability characteristics of a number of variables that affect the performance of braced truss web system were investigated. Based on the results, a factor relating the ratio of the lateral restraining force and axial load was established. This factor with adequate reliability was recommended as a web/bracing design amendment to Canadian Code on Engineering Design of Wood.For the investigated truss joints, SAMPC appears to be superior in terms of its ability to simulate MPC joints in elaborate detail. This detailed model can aid in developing a better understanding of joint behavior under realistic joint configurations and loading conditions. The ability of the model to accurately predict the behavior of the designed MPC joints brings up the potential of modeling joints composed of different wood species and truss plate types featuring more complex joint configurations and loading conditions. The body of information from modeling results can be used to evaluate the adequacy of a given structural design, to facilitate truss plate, truss joint and overall truss design.

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In this research, a series of experiments have been conducted, including mountain pine beetle attacked wood/plastic composite (MPB-WPC) prototype product development, dynamic mechanical analysis (DMA), short-term creep tests for master curve construction based on the time-temperature-stress superposition principle (TTSSP), and a long-term creep test. Moreover, a newly established stress-temperature incorporated creep (STIC) model, a modified Williams-Landel-Ferry (WLF) equation that incorporates the variables of temperature and stress, and a newly developed temperature-induced strain superposition (TISS) method were introduced.The MPB-WPC products showed definite potential as a value-added product option for MPB-attacked wood. The formulation affected the MPB-WPC products’ properties. The capacity of the products without a coupling agent was considerably inferior to the product formulations that included a coupling agent. The surface condition of the product was also influenced by the formulation. The dynamic mechanical properties were studied. The mechanical and viscoelastic behaviours of the MPB-WPC products were considerably influenced by the formulation of wood and plastic and the presence of a coupling agent, which can be attributed to modification of the interface property and the internal structure.The new STIC model smoothly introduced the effect of temperature into a conventional power law creep equation, and the model can be applied to predict the creep strain in which the effect of temperature is involved. Moreover, the temperature-stress hybrid shift factor and a modified WLF equation were studied; and, the parameters were successfully calibrated.Temperature-induced strain was observed in the results of the 220-day creep test. For a temperature-sensitive material like WPCs, the information obtained from conventional creep studies is not sufficient to predict long-term performance. The comparison between the long-term creep data and the master curves showed that master curves tended to overestimate the creep strain. Generally, the master curves constructed based on TTSSP cannot precisely predict the long-term creep strain, but can provide conservative estimations. To deal with the effect of fluctuating temperatures on the creep strain, the STIC model and the proposed temperature-induced strain superposition (TISS) method were established and employed. The additional temperature-induced creep strain and overall behaviour were successfully simulated.

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The current outbreak of Mountain Pine Beetle (MPB) in the province of British Columbia (B.C.) is the most extensive disturbance event occurring in North American forests in recorded history. The concept of converting the beetle killed wood into engineered wood products by defect removal and reconstitution is employed to maximize value recovery from the material. Cross Laminated Timber (CLT), which is produced in modular form and can be utilized as part of a structural system for floor, wall or roof elements, is considered as an excellent application of the concept. CLT originates from Europe. Such products have been developed as a proprietary product by individual companies aimed at servicing specific markets. There is a need to investigate different ways of making CLT and to define its structural performance suitable for North America. The main focus of this study is to investigate the structural performance of box based CLT system used in floor applications. Comprehensive three dimensional finite element models, which can be used to analyze the mechanical and vibration behavior of the plate and box type structures, were developed. Four prototype box elements, each having five replicates, were designed and manufactured locally. Third point bending tests were conducted on the specimens in the Timber Engineering and Applied Mechanics (TEAM) Laboratory at the University of British Columbia. The numerical analysis agreed well with experimental data in terms of vertical deflection and bending stiffness. Vibration, which is critical to floor serviceability, was also studied. Three types of excitation were applied to measure the fundamental frequency of the twenty specimens. Finite element analysis provided good predictions of fundamental frequency values comparing to the experimental results. A local built demonstration building, L41home, was presented and analyzed as an example using the tools developed in this study for CLT applications. As a pioneer research of CLT materials in North America, this work has contributed to the understanding of the structural performance of floor systems using CLT panels for the commercial and residential applications.

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British Columbia (BC) is in the midst of the largest outbreak of the Mountain Pine Beetle (MPB) ever recorded in western Canada. Technologies capable of converting stained lumber into market acceptable products are urgently required to reduce the impact of the growing volume of MPB killed lumber on the profitability of forestry in BC.New, thick MPB strand-based structural composite products can be produced and help absorb a large volume of MPB wood. With appropriate mechanical properties, such products can be used as beams, headers, and columns in the low-rise commercial, multi-family residential and single family residential markets.This work was focused on the duration-load and creep behaviour of thick MPB strand-based wood composite. The beam specimens were made in the Timber Engineering and Applied Mechanics Laboratory at UBC. A series of tests were conducted on the matched groups to investigate the creep-rupture behaviour. These investigations comprised of ramp load tests at three loading rates, long-term constant load tests at two stress levels and cyclic bending tests at six stress levels. A damage accumulation model was developed to study the creep-rupture behaviour. This model stipulates that the rate of damage growth is given in terms of the current strain rate and the previously accumulated damage, and a 5-parameter rheological model is applied to describe the viscoelastic constitutive relationship to represent the time-dependent strain, while the damage accumulation law acts as the failure criterion. The results of the long-term constant load tests were then interpreted by means of the creep-rupture model which had been shown to be able to represent the time-dependent deflection and time-to-failure data for different stress levels. The predictions of the model were verified using results from ramp load tests at different loading rates and results from cyclic loading tests at different stress levels. The creep-rupture model incorporates the short term strength of the material, the load history and predicts the deflection history as well as the time-to-failure. As it is a probabilistic model, it allows its incorporation into a time-reliability study of wood composites’ applications.

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This thesis presents a study to evaluate the seismic performance of post-and-beam(P&B) wood assemblies and buildings of Japanese style using computer modeling,experimental studies and probabilistic-based approaches.A numerical model called “PB3D” is proposed to predict the lateral response of theP&B buildings under static or dynamic loads. Special techniques are used to reduce theproblem size and improve computational efficiency with reasonable prediction accuracy.This model simplifies a P&B building into a combination of 2D assemblies (e.g. shearwalls, floor/roof diaphragms) while capturing the global structural responses of interest(e.g., inter-story drift and floor/roof acceleration). A mechanics-based wood shear wallmodel is implemented to represent the hysteretic properties of symmetric/nonsymmetricP&B walls. Roof/floor diaphragms are modeled as structural frames with calibratedequivalent diagonal braces in order to consider the influence of the diaphragm in-planestiffness on the building performance.Experimental studies have been conducted to study the behavior of 2D assembliesand buildings. The engineering characteristics of single-brace P&B walls have beenevaluated by monotonic and reversed cyclic tests. The contribution of additional gypsumwallboards to the wall lateral resistance has also been studied. An in-plane pushover testhas been conducted to study the in-plane stiffness of a floor diaphragm. Two one-storyP&B buildings have been tested under biaxial static loads and one-directional seismic loads, respectively. The established test database as well as a test database of a two-storyP&B building provided by a research institute in Japan has been used to verify the “PB3D”model.Using the response surface method with importance sampling and considering theuncertainties involved in seismic ground motions, structural mass, and response surfacefitting errors, seismic reliability analyses have been conducted to estimate the seismicreliabilities of a series of shear walls, a one-story building and a two-story building.System effect on the shear wall reliability has also been studied.The framework presented in this thesis provides a useful tool to assess the seismicperformance of the P&B wood buildings and to aid the performance-based seismic designof these structural systems.

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This thesis describes a study on the stability capacity and lateral bracing force of woodbeam-columns and metal plate connected (MPC) wood truss assemblies.A user-friendly computer program, SATA, was developed based on the finite elementmethod (FEM). The program can be used to perform three-dimensional nonlinear structuralanalyses by using the Newton-Raphson and arc-length methods. The Monte Carlo simulationand response surface methods have also been incorporated into the program for the purpose ofreliability analyses.Experimental studies were conducted to provide input parameters and verification forthe developed software. Material property tests were performed to consider a variety ofmaterials. Biaxial eccentric compression tests of wood beam-columns and full-scale tests ofMPC wood truss assemblies were also carried out to study the critical buckling load and lateralbracing force. The program predictions were in good agreement with the test results.A reliability analysis was conducted for a simplified MPC wood truss assembly usingthe developed program. The effect of the variation of the structural behaviour and externalloads on the critical buckling load of the truss assembly was studied. The adequacy of the 2%rule-of-thumb was also studied.This research bridges the knowledge gap that currently exists in the understanding anddesign of MPC wood truss assemblies and their lateral bracing systems. The test database andthe output of the developed program contributes to the development of more efficient designmethods for MPC wood truss assemblies and other structures where buckling failure is ofconcern.

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