The Dependency of Collapse Capacity of RC-MRF to Maximum Considered Earthquake

One of the situations that distorts the safety of concrete moment frame structures is the rare seismic events. Bam earthquake in 2003 was one of the rare seismic events that caused damages to many newly built structures. In this paper, the capacity of structures was evaluated according to the standard record sets of FEMA P695 and maximum considered earthquake (MCE). A comprehensive method should be used to express the seismic behavior of structures for assessing the collapse capacity. Incremental dynamic analyses proposed by Vamvatsikos and Cornell in 2002. This method is used for assessing the collapse capacity of structures in this study. The proposed methodology is used for collapse assessing of an individual as well as a group of buildings with due attention to rare seismic events and incremental dynamic analyses method. Illustrative results show that, if structures provide minimum acceptable requirements of FEMA P695, they would have been secured against rare seismic events.
Development of nonlinear models for collapse stimulation is the first step of collapse assessing methodology. All of the structures have been designed according to ASCE 7-05 code, and for expressing of nonlinear behavior of materials, Mander and Menegotto-Pinto model has been considered. Selection of ground motion record sets for collapse assessment of building structures is very important. Both far-field and near-field records have been considered in FEMA P695, but in this paper, the far-field records were used. Three analyses have been considered in assessing the collapse capacity. Eigenvalue analyses, incremental dynamic analyses and static pushover analyses are required for assessing the collapse capacity. Incremental dynamic analyses is one the suitable methods for expressing of seismic behavior of structures. The basic idea of this analysis was described by Bertero in 1997. In 2002, this method was accompanied with big progress by Vamvatsikos and Cornell. Illustrative results show where the incremental dynamic analyses curve slope is equal to 20% of the elastic while the point also belongs to softening branch defined as collapse point. Additionally, another candidate point is displacement ratio of 10%. Illustrative results show that where the incremental dynamic analyses curve lining to infinity is being defined as collapse point. The incremental dynamic analyses curves show record to record variability, thus it is essential to summarize such data. The fragility fitting approach has been used widely for defining the median collapse acceleration. Adjusted collapse margin ratio is the most important parameter for assessing the collapse capacity of structures. According to FEMA P695, the acceptable value of the adjusted collapse margin ratio for each individual model within a performance group should exceed ACMR (20%). Additionally, the average value of adjusted collapse margin ratio for each performance group should exceed ACMR (10%).
Finally, collapse capacity of 5 and 10 story concrete moment frame structures are defined. Both structures have acceptable adjusted collapse margin ratio and both of them have acceptable safety according to rare seismic events. Structures that could not satisfy the FEMA’s conditions must increase their lateral strength and re-evaluate.