EVALUATION OF SEISMIC FRAGILITY OF INFILLED REINFORCED CONCRETE FRAMES SUBJECT TO AFTERSHOCKS

Masonry infills strongly interact with primary framed structures in presence of seismic loads. Depending on the regularity of the distribution in plan and over the height, and on the geometrical and mechanical properties of the infill/frame system they can reduce or amplify structural damage due to mainshocks and aftershocks. The paper presents an assessment framework to determine fragility curves of reinforced concrete bare and infilled frames subject previous earthquake induced damage. Mainshock ground motions are defined using spectrum-compatible accelerograms having return period associated with different limit states. Aftershocks are defined as spectrum compatible signals and are scaled in amplitude to different intensity levels. Incremental dynamic analysis (IDA) is carried out in OpenSees using signals composed of a fixed part (mainshock) and a variable part (aftershock). Aftershocks are varied in amplitude in order to assess the response of the pre-damaged structure to earthquakes having different intensity levels. Fragility curves are derived for bare and infilled framed structures, considering the cases of no pre-damaging, low pre-damaging, and high pre-damaging. Results show that the response of infilled frames to the aftershocks significantly depends on the damage experienced during the mainshock. For the cases of low predamaging (low-intensity mainshock) seismic response has resulted less sensitive to further shakings. Conversely, infilled frames severely damaged by mainshock achieved collapse in correspondence of intensity levels f the aftershocks significantly lower than those of the mainshock