Wind- and rain-induced oscillations of cables of stayed bridges

Abstract

Abstract The purpose of this investigation is to study the oscillations of cables of a cable-stayed bridge under the combined effect of wind and rain, as during some observations rain was a probable cause for an unstable behaviour of an otherwise stable system. To find the source of instability is scaled model tests a part of the cable was simulated with a small rigid cylinder. The required freedom for motion was obtained by using thin cantilever bars. The amplitude of the oscillation was measured by means of strain gauges directly attached on the surface of the bars. The idea is that the instability was caused by droplets accumulating to a liquid film, rivulet, shape on the surface of the cylinder under the influence of the drag, gravitational and friction forces. Flow visualisation was made in order to see the formation of two rivulets. Then the rivulets were simulated by two bars placed at different locations on the cylinder surface as well as with bars of different size and shape. It turned out that the presence of the bars results in a strong increase in the amplitude of the oscillation of the galloping type. When repeating the experiments using oil as an efficient substitute for water, galloping oscillation was obtained implying that the source of instability is the presence of the rivulet at a certain position changing the cross-section of the cylinder into a non-symmetrical one. In an attempt to suppress the instability, metal wire was wraped around the cylinder which led to a clear reduction in the amplitude, suggesting the possibility for a solution of the problem.