Modal analysis after nonlinear time history 2

[iotoma]

Hello again,

I am trying to assess the rate of degradation of a RC building subjected to earthquakes and I'd like to do it in terms of fundamental period of vibration (the larger it gets, the lower the stiffness of the structure).
I read in the Help that one can run modal analysis after time history, provided that the time history analysis case is a non-linear case and it uses direct integration.
So I ran a modal analysis case before subjecting my structure to the earthquakes and then I ran two non-linear time history cases (the 2nd starting from the results at the end of the 1st time history case). At the end, I ran a modal analysis case using the stiffness of the structure at the end of the 2nd time-history case.
It seems as though the fundamental period of vibration stays the same, no matter how large the scale factor is for the time history-cases. In my opinion, the period of vibration should change (namely, get higher).
Any ideas how could I solve this problem?

Best wishes,
iotoma

 

[sebivaq]

Dear itoma,
in order to increase the period, the structure should: a) have more mass or b) be more flexible (lower stiffness). In your case, you want to subject the structure to an earthquake and simulate the damage that this one does to the structure.. and this damage should increase the flexibity (less stiffness) because of the formation of plastic hinges. This kind of problem should be modeled by the nonlinear behaviour of the material, that's not the same as the geometrical nonlinear case that your are taking about.

Acoording to the SAP2000 Help:

"Only nonlinear material behavior in Link objects is considered; frame hinge and geometric nonlinearity are excluded."

 

[iotoma]

Dear sebivaq,

You are right, meaning that I am trying to simulate the damage induced by the earthquake. I never considered geometrical non-linearities in my analysis.
I still got the same results for the period of vibration even if I considered nonlinear material behavior.

Best wishes,
iotoma

 

[sebivaq]

I think that the problem is that SAP2000 doesn't introduce plastic hinges where the section fails. As a result, you have the same stiffness, the mass will obviously be the same... and that's why your period is exactly the same. You need to define the non-linear properties of the material in order to make a plastic non-linear analysis... Did you define these properties for your materials? Watch the attachment.

 

[sebivaq]

Dear iotoma,
have you set the non-linear case in the modal analysis? Take a look at the attachment.

 

[iotoma]

Dear sebivaq,

I have set all the parameters for the non-linear analysis and modal analysis exactly as you described but still no difference in the periods of vibration.
I'm still looking into it and hopefully come up with a solution.

 

[sebivaq]

Dear iotoma,
the non-linear combination for the modal analysis (as seen in the last attachment), should have non-linear load cases. Take a look at the new attach.

 

[iotoma]

Dear sebivaq,

Thank you so much for opening my eyes ... it was a childish mistake on my part. I considered the DEAD load case as linear static instead of non-linear.


Thanx a lot!
iotoma

 

[sebivaq]

You are welcome!