Stacking Roof Deck 2

[Brad805]

We had a request to prepare the shop drawing layout for roof decking. The EOR has a unique design I have not seen where they indicate to stack roof deck in a snow drift zone in lieu of using an appropriate product or adding a interior bearing member. I opted not to get involved, but I am curious to hear others opinion of such a detail. It seems crazy without load testing to show how the sheets might load share, but maybe not?

Detail:

 

[JAE]

We've done this in industrial buildings where we may have to spread joists out further apart to accommodate an HVAC unit hung between joists.

The decks share load in proportion to their stiffness (assuming no shear bond). So two identical layers would each take 50% of the load.

Technically, the decks are either welded or otherwise fastened to the supporting steel so there is some composite behavior between decks, but assuming none is conservative.



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[Agent666]

They only share load for load acting towards/through the two sheets. Wind suction on either face for example only engages one sheet.

Never seen anything like this in these parts.

 

[CANPRO]

I had considered this once and I asked our deck supplier about it...they advised against it. No guarantee the deck fits nicely together. If you have the style of deck where the sidelaps are crimped together, not sure the next layer of deck will fit. If the deck is screwed/shot down, the head of the screws will keep the deck from fitting tight. The upper layer would have to be screwed down, because any gap at all would make a puddle weld useless. Long story short, seems like its more trouble than its worth.

 

[JAE]

I think we had no problems whatever on the many projects where we nested deck. The two deck profiles are essentially the same thing.

The connection from deck to steel might have to be welded as the screw heads or power driven fasteners would get in the way of the second layer - unless you placed both layers and then screwed them down at the same time...which is what our client usually did.



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[Agent666]

JAE, have you ever seen any issues with water getting between the sheets, I'd imagine if that was ever to happen the sheeting wouldn't last too long?

 

[Shu Jiang PEng SE]

Structurally, the deck is one-way slab system. Supporting beams perpendicular to the deck ribs with appropriate spacing shall work

 

[JAE]

Agent666,
No - the decking is always under insulation and a roofing system.

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[Brad805]

Thanks all. I was mainly concerned about the likelihood of an equal load share, and it seems more reasonable now after chatting with you. I still cannot see myself doing showing this, but now I would be less likely to object if asked to do this in the future. In this case a single sheet will be more than adequate for wind loads. The drift zone snow loads are very large in my area and here they had a single storey adjacent to a second storey.

 

[hokie66]

I can see doubling up as a simple way to handle that, provided the sheets nest.

 

[BUGGAR]

To assure that they nest, pull the sheets two at a time from the bundle that they drop off the truck. They should match, dent for dent.

 

[JAE]

I believe that the decking is usually shipped to the site in a nested bundle.

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[msquared48]

JAE:
No condensation between the sheets?

Mike McCann, PE, SE (WA)

 

[JAE]

I suppose it depends on how much insulation is used on top of the decking and the temperature/humidity conditions.

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[CANPRO]

The situation where I was advised against stacking the deck, I was trying to reinforce existing deck due to increased snow load...different question all together comparing to new construction.

On the question of load sharing due to wind uplift - my understanding of wind uplift is that it is due to pressure differential between the interior and exterior...so wouldn't the uplift load be applied from the bottom up and still engage both sheets?

 

[Ron]

Roofs will leak, condensation can occur and decks will corrode....if you can accommodate those, nesting is not an issue. If you want to make sure the decks act together, screw them, punch them or weld them at reasonable intervals in the flats, not the flutes. If you are working in the US (and perhaps under other codes as well), remember that welding for decking requires qualification to the number of sheets welded through, so with nested decking, you might have as many as 8 sheets at some laps, all of which must be welded through for attachment to the structure. See AWS D1.3, "Structural Welding Code-Sheet Steel". I believe Canada's requirements are the same or similar.

 

[Settingsun]

Canpro, I think you're right about wind engaging both sheets. I once had a teacher who stressed that there is no such thing as suction as most people think about it. Suction is a 'push' that is achieved by lowering pressure on the other side as opposed to raising it on the push side.

 

[Eagleee]

great thread, i am curious of the wind suction issue raised. i have always been under the impression that for suction on roof you have the (small) interior overpressure component acting on ‘inside’ sheet and the larger exterior suction component acting on the ‘outside’ sheet. interesting to see what others make of this as well.

 

[Settingsun]

Maybe there's a third pressure involved: between the stacked layers. Too late to think about it, hopefully solved by others by the time I come back.

 

[BUGGAR]

I'm thinking about that suction, but today is Saturday and my day off from thinking. Did you check with Bernoulli on that?