ANALYTICAL STUDY ON GROWTH-MECHANISM OF RAIN VIBRATION OF CABLES

The fundamental growth mechanism of rain vibration of cable in cable-stayed bridges are studied analytically. It has been reported by Hikami and Shiraishi (1987) that the rain vibration is a large amplitude oscillation caused by the combined influenced of rain and wind, and that the rain water rivulet formed along the upper surface of cable is the origin of rain vibration. Therefore, a cylinder of figure-8 cross section is adopted as an analytical model for the rain vibration of cable with upper rivulet. The characteristics of unsteady wind forces of the figure-8 section model are investigated first through wind tunnel experiments. Two possible mechanisms are, then, checked for rain vibration by means of quasi-steady galloping analysis; one is Den Hartog mechanism of cable oscillation and another is the two-degree-of-freedom instability, i.e. the torsional mechanism, caused primarily by the circumferential oscillation of the rivulet along cable surface. It is found that the oscillation of upper rivulet, the fundamental frequency of which could coincide with the natural frequency of cable motion at the wind speed around 10 m/s, is aerodynamically coupled with the flexural oscillation of cable, and that this coupled motion of rivulet makes the modal aerodynamics damping negative. That is, it is concluded that the circumferential oscillation of the upper rivulet is indispensable to the growth of rain vibration. © 1990.