Probabilistic seismic assessment and retrofit considerations for Italian RC frame buildings

In recent decades, the seismic assessment of existing buildings has developed significantly from traditional objectives that focused on ensuring life safety of the building. The economic impact of the 1994 Northridge earthquake in the US due to the extensive damage suffered by buildings, in addition to the overall disruption, highlighted the need for a paradigm shift in the way in which the performance of buildings ought to be defined. This paper considers the assessment of existing Italian reinforced concrete (RC) frame buildings with masonry infill, which were typically gravity load designed (GLD) prior to the introduction of seismic design provisions in the 1970s. The assessment of GLD RC frames with masonry infill is discussed within a setting similar to that of the FEMA P58 guidelines that aim to provide practising engineers with the tools and procedures; both advanced and simplified, to quantify the performance of existing buildings in a more meaningful way that can be easily conveyed to decision makers. In this study, extensive numerical analysis was carried out on a variety of case study buildings to quantify and benchmark the performance; both in terms of expected demand and overall collapse capacity, where the impact of incorporating the potential shear failure in column members was shown to result in a reduction of up to 10% of the median collapse intensity due to the interaction with the masonry infill. Furthermore, loss estimation studies were carried out on these case study buildings to not only quantify the expected losses but also investigate ways in which shrewd retrofitting of both structural and non-structural elements can have maximum impact on the overall performance. On the contrary, it was shown that by adopting structural retrofitting schemes involving strengthening and/or stiffening of the structure in compliance with NTC 2008 requirements in these situations may actually lead to a worsening of the building performance defined in terms of expected annual loss. Overall, this paper provides insight into the probabilistic seismic assessment and retrofit considerations for existing GLD RC frames in Italy.