A ndi Xhelaj

Abstract

The wind-excited vibrations of structures induce fluctuating stresses that lead to fatigue damage accumulation and can determine structural failure. This paper presents the findings of a research program assessing the fatigue life due to wind induced vibrations of a 30 m high slender and tapered lightning pole. The structure analyzed in this work is part of a group of lightning rods located in an electric power plant. Over the course of 10 years after installation, some of the lightning rods presented fatigue problems due to wind actions. One of these poles collapsed, while some others suffered from crack damage at the pole-to-base plate connection detail. A four-stage study has been conducted to understand the causes of fatigue damages. In the first step, a climatological analysis has been carried out, obtaining the probability density function of wind velocity in the area, starting from 49 years of anemometric records measured at the site. In the second step, a full-scale experimental investigation based on pull and release tests was conducted to identify the natural frequencies and modal damping ratio of one of the uncracked lightning rods under investigation. The results of the test show very low modal damping ratios, especially concerning the second mode of vibration. In the third step, results from the dynamic identification test were used to estimate the response of the structure induced by turbulence and vortex shedding induced vibrations (VIV). Critical resonant conditions on the first and second modes of vibration have been considered, adopting standard (Italian CNR-DT 207 R1/2018, Eurocode 1 and 3) and advanced calculation techniques from the literature. Severe VIV response in the second mode produced very high stress conditions at the base, however not exceeding the ultimate limit state of the structure. Finally, in the fourth step, the fatigue damage induced by each excitation mechanism was estimated. Calculation reveals that the degradation of the lightning rod is predominantly related to the VIV in the second mode, even if also the effects of turbulence and VIV in the first mode give non negligible contributions. The findings demonstrated that the limited fatigue life of the lightning rod depends primarily on the stress range at the base of the pole attributable to wind-loading effects. The paper also discusses the large uncertainties affecting the analysis, stressing out the limitations of current standard methods in VIV response and fatigue assessment, even in the case of a very simple structure.

Session

Room	Date	Hour	Subject
Room 10	Thursday 30th November	10:45-11:15	Andi Xhelaj S13-1 Vibration fatigue 44 - Fatigue life assessment of a slender lightning rod due to wind excited vibrations