Question about Example6.

[juanoviedo]

Hello Everybody!

Can someone help me clarify one thing about SectionAggregator Command in the example folder “example6.genericFrame2D” ?

The issue is:

In the folder there is one file called “Ex6.genericFrame2D.build.InelasticSection”. The element type used is “nonlinearBeamColumn” for members, beams and columns.
Also, for both members (beam and columns) the sections are defined with “uniaxialMaterial Elastic” and “uniaxialMaterial Steel01”, for axial-force-strain response and flexural response, respectively. And finally “section Aggregator” to combine both behaviors.
It is also written that no P-M interaction is defined by this “section Aggregator”, then, how does this nonlinearBeamColumn element work for column type member?

My case is nonlinear earthquake response of reinforced concrete frames, I am not interested in detailed response of reinforcing bars, I need structure response; then, how should I define the column members?

If I used “section Aggregator” command with “uniaxialMaterial Elastic” for axial behavior and “uniaxialMAterial ElasticPP’ for flexural behavior, what kind of interaction would I be getting?

If I use the above combination, this error keeps coming out:
“SectionAggregator :: getSectionFlexibility - - Singular Section Stiffness”
What does it mean?

Perhaps, I do not understand well the concept of the “section Aggregator” command.

I will appreciate so much any comment and suggestion,

Best Regards,

[fmk]

if you want P-M interaction in the beam-column you have to use a fiber section. If you use the section agregator the sections agregated do not interact. In your case if you use uniaxial fibers to define the P and M relations of an aggregated section there is no P-M interaction.

you are getting a failue because the ElasticPP has 0 tangent after yield, you cannot invert a block diagonal matrix if one of the diagonal blocks is 0. If you insist on using the aggregator you need to use another material, e.g. hardeining with a small slope after yield.

[juanoviedo]

Hello Frank!
Thanks for your reply!

I understood perfectly the problem with ElasticPP and at certain point the issue with the "section Aggregator" and P-M interaction.
I will try to modify my model by using fiber section to get P-M interaction; however there is still one doubt:
In the example6, as I explained in my last post, if there is no P-M interaction for the column, then, strictly speaking, the column should have P-M interacion, shouldn't it? If so, I ALWAYS have to define fiber sections for column type members, don't I?

Thanks again for your time,

Regards,

[fmk]

If you want P-M interaction in your element then YES you ALWAYS have to use the fiber section.

[juanoviedo]

Hello Frank!

Thanks again for your clear explanation. I will proceed accordingly.

One last general doubt:
When don't I need P-M interaction for COLUMNS? I mean, which case the P-M interaction is not needed?

Thanks again,

[silvia]

that depends on your model and what you are trying to achieve.
an axial force typically affects your strength. Non-prestressed beams typically don't have axial forces, so its effects can be neglected. So, if you have a large model and need to reduce the size of your model, you can put in M-curvature relationships into your beam elements.

[juanoviedo]

Hello Silvia.

Thanks for your comments.

And what about vertical elements such Columns? When the P-M interaction is not neccessary?
It is always necessary, isn't it?

Thanks,

[silvia]

if you have gravity loads, yes, they are.
look at your PM interaction diagram.