SEISMIC PERFORMANCE IN NORTHERN CHILE OF CHERENKOV LARGE SIZE TELESCOPES FOR GAMMA RAY ASTRONOMY

Traditional base isolation techniques can significantly improve the seismic performance of civ-il structures by reducing accelerations and hence earthquake-induced internal forces and stresses in structural members, and criteria for dimensioning the isolation system have been developed and endorsed in current design codes. On the other hand, the application of seismic isolation to non-civil structures may require the definition of specific performance criteria to be satisfied. The study is focused on the application of seismic isolation to a Large Size Telescope (LST) structure for gamma ray astronomy based on Imaging Atmospheric Cherenkov Technique, characterized by peculiar features such as a non-uniform distribution of masses and stiffnesses, and very high natural frequencies, due to its high stiffness and small mass. According to the design specifications, the telescope shall have only small damages, with maximum loss of operation of 1 week, for an earthquake with return period of 95 years, and moderate damages, with maximum loss of operation of 6 weeks, for the basic design earthquake (re-turn period 475 years). This implies that not only the structure, but also sensitive optical instrumentation, shall be protected during the ground motion. To this end, an isolation system is proposed which has the purpose of maintaining the seismic accelerations which the LST instrumentation is subjected to within the operating ranges of the instrumentation. The article presents the investigation conducted on a three-dimensional model of the LST in-stalled in a high seismicity area in Atacama Desert, in the North of Chile. Starting from the performance of the basic structure, characterized by a fixed base, the isolation system is dimensioned and the performance of the two configurations (not isolated and isolated) is evaluated through time history analyses in order to assess the actual reduction of accelerations at different control points.