Laminated plywood shelves

[joe1086]

I know its done...but you were overthinking this.  35" span and 12" wide shelf, even 3/4" plywood would hardly sag.  A few scattered drops of TB-II between the layers and a couple of spring clamps to keep them from sliding will be more strength than that ever needs.

You're probably right, it's just I can't help myself.

[Bill Holt]

You're probably right, it's just I can't help myself.

"Aint that the truth!!!!!"  If only I.............

[Edwin Hackleman]

Here's the frame I used for the strong torsion box shelf shown above. It's just 3/4" stock about 1/2" wide and it gets covered with 1/4" ply. The length is about 40" and the depth is 16". I believe I used glue and staples to attach the plywood because I knew I was going to eventually paint the shelves for a workshop cabinet.

   
Frankly, I was amazed at the bending strength these shelves have and was glad that I made the torsion boxes. The dovetails were simple a trial exercise for me to use a simple jig that I made for making them with a bandsaw. Any simple corner joint would have worked. 

   
Thanks for looking and the interest shown here.

[Edwin Hackleman]

I dug this out of the WN files:

"...a torsion box is a totally different animal than two pieces of plywood (glued or) screwed together.  It is similar to an I-beam joist https://www.apawood.org/i-joist   If you made a torsion box with the web or frame of 1/4 inch thick plywood, 6 inches wide, and spaced them appropriately, and glued 1/4 ply to the top and bottom (meaning the top and bottom were now 6 inches apart from each other ) it would hold far more weight than two 3/4 inch plywood pieces screwed together."

In addition, the torsion box likely takes less glue to hold it together that two sheets of plywood. The simple frame between the two sheets holds the secret to its bending strength, but I have never seen a research article that adequately explains it. Somehow it always works. The Pic showing me standing on the shelf is untouched, and I assure you that there is no angle iron inside the assembly. And, BTW, the torsion box is about one-sixth the weight of two thick sheets of plywood.

[abernat]

Its bend resistance is proportional to its thickness. For the top to bend (assuming the frame doesn't buckle) the bottom needs to expand to compensate - so you're turning the vertical force into the tear resistance of the bottom skin. It's trivial to fold 1/4" plywood in half but the same sheet has an enormous tear resistance.

So - you need enough frame and enough glue to keep the frame attached, but other than that you're golden. And the thicker the frame the stronger it is.

[kurt18947]

For a less then 3' long shelf - IMO you don't necessarily need a 1-1/2" thick shelf (unless that is the look you are going for). May wish to consider a single sheet of 3/4" furniture grade ply with a wider front edge treatment which should make it stiff enough. Also, no need to edge band the front of the shelf with this construction. If looking for something that thick I like the torsion box idea - stiffer and less weight.

There is a variation of the torsion box theme that I've filed away mentally. Unfortunately I don't have numbers for strength. The idea is to simply use a 3/4" shelf then place about a 1"X 6" flat in the center front to back on the lower side of the 3/4" shelf. Now take 1/4" ply finished on one side, probably the same species as the top of the 3/4" shelf.  Glue & nail/staple each end of the 1/4" ply to the underside of the 3/4" shelf. It will look a lot like a solid piece once the front is finished with a solid piece. As I said, I don't have strength numbers but they were a lot more than I'd have expected with only a 3/4" gap between the plywood pieces. The benefit of this construction is that the total thickness would be around 1 3/4" but it's stronger than doubled 3/4". I'm guessing this would be more appropriate on longer shelves, perhaps in excess of 4' or more? The 1/4" ply would be in tension and it takes a fair bit to pull a piece of 1/4" ply apart on a straight pull.