Partial vs. Full Composite Members

[structeng2]

Hello All -

I have a question about the limitations of composite action. The general understanding is that when your stud anchor capacity (∑Qn) < 0.85*f'c*beff (Cs,max), your section is partially composite because you can't transfer the full load into the concrete.

[ol 1]
[li]What happens if you have a very small steel section (or a large concrete area) such that As*Fy < ∑Qn < 0.85*f'c*beff?[/li]

[li]Running through the standard steps, you would find that Tmax < Cs,max (where Cs,max is limited by ∑Qn), which would typically indicate that your PNA is in the slab. However, my understanding is that when you are partially composite, your PNA has to be in the steel (such that at least part of the beam is in compression)?[/li]

[/ol]

Sorry if this seems rudimentary, but just trying to make sure I understand this correctly.

Thanks!

 

[BridgeSmith]

If the shear capacity of the studs exceeds the tension capacity of the steel beam, you can assume full composite action for that section. full composite action can also be assumed if the stud shear capacity exceeds the compression capacity of the concrete effective flange.

As far as partial composite action, I don't know about in building construction, but in bridge design, it is not allowed the way you're describing it. We are required to provide the number of shear studs necessary to transfer the full tension capacity of the girder, or the full compression capacity of the slab, or in the case of continuous spans, the sum of both between the max positive moment and the max negative moment.

In bridge design, partially composite girders have shear studs in the positive moment regions, but studs are omitted in the negative moment regions. It's rarely done anymore, since we are no longer allowed to ignore the stiffness of the slab in the negative moment regions.

 

[JLNJ]

The AISC Specification Commentary has some good info on partial composite action. In fact, a lot of what used to be in the Spec is now in the Commentary.

 

[structeng2]

Thank you for the responses.

Yes, I was just reading the commentary and found this bit that answers my question about when it is composite (which aligns with BridgeSmith's response):

It also appears there are restrictions on %composite action but still reading into it.