GNGTS 2013 - Atti del 32° Convegno Nazionale

but during the coda waves the frequency values return to the initial values of 5 Hz (periods of 0.2 s). The results obtained through the S-transform demonstrate that such high period elongation of about 60% (a frequency decrease of about 40%) has to be considered a purely temporary variation, since the fundamental period of the building has already been recovered during the coda waves. Finally, we analysed also the recordings of 39 aftershocks with magnitude Ml ≥ 2.5 acquired by the G3 GeoSIG accelerometer. We computed both the acceleration response spectra and the HVSR for all events. The response spectra of the analysed events have similar behaviour of foreshocks and aftershocks collected by ETNA accelerometer with fundamental peaks of about 0.2 s. Moreover, HVSR results give a fundamental frequency of the building of 5 Hz along the longitudinal direction and 5-6 Hz along the transversal direction, thus demonstrating again a purely temporary variation of fundamental frequencies during the mainshock. Another interesting finding concerns the comparison between the performances of MEMS and force-balance accelerometers. Comparing the aftershocks recorded by ETNA with those recorded by GeoSIG it is clear that the frequency detected for the fundamental modes of the structure are the same. However, for long periods the MEMS instrumentation shows clearly the presence of instrumental noise. For period longer than about 1.5 s the ETNA response spectra decrease while the GeoSIG become flat. Preliminary conclusions. The monitoring of the Rotonda school during the Pollino seismic swarm proved the importance of a monitoring system with real time data transmission that can be set up in a short time. It is then possible for Civil protection authorities to gain information useful in deciding if a building is safe for use, requires inspections or has to be abandoned. The building in Rotonda provided another very important results in the discussion about the importance of temporary vs permanent period elongation. The building suffered during the mainshock a spectral acceleration almost reaching 2 g, which caused a period elongation of about 60% (or a frequency decrease of about 40%) that was completely recovered and thus has to be considered a purely temporary variation. This suggests that to testify onset of damage, the presence of a permanent variation is much more important than the temporary variation even if this is a great one. References Calvi G.M., Pinho R. and Crowley H.; 2006: State-of-the-knowledge on the period elongation of RC buildings during strong ground shaking. First European Conference on Earthquake Engineering and Seismology, Geneva, 2006. CD Paper Number 1535. Ditommaso R., Vona M., Gallipoli M.R. and Mucciarelli M.; 2013: Evaluation and considerations about fundamental periods of damaged reinforced concrete buildings. Nat. Hazards Earth Syst. Sci., 13, 1903–1912. doi:10.5194/ nhess-13-1903-2013 Gallipoli M.R., Mucciarelli M. and Vona M.; 2009: Empirical estimate of fundamental frequencies and damping for Italian buildings. Earthquake Engineering and Structural Dynamics, 38(8), pp. 973-988. Masi A., Santarsiero G., Gallipoli M.R, Mucciarelli M., Manfredi V., Dusi A. and Stabile T.A.; 2013: Performance of the health facilities during the 2012 Emilia (Italy) earthquake and analysis of the Mirandola hospital case study. Bull Earthquake Eng. doi 10.1007/s10518-013-9518-4 Mucciarelli M., Gallipoli M.R., Masi A., Vona M., Ponzo F., Dolce M.; 2004: Analysis of RC building dynamic response and soil-building resonance based on data recorded during a damaging earthquake (Molise, Italy 2002). Bulletin of Seismological Society of America, 94(5), pp. 1943–1953. Rosenberg A, Beverley K, Rogers G.; 2004: The new strong motion seismic network in southwest British Columbia, Canada. 13th World Conference on Earthquake Engineering. Vancouver, B.C., Canada, August 1-6, 2004, Paper No. 3373. Stockwell R.G., Mansinha L. and Lowe R.P.; 1996: Localization of the complex spectrum: the S transform. IEEE Trans. Signal Process, 44, pp. 998-1001. 445 GNGTS 2013 S essione 2.3