Seismic risk assessment of code-conforming special concentrically braced frame buildings in Turkey

In the Turkish Seismic Code, Seismic Hazard Maps are used for the design of new buildings, which assumes uniform hazard over the country; design acceleration values for all locations have the same probability of exceedance in a defined period. However, even if buildings with similar structural systems located in different regions are designed to comply with the code requirements, they might have a different probability of collapse since the collapse capacity and the hazard curve shape are unknown. In this study, three buildings with different fundamental periods are investigated to confirm that the seismic risks are within acceptable limits. Special Concentrically Braced Frames (SCBFs) are preferred since they are widely used in seismic regions and have relatively low ductility capacity. Nonlinear time history analysis is performed for each building with 22 earthquake record pairs taken from the PEER. The corresponding fragility curves are developed for the risk assessment using Incremental Dynamic Analysis and the FEMA-P695 procedure. The risk integrals are utilized to determine the collapse probability of the structure. Sensitivity analysis of different parameters in the construction of the fragility curve is performed. This study shows that while all the buildings are designed according to the same hazard level, each building has a clearly distinct collapse risk. Collapse risks of the buildings in 50 years are calculated as 0.61%, 1.9%, and 0.62% for Building-1, Building-2, and Building-3, respectively. While the collapse risk of Building-1 and Building-3 stays within acceptable limits, Building-2 fails to satisfy the required structural performance. Deaggregation of the mean annual frequency of collapse emphasizes that small to medium acceleration intensities make the biggest contribution to collapse risk. Under high levels of intensity, SCBFs might undergo critically strength deterioration and some additional restrictions in the Code may be necessary to reduce risk.