Roof Metal Deck + Beam Unbraced Length

[ToadJones]

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

 

[ToadJones]

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.

 

[271828]

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.

 

[BAretired]

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

 

[csd72]

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.

 

[271828]

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.

 

[a2mfk]

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.

 

[csd72]