The issue of unloaded davit arms vibrating and fatiguing to failure has been prevalent in our industry for decades. At extra high voltages davit arms are fairly long slender structures with relatively high flexibility and low structural damping, they are susceptible to wind-induced vibrations. This is particularly true at larger voltages where clearances require longer davit arms than lower voltages (138 kV and less). Davit arms represent a bluff structure. A bluff structure is one in which the flow separates from large sections of the structure's surface. The area of re-circulation downstream of a bluff structure is called the wake. In most conditions, the wake of a bluff structure will be unsteady and will shed vortices. Under the right circumstances (dependent on wind speed and Reynold's number), the shedding vortices create an oscillating pressure differential. This differential leads to motion in the across-wind direction. If a resonant frequency of the bluff structure lies within the spectrum of vortex shedding frequencies, and there is not sufficient damping, large across-wind amplitudes are probable, resulting in fatigue damage and potential failure at the arm's weld.
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