عنوان مقاله [English]
According to the basic theorems of plastic analysis of structures, increasing the strength or stiffness of a part of the structure does not weaken it under a specific static load. This result is widely used to simplify the modeling and design of structures, but these theorems have not been proven under dynamic loading like earthquake excitations. This study applies the results of the safe theorem numerically in a two-dimensional steel moment-frame structure with five stories under nonlinear dynamic analysis with 29 different earthquake ground motion records. Because under transient dynamic loading condition, the mechanism or collapse does not occur in the structure, the safety of the structure is investigated by maximum rotational deformation of the members. By changing the characteristics such as local stiffness and strength of the members in the range of 0.8 to 1.5 times the initial value, the maximum deformation demand of the members has been compared. The results of the dynamic analysis show that with increasing strength, in most cases, the demand for ductility decreases, and with increasing stiffness, in almost all cases, the ductility demand increases. The results of nonlinear static analysis are compared with the nonlinear dynamic analysis in cases where increasing stiffness has increased the ductility demand. From this comparison, it can be concluded that with increasing the stiffness of the members locally, the demand for ductility in the dynamic analysis is less than its corresponding value in static analysis. As a result, it can be said that by increasing the stiffness of the structure, observing the limitations of the codes for the ductility capacity of members and connections, leads to the safety of the structure. Generally, it is concluded that the safety of the structure is not compromised by minor increasing the strength and also minor increasing stiffness of some members of the structure under dynamic loading; if regular limitations of the codes for the ductility capacity have been observed.