Elastic modal periods after nonlinear time history analysis!

[aminamini]

Hi

I modeled a 10-story steel moment resisting frame using fiber sections and nonlinear elements. Nonlinear time history analysis of structure under a special record was performed. An eigenvalue analysis was carried out after each step of nonlinear time history analysis and the variation of modal periods during the dynamic response of structure was recorded. My question is that, why the modal periods of the structure at the end of the ground motion are equal to the elastic periods, while several inelastic deformations occurred in the structure?
I would greatly appreciate if anyone help me.

with best regards,
Amini

[fmk]

steel building .. if the material is the typical steel01, steel02 and no deterioration in stiffness properties, the materials can go back to elastic state when the lateral loads have gone

[aminamini]

Thank you very much dear fmk!