Failure of a transmission line system due to extreme weather conditions such as derechos, hurricanes, and other extreme wind events has caused major widely spread outages. While determining the probability of failure for transmission line components due to such wind events is an important first step for assessing the risk associated with system failure and system resilience, development of fragility functions can be a tedious task because of uncertainties associated with structure, line span, and loading. This paper presents a novel moment-matching technique for handling such uncertainties and estimating the structural fragility of a transmission tower system. Limit states are identified by carrying out nonlinear buckling analysis. Wind-load models are capable of accounting for coherence along the horizontal and vertical directions, after which fragility analysis for the transmission tower system can be carried out by considering variability in structural parameters and wind loads. Realistic drag coefficients were employed for analysis based on wind-tunnel tests carried out for the case-study tower system. The effects of adjacent towers was also considered to account for more realistic boundary conditions. Fragility curves for different wind directions are presented for two states of a system that includes a transmission line system with balanced loads (i.e., intact) and one with unbalanced forces (i.e., with broken conductors).
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