Roof Diaphragm - Sheathing over T&G decking

[JAE]

There's been one other post before this on this topic (thread507-106592) but we just faced this situation and I thought I'd ask out there if there are further comments or input.

The IBC doesn't recognize 2x tongue and groove decking (we have 2x6's) as providing roof diaphragm shear behavior. I've heard in the past that placing Structural I plywood or OSB over the top of the decking can provide the necessary diaphragm action.

My questions, however, are this:

The IBC recognizes the sheathing but specifies nailing on the "supports" - edge nailing and field nailing. We have gluelam beams at 6'-9" o.c. and thus there are no roof members spaced at 16" or 24" o.c. to specifically provide for the nailing.

If we use the 8d nailing with 1 3/8" required penetration, there is no standard length of nail that works - 8d or 10d nails are too long and would poke through the decking - ticking off my architect.

Is the sheathing considered as "blocked" under the IBC diaphragm capacity table?

The IBC states that we can use the NDS nail capacities and manually calculate diaphragm shear capacities from them. Is there a specific technique that shows how to do this?

What have any of you done with respect to this?

 

[JAE]

swsengineer - you are correct. But we asssumed that the 8d nail from the diaphragm tables was into 2x lumber which means there is an edge distance reduction factor in the 8d nail vs. the 10d nails placed into the flat 2x6 decking, which theoretically has few or no edge conditions.

We assumed that there is a natural variation in the positioning of the 8d nails in 2x framing such that the nails can get as close as about 3/8" from the 2x edge - thus per NDS there is a 4D loaded edge distance requirement that must govern. With 3/8" we have less than 4D (.375/4D = .72 reduction factor on the 8d nails).

This equates the two nails.

Now I'm not 100% sure that the diaphragm tables relied on a variation of nail placement like that but the fact that the diaphragm values vary with 2x vs. 3x framing suggests an edge distance issue.

It would be nice if the IBC or someone could produce an example of how the diaphragm table values are calculated.

 

[SperlingPE]

The Western Woods Use Book addresses this situation.
Might be available in your local engineering library.
Can still be purchased.
I thought that T&G paneling (I know not 2x6's) was allowed by IBC.