Second‐mode tuned mass dampers in base‐isolated structures for reduction of floor acceleration

Abstract

SummaryTo reduce floor acceleration of base‐isolated structures under earthquakes, a tuned mass damper (TMD) system installed on the roof is studied. The optimal tuning parameters of the TMD are analyzed for linear base isolation under a generalized ground motion, and the performance of the TMD is validated using a suite of recorded ground motions. The simulation shows that a TMD tuned to the second mode of a base‐isolated structure reduces roof acceleration more effectively than a TMD tuned to the first mode. The reduction ratio, defined as the maximum roof acceleration with the TMD relative to that without the TMD, is approximately 0.9 with the second‐mode TMD. The higher effectiveness of the second‐mode TMD relative to the first‐mode TMD is attributed primarily to the unique characteristics of base isolation, ie, the relatively long first‐mode period and high base damping. The modal acceleration of the second mode is close to or even higher than that of the first mode in base‐isolated structures. The larger TMD mass ratio and lower modal damping ratio of the second‐mode TMD compared to the first‐mode TMD increases its effect on modal acceleration reduction. The reduction ratio with the second‐mode TMD improves to 0.8 for bilinear base isolation. Because of the detuning effect caused by the change in the first‐mode period in bilinear isolation, the first‐mode TMD is ineffective in reducing roof acceleration. Additionally, the displacement experienced by the second‐mode TMD is considerably smaller than that of the first‐mode TMD, thereby reducing the installation space for the TMD.