Difference in Time Period

[rashid]

Hi everyone,

I am analyzing a ten story reinforced concrete moment resisting frame in seismostruct. I am using Force-Based Plastic Hinge elements. Any idea what might be the reason for the change in time period, estimated using eigen value analysis, by changing the plastic hinge length ?

Best regards,
Rashid

[seismosoft]

This is not something unexpected. You may refer to the literature for localisation issues to understand this behaviour.
Seismosoft Support

[rashid]

Dear Seismosoft team,

As far as I have read about localization issues, they are mostly concerned with the post yielding behavior of a structural element. I'm estimating time period using eigenvalue analysis which is using the elastic model properties. In standard literature, localization issues are discussed for displacement and force-based formulations. Though there is an initial stiffness difference (which can affect time period) between the displacement-based and force-based elements, but here I'am concerned with the change in time period between force-based and force-based plastic hinge elements. Will you please elaborate ?

Best regards,
Rashid

[seismosoft]

The formulation of the infrmFBPH element is such that its initial stiffness is affected by the length of the plastic hinge (although it should not).
If you feel that you cannot calculate correctly the plastic hinge length, you might need to use the infrmFB or even the infrmDB element types (the latter with shorter member length).

Regards,
Seismosoft Support

[bahawise]

Dear Seismosoft team,
I have an question about natural time period,
Does the SeismoStruct consider the changes in natural time period due to the plastification of or stiffness deterioration of members during nonlinear analysis?

Thanks

[huffte]

Typical direct integration nonlinear schemes include no explicit computation of the varying natural periods of the degrading and softening structure. The effect is implicit in the solution procedure for nonlinear analysis.

[bahawise]

Dear Huff
thank you so much for your kind response,
For my research project, I need the value of natural periods considering both linear and nonlinear analysis. is it possible to screen that?

Thanks
Baha

[huffte]

It seems you will have to determine that yourself. Perhaps the reason typical nonlinear analyses do not explicitly report natural periods is that the stiffness is continually changing. Thee are many different definitions which could be used - tangent stiffness, secant stiffness, or some other measure - to calculate an instantaneous natural period.

[bahawise]

Dear Huff

thanks for the response,
I think the seismosoft team should consider this phenomenon for the next version of seismostruct. as far as i know there is a software that can report natural period at any level of plastification. but not all software perfect. there is always lack of something.

Sincerely,
Baha

[seismosoft]

Thank you for the suggestion. You will consider it for inclusion in one of the next versions of the program.
Seismosoft Support