Period calculation for cracked section

[jonade]

I wanted to calculate the period of a reinforced concrete structure using a fiber section. Since the cracked section property shall be used for calculating the period I wanted to know how I can specify it in Opensees. Moreover, I want to ask whether cracked section properties should be specified for nonlinear time history analysis or not.

[selimgunay]

Nonlinear time history analysis does not explicitly need the cracked section period. If you are using Concrete01, you will see some effect of cracking after gravity as the beams will crack and the moment of inertia will automatically reduce. If you are using Concrete02, you can run a small level earthquake that will lead to tensile stresses big enough to start cracking in beams and columns and do an eigenvalue analysis at some time that corresponds to the strong part of the ground motion

[jonade]

Thank you selimgunay that is really an interesting eye-opening answer. And I have recently read that the stiffness subroutine can be reprogrammed to write the stiffness as output in a dat file. Then through the use of Matlab to do and perform an eigen analysis according to this site. https://www.researchgate.net/post/When_ ... lculate_it .
So is there a script that has been done to do calculations like this? and if not according to your recommendation how can I calculate the fundamental period of a structure. I need to know the fundamental period to scale multiple earthquake records. I can run the time history for one specific earthquake record and try to calculate the period of the structure but that will depend on the time i had selected to calculate the period. To be methodical can you recommend a methodology that i can use always. is there any standard practice?

[selimgunay]

You need to use the eigen command for eigenvalue analysis and to calculate the natural periods

[jonade]

Yes and as u suggested I should do eigenvalue analysis at a time that corresponds to the strong part of the ground motion. Otherwise, I will have a period with the uncracked crosssection stiffness, which is small. So is there a way to know the fundamental period of the concrete structure without running time history analysis? As always thank you for your swift replies

[jonade]

I am proceeding by taking the period calculated by eigenanalysis using uncracked sections as a fundamental period. I am not sure if that is the correct way and i would appreciate it if someone can confirm it.

[selimgunay]

If you don't want to do time history analysis, you can use cracked section stiffness (which is around half of uncracked section stiffness) to compute the periods

[jonade]

Thanks, so how can I assign a cracked section stiffness for a fiber section? I have seen examples for an elastic section but I haven't seen any for a fiber section.

[jonade]

and if it's necessary to assign an elastic section to use cracked section stiffness, how will I be able to model the plastic hinges at the end? Thank you

[jonade]

I am going to use beamwithhinges element and assign a cracked section property for the middle section and use a fiber section for the two ends. I will also compare my results with the time period calculated during time history analysis. Thanks for the replies Selimgunay.