Seismic fragility assessment of infilled frames subject to mainshock/aftershock sequences using a double incremental dynamic analysis approach

The paper presents an assessment framework aimed at evaluating seismic fragility and residual capacity of masonry infilled reinforced concrete (RC) frames subject to mainshock/aftershock sequences. A double incremental dynamic analysis (D-IDA) approach is used, based on the combination of a mainshock (MS) signal at different intensities with a set of spectrum-compatible aftershocks (AS) scaled in amplitude with respect to peak ground acceleration. Limit state functions, specifically defined for infilled frames, are used to detect chord-rotation exceeding and shear collapse of RC members during standard and double incremental dynamic analyses. Intact and aftershock fragility curves are obtained for a reference full-scale RC frame specimen, by simulating seismic response with and without infills through a fully fiber section model developed in OpenSees. D-IDA results allow also defining aftershock residual capacity domains and loss diagrams, which are used to compare responses of bare and infilled frames subject to increasing MS intensities. Results show that masonry infills can drastically reduce seismic fragility of RC frame structures during main events and AS, and also limit and economic losses for the mid-low intensity earthquakes. Such beneficial contributions, however, depend on the capacity of RC members to support additional shear demand due frame-infill interaction and avoid sudden failures which conversely occur.