Fatigue life assessment of a slender lightning rod due to wind excited vibrations

Abstract

The wind-excited vibrations of structures induce fluctuating stresses that may result in the accumulation of fatigue damage, ultimately posing a risk of structural failure. This paper presents the findings of a research program assessing the fatigue life of a 30 m high slender and tapered lightning pole under wind induced vibrations. A three-stage study has been conducted to understand the causes of fatigue damage. In the first step, a hybrid numerical and full-scale experimental investigation was carried out to identify the natural frequencies, modal shapes and modal damping ratios. In the second step, results from the dynamic identification test were used to estimate vortex shedding induced vibrations. Critical resonant conditions on the first and second vibration modes have been investigated, adopting standards and calculation techniques from literature. Finally, in the third step, the fatigue damage induced by vortex shedding vibrations was estimated. The findings demonstrate that the fatigue issues of the lightning rod are mainly related to the wind induced stress at the base of the pole, highlighting the contribution of vortex shedding resonant with the second vibration mode. The paper also discusses the large uncertainties affecting the analysis, showing that errors in parameter estimates give rise to very large scatter in the fatigue damage assessment.