Perimeter Beam Bracing
Perimeter Beam Bracing
(OP)
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?
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.






RE: Perimeter Beam Bracing
-Huck
RE: Perimeter Beam Bracing
Not to say these don't work, they just aren't safe enough and don't meet minimum practice standards. Never seen one fail, but I'm not interested in stamping one or otherwise trying my luck.
RE: Perimeter Beam Bracing
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RE: Perimeter Beam Bracing
@Huck: I suspect that gravity / friction would usually prevent LTB. I haven't relied on that in the past however.
@CEL: I agree, I usually do not consider this condition braced. I'm curious about your requirement for rotation resistance. Why do you say that? I actually agree but I know of no code specified torsional restraint requirement. I've never understood why the tension chord of a simple span truss can buckle but the tension flange of a wide flange beam cannot. Also, would you change your answer if the beam in question was an interior beam rather than a perimeter one?
@JAE: based on your response, I think that you might be envisioning the deck flutes running in the wrong direction.
I see two scenarios:
1) If the beam tries to buckle toward the building exterior, the deck will engage the topping and then it is a matter of how well that topping edge is tied back to the rest of the slab.
2) If the beam tries to buckle toward the building interior, the deck may pull away from the topping and crinkle up accordion style.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
gjc
RE: Perimeter Beam Bracing
There may be additional detailing requirements for seismic of which I am unaware.
RE: Perimeter Beam Bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
RE: Perimeter Beam Bracing
While I still tend to think about this in terms of NZS 3404, I like the simplicity of this AISC answer:http://aisc.org/DynamicTaxonomyFAQs.aspx?id=1646
and the clarity of this bridge resource:http://www.academia.edu/6206943/Steel_Bridge_Desig...
RE: Perimeter Beam Bracing
And the perpendicular framing members is a good point, KootK.
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RE: Perimeter Beam Bracing
That bridge bracing manual is great -- thanks. Yura's pretty much got a monopoly on all things bracing these days. If he ever gets hit by a bus we may just have to settle for what he's figured out for us so far.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
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RE: Perimeter Beam Bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
However, if you want to be rigorous, you could calculate the capacity of the deck in weak axis bending (the webs will fail in double curvature).
DaveAtkins
RE: Perimeter Beam Bracing
BA
RE: Perimeter Beam Bracing
DaveAtkins
RE: Perimeter Beam Bracing
You could make a nearly identical argument for an un-topped steel deck. Of course, you never would. Is there not an inconsistency there?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
DaveAtkins
RE: Perimeter Beam Bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
RE: Perimeter Beam Bracing
DaveAtkins
RE: Perimeter Beam Bracing
BA
RE: Perimeter Beam Bracing
Link
Link
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
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.
RE: Perimeter Beam Bracing
RE: Perimeter Beam Bracing
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Perimeter Beam Bracing
RE: Perimeter Beam Bracing
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.
RE: Perimeter Beam Bracing
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.
RE: Perimeter Beam Bracing
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
RE: Perimeter Beam Bracing
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.
RE: Perimeter Beam Bracing
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
RE: Perimeter Beam Bracing
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.