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Roof Metal Deck + Beam Unbraced Length

Roof Metal Deck + Beam Unbraced Length

Roof Metal Deck + Beam Unbraced Length

(OP)
Would you consider metal decking to brace the compression flange of wideflange roof purlins if the roof deck is screwed down with Tek Screws?  

RE: Roof Metal Deck + Beam Unbraced Length

Yes.

RE: Roof Metal Deck + Beam Unbraced Length

Yes.

RE: Roof Metal Deck + Beam Unbraced Length

(OP)
Yes.

RE: Roof Metal Deck + Beam Unbraced Length

(OP)
oops, answered my own post.

RE: Roof Metal Deck + Beam Unbraced Length

Yes.

RE: Roof Metal Deck + Beam Unbraced Length

Okay but what if the ribs run parallel to the purlin?

I know what the answer is, but I've never been very comfortable with it.

RE: Roof Metal Deck + Beam Unbraced Length

On second though - unlikely that the deck will do anything but span between purlins, but the general question remains - do you count on deck running parallel with a member to brace the compression flange? Say in an edge beam?

RE: Roof Metal Deck + Beam Unbraced Length

Ribs parallel to deck does not brace compression flange. If it is for a girder, then the unbraced length would depend on the spacing of the beams.

RE: Roof Metal Deck + Beam Unbraced Length

You're the first licensed (I assume) person I've heard take that stance.

A company I had multiple interviews with and ultimately a job offer from gave me an 'engineering test' which, among other things, involved sizing a beam. I sized it larger than the Manager with whom I reviewed the test did, finding that LT buckling would control. He informed me that I was wrong because the parallel ribs did indeed brace it.

Most engineers at my current office take the same stance, although it's a condition that comes up more in conversation than in actual projects.

RE: Roof Metal Deck + Beam Unbraced Length

Agree with slick, but the deck would not be parallel to the purlins.

RE: Roof Metal Deck + Beam Unbraced Length

(OP)
what if the decking had substantial topping material, like concrete?  

RE: Roof Metal Deck + Beam Unbraced Length

Assume you have a edge girder spanning 30' with beams framing in at 5' o.c.

The deck (no topping) will get attached to the beams with puddle welds (typically) and also to the edge beam with puddle welds (typically).

There is an argument to be made that this panel which is rigidly attached to the beam and girder will offer the required flange brace force. I haven't come across any myself.

I am sure others will chime in.

RE: Roof Metal Deck + Beam Unbraced Length

If it has topping, then I would be inclined to assume it to be braced.

RE: Roof Metal Deck + Beam Unbraced Length

CSA S16-01 Article 9.2.7 stipulates that the deck must resist a force equal to 0.05 times the maximum force in the flange or chord.  The force may be considered uniformly distributed along the length of compression flange or chord.

BA

RE: Roof Metal Deck + Beam Unbraced Length

VTEIT- read through slick's referenced post, I didn't need my mind changed, but I wonder if it would change you or your coworker's minds... A deck perpendicular to the deck flutes in my mind has very little in-plane compressive strength. It would most likely fold like an accordion with most common gauge sizes. Think about it on a micro-level, one rib, this is like a cranked-moment-frame made out of very thin steel with little bending strength...  Luckily when deck ribs are parallel to a beam in most instances there are beams perpendicular to, and bracing, the girders.

Its amazing how much beam bracing is brought up as a topic on this message board. Maybe AISC should do a webinar or technical article or something specifically on this subject and address several common situations and methods of bracing.

RE: Roof Metal Deck + Beam Unbraced Length

a2mfk - my mind doesn't need to be changed - I'm of the opinion that even using the '5% rule' for which BAretired actually sited a source, parallel ribs won't provide that. As mentioned, a slab poured on that deck will. So we're on the same page.

The reference thread was a good read though - thanks.
 


 

RE: Roof Metal Deck + Beam Unbraced Length

If the deck is connected to the purlin, it braces it.  If the deck is parallel to the purlin, the member is not a purlin.

RE: Roof Metal Deck + Beam Unbraced Length

If the member is a girder with beams at five or ten foot centers, it doesn't need lateral bracing between beams so the whole question seems academic.

BA

RE: Roof Metal Deck + Beam Unbraced Length

(OP)
its my post....and I'm with Hokie here.

I simply would not lay decking parallel to purlins.
It would be more than semantics. It would be asinine by design.  

RE: Roof Metal Deck + Beam Unbraced Length

WRT to bracing of a roof girder with deck running parallel to the girder:

The SDI stance is that such a girder is braced equally as well as if the deck is going the other way.  They point out that this is a deck shear deformation issue and that basic mechanics of materials dictates that direction doesn't matter.

I've heard several structural steel experts say that they're not so sure it's as simple as given in the previous paragraph.  For example, is there local "accordian like" behavior that lets the flange get moving a little?  If so, then the initial displacement is much larger than assumed in the Appendix 6 beam relative lateral bracing equations.  

Best I can tell from asking people who should know, there has never been research performed to answer this question.  I know what I'd do: the girder is braced at the purlins and not in between.

RE: Roof Metal Deck + Beam Unbraced Length

I agree with 271828 that the girder is braced at beams or purlins and not in between.  I cannot think of an instance where bracing of a girder would rely on deck spanning parallel to it.  Is there such a case?

BA

RE: Roof Metal Deck + Beam Unbraced Length

My opinion regarding bracing from parallel deck would be:

Plain steel deck = no bracing
With concrete topping = bracing if there is sufficient connection between.

The SDI handbook would be aimed at the type of decking that would always have topping and I would therefore agree with the comment in that context.

But if you go back to first principles and apply the required brace force as a line load you will find that the eccentricity at the ribs would exceed the bending capacity of a plain steel deck.

RE: Roof Metal Deck + Beam Unbraced Length

Quote:

The SDI handbook would be aimed at the type of decking that would always have topping and I would therefore agree with the comment in that context.
Not sure if this is directed at my reference to SDI.  The SDI stance includes B deck with board insulation, not just form deck with some type of concrete, either regular or lightweight insulating.

RE: Roof Metal Deck + Beam Unbraced Length

This is a rare case indeed, because the deck is there first for gravity loads, and it must span perpendicular to the framing member to serve this purpose. Even when decks must change direction due to odd framing plan geometries, this is usually where you note on your drawing a change in the deck span orientation. I could see accidentally assuming it provides bracing more on light-gage steel framing applications then WF/joist framing members.

Something to be aware of should it crop up I suppose.

When rereading Toad's original post, I'd say this dead horse has been thoroughly re-beaten.

RE: Roof Metal Deck + Beam Unbraced Length

deadhorse

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