Fragility analysis of a transmission tower under combined wind and rain loads

Frequently, only one parameter is used in fragility evaluations for engineering structures under environmental loads. However, there are two intensity measures (IMs) for combined wind and rain loads. Consequently, this study presents a process for constructing the fragility surface of tower structures under combined wind and rain loads, and an alternative IM based on basic wind speed and rain intensity is proposed to satisfy the requirement of fragility surface with two IMs. Then, a fragility evaluation is performed for an actual transmission tower-line system. The uncertain parameters and corresponding probability distributions are provided to establish the uncertainty numerical models. Intensity measure groups of wind and rain loads are generated, followed by a nonlinear dynamic analysis. The fragility surfaces are consequently obtained, and meteorological data of typhoon Hagupit are used to calculate the failure probabilities under wind and rainfall excitations. Wind attack angle is also found to exert great influence on the fragility surface, with the most unfavorable wind attack angle being 90 degrees. For lighter rain intensities, the fragility surface changes abruptly. Finally, the concept of critical collapse surface, a quick and convenient tool for judging the structural state at varying wind speeds, directions, and rain intensities, is proposed.