ENHANCING SEISMIC RESILIENCE IN ISTANBUL: RAPID ASSESSMENT AND COST-BENEFIT ANALYSIS OF RETROFITTING REINFORCED CONCRETE BUILDINGS

A significant portion of the existing building stock in Türkiye consists of substandard reinforced concrete (RC) buildings. The destructive 1999 Marmara earthquakes have shown that the large majority of losses are due to poor seismic performance of such buildings. Moreover, the catastrophe of the 2023 Kahramanmaraş and Hatay earthquakes revealed that RC buildings constructed after 2000 have shown significantly better performance in terms of seismic resistance than buildings constructed before 2000. Approximately 70% of the building stock in Istanbul consists of RC buildings constructed before 2000. Conducting seismic safety assessments for many buildings using code-based detailed assessment approaches is not practically feasible. In order to tackle this issue, a reliable and rapid seismic assessment methodology is required. The Istanbul Metropolitan Municipality (IMM) has recently initiated a comprehensive investigation to assess the seismic safety of approximately 25,000 existing RC buildings located in Istanbul. For this purpose, the PERA2019 rapid assessment methodology has been employed for the seismic risk classification of these buildings, considering the actual structural data gathered from site investigations. In this study, firstly, the fragility curves are derived based on the seismic safety ratios (SSR) obtained for those buildings through the use of the PERA2019 methodology. Then, by considering that the buildings with relatively high seismic risk are retrofitted to code-demand level, a preliminary cost-benefit analysis is performed. For this purpose, a probabilistic approach realized by integrating the structural fragility with the site-specific hazard curve is followed. In the cost-benefit analyses, the initial construction cost (structural, non-structural, and other costs), pre-earthquake retrofitting cost, and expected seismic damage cost (the value of human life and retrofitting or repairing costs) are taken into account. The results indicated that viable and rational mitigation strategies can be selected through the implementation of rapid seismic performance assessment methodologies, thus leading to a rational and properly prioritized reduction in seismic risks and a more resilient building stock.