ANIDIS - L'ingegneria Sismica in Italia, ANIDIS XIX & ASSISi XVII - 2022

Dimensione del carattere:  Piccola  Media  Grande

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

Paola Sorrentino, Laura Giovanna Guidi, Giuseppe Brandonisio, Antonio De Luca

Ultima modifica: 2022-08-27


In recent years, several studies have shown an increase of maximum PGA and PGV values that can be associated to the rapid increase of the archived strong motion records in databases. Nowadays, the worldwide largest PGA values are 3,94g (vertical) and 2,75g (horizontal), from the 2008 Iwate-Miyagi and 2011 Tohoku earthquakes, respectively, and the PGV current maxima are larger than 2.0m/s (horizontal and vertical), both from the 1999 Chi-Chi earthquake (Suzuki and Iervolino, 2017).

The awareness of the existence of larger values, also for long period, resulted in a new approach in BIS design. With these values to be considered in design, it was necessary to move to larger design periods and to face greater design displacements.

Changes occurred in the history of seismically isolated building, as indicated also in Design Recommendations for Seismically Isolated Buildings by Architectural Institute of Japan (2016) and in De Luca and Guidi (2018). In particular, AIJ classifies four periods, tracing the main changes in seismically isolated building design and technology in Japan; De Luca and Guidi propose a classification of BIS design in three successive generations in which the passage from a generation to another is marked by earthquakes, read as turning points in the knowledge of earthquake engineering.

A significant contribution to the evolution of BIS design has been given by Miyazaki (2008) that defined a "Next Generation of seismic isolation" to counteract these unprecedented spectral values of displacement, velocity and acceleration. He indicated the following reference values: 1.20 g peak spectral acceleration at T=2.0 s recorded during 1999 Chi-Chi Taiwan earthquake, or 180 cm peak spectral displacement recorded during 2007 Chuetsu-oki earthquake.

This paper aims to analyse the acceleration, velocity and displacement spectra of different set of strong motion records and to provide suggestions for seismic design of base-isolated structures in the light of higher spectral values. In particular, we will consider: the set of FEMA307 (1998), in which three categories of ground motion are considered (short duration, long duration and forward directivity); the set defined in Trevor Kelly (2001), that divided motions into pre-1971 and post-1971; the set used by Miyazaki (2008) to define the Next Generation of Seismic Isolation; the set defined by Suzuki and Iervolino (2017) to trace the history of largest PGA and PGV. A comparison between results, for different periods, will lead to possible design indications for BIS. 

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