Research On Galloping Stability Of Iced Stay Cables

In cold weather, ice accretions can change the original cross-section configurations of stay cables, which will lead to galloping vibrations of stay cables due to the aerodynamical instability. Galloping is a type of wind induced vibration with the characteristics of large amplitude and low frequency and plays an important role on the safety of cable-stayed bridges. Therefore, it is necessary to investigate the galloping stability of iced stay cables.Three types of typical ice accretions on stay cable surface are determined based on the results from literatures. The mean lift and drag coefficients are obtained by wind tunnel tests on six iced stay cable models with a geometrical scale of 1:1. In the present thesis, the effects of wind attack angles on aerodynamic forces is carefully investigated, and the possible attack angle range that may result in the galloping vibration is determined in terms of galloping force coefficients. The results show that the mean lift coefficients of six iced stay calbe models exhibit sudden decrease region under certain attack angles, which is an eccential requirement for galloping. For these six models, the minimum galloping force coefficients are much less than zero. Consequently it is possible that galloping may occur.In this study, a single-degree-of-freedom model is also established and based on the aerodynamic coefficients obtained from the wind tunnel tests. The aerodynamic coefficients are expressed as polynomials. Taking the Yangpu Bridge in Hainan province as an example, the galloping responses are obtained by numerical computation. The critical galloping wind speeds of the first mode are analyzed in detail and compared with the theoretical results. The effects of the damping ratio of the cables, the length of iced accretion on the critical galloping wind speed are studied. The results show that:the longer the cable, the smaller the critical galloping wind speed. For the longest calbe on Hainan Yangpu Bridgethe critical galloping wind speed of the first mode is far lower than the local design wind speed; and the critical galloping wind speed will be heightened if the damping ratio of the cable increases.In Chpter 5, the flow around two kinds of iced stay cable models (the crescent shape and the D shape) is simulated by use of commercial software Fluent. The critical galloping wind speeds are computed in terms of the mean aerodynamic coefficients from CFD, and is compared with those of the wind tunnel tests. It can be found that the CFD results can catch the main features of the mean aerodynamic force coefficients.