Dynamic Characteristics of a 55-Story Building Before and After Retrofit
A sparsely instrumented 55-story building in Osaka, Japan, recorded unprecedented severe and long-duration, long-period resonant responses during the March 11, 2011, M9.0 Tohoku earthquake that occurred at 767 km distance. Thereafter, studies of the records resulted in implementation of a significant retrofit design, comprising dampers and buckling restrained braces (BRBs). The responses of the retrofit building were subsequently recorded during the April 24, 2016, M7.3 Kumamoto earthquake that occurred 478 km away, but at an azimuth nearly opposite to that for the Tohoku event. The earthquake records and ambient response data analyzed in this study represent a rare opportunity to assess the impact of retrofitting on an instrumented tall building subjected to strong long-period shaking from distant earthquakes. As expected, the fundamental frequency and critical damping ratio of the building increased, albeit by small amounts, after the retrofit. Increased damping percentage is a positive finding, and suggests that even larger percentages might be attained for shaking stronger than occurred following the 2016 event. The records indicate that the building still experiences significant resonance and torsion, as well as a beating effect.
KeywordsBuilding instrumentation Frequency Damping BRB Dampers
Data used in this paper was provided for cooperative studies between USGS and Building Research Institute of Japan as part of activities of Wind and Seismic Effects Panel of United States Japan Panel on Natural Resources. The authors thank USGS internal reviewers Chris Stephens and Kishor Jaiswal for their valuable comments.
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