RC beam stiffness

[Francesco Schiava]

Hi,
I’ll go straight to my issue. I can’t see the influence of the steel rebars in my concrete section’s stiffness. In terms of “Jy”, the value seems to be referred only to the concrete area, and doesn’t change when I add or modify rebars. (Also I wonder if that “Jy” is referred to non-cracked section.)
In this way, for instance, I’m unable to get the proper diagram for stresses in a continuous beam with different rebars (and thus stiffnesses) in the subsequent spans. The output is the same as if I had the same section throughout all the spans.
Am I doing something wrong in setting the sections? I thought that the proper way was:
-set the geometry of the section, assign it to the elemente, and assign material “concrete”
-then “add rebars” to the section with the specific tool

Sorry for my poor English

[NextFEM Admin]

Hello,
thanks for your inquiry. By default, stiffness of RC members does not change when adding/varying rebars. This is a common assumption in all civil design programs.

You add rebar to the cross-section (Section properties) if you want to calculate its strength; to add rebars on elements (this does not affect the analysis results, as told before) use the Assign / Rebar command. This will affect only RC checking.

[Francesco Schiava]

I see, thank you.
So I guess that if I’d need to see the rebars stiffness’ effects on the analysis, as well as the effects of cracked concrete and partialized section, I will manually modify the Jy, Jz directly in the “section properties”, accounting the wanted contribution…

[NextFEM Admin]

Yes, this is a valid option.
In practice, rebar stiffness is always neglected, while to account for cracked concrete the Young’s modulus can be reduced (up to 50% as per Italian code – other codes can have different limits).

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