Composite action on beams with deck parallel to them 1

[MonsieurR]

Hello everyone,

I typically design floor decks with studs in all beams and considering composite action for both the perpendicular and parallel to the deck beams, at least for the purpose of deflection checks.

Nonetheless I have seen people who consider composite action only on the secondary beams. I'm currently in an argument with a senior about this topic. It appears AISC allows to consider composite action for the primary beams, but why do some still neglect it?.

What are your thoughts on the matter?

Kind regards.

 

[jike]

I am with you and have always done composite design for primary beams.

Of course some beams (Primary and/or secondary) may be part of the lateral resisting system but still may be designed as composite. An of course openings should always be taken into account. In fast track design the Mech Engr/Elec Engr/Architect should locate openings/trenches early on so as not to cause alot of rework/change orders in the structural design.

 

[MotorCity]

It is allowed, but I avoid composite design when the deck span is parallel to the beam span.

The reason is that the deck flutes and stud rows rarely align such that the stud row is centered on the beam (or close to it). You could have a condition where the stud ends up being near the flange tip of the beam. Of course you can select a beam with a wider flange, I would rather avoid field issues and just pick a heavier beam.

 

[271828]

In every office I've worked for, we used girder fillers to avoid the issue of the deck rib potentially not aligning with the center of the girder.

I've seen drawings from many other offices and can't recall ever seeing a job with noncomposite girders and composite beams. I think your senior combatant is just wrong. LOL

 

[JoshPlumSE]

Only when they're part of a moment frame or cantilever back span (i.e. they have negative moment along their length) do I ignore the composite behavior for the girders.

 

[BAGW]

Only when its part of moment frame I have ignored composite action on girders.

 

[wannabeSE]

In addition to lateral frame beams, we typically design collectors as non-composite. Otherwise, they are composite

 

[MrHershey]

We typically consider composite action on girders unless we’ll have negative moment or for whatever reason the girder size is heavy enough that it works noncomposite (and then we’ll still usually provide studs, they’re up there with stud gun anyways and need to attach deck down).

The reason is that the deck flutes and stud rows rarely align such that the stud row is centered on the beam (or close to it).

Just happened to be looking at this the other day. For what it’s worth, AISC commentary implies this really isn’t a concern as long as stud diameter is no more than 2.5 times the flange thickness.

 

[Hokie93]

I agree with 271828. Girder filler plates are a standard item utilized at girders to permit the shear studs to be centered on the girder flange.

 

[jike]

I have always terminated the metal deck on each side of the girder and used "girder fillers" as others have referred to. It just takes drawing one detail.

Even for moment frames, I use composite beams adding negative steel in the concrete deck where necessary and taking into account the different moments of inertia.

Collectors can also be designed compositely as along as you add separate studs for bending and collector shear.

 

[WillisV]

Don't need to add separate studs for bending and collector - they serve double duty.

 

[MrHershey]

Collectors can also be designed compositely as along as you add separate studs for bending and collector shear.

This typically isn't necessary. Quote from a MSC article from about ten years ago:
"Assuming the shear studs have sufficient ductility to distribute the horizontal shears evenly along the beam, a composite beam can transfer a horizontal shear due to lateral loads between the floor diaphragm and steel beam that is equal to the summation of the strengths of all the shear studs on the beam regardless of demand on the shear studs from the gravity loads"

Basic idea is you have to think of shear direction. For a composite beam under uniform load, all shear in the studs on one half of the beam is to the left and all shear in studs on the other half of the beam is to the right. In a collector all shear is in the same direction for all studs.

So say studs have 1000# capacity and I need both 1000# shear per stud for both gravity loads and 1000# shear per stud for transfer lateral loads. Would seem as though you don't have the capacity, but you'll find it when you consider the direction of loading. For gravity, half the studs are at -1000# under gravity with other studs being at 1000#. That means you can add 2000#/stud of shear to the -1000# studs and -2000#/stud of shear to the 1000# studs and still meet your capacity.

Edit: Forgot Link (PDF) to article.