Design spectra to be used in Base Isolation Design in light of recent strong motion records

Abstract

Strong motion records have shown an increase of the maximum values of PGA and PGV, also associated to the increase of the archived strong motion records. Nowadays, the worldwide largest PGA values are: 3,94g (vertical) and 2,75g (horizontal), from the 2008 Iwate-Miyagi and 2011 Tohoku earthquakes, while the current PGV maxima are larger than 2.0 m/s (horizontal and vertical). These unforeseen values have encouraged a more conscious approach in BIS design, being necessary to move to larger design periods and to face wider design displacements. Many changes have occurred in the history of BIS, as indicated in Design Recommendations for Seismically Isolated Buildings by AIJ (2016), tracing the improvements of BIS in Japan, and in De Luca and Guidi (2019). In this excursion, a significant contribution has been given by Miyazaki (2008) that defined a “Next Generation of seismic isolation” to counteract the unprecedented spectral values of displacement and acceleration. In this perspective, the paper analyzes the acceleration, velocity and displacement spectra of different sets of strong motion records to provide suggestions for seismic design of base-isolated structures in the light of higher spectral values. In particular, we will consider: the set defined in Trevor Kelly (2001), that divided motions into pre-1971 and post-1971; the set of FEMA307 (1999), in which three categories of ground motion are considered (short duration, long duration and forward directivity); the set used by Miyazaki (2008) for the Next Generation of Seismic Isolation; the set defined by Suzuki and Iervolino (2017) to trace the history of largest PGA and PGV; the set used by De Luca and Guidi (2019) to trace the improvements in BIS design and propose a classification of BIS in three successive generations. A comparison between results, for different periods, will lead to possible design indications for BIS.