عنوان مقاله [English]
In this paper, the seismic performance of vertically irregular steel buildings is evaluated. A ten-story steel building with special moment resisting frames is considered as the superstructure. The sufficiency of structural elements was evaluated by the earlier Iranian Seismic Codes. The three-dimensional finite element model of soil with nonlinear material behavior was attended below the superstructure to incorporate the foundation flexibility effects into the analytical models. The irregularities of mass, stiffness, strength, and concurrently stiffness and strength were assigned to different stories. Irregularity position was limited to the 1st, the 5th, and through the bottom half floors (stories 1 to 5). Through a series of nonlinear incremental dynamic analyses, the flexible-base structural performance levels were evaluated in the framework of probabilistic performance-based earthquake engineering. The confidence level curves of both regular and irregular structures were developed at two performance objectives: collapse prevention and global instability.It is observed that the mentioned non-geometrical irregularities reduce the confidence levels of irregular structure as the intensity of the ground motion and related displacement demands increase. In comparison to the regular structure, depending on the position of irregular stories through the structure height, the seismic intensity and corresponding demands to a specific confidence level, change. As the structures pass through the collapse prevention performance level 40\% to 60\% variation is observed in the seismic capacity of no uniform distribution of strength along the structure height. Meanwhile, mass irregularity of the bottom floor has low variations in both demands and seismic intensity compared to the regular model. Generally, at rigorous nonlinear phase of structural behavior, the irregularity effects are highlighted. Among all the irregular structures, an appropriate confidence level of strength irregularity models is obtained at low seismic intensities. The displacement demands are also decreased so that in comparison to the regular structure, 85\% differences are observed.