Advice needed on building a break down torsion box frame

[John W Johnson]

I plan to build a torsion box frame that fits on top of some Krenov sawhorses I built a few hears back. Due to shop size I need something that I can break down and store when not in use. The rough size will be about 27" x 66" and I'll probably use a similar size piece of 1/2" plywood as the top skin. If it means anything I plan to make the pieces approximately 8" OC both axes.

My first question is this, if I use 3/4" plywood will 4" wide pieces be sufficiently strong. And secondly, should the notches on the long pieces and cross pieces be of equal depth? Or should I make the notches on the long pieces say, 1 1/2" deep with 2 1/2" notches on the shorter cross pieces? I cannot imagine I'd put anything heavier than a 20" x 36" x 48" carcase on this frame.

[Art Mann]

Let me describe the torsion box I built as a shelf in a storage room and you can judge from that. The shelf spans 8 feet and is 24 inches wide. The top and bottom skins were made of 3/8" nominal B/C grade SYP plywood. The interior was constructed of 3/4" X 1 1/2" (1X2) pine strips forming an 8" by 8" grid. The overall thickness of the shelf was 2 1/4". After completing the project and allowing the glue to thoroughly cure, I tested the strength of the shelf by supporting it on the ends with short pieces of 2 by 4 and applying a 300 pound dead load (me) in the center. I measured a deflection or sag of about 1/8". In other words, your current design is a huge overkill and will be much heavier than it needs to be to support a mere cabinet carcass. You should be able to easily carry a torsion box that meets your requirements for strength under one arm.

Now, as for the material used for the interior grid, the thickness is not very important. I used 1X2s because I had a bunch of them on hand. If I were buying the material, I would have used 1/2" MDF instead because it is cheaper and easier to work with. It is the height of the grid, rather than the thickness or strength of the construction material that is important. The interior material mostly acts as a spacer to hold the 2 skins apart and keep them in tension/compression when a load is applied.

You don't have to construct the internal grid as long pieces with notches cut in them at all. I just cut the long pieces on my torsion box to the correct length and cut little 8" long pieces as the cross pieces and attached them to the long pieces with a brad nailer. I did this because I had a bunch of short pieces of 1X2" material that I wanted to make use of. It may be easier to assemble if you cut notches in longer pieces as you have proposed but notch depth or snugness of fit is not important.

Now the important part. In order for the torsion box concept to work, you must securely glue the edges of the grid to the skins at all points of contact. I probably used 200 or more brads to hold the grid tightly to the skins until the glue dried.

A torsion box is a remarkable piece of engineering, the principles of which are used in the construction of the wings of jet fighters and airliners. I think this design is one of the most under used techniques in woodworking. It allows one to construct a surface that is very flat and is almost as strong as the same surface constructed of solid material.

Edit: That last statement may be a little bit of an exaggeration.

[Jim Matthews]

Have you seen the design used by Joshua Finn?

It uses two torsion box beams, with soft homasote insulation on top.
They can be any length you like, and you could screw a sheet of plywood across the span for a larger surface.

http://www.woodfever.net/2012/12/a-josh-finn-workbench-update.html

Torsion boxes require all four exterior "skins" to be glued to a central framework, to resist twisting as mentioned by Art, above.

I think you need at least twice the thickness of a skin layer in the latticework for it to work best.
(But that's a SWAG)

[Richard Coers]

I don't understand the question or idea of a break down torsion box. If you want to disassemble the torsion box, then it's not a torsion box. It's a frame with a piece of plywood on top. Your shop is tight if you can't lean something 27x66 somewhere! As mentioned, the torsion box gives you great strength from gluing thin frame pieces to thin skins. A couple of 2x4s will do what you want to do.

[Art Mann]

I don't understand the question or idea of a break down torsion box. If you want to disassemble the torsion box, then it's not a torsion box.

I was under the assumption that the original poster meant that he could use two sawhorses and the torsion box to construct a temporary table and then stow the torsion box and saw horses separately when he is done. If he is talking about disassembling the torsion box then I totally agree with you. All the strength of a torsion box comes from its permanently interlocked skins and grid.

[John W Johnson]

Sorry guys, I meant to say I want to build a temporary grid whose long pieces will fit over the tops of my sawhorses. I was thinking torsion box when I only wanted a grid locked frame to hold a sheet of ply. I know better, but in my haste posted a misleading title and description. What I was thinking about would not need to be as strong as a torsion box. I had read something a month or so ago that had photos of what I was trying to describe. In the end I found the forum with the information I was looking for.

But Art Mann's post and Jim Matthews link have me thinking in terms of a fairly thin, lightweight torsion box instead. I have built three torsion box table tops in my shop, but they are all 4-5" thick, with mdf cores and skins, heavy as lead, and certainly overkill after reading Art's post. In truth building something 2" thick with wood grids should be light enough, would not take up that much more room, and would be much more stable for glue ups. If I have enough thin ply I'll probably take that route. Thanks to all, and sorry for the confusion.

[Judson Green]

One thing that's very very extremely important and goes without say really BUT it happened to this guy...

Make sure whatever your building your torsion box on is FLAT.

Otherwise you're gonna have a very ridged potato chip.

[Jim Matthews]

Gotcha - I probably saw the same article you're thinking of - once apon a time.

You'll need three long spars to span the sawhorses. Whatever size top "sheet" will determine their length.
FYI - A full sheet of 1/2 or 3/8 ply is pretty heavy - I would make the top layer from two smaller sheets, for easier handling.

All you need is a series of half-lap joints to lock the pieces together, side to side.
I figure four spars and a couple well placed screws (or through bolts) through the top of the sawhorses would be sufficient.

The version I recall is about knee height.
If I can find it, I'll post the link.
(Found it here (http://www.youtube.com/watch?v=s5PDmNqdg_o).)

FWIW - I bought a pair of VIKA folding tables (http://www.youtube.com/watch?v=ImL-od-ilbc) for just this purpose - they fold flat and hold plenty.
They have two standard height settings.

Not cheap, but built like tanks.