More than 50 cable-supported bridges have been built in Korea since 1980. While maintaining those infrastructures for last 30 years, we have experienced unexpected problems from operational field monitoring of long-span bridges. This paper discusses on what we have observed, learned and how we responded to those events. As an example, a vortex-induced vibration was observed in 2011 in parallel-disposed twin cable-stayed bridges. Operational monitoring and wind tunnel tests successfully

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... d that the close distance between two bridges was one of the main cause of the vibration. However, an operational modal analysis revealed that unexpected low damping ratio of the bridge was also contributed to the realization of this serviceability problem in an operating bridge. Another observation of vibration in a long-span suspension bridge also attracted our attention on the potential low damping ratio, which may invoke serviceability issues in assessment of bridges. This paper introduces the details of the cause investigations, assessment and mitigation of the serviceability problems, validation through field tests and operational monitoring, and concluding suggestions to bridge owners and engineers.