Perimeter Beam Bracing 1

[KootK]

This should be an easy one. Here's the situation:

1) Perimeter wide flange beam running parallel to steel deck flutes.
2) Steel deck is topped with concrete (typical composite Canam / Vulcraft set-up).
3) Steel beam is not composite with concrete topping (no nelson studs, only puddle welds).

Is the top flange of this beam continually braced?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

Are you saying that untopped steel deck is able to brace the top flanges of beams running parallel to the deck flutes Dave? If so, that would run contrary to the conventional dogma.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

I have always operated under the assumption that the steel deck adequately braces the top flange of the beam.

 

[DaveAtkins]

Like jayrod12 stated, I have always assumed an untopped steel deck braces a beam top flange, regardless of the direction of the deck. And as I stated earlier, you will never have a steel deck spanning 30' (as an example) parallel to a perimeter beam because the deck cannot span that far for gravity loads.

DaveAtkins

 

[BAretired]

I would consider the beam braced by the open web steel joists, not by the deck. In most cases it won't make any difference because Lu for the beam is usually greater than or equal to the spacing of the joists.

BA

 

[KootK]

@Jayrod / Dave: I disagree strongly. Given that you'd be dealing with a long length of deck that would be prone to bunching up accordion style, I don't see how one could ever expect the deck to satisfy AISC stiffness requirements for bracing. We'll just have to agree to disagree however. This aspect of things has been discussed at length here and probably doesn't need rehashing. Also, as we all agree, the practical significance is limited as one usually has transverse framing members tying in at 5-6' on centre.

Link
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The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

I guess I could agree with you. I've never had a situation where a beam was parallel to the flutes and didn't have supported members at a spacing greater than lu.

So as BA mentioned it's not so much considered braced by the roof deck but rather by the joists.

I'll concede this point. And probably keep a close eye out for a situation like yours KootK.

 

[hokie66]

I think "accordion" bunching in a steel deck is an unrealistic assumption. If the deck is adequately supported and connected, it won't bend in that manner.

 

[KootK]

What is it that keeps the deck from accordion bunching? Just the infill beams and joists, right? So for 5-6' between those joists, accordion bunching. The diaphragm shear analogue to this, usually called deck warping, is precisely why steel deck is pretty flexible stuff and shear deformations matter in diaphragm design.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

I should stay out of this, as I don't use steel deck diaphragms. But I do know that it doesn't take much to brace a steel beam laterally. But then, yours is a channel, and I don't use many of them either.

 

[KootK]

If I could work shear friction into this somehow, I could have a trifecta of Hokie-isms. Hokie the elder that is.

It should be noted that this is not "my" channel per se. I claim existing condition immunity.

In the past, there has been much debate over whether or not wood infill framing could be used to brace a steel beam. The conclusion has usually been that it's dubious and, if it is to be used, it should be checked against AISC stiffness requirements. Surely, 2x framing and plywood would be stiffer than steel deck oriented accordion style. Just sayin'.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Well Kootk, I seem to be the only out and out nay sayer... But I did say I didn't think it would fail. It is simply not good enough for my stamp.

I still think this "stinks", particularly given just how poorly made many of the puddle welds I've seen have been. Shop conditions, sure, but the real world is not all a shop.

 

[DaveAtkins]

OK--in order for the deck to fail in buckling, it must bow upwards or downwards. It cannot do this, since it is supported by the perpendicular framing members.

So, if the deck cannot move upward or downward, the only way it could fail like an accordion is if the deck flanges and/or webs fail in weak axis bending. I still say the strength in weak axis bending is adequate to brace the beam.

I guess I am saying for this situation, there is two way action which you are not accounting for.

DaveAtkins

 

[KootK]

I'm starting to feel like a missionary wandering the inter-web looking for converts Dave. Expect me to show up on your doorstep on Sunday afternoon. I'll be dressed sharp and carrying a handful of Vulcraft catalogues for your family to peruse.

Check out the attached sketch. It's my best attempt at communicating how I see things.

The deck issue that I'm worried about is not buckling. Nor is it strength. Rather, it's simply in plane flexibility. Maybe the deck yields in weak axis bending, maybe it doesn't. Either way, in my opinion, the system is not stiff enough in-plane to serve as effective bracing.

Folks are often fond of saying something to the effect of "it doesn't take much strength to make a brace effective!". Less popular, but equally important, should be the sentiment "it doesn't take much movement to make a brace utterly useless". Stiffness matters every bit as much as strength, as reflected in the AISC bracing provisions.

Lots of your friends and neighbours, including many attractive marriageable women, share this belief Dave. Come towards the light...





The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[DaveAtkins]

I like the way you word things, KootK! The gospel according to Vulcraft?

I can see your point. I think in the future I will assume a wide flange parallel to a deck is braced only at the perpendicular members, not continuously. But...as a few people stated earlier, I doubt it makes any difference, since typically perpendicular members will be spaced 5' to 6' apart, and the member being braced will probably be fully effective with that unbraced length.

DaveAtkins

 

[KootK]

Agreed, almost no practical significance at all.

Continuing in that vein, an issue that I struggled with for over a decade is how corrugated decks resist shear. If you do the Mohr's circle bit, you come to the conclusion that the deck must resist in-plane tension and compression stresses perpendicular to the flutes. It took tinkering with a cardboard model for me to get it sorted out in my head: at a macroscopic level, deck section warping contains a component of axial stress perpendicular to the flutes.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.