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Perimeter Beam Bracing

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?

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

RE: Perimeter Beam Bracing

I'll bite. Sure, unless the beam is massively big with a huge required bracing force - The cast in slab braces the top flange due to gravity load. 9/10 it should be fairly easy to develop the required bracing force for the flange (~.02Mn) - even when using unconfined compression elements in the slab (using less than 0.35F'c of the slab).

-Huck

RE: Perimeter Beam Bracing

Nope. You need a positive attachment with at least minor rotation-resistent capacity. It may behave as stable and braced, but is not reliably so at Ultimate. Even for a lean-on style bracing solution you still require positive attachment.

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

It is braced in my view. The puddle welds or deck screws can be checked if you are unsure - use AISC's beam stability section to verify strength of the deck-to-beam connections as well as stiffness (which I'm sure the deck is stiff enough). The main question is whether the lateral bracing force required is more than the deck connectors.

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RE: Perimeter Beam Bracing

(OP)
This is usually a moot point, of course, as such beams are often sufficiently braced by perpendicular framing members.

@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

I typically took them as braced. It may well be a moot point in that something has to support the deck perpendicular to the beam in question and those beams/purlins/joists would provide the lateral bracing at their spacing(s). These are generally close enough to provide the bracing such that the full capacity of the edge beam can be achieved.

gjc

RE: Perimeter Beam Bracing

I agree with JAE, if the deck is a composite type, and filled with concrete, even if the deck runs parallel with the beam. The only consideration is the connection of the deck to the beam.

There may be additional detailing requirements for seismic of which I am unaware.

RE: Perimeter Beam Bracing

(OP)
@Hokie / JAE (maybe): The only thing keeping the deck from going "accordion" is the ability of the deck deformations on the sides of the flutes to engage the adjacent concrete in tension, right? If so, how on earth would one come to know that capacity? Let me know if my use of accordian doesn't make any sense. It may be sketch time.

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

RE: Perimeter Beam Bracing

(OP)
Oh, application is a very small, 10' girder that only has a heavily loaded transverse framing member coming in at midspan. It would be an academic problem if it didn't actually exist.

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

RE: Perimeter Beam Bracing

Then why are you concerned? Short member, braced at midspan. But I still think the deck will brace it, provided it is connected well enough. Sometimes, I think we get too picky about what constitutes bracing, when the true force required is very small.

RE: Perimeter Beam Bracing

Yup, now I'm with Hokie... You have a nodal brace at mid span on a 10' span. Not likely to fail, particularly with the minor attachment to a slab element along the length. At that point I'd not be worrying about the puddle welds because they are secondary to the nodal brace.

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

With concrete on the deck there is no accordion action that occurs in my view. There is natural bond on a deck (even non-composite types).

And the perpendicular framing members is a good point, KootK.

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RE: Perimeter Beam Bracing

(OP)
I have a rather special situation that has resulted in LTB needing to be checked between the points of nodal bracing (5' 1/2 span). I've already counted on the supported framing as nodal bracing. I only shared the particulars of the situation to provide context. My question, which you've all kindly weighed in on already, is whether or not this deck assembly, as described, could be counted on as beam top flange bracing in general. I'm still not so sure. Check out the attached sketch for a more explicit depiction of the failure modes that nag at me.

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

I can't see Mode 1 ever happening. Mode 2 perhaps but I'd have to think about it - there is some connectivity between the beam and concrete directly above the beam via irregular welds and friction (I know - you can't count on friction). And the bending stiffness of the deck itself might be adequate - I'd have to put numbers to it to see.

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RE: Perimeter Beam Bracing

(OP)
Normally, having a hot rolled angle as your deck edge element would preclude mode number two.

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

RE: Perimeter Beam Bracing

I don't believe what you show in your sketch is a realistic failure mode. If there is a potential for that, there are a lot of untopped decks in this world that are in trouble. My opinion--you are overthinking this.

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

Personally, I would consider it unbraced between the midspan joist and the end supports, recognizing that I am being somewhat conservative.

BA

RE: Perimeter Beam Bracing

There is a limit to how far a deck can span, based on gravity loads (bending and deflection). So there is a limit to how far apart perpendicular members can be spaced. I just think that for typical deck spans the deck is able to brace the perimeter beam.

DaveAtkins

RE: Perimeter Beam Bracing

(OP)
Thanks for the input Dave. I deny the existence of over thinking. Either I've thought about something to my satisfaction and it has become rote, or I haven't and further thinking is required. Nothing in between.

Quote (DaveAtkins)

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).

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

Again, we don't check an untopped deck in weak axis bending because we assume it provides adequate bracing for a beam top flange.

DaveAtkins

RE: Perimeter Beam Bracing

(OP)
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.

RE: Perimeter Beam Bracing

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

RE: Perimeter Beam Bracing

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

RE: Perimeter Beam Bracing

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

RE: Perimeter Beam Bracing

(OP)
@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
Link

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

RE: Perimeter Beam Bracing

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.

RE: Perimeter Beam Bracing

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.

RE: Perimeter Beam Bracing

(OP)
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.

RE: Perimeter Beam Bracing

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.

RE: Perimeter Beam Bracing

(OP)
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.

RE: Perimeter Beam Bracing

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.

RE: Perimeter Beam Bracing

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

RE: Perimeter Beam Bracing

(OP)
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.

RE: Perimeter Beam Bracing

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

RE: Perimeter Beam Bracing

(OP)
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.

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