A IDA based Framework for the Assessment of Seismic Fragility of Infilled Frames with and without prior Seismic Damage

Seismic events are generally followed by a number of secondary shakings (aftershocks). Buildings which have resisted mainshocks are more or less vulnerable to further shakings as the have experienced damaging during the first event. Masonry infill walls are commonly used in reinforced concrete structures and it is well known that they strongly interact with primary structures in presence of seismic loads. Infills may be beneficial or not to the capacity of buildings to resist earthquakes, but since they have a role in seismic response in mainshocks, they also influence the structural behavior during aftershocks. An assessment framework aimed at determining fragility curves of bare and infilled frames subject to previous seismic damage is presented in this paper. Incremental Dynamic Analysis (IDA) has been used by adopting a set of ground motions defined by combining a fixed (mainshock) and a variable (aftershock) part of the signal, which has been scaled in amplitude with respect to increasing intensity levels. Fragility curves are finally derived considering the cases of no pre-damaging, low predamaging, and high pre-damaging for both bare and infilled framed structures. Results show that masonry infills can reduce seismic vulnerability to aftershocks if they are regularly distributed and if local shear failures are avoided.