Curved Roof Deck

[Gumpmaster]

I have a curved roof (62 foot radius) constructed of bowstring LH trusses and metal roof deck.

I'd like to use 18 gage deck but I have no idea if it's possible to curve it (the easy way) at a radius like this.

On a related note, the Architect wanted to use straight joists framed the opposite direction and roll the roof deck the hard way. I told him no because I was worried about diaphragm capacities. Has anyone seen any design guidance for using curved roof decking? Also, what is your favorite method of getting a curved roof?

 

[hokie66]

Just to clarify what you are asking, is the steel deck the roofing or is it the substrate for another type of roofing? We build lots of curved roofs in Australia, but the deck is actually the roofing, and we don't use diaphragm action of the deck.

 

[Gumpmaster]

The deck is below the actual roofing. The deck sits right on the joists and above it is the insulation and a thin standing seam metal roof.

 

[dik]

Talk to your local deck suppliers. The radius is easily attainable about the weak or strong axis. Recently did a water tower and the deck was rolled to an 11'-6" diameter. They put 'side dimples' on the stiff part to permit the curvature.

Dik

 

[JAE]

Yes, you can roll the deck the easy way.

One thing you can do if you have a lot of time, is calculate the distance of bend that the deck sheet metal is put under. The length of the effective bend is perhaps the 6" rib spacing. Essentially you would construct a pseudo beam (say a 12" wide piece of deck, 6" long) that would bend some distance.

Then determine the moment of inertia of the sheet metal (very small) and calculate the flexural stress in the sheet metal as it is bent. You'll most likely have a very small stress.

 

[BAretired]

What is the design snow load and what is the spacing of the bowstring trusses? What depth of steel deck are you contemplating?

I don't think there should be any problem getting the curvature the "easy way" but if it is 3" or deeper deck, it might be a little tough getting it the "hard way".

BA

 

[Gumpmaster]

Loads are pretty minimal (25psf snow). Trusses 7'-6" on center. The only reason I need 18 gage deck (1-1/2") is the client required the deck to meet factory mutual requirements. It's something they think sounds like a good idea even though they're self insured (Military client).

I just have no concept as to how thick 18 gage material is and how hard the contractor is going to have to work to lay it down on a curve.

Thanks for the info.

 

[BAretired]

You should not have any trouble with installing the deck. It is mostly a matter of cost. 18 ga. deck is 1.21 mm thick compared to 0.76 mm for 22 ga. which would be adequate for that span and load, so it is about 60% more in weight (and cost).

BA

 

[Gumpmaster]

You know federal government jobs. Money's no object. Plenty of it floating around in peoples pockets free for the taking.

I agree. I'd love to use 20 gage deck which is the minimum allowed by the unified facilities criteria, but the Air National Guard has a technical memo stating that all roof structures must meet Factory Mutual approval criteria, which also, incidentally, rules out 3" deck.

 

[dik]

Can you reduce the truss spacing and get to 22 GA? For that loading 22 GA should be OK for about 6'-8 and should be OK for FM Global requirements. 18 GA will be a little bouncy for installation on 7'6... it works, but bouncy. Check with a local roofer.

Dik

 

[vandede427]

from my understanding, Factory Mutual limits joist spacing to 5'-0" for 1-1/2" deck.

 

[JAE]

Thickness of an 18 gage deck = 0.0478 inches

So if you were to magically "fix" the edge of a deck and let it bend about its weak direction with a cantilever = 7'-6" ft, the bend that results under its own self weight alone is huge - the deck is essentially a wet noodle.

 

[Gumpmaster]

According to Verco's catalog, their 18 gage deck is Factory Mutual approved for a 7'-9" span. The allowable uniform load is 91 psf for a double 7'-6" span.

I actually have 6'-0" truss spacing, except for the last bay. The front of the building curves creating a longer end bay. There's no practical way to decrease the end bay length because of the curve. The Architects put all these curves on a building and then are bewildered when the construction estimate is over budget.

I'd actually planned on using 3" deck before finding out that there aren't really any Factory Mutual approved 3" decks.