Developments in Seismic Safety of RC Building Stock of Istanbul According to the Years and Seismic Design Codes

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Turkey has been exposed to catastrophic earthquakes in the last decades. Consequently, seismic design codes and maps have been continuously improved with the experiences gained from these seismic events. These earthquakes have demonstrated that the RC (reinforced concrete) buildings collapsing and causing casualties and financial losses were generally built with poor workmanship and materials, without receiving a proper engineering service. However, properly designed buildings with regular structural systems, built according to their projects, have generally shown a satisfactory seismic performance even if they have been designed and constructed according to older and much less demanding versions of seismic design codes, with respect to their more recent versions. Towards planning of mitigation activities for improved seismic resilience, Istanbul Metropolitan Municipality investigated nearly 25000 reinforced concrete buildings to sort them based on their seismic risks. In this study, the examined buildings are grouped according to the seismic design code they were subjected to in their construction year. Then, several structural features and the seismic risks of the examined buildings constructed within each time period are investigated in detail. In addition, the change in the seismic risks of these buildings, if they had satisfied the minimum requirements in the seismic design code they were subjected to, is examined. No significant improvement has been seen in the structural features and seismic safety of the existing buildings from mid 20th century until 1999–2000, which marked a milestone due to Kocaeli and Düzce earthquakes (1999). After this period, a remarkable improvement was seen in terms of structural features leading to significantly enhanced seismic performance. Furthermore, if the buildings constructed before 1999–2000 had at least satisfied the minimum requirements in terms of reinforcement quantity and details, and concrete strength of their era's seismic design code, the buildings constructed since 1975 would have had a significantly higher seismic safety with respect to their current situation.