Dynamic characteristics of a 55-story building before and after retrofit

Abstract

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.