Steel roof deck on glulams

[kc9508]

We're working on a project that's just in schematic design and the architect wants to look at using glulam beams combined with a 3" metal roof deck (glulam spacing will be about 8'-0" on-center). The framing at the perimeter of the roof will be steel beams, not wood; i.e. the glulams are just the "interior" beams. I'm taking a look at the diaphragm capacity tables which of course are based on support fastener spacing, sidelap spacing, and deck span. It doesn't say specifically in the tables that the deck supports are to be steel. I'm thinking that as long as I specify wood screws with an embedment into the glulam that can develop similar uplift capacities as a tek screw, then the diaphragm shear values should be similar. Based on my understanding, the diaphragm connections are most critical along the perimeter of the roof (where we will have steel) and the diaphragm anchorage to the glulams would be to resist uplift and buckling of "deep beam" diaphragm. Any thoughts on this approach?

 

[Ron]

I see at least two problems with this. First, spanning 8 feet with metal deck is difficult. You don't say what type of roof system you will have but from a serviceability perspective, you will likely have a lot of deflection, thus placing the roof system under an inordinate amount of stress.

Next has to do with material compatibility, fastening and inevitable roof leaks. When you fasten light gauge steel to wood, they must be isolated from each other. You will have moisture absorption in the wood which will corrode the steel with time, even on the interior. This condition is exacerbated where fasteners penetrate the decking into the wood. Since fastening will be done in the bottom of the flutes, water from roof leaks and condensation will collect there and have a path to the wood, leading to deterioration and loss of fastener shear and uplift capacity.

Think beyond the overt structural design and consider serviceability and long-term efficacy.

 

[kc9508]

Thanks Ron. For the first point, the 3" Type N deck has sufficient capacity to span 10'-0" based on the loads per Canam/Vulcraft, so 8'-0" doesn't seem like much of a problem. Maybe I wasn't clear enough on the type of decking we were proposing to use.

The second point is an interesting one, however the deck is being covered with recovery board, polyiso insulation, and membrane. It won't be exposed deck. The decking and anchors will be galvanized to limit deterioration, but perhaps we should specify stainless steel screws?

 

[kc9508]

Sorry, hit post too quickly. Maybe the architect could specify an ice/water membrane or similar covering to separate the two materials as well?

 

[BAretired]

There was a time when 1.5" deep steel deck was used for maximum spans of 8'-0". That was too limber, but with 3" deep deck, I would not expect a problem.

The screws should be corrosion resistant, either galvanized, cadmium coated or stainless. I can't see any need for a separator between the deck and the glulam.

BA

 

[Brad805]

We have done this. Simpson is the only fastener we found that had investigated the diaphragm attachment question. SDS screws worked well. I recall we had to email Simpson to get their report summary. Our roof area was quite small and we had a torch down membrane.

 

[KootK]

I'm thinking that as long as I specify wood screws with an embedment into the glulam that can develop similar uplift capacities as a tek screw, then the diaphragm shear values should be similar. Based on my understanding, the diaphragm connections are most critical along the perimeter of the roof (where we will have steel) and the diaphragm anchorage to the glulams would be to resist uplift and buckling of "deep beam" diaphragm. Any thoughts on this approach?

Logically, I agree completely. Between boundary members, the diaphragm fasteners perform these following functions as you've suggested:

1) Uplift resistance which should be no problem.

2) Panel to panel shear transfer which should be largely unaffected by substrate.

3) Bracing against panel shear buckling which should be no problem.

My only concern would code compliance. Not sure if you're drifting off reservation with respect to ICC reports etc. Might be worth a call to Vulcraft etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kc9508]

Thanks all, very helpful information. BA - I hear you on deck depth, but with snow load increases in recent years here in the northeast, 1.5" deck doesn't fly for those spans. I'll contact Simpson and Vulcraft as Brad and KootK have suggested.

 

[XR250]

FYI, here is a carport I recently did with 3N 20ga deck spanning 14 ft. supported on W10x22 beams with wood nailers.
I have walked on top with the 200 lb owner and it is extremely stout (meets code too)
The SDI construction span is 16'-6"

 

[KootK]

With cantilever no less. Dude on the left looks pretty excited about it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

It actually has a 5 ft. cant. at the rear which, surprisingly, did not feel like a diving board.
Not sure why he is so excited.
Here is my personal pole barn using the same decking. It really makes an efficient system.