plywood two way action

[structSU10]

I am vetting the use of a 'non structural' ventilated roof panel. It is supported by a metal roof deck, but the panel itself is polyiso insulation, 2" air space, and 3/4" plywood. The airspace has 2"x6" blocking spaced at 18" O.C. one direction and 12" O.C. in the other direction, forcing the plywood to act as a two way plate.

I have significant snow loads on my roof - up to 200 PSF in drift zones - and the manufacturer does not have engineering / load data for capacity available. Does anyone have experience analyzing plywood for two way action?

 

[jayrod12]

You may need to provide some more information. The blocking must be continuous in one direction only, and if you have blocking in both directions, I wouldn't worry about it too much. I'd bet that 3/4" plywood can easily support 200 PSF at 16" centres. Where things get crazy is how is this ventilated panel supported?

 

[structSU10]

Maybe I do need to clarify - the blocking is not continuous in any direction - its 2"x6" individual blocks at the spacing noted.

Also, I thought plywood had a 'strong' and 'weak' axis. Digging though the APA plywood design manual, they give various properties in each direction which I suppose can be a simple f/fb in each direction combined. they also have shear between plys and such, which I am unsure if it come into play for this situation.

 

[DTGT2002]

I think we are a bit lost here - as noted above, we have not touched on the support conditions for the plywood - that is what will determine its bending behavior.


If you primary concern is the potential for the individual blocking members to act as concentrated loads and potentially transfer load to the midspan of a plywood region, I can see where you are going, but again, how the plywood is supported on the roof deck is core to all of this.


For my purposes, supports at 12" to 18" are hard to distinguish from uniform loads, but that is not permission to dismiss the differences in uniform loading vs. what you have, but a rational approach analysis on a uniform basis vs. the capacity may give a feel for how much space you have between your approximation of uniform and the material limit - 15-25% of the capacity remaining probably means you are well away from an issue...

Plywood does indeed have a strong and weak axis. Some layups have little difference, some great. Do you have specifications on plies, type and such?




Also, tell us more about what supports the plywood.

 

[structSU10]

The plywood is supported on the intermittent blocking, which is supported on polyiso insulation, which is supported by 3" metal deck as noted in the OP. See below:

 

[jayrod12]

a picture is worth a thousand words, you're definitely into two way plate area in a configuration like that. I would probably still be breaking it down into two components. My first kick at the can would be to have the plywood span up and down the roof to the lines of blocking, then I would check the plywood spanning between the blocking for the increased load. i.e. if up and down the roof the blocking has 16" spaces between pieces, then I would analyze the plywood for 16" span. Then left and right I would check a piece the width of the blocking, for a load equivalent to the blocking with plus one gap.