Real-scale observations of vortex induced vibrations of stay-cables in the boundary layer

Abstract

This paper shows some records and analysis of evidences of vortex induced vibrations of very long stay cables in the atmospheric boundary layer. The considered structure is a 254-m high telecommunication antenna whose structural system is made of a light flexible metallic truss, approx. 10×10m square, which is stayed at five different levels by a set of 20 pairs of stay-cables. The exposure of this antenna is category I with a low-turbulence wind flow which results in a high sensitivity to vortex induced vibrations. The fundamental frequency of the longest cable is 0.3 Hz so that several dozens of regularly spaced natural frequencies could be observed with a sampling frequency of 500 Hz. Sufficiently long recordings allowed to distinctly observe the separate peaks corresponding to the various modes of the cables. Among them many are excited by the detachment of vortices, taking place at different frequencies (because of different cable diameters and the change of wind velocity along the height of the antenna) in the range 35-55 Hz. Referring to the so-called spectral model proposed by Vickery and Clark, the detachment of vortices takes place in a certain bandwidth, whose extent is not completely understood yet. This bandwidth is clearly observed with our long stay-cables because of the large density of mode shapes in the range [0, 200] Hz and because of their regular spacing in the spectral domain. To the author’s knowledge this constitutes a first attempt at identifying this parameter of the Vickery and Clark model from full-scale measurements.