The Influence of Ice Accretion Thickness on the Aerodynamic Behaviour of Stay Cables of Bridges

Several cable-stayed bridges in the world are exposed to freezing rain conditions. Ice that accretes at the surface of the stay cable modifies the circular cross-section of the cable sheath. Storms resulting in a few millimeters of ice accretion occur more frequently, but thick ice accretion on the order of 60 mm of precipitation can be associated with freezing rain events in Eastern Canada with a return period of 50 years. Both cases could affect the aerodynamics of the stay cables and could lead to potential cable instability. A wind tunnel study was carried out to investigate the influence of ice accretion thickness on the aerodynamic behaviour of stay cables. Two freezing rain precipitation levels were investigated: 2.5 and 60 mm. Four stay cable models were fabricated in SLS: a baseline cable model without ice, cable models with ice accretion corresponding to 2.5 mm and 60 mm of freezing rain at -1.5 degrees C ambient temperature and a fourth cable model with 60 mm of freezing rain at -5 degrees C ambient temperature. The ice accretion shapes were generated based on the National Research Council Canada (NRC) morphogenetic numerical simulation model of ice accretion on stay cables. Aerodynamic forces acting on the static models were measured for different wind speeds, cable-wind angles and cable axial rotation angles, for Reynolds numbers (Re) up to 5 x 10(5). The wind-tunnel test was done in smooth flow and in turbulent flow. The results indicated that the drag coefficient increased in magnitude with the ice thickness from 0.75 to 1.2 at high Re but the highest drag coefficient value of 1.38 that was observed in the study occurred at Re of 0.5 x 10(5) for the cable with thick ice accretion. The aerodynamic response of cables with thick ice was largely dictated by the shape of the ice and the cable-wind angle but a limited effect of the wind turbulence was also observed. Exposing the cable without ice to wind turbulence or adding thin ice accretion had a similar impact with a shift of the drag crisis to lower Re. Large negative values associated with the variation of the lift coefficient with the cable axial rotation angle suggest that cables with thin and thick ice accretion may show the potential for cable instability.