Composite deck P.N.A location

[kruq]

Hi, I'm tring to understand how to locate Plastic Neutral Axis in composite beam by Design Example v14.1 Example I.2. Why in choosing shear transfer C=ΣQn there is assumed 50% of minimal C or T? It can't be take into considiration minimal value of C=0.85*fc*Ac and T=AsFy? Thanks for any reply.

 

[structSU10]

I'm not sure I completely follow your question, but the amount of compressive force that can be developed in the concrete is limited by the capacity of the shear connectors. A typical composite beam in a building will range from 25%-50% composite most of the time. If you can get enough shear connectors to get to 100% composite action, then you use the full capacities you have noted for tension and compression.

 

[kruq]

So in this example author assumed that beam will be partially composed, am I right? If I want fully composed then I use minimum of C or T, but then I need provide adequate amount of steel connectors on half of beam to get shear capacity bigger than T or C?

 

[structSU10]

That is correct. If you are using composite metal deck it will be difficult to get enough studs on the beam since you are limited by the spacing of the flutes.

 

[kruq]

Now it's look I understand. Could You suggest me, because it is my first composite slab, what percentage of composite is frequently used?

 

[structSU10]

If you first calculate C & T, and then determine Qn, start by assuming one stud per deck flute (look at the deck catalog to check typical flute spacing) and you can see what % composite action you can get. Most decks I work with have flutes at about 1' O.C. so if you take half your beam length as the number of shear connectors it might be a decent starting point. This assumes you are doing a uniformly loaded beam, and are uniformly spacing your shear connections.

 

[JoshPlumSE]

The number of shear connectors is all about cost.

1) You start with shear connectors at maximum spacing. Calculate strength. Calculate deflection. If it works, then you're good.
2) If not you add more shear connectors until they're at minimum spacing. Does it work then? If not you either go to multiple rows or go to a larger beam.... And start over again.

 

[kruq]

Thanks for answers. One more question, when PNA is located in concrete slab I should use as compresive force C=Fy*As or minimal force of this two: Fy*As and n*Qn? I asume that first one, but I want to be sure.