## eigen analysis with or without gravityloads

**Moderators:** silvia, selimgunay, Moderators

### 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.

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.

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.

### 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.

### 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.

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.

### 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.

### Re: eigen analysis with or without gravityloads

Thank you, Vesna.

I am very clear now.

I am very clear now.