Steel deck roof diaphragm plus sheathing

[HeavyCivil]

My current project has two buildings that are apparently required to be 'non-combustible' and as such have CMU walls, l.g. steel roof trusses and metal roof deck. The architect wants asphalt shingles which require 3/4" sheathing for a nailing surface.

I'm comfortable designing for lateral forces assuming the wood sheathing is my diaphragm. However, I am not sure how to fasten the sheathing through the decking to ensure diaphragm action, let alone what to use for the decking as its only purpose here is to shield the sheathing from fire.

This, in my opinion, could be easily accomplished by 2 layers of sheetrock on the roof truss bottom chords or cement board, but the architect says it is required.

The questions are:

1. How should the sheathing be fastened? (I'd like to emulate typical 6"oc/12"oc with self tapping screws, but that would seem quite a bit more labor intensive than using 16d nails driven pneumatically).
2. Can the steel deck serve as the diaphragm? I assume it can, but where can shear values be obtained for this? Obviously they are not in the NDS.
3. If it is easier to use the sheathing as the diaphragm as it is required anyway, what should be required for the deck if it is simply serving a purpose of non-combustibility?

 

[ToadJones]

My parents are yinzer's from the 'Burgh...I'm not too far way. It slips out sometimes.

I'm not familiar with the heel blocking details or Shear transfer trusses.

 

[HeavyCivil]

I wasn't familiar with shear transfer trusses until I recently got a truss shop drawing submital with a note that said "trusses not designed to function as part of the lateral force resisting system". It was a medium sized, rectangular stick framed building and the only shear walls were the load bearing exterior ones and the gable walls.

I marked it "Revise and Resubmit" and noted that the trusses had to act as part of the LFRS.

I got a call from the fabricator and he educated me that he could easily accommodate this, and on smaller buildings the design may not even change, but that it had to be noted on drawings (it was in the spec anyway). I think that designing trusses for transfer of in-plane forces is pretty standard. Out-of-plane is another story and is more complicated as it must be achieved with bracing.

In terms of heel blocking - a lot of builders use 2x blocking cut to the truss/rafter spacing (minus .75") because it makes layout really easy as they are setting trusses.

 

[hokie66]

Mike,
Codgitating? I assume that is deep thinking by old codgers.

 

[ToadJones]

I see.
Yes, we used to use blocking on trusses with taller heels (when I worked as a nail bender) only up to the point where they didn't suffocate the soffit/ridge vents.

I also never cared for "gable" trusses that only had vertical web members.
To avoid using them, I simply used regular trusses at the ends but set them back the wall thickness and balloon framed the gable end wall.
Then, the last truss nailed directly to the wall studs and also served as a drywall nailer.
I hated the gable trusses because they made the gable sheathing very wavy and screwed the siding up.