Roof Framing sloping in two directions

[txeng91]

Is it practical to slope a steel framed roof (bar joists w/ wide flange girders) both parallel and perpendicular to the joists? In other words the joists would be at a slope parallel to their length while also having a cant. The slopes range from 1/4” to 1/2” per foot. My concern is that this would present difficulties in attaching the roof deck, similar to the issues that having a cambered member close to a non-cambered member creates.

 

[jayrod12]

I've done it, but definitely the steel deck fastening becomes tricky at pitch breaks. Pay super close attention to each scenario.

 

[txeng91]

Doesn’t sound fun. Luckily with the way it’s framed I don’t really need much, if any diaphragm shear. I guess since the metal deck is flexible in the week axis it can kind of warp to the slope?

 

[jayrod12]

Yeah more or less. But you've just got sloped in two directions, which means to the deck it's sloped one direction at an angle to square. Unless you've got a curved roof, the deck itself doesn't have to bend.

 

[txeng91]

Never mind, for some reason I guess I was imagining trying warp the decking to the framing at the slope breaks but I'm realizing now that I need to have some sort of angle bridging or something between the joists at the pitch breaks and have the deck sheets split at those locations.

 

[txeng91]

I spoke with the joist manufacturer and they recommended not having “hard valleys” and letting them warp the deck to the structure.
They gave me a rule of thumb on being able to fasten the deck between adjacent members with around 1 inch differential from the deck slopes over a 5 foot span.

 

[KootK]

Thanks for reporting back TX. Are you sure that the 60:1 recommendation wasn't for curved deck rather than warped? As you and Jayrod mentioned, the case of warping is quite a bit easier to deal with. 60:1 doesn't even get you to 1/4" per foot cross slope. I've done gobs of big box work between 1/4" per foot and 1/2" per foot without issue. Although, that's without reported issues. Who knows, maybe all my deck welds are popped and my diaphragms are shot.

From a practical standpoint, if a system can't handle 1/2" per foot warping, I'd say that the whole warping concept is a bust. Other than straight mono slope roofs, it's pretty hard to always stick to both 1/4" per foot minimum AND 1/4" per foot maximum.

For the structure shown below, which was true deck bending, Canam advised us that it could work so long as it didn't take more than 50 lbs effort per sheet to push the deck down for fastening based on Ix etc. Even that is a a bit uncomfortable in my opinion. At that level of force, I'd have to think you'd be dealing with some substantial residual forces in the deck and fastenings.

 

[txeng91]

If I understood him correctly, he was basically talking about forcing a milti-span deck in the strong axis to an adjacent member that deviates from the deck slope. I vaguely remember him saying something about sledgehammers, so your concerns about the residual tension in the fasteners is probably warranted. My thought is that any concerns with warping in the weak axis should be negligible, I just need to look through the extreme conditions in the strong axis and make sure they’re manageable. It might make sense to just have them use single span deck sheets in some areas to alleviate any induced tension on the fasteners.

On the building in the picture, the deck is spanning parallel to the roof curvature?

 

[KootK]

If I understood him correctly, he was basically talking about forcing a milti-span deck in the strong axis to an adjacent member that deviates from the deck slope.

I've yet to coax you into saying the words yet but I get the impression that your situation is deck warping rather than deck bending. If that's correct, I really don't think that you need to worry about any of this unless you're getting into very steep cross slopes.

It might make sense to just have them use single span deck sheets in some areas to alleviate any induced tension on the fasteners.

I almost wonder if this might be worse for the deck fasteners. I'd think you'd always be fastening through a gap at each end. Although, perhaps the gap is small enough to not cause problems. It would surely depend on the fastener style as well (pins vs welds etc).

On the building in the picture, the deck is spanning parallel to the roof curvature?

It is, bending the deck the hard way. It's another story yet again it's actually two layers of deck for diaphragm reasons which made for lots of fun at the sheet corners where you've got heaps of deck overlapping in one place. Pretty good argument for discrete roof bracing when things get exotic.