Seismic response analyses of an asymmetric base-isolated building during the 2011 Great East Japan (Tohoku) Earthquake

SUMMARY Analysis of strong motion recordings of a base‐isolated building during the March 11, 2011, Great East Japan (Tohoku) Earthquake is reported in this paper. The building, located in Tokyo Bay area, is an asymmetric L‐shaped structure consisting of seven‐story and 14‐story building with vertical opening. Vibration monitoring system was installed on the building in 2010, and seismic responses were recorded including the strongest shaking (peak ground acceleration 0.80–1.40 m/s2) experienced during the main shock of March 11, 2011, Great East Japan Earthquake. The building survived the earthquake without structural damage. The study in this paper includes response analysis, system identification, and seismic performance evaluation of the structure, especially performance of base‐isolation system. The study shows that despite considerable shift in effective frequency of the building due to the increase in flexibility of isolation system during the main shock, large acceleration was recorded on the superstructure with the peak floor accelerations of nearly 300 cm/s2. Two factors contribute to this cause: one is the characteristics of the building where torsional modes dominate the seismic response of upper stories and the other is resonance, where dominant frequencies of ground motions coincide with the natural frequencies of torsional modes. Moreover, analysis shows that torsional modes were not significantly influenced by performance of base isolation, so that even though the base isolation has functioned properly, the upper stories still experienced large floor accelerations. The paper also discusses long‐term observation of seismic responses during aftershocks and various levels of earthquakes between 2010 and 2012. Copyright © 2014 John Wiley & Sons, Ltd.