# Strength of multiple plywood layers



## novemberechoes (Oct 1, 2012)

Hi all,

Can anyone out there tell me how to determine the moment of inertia for two sheets of plywood that have been joined together? I've looked through the APA website and can't find anything specific. I'm not sure if I should calculate it as roughly the same as a single sheet of plywood of comparable thickness (best-case scenario), as simply double the value of a single sheet (worst-case scenario), or something in between. If specifics are helpful, I'm looking to combine two sheets of 19/32 CDX ply with face grains running in perpendicular directions. 

Long story short the designer wants a platform that's thinner than standard framing allows (nothing new there, right?), so I'm looking at simply layering ply, as I've done before for certain turntables. On top of that however she also wants it cantilevered a minimum of 6" the entire way around, so I'm trying to do a calculation for a cantilever beam with a concentrated load at free end as a worst-case scenario.

Long story long, the designer wants a raked, circular platform, 18' in diameter, 2" in thickness, with 80+ 5" circles in the platform that light up. My plan is raked studwalls/joists with the cross-layed plywood attached like flooring. On top of that I'll do 3/4" foam that will have channels carved in it for rope lights. The whole thing will be lidded with 1/4" masonite with plexiglass circles inset to allow the rope light to shine through. All on a tight budget, of course. I think it'll look really neat when it's all done, but I'm guessing the director will want to stage people on the cantilevered sections quite frequently, and I'd like to give a firm answer in terms of number of cast members he can stage within a certain area of the cantilever.

Thanks in advance to any and all who might help. I've been reading posts on here for years but never posted.


----------



## Footer (Oct 1, 2012)

How you attach the pieces together can really change how this is going to work. If you just plain screw it, I would say go with the worst case two pieces of ply. If you glue and screw, you could probably go with the best case single piece minus 25%. You won't find any tables on this mainly because general industry does not do something like this and the attachment method can be so varied. Also, the surface of CDX does not really lend it self to perfectly joining it with another piece of ply. 

Sounds like to be you should just build a giant triscuit. You'll get a stronger deck.


----------



## FatherMurphy (Oct 1, 2012)

novemberechoes said:


> ...she also wants it cantilevered a minimum of 6" the entire way around... ...and I'd like to give a firm answer in terms of number of cast members she can stage within a certain area of the cantilever.



How many actors can a director put on a 6" cantilever? This might be a self-limiting concern, unless you really meant a 6' cantilever.


----------



## MPowers (Oct 1, 2012)

There are quite a few pieces of info that need to be supplied before we can even begin to provide an adequate answer. Quality of the product, quality of the construction. As mentioned above, the bonding method between various layers will greatly affect the strength (should be 100% continuous adhesive bond) as will things like where are the 80+ circles? Can you arrange them to allow continuous "beam" areas to exist? Do any of them have to be in the cantilever or along the edge that supports the cantilever? Can you up grade your plywood choice to at least AB? Cabinet grade would be better. Why rope lights rather than 80 discrete light sources? If you figure the # of rope light lamps that will be exposed under each circle vs: the number that will be lighting closed covered "channels", you might be better off buying a bunch of raw LEDs and poke wire through the bottom of each hole. No channels to weaken the structure, all wiring accessible between the stud walls (if the deck is high enough). Then you can make the structure stronger by using one of your 5/8" (19/32) ply sheets as the top surface under the maso floor and the lower surface, then use a 5/8" thick layer of "beams"/"Frames" in between to create a stress skin scenario. Veto any lights in the overhang of the cantilever. Move or shift any lights that split the line directly above the front edge support of the cantilever. 

To truly provide accurate structural answers, I would have to be part of the design team and the engineering would take a reasonable amount of time for which we would have to charge a fee. You don't want that and I don't want that. Take a look at the suggestions above, some may work for you some may not. Supply more info and ask more questions and I'll try to answer as best I can.

Hope this helps a little bit.


----------



## novemberechoes (Oct 2, 2012)

Hey guys,

Thanks for all the input so far. To respond to various points:

I'd have loved to be able to use triscuits, as that's what we have as stock platforms. Unfortunately the lighting demands have nixed that. The light sources are arranged in concentric circles (it's a production of These Shining Lives, so essentially we're creating the electron cloud for radium), and the lighting budget doesn't allow for individual instruments so they've chosen rope lights, partly because they have a large amount in stock already. As such, I didn't think triscuits were a viable choice since the rope would be running through stringers, the skins would have a number of large holes in them, and I'd have to look at getting structural plexi/acrylic or building in some kind of support to make up the space between the skins and the rope lights. I've had success with LEDs other places before, and agree that may be a better answer, I'm going to try and urge them in that direction. The platform is pretty low, which might make running the wires for LEDs difficult, but the good news is that given everything else I'm dealing with the actors are never even 2' above the deck. Thankfully, none of the lighting sources are in the cantilevered sections, so that's not a concern at this point. 

The 6" cantilever is the shortest span I'll have unsupported. Due to the joist method I'm looking at using (spaced every 2' on center) and the circular shape of the platform there are instances where the distance the ply is cantilevered out is a foot or so. I can add extra supports in to cut that back down to 6", I just thought this was a good time to think about a somewhat unique structural problem.

My sense is that this design is probably beyond our budget, hence looking at using CDX or maybe at best something like BC. Most of this design on my end was just a series of problem solving trying to work around the constraints. If I can't get more money out of the budget to either up our materials or lighting options than we'll just have to send the designer back to the drawing board.

Again, I appreciate all the info and ideas. Maybe some day if I ever feel like publishing an article or going for a PhD I can look at examining this problem in depth. I'll just need to find a grant for all that plywood...


----------

