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Izuru Okawa

Publications and source records attributed to Izuru Okawa.

4 recordsLinked to original sources
USGS2017

Before and after retrofit behavior and performance of a 55-story tall building inferred from distant earthquake and ambient vibration data

A sparsely instrumented 55-story building in Osaka, Japan had recorded unprecedented, severe and long-duration long-period resonating 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 retrofitted building were also recorded during the April 24, 2016 M7.3 Kumamoto earthquake that occurred at 478 km. The earthquake and on-demand acquired ambient response data are analyzed in this study to assess the impact of this rare, retrofitted and instrumented tall building subjected to long period strong shaking from events originating at far distances. As expected, the fundamental frequency and critical damping ratio of the building increased, albeit small, after the retrofit as compared to before the retrofit. Increase of damping percentage is a positive finding and indicates that even larger percentages may be attained under shaking stronger than the 2016 event. The records indicate that the building still experiences significant resonance, torsion, as well as a beating effect.

Osaka↗
USGS2014

Drift issues of tall buildings during the March 11, 2011 M9.0 Tohoku earthquake, Japan - Implications

One of the most significant effects of the M9.0 Tohoku, Japan earthquake of March 11, 2011 is the now well-known long duration (>10 minutes) shaking of buildings in Japan – particularly those in Tokyo (~350-375 km from the epicenter) and in places as far as Osaka (~770 km from the epicenter). Although none collapsed, the strong shaking caused many tall buildings not to be functional for days and weeks. The purpose of this paper is to discuss the behavior and performance of four tall buildings, considered to be representative of most tall buildings in Tokyo and other locations, and from which shaking data were retrieved. Of particular interest is a building in Osaka that almost reached an average of 0.5% drift ratio – this, with a ground level input motion of ~3% g is significant. What might have happened during an event with input level motions with similar low frequency content and in 10-20% g range is a legitimate question that must be pondered. The particular building had serious site effects and was in resonance. The other three examples are from Tokyo. For example, based on records obtained from a 54-story building retrofitted with 288 oil dampers on 24 floors, computations show that average drift ratio may have reached ~0.3% and maximum drift ratio likely was > .3%. The maximum allowed by Japanese practice for buildings taller than 60 m and for collapse protection (level 2) motions is 1% (The Building Center of Japan, 2001). Performances of tall buildings in many seismically active regions of the world (e.g. Chile, Turkey) or those tall buildings affected by long distance long period effects by sources at a distance (e.g. Abu Dhabi, Dubai) are of interest to the earthquake engineering community. Chile imposes 0.2 % drift limit that result in elastic design. USA and Turkey impose 2% drift limit. Such wide variations of drift limits in design practices deserve discussion in light of functionality and performance of tall buildings during the 2011 Tohoku event.

Conference Paper↗
USGS2012

Response of a tall building far from the epicenter of the 11 March 2011 M 9.0 Great East Japan earthquake and aftershocks

The 11 March 2011 M 9.0 Great East Japan earthquake generated significant long-duration shaking that propagated hundreds of kilometers from the epicenter and affected urban areas throughout much of Honshu. Recorded responses of a tall building at 770 km from the epicenter of the mainshock and other related or unrelated events show how structures sensitive to long-period motions can be affected by distant sources. Even when the largest peak input motions to the building is about 3% g , the strong-shaking duration was about 140 s. The 300- to 1000-s prolonged responses of the building are primarily due to a combination of site resonance (e.g. structural fundamental frequency ~0.15 Hz and site frequency ~0.13–0.17 Hz) and low damping (~1–2%) of the structure. Response modification technologies can improve the response of the building during future earthquakes. The need-to-consider risks to such built environments from distant sources are emphasized.

Honshu↗