Seismic retrofit solutions using base isolation for existing RC buildings: economic feasibilty and pay-back time

The existing reinforced concrete buildings typical of the Mediterranean area commonly have poor seismic performance and high vulnerability to the seismic events. As demonstrated by recent post-earthquake reconstruction processes, they commonly exhibited significant damage to structural components and to infills and partitions resulting in very high repair costs. This suggests that effective seismic strengthening interventions should aim at both improving the safety and reducing the expected annual losses. Nowadays, the seismic retrofitting of existing RC buildings by using base isolation is becoming popular because of the high effectiveness as seismic protection strategy. However, the high costs of installation are limiting the widespread in the common design practice. In this context, a unique design parameter capable of combining the increasing seismic safety, the cost of installation and the reduction of the expected losses can be useful to draw simple cost-benefits considerations. This research work proposes a PBEE-based methodology to quantify the Pay-Back Time (PBT) of seismic retrofit solutions for existing RC buildings. The non-linear response of base isolated building is assessed and a comparison with different strengthening solutions is proposed to show the applicability of the PBT as a unique design parameter to select the most effective retrofit solution. A database of 59 RC buildings retrofitted by using base isolation during the L'Aquila reconstruction process, where actual retrofit costs are available, is used for the validation. Finally, these data are used to calibrate a simple formulation of the PBT to be used in the design practice.