New Methodology for the Assessment of Decayed Utility Wood Poles
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Wood is one of the oldest and most common material used in construction. Since the beginning of the electrification in Canada in late 19th-century, wood poles have been widely used to provide structural support to electric transmission and distribution lines. For example, electrical network in Ontario has over 2 millions distribution poles across the province. Wood poles are typically exposed to severe environmental conditions, which cause deterioration due to wood rotting, insect attack, and weathering. The wood deterioration resulting in loss of strength can compromise the structural integrity of poles. Typical life expectancy of wood varies from 35 to 50 years depending on the environmental condition and type of wood. Electrical distribution infrastructure in Canada is aging. For example, the average age of in-service wood poles in Ontario is estimated to be 29 years with a standard deviation of 15 years. About 300,000 wood poles have been in-service for more than 45 years, which are rapidly reaching to end of expected service life. Different types of non-destructive testing (NDT) methods have been historically used for the condition assessment of wood poles. However, current methods are based on simple concepts that do not consider the variations of wave velocity and wave attenuation in an orthotropic material. The goal of this research investigation is to develop an advanced and reliable NDT technique for in-situ inspection and assessment of wood poles in order to remove unsafe poles from service, extend the service life of sound poles, and support optimum replacement strategies for the renewal of wood pole infrastructure. The thesis presents a new methodology for condition assessment of wood poles using ultrasonic testing based on theoretical, numerical, and experimental studies. The research covers areas such as signal processing, dynamic characterization, statistical reliability analysis, numerical simulations, and laboratory testing. Wood is modeled as a cylindrical orthotropic material with uncertainties in its elastic and mechanical properties. The arrival time of compressional waves as well as full-waveform analysis are used for an integrated evaluation of wood pole. A simplified model of P-wave propagation in pole cross-sections is developed; which allows to (a) estimate the elastic moduli in the radial and tangential directions by solving the inverse problem, and (b) compute the probability density function of P-wave velocity. Both of these parameters are critical for condition assessment; however, they are not available in the literature because of the complexities associated with modelling wood as an orthotropic material. A new specialized software is developed for (a) general signal processing, (b) non-destructive condition assessment of wood poles, and (c) management of a statistical database for the assessment of wood poles. Based on the proposed methodology, a new clamping device is designed and built for the ultrasonic testing of wood poles in the field. The basic background for signal processing covering Fourier analysis, frequency response and impulse response functions, and the complex exponential method for dynamic system identification is reviewed and summarized. The elastic and mechanical properties for most common species of wood used as poles are summarized from the literature, including the main statistical distributions used for their probabilistic characterization. The calibration and basic assumptions for the simulation of wave propagation in orthotropic media using finite element analysis are explained in detail. Numerical modelling is based on finite element method under plain strain condition. The numerical model is calibrated using theoretical results and validated using experimental results from laboratory testing of a new red pine pole. After calibrating the model, numerical simulations were performed to understand ultrasonic wave propagation in cross-sections of sound and decayed wood poles sections. Results of numerical simulations of ultrasonic wave propagation in pole cross-sections are presented. The effect of a void in the cross section on the ultrasonic measurement is discussed. A sample of 8 wood pole cross-sections were subjected to laboratory ultrasonic tests. In the testing, a transmitter was placed at four positions around the pole circumference. For each transmitter position, five receivers were used. The transmitter-receiver system was calibrated to evaluate its transfer function and thus eliminate the inherent characteristics of the transmitter-receiver system from the actual measurements. The experimental results of the condition assessment of new and decayed pole samples are presented in the thesis. The effect of a hole in a new pole was studied and the results were compared with the numerical analysis. A blind test is performed on an aged red pine pole. The predicted areas of decay from the ultrasonic measurements are in good agreement with the actual decay observed from dissecting the pole sections. In summary, the experimental and numerical results presented in this thesis show that the proposed methodology can be successfully applied for condition assessment of in-service wood poles in the electrical network. This method will contribute to cost-effective life cycle management of energy infrastructure as a whole.