Significant seismic behavior features of two tall buildings inferred from response records

In this paper, recent studies of recorded responses of behavior and performances of two instrumented tall buildings subjected to long-period motions from events that originate at far distances (e.g. 100-800km) are presented. Significant results indicate that (a) computed average drift ratios are substantial (~0.5%), and (b) there is permanent shift of fundamental frequencies for a tall building which was hundreds of km away from the epicenter of a large (M9.0) earthquake. In addition, (c) there are significant local site effects and basin effects, some causing resonance of buildings, (d) beating effect are observed particularly in elongated responses whereby elongated responses can contribute to low-cycle fatigue, and significantly, and (e) identified critical viscous damping percentages are low (<3%). This is consistent with recent recommendations of the Los Angeles Tall Buildings Design Council (LATBDC) 1 and the Tall Buildings Initiative (TBI) of Pacific Earthquake Engineering Center (PEER)2, and (f) beating effects are observed particularly in elongated responses whereby elongated responses can contribute to low-cycle fatigue.

Analyses of one tall building from Japan affected during the 11 March 2011 M9.0 Tohoku earthquake, and one in Los Angeles, California during the 17 January 1994 M6.7 Northridge earthquake are presented. A variety of methods including spectral analyses, system identification, and time-frequency functions are used to extract dynamic response characteristics (modal frequencies and damping), drift ratios, and effect of site conditions including basin effects.

In general, data-driven analyses show that, the two tall buildings (as well as many others not reported herein) exhibit (a) lower damping than those used in current design process analyses (<3%) and (b) a beating effect and significant basin effect.

These are significant: (1) Additional damping generating elements can be considered during design processes to decrease the prolonged and amplified responses. (2) Basin effects are not considered during design, it is important to at least consider looking into such effects as these can result in resonance and amplified responses as shown in recent studies.