Seismic response analyses of an asymmetric base-isolated building during the 2011 Great East Japan (Tohoku) Earthquake Dionysius M. Siringoringo 1, * ,† and Yozo Fujino 2 1 Department of Civil Engineering, University of Tokyo, Tokyo, Japan 2 Institute of Engineering Innovations, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 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/s 2 ) 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/s 2 . 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. Received 17 October 2013; Revised 4 March 2014; Accepted 10 March 2014 KEY WORDS: building seismic monitoring; asymmetric base-isolated building; building torsional response; 2011 Great East Japan Earthquake; system identification 1. INTRODUCTION Base isolation as a seismic mitigation technique for building and bridge has gained popularity in Japan especially after the 1995 Hyogo-ken Earthquake. The favorable response during an earthquake, ability to limit structural damage to a low and repairable level, and ability to maintain functionality after a large earthquake are some advantages that have increased the acceptance of base-isolation technology among engineers and building owners. Nowadays in Japan, base- isolation system is widely used for hospital, school, university, and office buildings [1]. The system usually consists of seismic isolators such as elastomeric or sliding type combined with energy absorbing dampers. The isolator creates a structure with longer natural period, whereas the damper provides higher damping to reduce the structural response. According to a recent survey conducted in various locations in Japan, seismically isolated buildings generally showed *Correspondence to: Dionysius M. Siringoringo, Department of Civil Engineering, University of Tokyo, Tokyo, Japan. † E-mail: dion@bridge.t.u-tokyo.ac.jp STRUCTURAL CONTROL AND HEALTH MONITORING Struct. Control Health Monit. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/stc.1661 Copyright © 2014 John Wiley & Sons, Ltd.