The necessity of seismic collapse assessment of existing mid-rise buildings of urban areas in high seismic regions

Many megacities in the world are located in high seismic regions at which severe earthquakes are completely probable. Mid-rise building constitutes a significant part of existing buildings in these cities. In severe earthquakes, the structural collapse causes extensive fatalities and financial damages and considerably decrease urban resilience. So the collapse risk assessment and the recognition of high-risk buildings is a crucial matter which should be one of the major concern of urban disaster management organizations. In this paper, the necessity of developing a simplified methodology for seismic collapse risk assessment of the existing mid-rise building is investigated. Furthermore, a framework is suggested to estimate the collapse risk of these buildings. Nonlinear dynamic analyses are the most accurate and reliable methods for seismic collapse assessment processes which need a significant number of nonlinear time-history analysis. So, these methods are time-consuming and need high calculation costs. In this paper, the nonlinear static analysis results are used to rapidly evaluate the structural responses. According to the seismic design and seismic rehabilitation instructions, the building's performance level should not exceed the collapse prevention (CP) performance level under the maximum considered earthquake (MCE). This performance level is accompanied by a significant degradation in the stiffness and strength of the lateral resisting system so that the structure has no margin against total collapse. However, the gravity resisting system continues to support the gravity loads. In this paper, the CP performance level is detected by a criterion based on the structural characteristic of the original frame and load pattern of the pushover analysis. Furthermore, the seismic hazard and the soil type of the site are considered in the seismic design process of the structures and indirectly affect the CP performance level. A series of intermediate steel moment-resisting frames (MRFs) and ordinary concentrically braced frames (OCBFs) are designed according to Iranian seismic code and are used as case studies. These frames which are located on the stiff soil and far from active faults are two-dimensionally modeled by the OpenSEES program. The analysis results demonstrated that the proposed methodology can predict the collapse risk of the mid-rise steel frames with high precision and easy implementing way.