Abstract Summary
Load-bearing cables in bridges frequently have problems with excessive vortex-induced vibrations, requiring intervention by additional damping devices to reduce vibration amplitudes. This contribution presents results from monitoring of acceleration responses in hangers on the Hålogaland suspension bridge, a long-span bridge in northern Norway. Since its construction, this bridge has had problems with hanger vibrations believed to be problematic for long-term service life. The hangers have later been equipped with Stockbridge dampers, consisting of two rigid masses suspended from two steel-coil wires. Surprisingly, several of the dampers have been damaged during their first year for reasons largely unknown. This further strengthens the need to verify the combined dynamic behavior of the hangers and the dampers. This contribution represents a first characterization of the monitoring results. The hangers considered are between 120 and 100 m long. Data from two configurations are presented: hangers with and without dampers. Thus, the effectiveness of vibration mitigation is quantified. It is shown that the dampers reduce the vibration amplitudes by a factor of 2 to 10 – depending on the wind conditions and the dominant frequency range of vibration. Secondly, an analysis using measured wind velocities is carried out. By using spectrograms of the accelerations of hangers without dampers, it is clearly shown that the time-varying vibration frequencies strongly correlate with the mean wind speed. This is further evidence that the vibrations can be characterized by a shedding frequency (fs) commonly given by the relation fs=St*V/D, where St=0.18 is the Strouhal number, D is the diameter, and V is the wind speed. It is also clear that the vibrations distinctly shift from one cable vibration mode to the next concurrently with variations in the mean wind speed. Still, there are also cases where additional higher-order modes are excited, occurring at twice the shedding frequency. Lastly, it is presented some future work further characterizes the in-situ behavior of the damper: accelerometers have been attached on the Stockbridge damper masses. To the best of the authors' knowledge, none or few existing studies of dampers have combined data from wind, cable vibrations, and damper mass vibrations. The work is expected to reach recommendations for the effective and durable use of Stockbridge dampers on cables.
