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Numerical simulation of seismic collapse mechanisms of vertically irregular steel high-rise buildings Reza Ramezani Azghandi a, Hamzeh Shakib a,⁎, Maedeh Zakersalehi b a Tarbiat Modares University, Tehran, Iran b Niroo Research Institute, Tehran, Iran a b s t r a c t The effect ofmass and stiffness vertical irregularities on the collapse mechanismand resistance of steel high-rise structures is investigated using 3-D numerical simulations. Simulations are carried out utilizing a verified modelling approach in which the collapse processes of a regular and four irregular structures are studied under 11 earthquake records. It is indicated that the utilized finite element model, related considerations and calibrations present a reliable and efficient technique for collapse simulation. The findings showthat vertical irregularities adversely affect the collapse resistance and mechanism of steel high-rise buildings. Especially, mass vertical irregularity with concentration in the lower half of the structure is the most vulnerable one; while the stiffness irregularity with distribution in the top and bottom parts of the structure is the least affecting one. Studying the simultaneous effect of frequency content and vertical irregularities shows that, generally, records with wide frequency content mostly reduce the collapse resistance of the structure and have no significant effect on the collapsemechanismof irregular structures in comparisonwith regular structure;while records with medium frequency content are most effective on the collapse mechanism. منبع 10.1016@j.jcsr.2019.105914.pdf