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?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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Roof Metal Deck + Beam Unbraced Length
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RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
I know what the answer is, but I've never been very comfortable with it.
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
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
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
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
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
BA
RE: Roof Metal Deck + Beam Unbraced Length
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
The reference thread was a good read though - thanks.
RE: Roof Metal Deck + Beam Unbraced Length
RE: Roof Metal Deck + Beam Unbraced Length
BA
RE: Roof Metal Deck + Beam Unbraced Length
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
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
BA
RE: Roof Metal Deck + Beam Unbraced Length
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
RE: Roof Metal Deck + Beam Unbraced Length
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