Probabilistic Evaluation of the Seismic Behavior of Reinforced Concrete Structures with Stiffness Irregularity Based on the FEMAP695

The use of aesthetic parameters in the architecture of the building as well as the existence of various applications in different stories of a building will lead to irregularities in the structure. Non-uniform distribution of dynamic characteristic, e.g. mass, stiffness and strength of these structures causes the focus of nonlinear deformation, and consequently, the global or local destruction of the structure during an earthquake. For this reason, many current world-wide codes have provided definitions and limitation for irregular buildings. One common type of irregular structures that is important to investigate its seismic behavior is stiffness irregular structures. ASCE 7-16 defines stiffness irregularity as:
Stiffness-Soft Story Irregularity: Stiffness-soft story irregularity is defined to exist where there is a story in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above (ASCE 7-16).
Stiffness-Extreme Soft Story Irregularity: Stiffness-extreme soft story irregularity is defined to exist where there is a story in which the lateral stiffness is less than 60% of that in the story above or less than 70% of the average stiffness of the three stories above (ASCE 7-16).
It should be noted that this definition is based on the irregularity magnitude and the effect of the location of irregularity in height of structures has not been mentioned, which can be expressed as a defect in the definition of the stiffness irregularity. The major previous researches on the seismic response of the structures with vertical irregularity have been deterministic that they generally have aimed to calculate the mean values of peak responses. The seismic behavior of the structure due to available uncertainties is probabilistic rather than deterministic. It should be noted that research on the effect of irregular structures on the seismic capacity of structures is less than the effect of irregular structures on the seismic demand of structures. In order to assess the collapse capacity of irregular structures, the use of a collapse behavior model of the elements with respect to the probabilistic process as well as Incremental Dynamic Analysis (IDA) can be effective. Various methods have been developed to evaluate the probabilistic response of structures.
In this study, the effect of the stiffness irregularity on the probabilistic seismic performance of RC moment resistant frames has been studied according to the FEMAP695 (2009) procedure. To do so, nonlinear static analysis and the Incremental Dynamic Analysis (IDA) are conducted. According to the fragility curves, the collapse capacity of the structure is calculated for each model. The CMR for the models is compared to the acceptance collapse criteria based on the FEMAP695 (2009) guideline.
The obtained results show that the irregular structures have different seismic performance than regular one. The results show that both the level of irregularity and the location of irregularity in height affect the seismic responses
of these structures. It seems that the reconsideration of seismic code requirements for stiffness irregularities needs to be necessary to provide more accurate for structures with stiffness irregularities especially for those with an irregularity in the critical stories.