RISK ANALYSIS OF RC BUILDINGS EQUIPPED WITH HYSTERETIC DAMPERS DESIGNED BY THE ITALIAN SEISMIC CODE

Among the modern techniques envisaged to improve the seismic performance of structures there is energy dissipation, which consists in dissipating part of the seismic input energy by means of special devices, i.e., the dampers, which are not part of the structural frame supporting gravity loads. Generally, these systems are designed to be engaged during the design earthquake and to satisfy the structural safety requirement at the ultimate limit state to protect the life of the occupants. However, the Italian Seismic Code prescribes that these devices shall also be able to resist earthquakes of higher intensity, which correspond to the non-collapse limit state. While the behavior of structures equipped with dampers under design conditions and the effectiveness of different systems have been widely investigated in the literature, there is a lack of information about their behavior under collapse conditions. The present study aims at identifying critical aspects of the effectiveness of energy dissipation systems in limiting building damage for high seismic intensities and to provide the basis for the implicit risk of damage assessment and global collapse considering over-stroke displacement of the dampers. In this light, the retrofit of two existing reinforced concrete frame structures of 3 and 6 stories is designed in order to guarantee the Immediate Occupancy structural performance level, i.e., the condition in which the structure, after the design earthquake, is immediately accessible as it retains its original strength and stiffness. The retrofit is performed by means of steel braces equipped with hysteretic devices characterized by different ductility levels. In order to verify the performance of the dampers and the retrofitted structures, non-linear dynamic analyses are performed considering three sets of bidirectional natural ground motions, correspondent to seismic events characterized by return periods of 475, 975, 1950 years respectively.