Retrofit of Aerodynamic Cable Instability on a Cable-Stayed Bridge: Case Study

Elegantly poised over the Mobile River, the twin pylons and the semi-harped cable stays of the Cochrane Bridge subtly complement the vast and undulating landscape of the Mobile Bay as the bridge carries US Route 90 over the Mobile River in Alabama. In February 1998, light rain drizzled on the bridge, and a weather station nearby recorded wind speeds of about 48 km/h (30 mph). Under these seemingly mild weather conditions, the normally immobile cable stays started to vibrate, and within moments, these nascent vibrations reached amplitudes of more than 1.2 m (4 ft). Alarmed by this event, the Alabama Department of Transportation (ALDOT) took immediate action to ensure the continued safety and serviceability of the bridge. A team of consultants was selected by ALDOT to investigate mitigation measures for the large-amplitude cable-stay vibrations. The fast-tracked comprehensive program planned and implemented to inspect, test, document, and evaluate the effects of the large-amplitude vibrations and the recommendation of retrofit measures that would limit future occurrences of such cable-stay vibrations on the Cochrane Bridge are described in detail.