PSI - Issue 44

Paola Sorrentino et al. / Procedia Structural Integrity 44 (2023) 1300–1307 Paola Sorrentino et al. / Structural Integrity Procedia 00 (2022) 000–000

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By way of example, in the case of the 2011 Tohoku Earthquake, at Tsukidate Station, the response acceleration spectrum exhibits a maximum of 12.53g at 0.225s, while a maximum value equal to 0.92g at 0.25s was recorded for the 1940 Imperial Valley Earthquake, at El Centro Station. On the other hand, despite the very high values in terms of acceleration, the response displacement spectra (Figure 7b) show values lower than 50cm. In particular, the highest value is equal to 48.30cm at 1.80s (1971 San Fernando Earthquake, Pacoima Dam Station). 2.5. De Luca and Guidi (2019) De Luca and Guidi have proposed a classification of BIS design in three successive generations, looking at the main improvements occurred since the end of 1980s. Referring to an approximately 10 years long periods, the passage from a generation to another is marked by humongous events, read as turning points in the knowledge of earthquake engineering. Considering the effects of these paradigmatic events, the changes occurred in BIS design have been read in terms of design vibration period, T iso , design displacement, δ , and device diameter, ϕ . In particular, the first generation (1984-1994) precedes Northridge and Kobe earthquakes, being characterized by the earliest pioneering U.S. applications, that opted for low design vibration periods (T in the range 2.0 s - 2.5 s) and small design displacements ( δ nearly 10 – 20 cm), ensured by devices having f = 400 – 600 mm. Events occurred until 1995 showed increasing demands in terms spectral acceleration and displacement: 1985 Mexico City firstly showed a spectral acceleration Sa(g) = 0.658 at T = 2.06s; 1994 Northridge Earthquake resulted in a spectral displacements of 60,48cm at T= 2.58 s. These unexpected demands have pushed towards a second generations of BIS (1995 -2005), characterized by a conscious effort in BIS design: this led to the use of larger devices ( f = 800-1000 mm) to get longer vibration periods (T in the range 2.5 s – 4.0 s) to cover wider displacements ( δ nearly 20-40 cm). Finally, the third generation (2005-2018) succeeded 1999 Chi Chi Taiwan EQ and other recent catastrophic events, looking towards the “Next Generations of seismic isolation” proposed by Miyazaki (2008). To face 2011 Tohoku spectral acceleration Sa(g) of about 13 or 2011 Christchurch spectral displacement greater than 80cm, more effective BIS solutions are required. To face these demands, recent applications opt for the use of wider devices ( f = 1000-1300 mm) able to cover very large displacements ( δ greater than 40 cm), pushing towards vibration periods higher than 10s.

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1980, Irpinia - Calitri 1985, Mexico City - SCT1850919BL.T (S00E) 1989, Loma Prieta - Corralitos 1994, Northridge - Sylmar 1995 Kobe -KJMA 1999, ChiChi Taiwan - CHY028 2010, BioBio 2011, Christchurch 2011, Tohoku - Tsukidate Envelope

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40

4 acceleration (g)

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displacement (cm)

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

T (s)

T (s)

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(b)

Figure 8. De Luca and Guidi (2019) spectra. Spectral accelerations (a); Spectral displacements (b).

3. Comparison between different sets

In the current section, the abovementioned sets are compared. The envelopes of the response acceleration spectra of each set (Figure 9a) exhibit a great variability in terms of forms and values. In particular: • The envelope of pre 1971 spectra (Trevor Kelly, 2001), in comparison to the others, shows values lower than 0.9g in the amplification zone, not exceeding 0.2g in the de-amplification zone. The amplification range extends up to the period of 1.10s. • Compared to the previous one, the envelope of post 1971 spectra selected by Kelly, revealed unexpected