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Partial Composite Beam Design - PNA

Partial Composite Beam Design - PNA

Partial Composite Beam Design - PNA

(OP)
I'm currently working on a project with existing composite beams (which i haven't looked at in years). I am using enercalc to check some of my work and have come across some VERY basic disagreements with some of enercalcs results.

I have a W24x55 with a lot of concrete area (my effective width is 12' with a 4 1/2" on 1 1/2" slab) so the steel controls. It's partially composite with 32 shear studs (somewhere around 36% composite).

So here are my questions:
1) It is my understanding that for all partial composite beams, the PNA is located within the steel member. Since there is not enough shear studs to transfer the total compressive force (0.85f'c*t*b or as*fy) into the slab, there must be some steel in compression and therefore making the PNA within the steel. Is this correct?

2) As percent shear capacity increases, the distance from the bottom of steel to the PNA also increases. In essense, the more compressive force transferred into the slab, the higher the PNA from the bottom of steel. Correct?

3)Is there any time where the moment capacity of a partial composite beam is equal to the moment capacity of the non-composite member? Enercalc is spitting out that for a W24x55, the composite action must be greater than 47% before any additional moment capacity exists. A W24x55 member with 46% composite action has the same moment capacity as a non-composite W24x55? This doesn't make sense to me.

For all who want good a good composite beam reference, i've found this article to be very effective.
http://www.eng.mu.edu/foleyc/paper_pdfs/vinnakota_foley_vinnakota_1988_ej_aisc.pdf

I hope these questions don't appear too basic, but i wanted to verify my own thoughts before i ask enercalc for clarification on their internal calculations.
Thanks to all responses.

RE: Partial Composite Beam Design - PNA

1). True.
2). Yes.
3). Nope - doesn't make sense.   

RE: Partial Composite Beam Design - PNA

A star to WillisV for simple answers to complicated questions.

RE: Partial Composite Beam Design - PNA

1) Technically speaking the PNA does not need to be within the steel member.  It can certainly be at the very top of the steel member.... That's one of the situations that tabulated in AISC's design tables.

I believe it is even technically possible for the PNA to be located within the concrete slab.... Meaning that the 0.85f'*a*b > As*Fy of the member.  To make this work, you have to start reducing the depth of the compression block.... Or, consider the concrete to be at something less than it's compression failure strain. Make sense?

That being said, you'd have to have a monstrous slab to make that happen. I'd say that it is so unlikely that you could ignore it for all practical purposes.  

2) Yes, the larger the compression in the slab (due to shear transfer from the studs and due to strength of slab)then the higher the PNA is in the section.

3) That doesn't make sense to me.  If the steel section is still assumed to yield fully, then how can adding a slab result in a Mn that is less than Mp of the original section?  

Back in the 9th edition days, I believe this was possible because of the difference in the way the code checks were done. So much was based on extreme fiber stresses rather than overall plastic yielding of the section.  



   

RE: Partial Composite Beam Design - PNA

I'm not up-to-date, but if everything is ultimate now, could item 3 in the OP be because of sudden failure of the concrete rather than slow failure of the steel?

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Partial Composite Beam Design - PNA

(OP)
If you scroll down to the bottom of the page of the documentation for enercalc (go to beam module and then composite beam) and you will see the "properties tab". The one at the top is 100% composite action with a distance from the bottom flange to the PNA equal to 27.31" (which might make sense for a W21x44 with a 7 1/2" slab). However as you decrease the composite action, to say the bottom (88%), the PNA is not only in the steel section but moved up further (to 27.43" from the bottom). This appears to contradict my first 2 thoughts.

Part of me is wondering if enercalc is using the PNA as the distance from the bottom flange to the bottom of 'a'. But that isn't the PNA.

RE: Partial Composite Beam Design - PNA

Are you using old ASD, new ASD, or LRFD? I'm assuming not old ASD since you are asking about the PNA.

That being said, 100% composite action means there are enough studs to transfer the lesser of AsFy and 0.85f'c*beff*h. If the shear studs are not adequate to transfer that shear force (which is partial composite action) then the PNA MUST be below the top of the top flange (ie in the steel section). It's not possible to have it at or above the top o the top flange in partial composite action.

2) True

3) is probably possible using old ASD depending on the steel stress before composite action.

One last thing to add is that there is a large difference in moment capacity between old ASD and new ASD because of the design approach.

RE: Partial Composite Beam Design - PNA

My experience with Enercalc has not been good.  It is so riddled with errors, it makes it hard to trust just about any solutions that it comes up with.  I really only use it as a sanity check.

I say that just to say: trust yourself over Enercalc.  Composite beam design isn't difficult, especially when selecting values from a table using AISC 13th.  You essentially "choose" the PNA location based on how many studs are there.  Determine Mu by hand, and go into the table and choose the first value of studs that meets the capacity.  Don't forget to double the studs (assuming you have a uniformly distributed load) because the table value only covers studs for 1/2 the beam.

RE: Partial Composite Beam Design - PNA

(OP)
Steellion,
I couldn't agree with you more on the enercalc thing. Enercalc fixed this problem with their latest update (which i was having problems downloading due to IT issues). But this definitely forced me to re-think composite design and understand it a lot better. I just don't understand how you can put a product out with errors like this.
 

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