## eigen analysis with or without gravityloads

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caosasa
Posts: 26
Joined: Sat Aug 07, 2010 5:31 am
Location: tongji

### eigen analysis with or without gravityloads

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.
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
Posts: 3033
Joined: Tue May 23, 2006 11:23 am
Location: UC Berkeley

### Re: eigen analysis with or without gravityloads

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
Posts: 26
Joined: Sat Aug 07, 2010 5:31 am
Location: tongji

### Re: eigen analysis with or without gravityloads

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
Posts: 3033
Joined: Tue May 23, 2006 11:23 am
Location: UC Berkeley

### Re: eigen analysis with or without gravityloads

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
Posts: 26
Joined: Sat Aug 07, 2010 5:31 am
Location: tongji

### Re: eigen analysis with or without gravityloads

Thank you, Vesna.
I am very clear now.