eigen analysis with or without gravityloads

[caosasa]

Dear all,
i find a strange phenomenon.
for the eigen analysis,
the period is smaller when the eigen analysis of a structure is conducted with gravityloads,
compared with the period when the eigen analysis of a structure is conducted without gravityloads.
the "Ex3.Canti2D.build.InelasticFiberSection" is adopted.
i think when the gravityloads are included,
the structure should be stiffer.
consequently,
the period should be smaller, not bigger.
can anyone explain this problem?
Thanks.

[vesna]

From what you are saying I am not sure what was your observation. However, some of materials need some load to correctly configure the stiffness. I would relay on periods calculated after gravity load was applied on the structure.

[caosasa]

Dear Vesna,
Thank you very much. You are very warmhearted.
The phenomenon i am saying is:
The periods that calculated after gravity load is longer than the periods that calculated before gravity load.
I think after the gravityloads are counted,
the structure should be stiffer.
consequently,
the period should be shorter, not longer.
I think you already explained the problem.
The reason is"some of materials need some load to correctly configure the stiffness".
Thank you again.

[vesna]

Irrespective of how OpenSees handles materials and establishes the stiffness here is my general comment on change of stiffness with a load applied on the structure. After you apply gravity load on the structure minor cracks may develop in its elements (especially if it is a RC structure) which will lead to structure softening and thus smaller stiffness and longer period.

[caosasa]

Thank you, Vesna.
I am very clear now.