Seismic performance of Italian pre-code RC buildings strengthened via local interventions: Fragility curves and failure rates

The existing Italian Reinforced Concrete (RC) building stock exhibits a significant seismic vulnerability. Previous works conducted under the national project Implicit Risk according to the Italian Building Codes (Rischio Implicito secondo le Norme Tecniche per le Costruzioni, RINTC) have demonstrated that seismic failure rates of pre-70 structures are higher than those of ‘70 s or ‘80s-‘90 s buildings, especially at collapse and mainly due to the occurrence of shear failures. Extensive retrofitting efforts are, thus, required for these buildings. This work investigates the potential improvement in seismic performance of pre-70 RC buildings by particularly addressing all the (very) limiting shear failures in RC members, including beam-column joints and squat columns. The retrofitting design aims at increasing the as-built capacity to meet that requested for newly designed buildings at the Life Safety Limit State, only, according to the Italian code prescriptions about existing buildings. In order to provide valuable insights into the seismic mitigation framework, a commonly adopted, cost-effective strategy, compliant with the current code requirements, has been considered. This strategy is based on local interventions designed to strengthen the considered buildings without significantly modifying their lateral stiffness. The effectiveness of this strategy is evaluated through nonlinear analyses of 3- and 6-storey buildings designed for gravity loads only (GLD) or according to obsolete seismic codes (SLD). Refined modelling strategies have been implemented, also including the presence of masonry infills. Fragility curves and failure rates have been lastly obtained for two performance levels (i.e. Global Collapse, GC, and Usability-Preventing Damage, UPD), and compared with the relevant as-built buildings to quantify the seismic improvement. As a result of the adopted local strategy and the selected retrofitting techniques, it is found that failure rates at UPD do not change significantly. However, the seismic performance at GC improves remarkably, particularly for buildings designed in compliance with outdated seismic codes.