Seismic performance assessment of as-built and retrofitted RC buildings considering the influence of infills: pre-‘70 low-mide rise case-studies

The seismic assessment and retrofit of non-conforming Reinforced Concrete (RC) buildings still represent a challenge for researchers and practitioners worldwide due to several aspects. On one hand, low standards RC buildings typically present some failure typologies very different from newly designed buildings, and such failure modes should be properly modelled for a reliable seismic performance assessment. On the other hand, retrofitting strategies should be chosen carefully looking both at safety at collapse and at seismic capacity at less severe performance levels, considering nonstructural components as well. Masonry infills, typically defined as non-structural components despite their structural role, significantly affect the seismic behavior of existing RC buildings, especially at low-medium intensity measures. Therefore, when the seismic performance of as-built or retrofitted RC buildings is assessed, their contribution should be considered. In this work, a preliminary analysis of two 4-storey low-standard RC buildings, different for design typology, is presented, based on static nonlinear analyses. First, case-study buildings, representative of Italian pre-70 residential buildings, have been assessed in their “bare” configuration (without infills), as common in design practice, in a code-based approach (according to Italian code). Based on the asbuilt assessment outcomes, their retrofit has been designed by means of two main strategies: (1) the complete resolution of all the shear failures (in columns or joints), if any; (2) the reduction of the displacement demand by means of RC column jacketing, when necessary, along with the resolution of shear failures (as for the previous strategy). Then, both the as-built and retrofitted building configurations have been modelled and analysed explicitly considering the presence of infills and shear-critical elements. Fragility curves have been obtained at different Damage States according to European Macroseismic Scale EMS-98, considering record-to-record variability, and compared to each other. Remarks about the choice among strengthening strategies are carried out, based on these outcomes and on a preliminary estimation of the expected mean annual losses for the investigated buildings.