Statistical study on damping modification factors adopted in Taiwan’s seismic isolation design code by using the 21 September 1999 Chi-Chi earthquake, Taiwan

The damping modification factors (B)(B) are used in seismically isolated building design codes in order to estimate the highly-damped design displacement of isolation systems from its 5%-damped counterpart. The purpose of this paper is to derive the damping modification factors from 338 acceleration ground motions recorded during the 21 September 1999 Chi-Chi earthquake, Taiwan. These records are classified into four groups characterized by the local site conditions at the recording stations in accordance with Taiwan’s seismic isolation design code. The BB factors derived are then compared with those adopted in the current seismic design code of isolated buildings in Taiwan, and with those obtained from the earthquakes recorded in the USA. It is shown from the study that the damping modification factors presently used in Taiwan are obviously smaller than those calculated from the Chi-Chi earthquake. It means that the current BB factors used in the code will lead to very conservative estimation of the design displacement of isolation systems. In addition, it is also shown that the BB factors derived from the earthquakes recorded in the USA are not appropriate for use in Taiwan. For more accuracy estimation of the BB factors in Taiwan, expressions obtained from the 338 ground motions by nonlinear regression analyses are proposed for various site classes.