Wind fragility modeling of transmission tower-line system based on threat-dependent structural robustness index

Transmission tower fragility models play a crucial role in analyzing the resilience of power grids subjected to various environmental loads such as wind, earthquake, and ice. The limit state of the dominant transmission tower-line fragility model relies on the threshold of tower tip displacement. This paper proposes a transmission tower fragility model based on a threat-dependent structural robustness measure. The proposed methodology evaluates and quantifies the robustness of the transmission tower in the tower-line system under gravity after removing the local failed elements identified from the dynamic wind analysis. Different limit states of the transmission tower are identified by the distribution of the robustness index and the deformed tower configuration. By comparing the developed fragility curves of the tower for different limit states with the tower tip displacement-based and the element failure-based methodologies, the results indicate that the tip displacement-based fragility model overestimates the failure probability of the tower. The proposed methodology, on the other hand, leads to a probabilistic assessment of transmission tower failure subjected to wind loading with a more distinctive physical meaning.