Effect of minor variations in characteristics of beam on the safety of 2D steel moment frame under dynamic loads

Increasing the resistance of members or adding constraints to the structure does not jeopardize the overall safety of the structure. This can be proved under certain static loads by the safe theorem which is one of the fundamental theories of plastic analysis. Although this theorem has not proved in the condition of dynamic loads, its results are used in the everyday design under dynamic loads, so this paper attempts to numerically study these results under dynamic loads. Since the concept of mechanism and collapse cannot be investigated under transient loads, the level of ductility demand of structural components is considered to ensure the safety of the structure. For this purpose, the plastic rotation of the members was extracted after a minor variation in resistance and stiffness of the beams in a 2D-five-story steel moment frame by performing dynamic analysis. To compare ductility demand in dynamic analysis with criteria values, the evaluation was surveyed under nonlinear static analysis. The results analysis shows that with the increasing resistance, in most cases the ductility demand decreases, and in some cases the maximum increases by 7.3%. With increasing stiffness, the ductility demand has increased by a maximum of 16%.By comparing the results of dynamic and static analysis, it can be concluded that the required ductility in the dynamic analysis by increasing characteristics has been lower than the corresponding value in static analysis. Therefore, the increase in the partial stiffness and resistance of the beams under dynamic loads has not caused the structural collapse.