Lb and composite action between I beam and deck parallel

[AskTooMuch]

On pic attached, compression flange of I beam is continuously braced, right?
Can I have composite action between existing beam and deck?
The beam is failing without composite action.

 

[SlideRuleEra]

1) compression flange of I beam is continuously braced, right?
2) ...composite action between existing beam and deck?
3) The beam is failing without composite action.

1) Yes.

2) No.

3) Depends on definition of "fail". If:

allowable bending stress < actual bending stress < yield bending stress

Beam "fails" on paper, but just deflects (elastically) more than it should... not acceptable, but beam does not collapse.

Also, beam may never have been actually subjected to theoretical design loads.

In either case, believe your calculations, figure out what to do about the situation... then do it.
Examples: Add another beam.

Modify existing beam or supports.​

Change how load is applied to the beam.​

http://www.SlideRuleEra.net

 

[AskTooMuch]

I'm still on very early design. Existing beam failed in bending with a quick modelling in Risa (no composite).

 

[JAE]

Existing beam failed in bending with a quick modelling in Risa (no composite

OK - not composite but did you depend on the concrete floor to brace the beam? In other words - what was your Lb (unbraced length)?
Do you positively know what Fy is?
Do you know your beam size (I presume so as your attachment looks like an original design drawing).


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[Ron247]

I agree with others. Can be full braced on top flange. I see no mechanism to provide composite of deck to beam.
I would also question slight torque since the load on one side of CL appears notably higher than other side.

 

[JoshPlumSE]

I don't think this beam is composite based on that detail. What I want to see to call it composite is:
1) Studs welded to the beam flange that extend into the deck. Just some way to transfer shear between beam and concrete.
2) I would want to see the concrete extend past the center of the beam. Hopefully cover most of the flange.

Now, there are other ways to transfer shear other than studs. Awhile ago I reviewed some products that used SJI type joists and composite action with the slab. I don't believe any of these products used studs like we normally do with WF beams. One relied on friction (if I remember correctly), but that may have been based on some specific finishing / roughening of the top chord. Others bent / distorted the top leg of the chord angle so that it would provide better shear transfer.

Also, I saw a project where someone created "after the fact" composite action by chipping out the concrete every so often, adding a stud and then repairing the slab.

 

[BAretired]

The flutes of the deck appear to be running parallel to the beam. Is that correct? Or should the flutes be running perpendicular to the beam? If parallel, I would be leery about saying the beam is continuously braced by the deck.

BA

 

[Ron247]

BA, once the concrete has cured, you would not consider it a brace or just when you are pouring?

 

[BAretired]

Ron247, If the flutes run parallel to the beam, there may be a few spot welds connecting the deck to the beam...who knows? If there are, the flange is braced to a slight extent, otherwise it is not braced by the deck but would likely be braced by the beams carrying the deck.

BA

 

[JoshPlumSE]

BARetired -

That is a really interesting point! I have always taken for granted that the beam would be continuously braced. Granted I'm usually talking about composite beams, where you know you're connected to the slab.

My tendency is to go ahead and assume it's fully braced even when the flutes run parallel. The idea being that the whole system (joists framing into girder connected to slab) behaves together. But, you've got me re-thinking this a little bit. There would probably be a point (longer spacing of joists) where I wouldn't be comfortable with calling it braced. I'm curious if anyone else follows BARetired's more conservative method.

 

[BAretired]

Josh,

If shear studs are used in the first parallel detail below, the deck does not need to be spot welded to the beam because the beam is laterally supported by the studs and develops composite action. If the studs are omitted and the deck spot welded to the beam, there is lateral support but no composite action.

In the second parallel detail, if the studs are omitted and both pieces of deck are spot welded, there is lateral support because the concrete rib is developed no matter which way the beam tries to buckle.

If the studs and half of the deck are removed from the second detail, even if the remaining half is spot welded, resistance to lateral buckling of the beam flange relies on bending of thin (20 gauge?) sheet over the height of the deck. In my view, that is not a sufficiently positive connection to consider the beam laterally braced.

BA

 

[JoshPlumSE]

resistance to lateral buckling of the beam flange relies on bending of thin (20 gauge?) sheet over the height of the deck. In my view, that is not a sufficiently positive connection to consider the beam laterally braced.

Agreed, for cases where there it is only metal deck. However, once the concrete deck is poured, the concrete slab will provide better resistance than the 20 gauge sheet metal. Stiffness wise this should be sufficient for bracing the top flange. Now, it's a matter of force resistance.... Because the load path would either be friction or the spot welds (when we don't have studs, obviously). Would that be sufficient to resist lateral buckling? I'm guessing it wouldn't be. So, maybe you've converted me and I'll look at the joist spacing as the unbraced length of compression flange of a concrete slab from now on.

 

[BAretired]

Would that be sufficient to resist lateral buckling? I'm guessing it wouldn't be. So, maybe you've converted me and I'll look at the joist spacing as the unbraced length of compression flange of a concrete slab from now on.

Exactly! It usually doesn't matter whether the beam is continuously braced or braced only at joist locations because l_u for the beam is usually about the same or greater than the joist spacing, but that may not always be the case.

BA