In this study, steel frames subjected to horizontal and vertical ground motions simultaneously were analyzed dynamically with considering the strength deterioration and the effect of vertical ground motion on the collapse behavior of steel frames was investigated. The axial force and the natural period of the frame in the vertical direction were specified as main parameters and it was found that a critical axial force for converging the accumulation of damage and a relation between the natural period of frame and the predominant period of ground motion had an important role in the response under horizontal and vertical excitations.
