Wind Tunnel Investigations of An Inclined Stay Cable With A Helical Fillet

EXECUTIVE SUMMARY

Experiments on a 1:1 scale sectional model of a stay cable were conducted in the 3-by-6-m Propulsion and Icing Wind Tunnel at the Institute for Aerospace Research, National Research Council Canada (NRC-IAR). The purpose of the experiments was to study the effect of adding a helical fillet to the surface of a stay cable and monitor the response of the cable during wind-induced excitation.

The experiments were carried out on a 6.7-m-long sectional model with a diameter of 0.162 m composed of a central steel core covered with a high-density polyethylene (HDPE) tube obtained from a bridge construction site. A double parallel helical fillet with a rectangular cross section of 2.3 by 2.4 mm and a right-handed helix angle of 45 degrees was glued to the surface of the model. The geometry of the helical fillet was selected to represent what is currently used on stay cables of long-span cable-stayed bridges to mitigate rain/wind-induced oscillations.

The sectional model was mounted in an eight-spring suspension rig allowing along-wind and across-wind vibrations of the model at a frequency of 1.4 Hz in both directions. The total sprung mass was 406 kg, and the inherent structural damping of the suspension rig was low, between 0.07 and 0.15 percent of critical damping for vibration amplitudes observed during the tests. Simultaneous measurements of fluctuating surface pressures on the circumference of the model and response at its extremities were carried out for wind speeds ranging from 4 to 36 m/s covering the entire critical number regime and beyond (43,000 < Re < 391,000), where Re represents Reynolds number.

The wind-tunnel investigation included experiments with the cable model at 60- and 45-degree inclinations in smooth flow with the helical fillet, at a 60-degree inclination in turbulent flow with and without the helical fillet, and at a 60-degree inclination in smooth flow without the helical fillet. The tests were conducted for several rotations of the cable model on its longitudinal axis.

The experiments revealed that a stay cable with a helical fillet inclined at 60 degrees to the flow can experience wind-induced vibrations with large amplitudes in smooth or turbulent flow for a low level of structural damping. The oscillations observed appeared to be self-limited in amplitude and could be mitigated with an increase of structural damping. The cable model with the helical fillet experienced a drag crisis almost as pronounced as for the smooth cable. The experiments also revealed that the aerodynamic forces at the source of the vibrations were highly sensitive to a rotation of the cable model on its axis. Measurements of the external diameter of the smooth cable model have shown a maximum eccentricity equivalent to 1 percent of the mean diameter, which appeared to be sufficient to influence the aerodynamics of the cable model, even with the helical fillet in place.