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Torsion Benchtop Longer than my Sheet Stock

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Forum topic by skydiver posted 12-04-2016 07:27 PM 438 views 0 times favorited 6 replies Add to Favorites Watch
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skydiver

15 posts in 2044 days


12-04-2016 07:27 PM

Topic tags/keywords: torsion box design

I am building a couple of drawer modules that will be separated by a rolling tool chest on a wall in my garage. I am planning on building a torsion bench-top for this work area. I am planning on building everything out of 3/4” birch plywood. The torsion box needs to be 3” tall (before framing out the box to glue down 3/4 plywood and a replaceable 1/4” melamine work surface). The width is going to be 24” deep and the length will be 11’ 4”.

The question I have is what design of the torsion box will offer the best way to have a solid, flat surface the entire length when I only have access to 4×8’ plywood stock? I would normal plan on the longer length be built out of single pieces spaced about 12” apart with spans spread between the runs staggered at another 12” apart.

I don’t know what the best way would be to break the almost 12’ run long lengths to ensure that the surface will be consistent across the whole length.


6 replies so far

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ArtMann

688 posts in 655 days


#1 posted 12-04-2016 11:25 PM

One essential element of the design of a true torsion box is that the top and bottom skins (not the Melamine work surface) must be constructed of a material that won’t stretch and isn’t pieced or spliced. That is more important than the thickness of the skins. Any other design will compromise the stiffness and strength of the assembly. I suggest you look for a source of 12 foot long plywood. It can be special ordered and you only need one sheet. If 3/4 inch is too expensive, then use 1/2 inch.

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Roy Turbett

137 posts in 3419 days


#2 posted 12-06-2016 12:41 AM

You can buy 12’ x 25” lengths of particleboard for countertops that should work well on a torsion box. My supplier in Lansing, MI has them in stock.

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skydiver

15 posts in 2044 days


#3 posted 12-06-2016 02:20 AM

thanks for the feedback. I think that I am going to try some 4×10 OSB and reduce the length a bit. Do you think that top and bottom skins are required? I am thinking of 1/4 plywood in two sheets on the bottom a a single piece for the top.

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clin

751 posts in 835 days


#4 posted 12-06-2016 05:46 AM

As mentioned, you need a continuous surface skin for a torsion box to work. The internal webs do NOT need to be continuous or even connected together. Theirs job is just to hold the skins a fixed distance part.

Now, this does not mean the skins have to be one piece. You could make the skins from two, thinner layers of material. In your case you could use two layers of 3/8” plywood to form a net 3/4”. You could do this in an 8’ section and a 3’ section. Then put the other layer on, but of course offset the joints. Glue the layers together, probably some screws to pull the top layer flat while the glue dries, I think you’d be all set.

While laminating two layers is more work, I think it’s a perfectly acceptable alternative if you can’t get longer material easily. Also, I would just put one layer on the box, than attach the other. No need to laminate them before making the box.

Bottom skins are required, else it’s not a torsion box. Most of the strength comes from the material on the outer edge (top and bottom), with the inner web just holding the outside in place.

If you place a weight on the table, the top layer will have compression forces, and the bottom layer tension. If you didn’t have the bottom layer, the top will just bend like a 3/4” piece of plywood. Add the bottom, and the whole thing will now act more or less like a 3” think piece of plywood (or however thick you make the box).

Again, the inner webs only hold the top and bottom a constant distance apart. These webs do not need to be continuous. They do not need to be nailed, glued or screwed or have ANY joint at all where they cross each other. The only reason to attach the web pieces together is to hold them in position while attaching the skins.

The critical joint is the connection of the web to the skin. The torsion box fails if the skins are not firmly attached to the webs. In this case, glue is fine for this, just be generous with the glue. Nothing hurt by lots of glue squeeze out and drips inside the torsion box. As with most glue joints, nails or screws are only needed to hold things together until the glue sets.

In your case, I’d consider doing the skins in two layers of different materials. I’d think about using MDF for the first or inside layer. The reason for this is MDF comes very flat. Then consider using plywood for the outer layer since plywood will hold up better to impacts from hammers and such. I realize you are planning on a separate wear layer on the very outside.

-- Clin

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ArtMann

688 posts in 655 days


#5 posted 12-06-2016 04:41 PM

I like Clin’s solution of using two thinner layers. I just want to emphasize that they need to be glued together over the whole surface.


thanks for the feedback. I think that I am going to try some 4×10 OSB and reduce the length a bit. Do you think that top and bottom skins are required? I am thinking of 1/4 plywood in two sheets on the bottom a a single piece for the top.

- skydiver


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skydiver

15 posts in 2044 days


#6 posted 12-06-2016 10:13 PM

Great input. Thanks for the excellent feedback. I will post pictures under my projects when I am done. I will need t update my Sketchup diagram and then add the new pieces to my Cutlist Program to calculate the needed lumber.

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