Damping measurements and observation of the large amplitude vibrations revealed that the cable stays were highly susceptible to wind induced vibrations. The vibration problem was identified as the rain-wind vibration of the cables stays, and was caused due to low damping in the cable stays coupled with ideal aerodynamic conditions for the propagation of the vibration problem. The damping measurements showed that the cable stays on the Cochrane Bridge had very low damping ratios, with a general trend of decreasing damping ratios with increasing cable length. Also, the anchorage detailing showed that the neoprene washers were ineffective in providing any damping to cable stays. Even though no damage had been caused to the bridge due to the large amplitude vibrations, the repeated occurrence of such vibrations would eventually cause fatigue and other damage to the bridge in the future. Rehabilitation was therefore necessary to curtail the occurrence of rain-wind vibrations in the future. Based on the evaluation of the various mitigation alternatives, external damping of the cable stays using new and more effective washers and external hydraulic piston type viscous dampers were designed for mitigation of future rain-wind vibrations on the Cochrane Bridge. The construction of the rehabilitation is currently under way, and is expected to be complete by end of year 2000. An extensive program of long term monitoring is also included as part of the rehabilitation project to monitor and evaluate the performance of the mitigation measures.
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