Gli effetti dell'amplificazione di sito sul comportamento degli edifici isolati nel centro storico dell'Aquila

In the last few years in Italy, particularly in L'Aquila after the earthquake which struck the town April, 6 2009, seismic isolation system was adopted in many new public (C.A.S.E. project, ANAS headquarter etc...) and private buildings or to retrofit existing structures. In the historic center of L'Aquila, many seismic isolation works have been carried out, also on historical masonry buildings. Seismic isolation allow to curtail the seismic forces on the superstructure, significantly reduce interstory drift and damage and keep the structure full operational also after severe earthquake. Considering the ordinary shape of the response spectra, this purpose is achieved by shifting fundamental period of the structure to the range from 1,5 to 4 sec, where spectral accelerations are generally smaller. The presence of particular site effects, which produce local amplification in the range of fundamental frequencies of isolated buildings, could significantly increase seismic forces on the superstructure and displacements in the devices, also beyond maximum design displacement. The Italian Seismic Regulation (NTC 2008) requires specific studies for assessing the local seismic response; alternatively, site effects amplification can be evaluated, according § 3.2.2 NTC 2008, by a simplified method who provide 5 conventional soil categories depending on average shear wave velocity in the first 30 m depth. In the geological context of the historical center of L'Aquila, the simplified method to evaluate the stratigraphic amplification, proposed by Italian seismic regulations (§ 3.2.2 NTC 2008) could provide a poor realistic assessment of the site amplification and lead to an incorrect structural design. The paper analyzes the effects of the August, 24 and October, 30 2016 recent earthquakes on seismic isolated buildings located in L'Aquila downtown also by support of numerical analysis and provide some ideas to improve seismic regulation.