عنوان مقاله [English]
A building with a sudden change in the geometry of the frame over the height is called a setback building, stepped building or vertically geometrically irregular building. This kind of irregularity causes an abrupt discontinuity in the stiffness, strength and mass of the frame, creating complex behavior in the structural system. This paper aims to study the seismic behavior of
geometrically irregular buildings with setbacks, the irregularity of which can cause damage during earthquakes. In this study, a total of 20 geometrically irregular steel moment resisting 9-story frames were designed. An eigenvalue analysis was performed to evaluate the dynamic characteristics of the structures. The seismic behavior of the frames was examined by nonlinear time-history analysis using seven ordinary far-field ground motion records. Due to the rapidly increasing use of pushover analysis in recent years, and to account for the effect of higher modes in pushover analysis, enhanced pushover analyses, including modal pushover analysis (MPA), upper bound pushover analysis (UBPA), consecutive modal pushover (CMP) and extended N2 (EN2) methods, were implemented. The results indicate that the effects of higher modes for geometrically irregular frames can be considerable. Also, two basic parameters have an influence on the seismic response of these structures: the position of the setback and the reduction in the area of the setback. The seismic responses of geometrically irregular frames, which have a larger
effective modal mass ratio for the second and third modes, are large, and the responses increase by a reduction in the area of the setback. The results show that peak seismic responses occur when a setback is in one second, and, then, at one third of the height. At the level of the setback (the level at which the tower is joined to the base) a large jump in drift diagram is observed when compared to the referenced regular frame. Also, enhanced pushover analysis methods can estimate the seismic demands accurately for geometrically irregular frames with setback. The seismic responses of the frames with setbacks and the accuracy of the enhanced pushover analysis methods generally depend on dynamic characteristics and geometrical configuration.