Wood diaphragm nailing and capacities

[bnickeson]

I'm designing a wood roof diaphragm that is taking a significant amount of shear load using the 2008 SDPWS manual. On Table 4.2A, it gives nominal capacities for blocked wood diaphragms. I'd like to use just a single nailing pattern for the entire roof but it appears this table makes you use a different nailing spacing at the perimeter of the diaphragm than at the other panel edges in the diaphragm (unless you use a standard 6" nailing). For instance, it seems that you can do a 4" perimeter nailing with 6" nailing at the remainder of the panel edges, or a 2 1/2" perimeter nailing with 4" spacing at the remaining edges. Am I interpreting this correctly? Is there a reason they don't give values for just a standard 4" nailing throughout the entire diaphragm? I barely trust framers to use the correct nails let alone use the correct nailing pattern when it's different in two separate areas. Is there any literature that gives diaphragm capacities with, say, a 4" nail spacing everywhere?

My roof shear is 540 plf so it's getting up there. I suppose I should use 2 1/2" perimeter nail spacing and 4" nail spacing at all other edges, but I'm not sure I trust the framers to remember the 2 1/2" perimeter requirement.

Thanks.

 

[jayrod12]

The diaphragm capacity with nails at 4" everywhere is the same as with them at 4" at the perimeter of the diaphragm and 6" at remaining panel edges. Because the nailing at the perimeter governs the design.

What they're doing is allowing you to reduce the nail spacing if so desired.