عنوان مقاله [English]
Increasing the strength of members or enhancing the redundancy does not jeopardize the overall safety of the structure. This can be proved under static loads by the safe theorem, which is one of the fundamental theories of plastic analysis. Although this theorem has not been proved in the case of dynamic loads, it has been widely applied to the design of systems under dynamic loads. Therefore, this paper aims to make use of the results of this theorem in the numerical analysis of structures subjected to dynamic loads. Since the structural instability mechanism and collapse do not occur under transient loads, an adequate level of ductility demand has been assigned to the structural components to ensure the safety of the structure. For this purpose, the plastic rotation of the members is determined after a minor variation in strength and stiffness of the beams in a 2D five-story steel moment-frame structure by performing dynamic analysis. To compare the ductility demand obtained by the dynamic analysis with the criteria values, the performance of the structure is also evaluated by conducting nonlinear static analysis. The analysis results showed that the increase in the strength of the beam members generally leads to a lower ductility demand; however, in some cases, the maximum ductility demand increased by about 7.3%. With the increase in the stiffness of the beams, the ductility demand increased by up to 16%. It can be concluded that with the increase in the stiffness and strength of the beams, a lower ductility demand is obtained by the dynamic analysis compared to the static analysis, and thus the structural collapse has not occurred under dynamic loads.