The Sosokan building of Keio University located in Yokohama, Japan, is a base‐isolated building with a semi‐active control system. The semi‐active dampers’ damping coefficient is changed to best reproduce the optimal control force in linear‐quadratic‐gaussian control. However, the implemented control gain was calculated on the basis of the structural parameters applied at the design stage. In this study, we identified the building's actual structural parameters using observational records from the 2011 Tohoku earthquake. Using the updated structural model and 1000 simulated ground motions, we conducted a dynamic response analysis, which resulted in a fragility curve. Then, we evaluated the damage probability using the fragility curve and a seismic hazard curve. Finally, we determined the optimal control parameters to minimize the damage probability and used these parameters to implement the semi‐active control system.

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