Fragility modelling framework for transmission line towers under winds

Extreme winds may expose overhead transmission lines to loads that may result in the collapse of transmission towers leading to severe economic losses. This paper develops a modelling framework to assess the fragility of transmission towers under extreme winds such as tropical storms. To this end, the wind loading on a transmission tower is represented by a combination of three wind load components (i.e., the transverse and longitudinal tower loads as well as the wire load) expressed in terms of three key wind load-related input parameters (wind speed, wind angle of attack and horizontal span of the transmission line). The wind load-carrying capacity of towers is accordingly defined by a three-dimensional convex-curve surface with the three wind load components as the principle axes. A kriging surrogate model is established for the tower capacity surface as a function of uncertain structural parameters, based on the results from the nonlinear static analysis of the tower and Latin-hypercube sampling. Accordingly, the limit state function would suggest tower failure if the wind load effect exceeds the tower capacity defined by the surface. In this framework, the tower fragility is established by the Monte Carlo simulation. If the wind intensity, wind direction and line configuration are known, the wind load components can be evaluated. Utilizing the surrogate model, the capacity surfaces of sample towers are also obtained in terms of the simulated uncertain structural parameters simulated. Comparison between the estimated wind load components and the simulated sample surfaces yields the fragility curve. The proposed framework is demonstrated by a numerical example, as well as an application to a typhoon scenario analysis in which the failure probability of a real-world long-distance transmission line is assessed.