TORNADO HAZARD ASSESSMENT

Tornadoes are one of the leading causes of property damage and death in North America, especially in the United States. Although the average number of tornadoes per year in Canada is far less than that in the US, in some regions such as southern Ontario, tornadoes are one of the most destructive natural hazards in terms of property damage and death. However, quantitative tornado hazard assessment has not been reported for this region. Such an assessment for normal buildings, infrastructure systems and critical facilities is needed to better understand and reduce natural disaster and to promote public health, safety and prosperity. The assessment should consider single as well as multiple facilities experiencing the same tornado event; it should also consider the possibility of a tornado outbreak. To carry out tornado hazard assessment considering a single tornado, in the present study, a statistical characterization of tornado parameters in southern Ontario is developed using the tornado database of Ontario as well as that of the neighbouring regions in the United States. These parameters include the tornado occurrence rate, the intensity, path length and width, and direction of motion. Using the developed statistics and an existing wind field model, a probabilistic assessment of the tornado hazards for point-like structures and line structures or elongated systems such as transmission lines in terms of wind speed are obtained for southern Ontario. A probabilistic assessment is also carried out for multiple critical facilities or infrastructural systems such as power plants, hospitals, power transmission lines etc. experiencing the same tornado. The assessment is focused on a sensitivity analysis of the tornado hazard considering multiple ‘facilities’ with different orientations, footprints and spatial separations. It is hoped that such an assessment will facilitate and guide disaster mitigation planning. Furthermore, an approach for characterizing tornado outbreak and extreme wind hazard due to tornado outbreak is also developed. The statistical characterization of tornado outbreaks is obtained using the tornado database of the neighbouring regions of southern Ontario in the United States. Using the proposed approach and developed statistics, an assessment of hazard due to tornado outbreaks is carried out for an area representing a city or any urban or suburban area in southern Ontario. Finally, a procedure for assessing global structural capacity of a latticed transmission line tower under extreme wind load is given by considering that the tower could be adequately treated as two-dimensional model. Both the nonlinear static pushover analysis and incremental dynamic analysis are incorporated in this assessment procedure. The procedure is implemented to evaluate the capacity of a tower designed according to design code. It is expected that such a simplified analysis for transmission towers will facilitate possible future quantitative reliability and risk assessment of transmission lines under the tornado hazard.

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