Most of the failure incidents of transmission line structures during natural disaster events are shown to result from localized high intensity wind loads associated with downbursts. As an example, in 1996, Manitoba Hydro reported a failure of 19 towers occurring in Winnipeg, Manitoba, Canada, during a strong downburst event. The current codes and standards for transmission towers do not account for downburst loads as they only specify design loads associated with large-scale wind events, such as hurricanes and typhoons. The studies reported in this thesis represent a part of an extensive research program aiming to address this short fall in the current codes and also to understand the failure mechanisms of transmission towers under downbursts. Details of a finite element program developed in-house specifically for the analysis of transmission lines under downburst loading are reported. The developed numerical code represents a comprehensive tool that considers the velocity field for downburst based on computational fluid dynamic analysis conducted by others, transforms these velocities into forces, models various components of a transmission line, and predicts the internal forces and deformations through time-step analysis. One of the challenges in studying the response of structures to downbursts is that the acting forces vary significantly with the downburst configurations, which are defined by the jet velocity, the jet diameter, and the location of the centre of the downburst relative to the structure. An extensive parametric study is conducted using the developed numerical code by varying the downburst parameters. It provides an insight about the variations of the downburst wind field as well as the internal forces in various regions of a transmission tower with those parameters. The critical downburst configurations leading to maximum internal forces in various regions of a transmission tower as well as the structural response under those critical configurations are then identified. The numerical model is extended by including failure criteria for the tower members and is then used to conduct a progressive failure analysis for one of the towers that failed during the Winnipeg downburst. Various modes of failure as well as the downburst configurations associated with those failures are described. Finally, the efficiency of the numerical model is improved by incorporating an optimization technique, based on the genetic algorithms, into the finite element model. This coupled finite element/optimization tool is capable of predicting in an efficient way the critical downburst configurations leading to both maximum internal forces and progressive failures of transmission lines subjected to downburst loading events.;Keywords. Downburst, Microburst, High Intensity Wind, Transmission Line, Transmission Tower, Finite Element, Failure, Progressive Failure, Optimization, Genetic Algorithms.
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