Thread: How Thin Can I Make a Torsion Box?

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How Thin Can I Make a Torsion Box?

Hello everybody. Without getting into a lot of extraneous detail, I'm thinking of making a torsion box frame using 3/4" plywood (turned flat) for the core. It would be skinned by 1/4" plywood or similar material on the outside. I would use pocket screws to join the pieces of the 3/4" core together. So the total thickness would be 1 1/4" (1/4 + 3/4 + 1/4). Would this be much stiffer than the 3/4" plywood by itself?

Thanks.

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Assuming that you're looking for an overall thickness of 1-1/4, is there any reason why you don't just glue a 3/4" plywood sheet to a 1/2" plywood sheet? It would be stiffer and stronger than your torsion box idea, not to mention much easier to build. Also, unless you've got good-quality plywood, you might have better luck gluing a couple sheets of MDF together if flatness is your primary concern.

Keith
Last edited by Keith Weber; 10-17-2011 at 12:44 PM. Reason: spelling

3. The stiffness of a torsion box is directly related to the height of the webs (the "thickness" of the box). What you're doing will not give you much stiffness. You'd do better to buy some flat hollow doors and glue them side by side to get the width you want. It'd be lighter, also.

Mike

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Originally Posted by Charlie Barnes
... So the total thickness would be 1 1/4" (1/4 + 3/4 + 1/4). Would this be much stiffer than the 3/4" plywood by itself?..
Yes, a 1 1/4" torsion box will be a lot stiffer than a 3/4" sheet of plywood. Stiffness of a beam goes up with the cube of the thickness, so a 1 1/4" piece of plywood would be almost five times the stiffness of the 3/4" sheet. A torsion box is not quite as stiff as a solid panel of the same thickness, but with the layer thicknesses you cite, it will be close. So I'd guess the 1 1/4" torsion box would be 3-4 times stiffer than the 3/4" plywood.

That said, unless you really need the lighter weight from the torsion box construction, it'd be easier to make the panel solid, like the other guys have suggested.

5. "Would this be much stiffer than the 3/4" plywood by itself?"

Judge for yourself, I'm very happy with mine, it sees daily use:

- Beachside Hank

6. I would think (note: not know) that ply sitting flat would defeat the purpose and negate the effects of the torsion box. In my mind it would be better to cut 3/4" wide strips and put them on their edges so that the wood grain direction is different than the skins. If you want flat without the height of a normal torsion box (I think Mike is right about the stiffness coming from the height of the ribs inside) then glue two sheets of MDF together. Jim.
Last edited by Jim O'Dell; 10-17-2011 at 9:32 PM.

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You haven't given us enough info! What are you going to use the torsion box for? What will be its span? What weight will it need to support. Is a flat surface important? Is a solid, resilient, durable top important?

Torsion boxes are used when you need a very, light, but strong structure that spans a distance where a solid beam would be too heavy. Flatness is a just a side benefit, but only if the top skin is properly made and supported. A torsion box works on the same principle as an I-beam- it is essentially multiple I-beams. The flanges are the skins. They get their strength from their height (height of the shear web) and resistance of the skins to stretching along their plane. It is possible to make a very strong, but very light weight torsion box totally from 3/16" thick hardboard held together with glue only.

Here is one torsion box I built from 3/16" hardboard for a two-part article I wrote which was published in AWW magazine two years ago. It was 8' long, 14" wide, and 2" thick. The 3/16" thick skins won't be very resistant to point loads (hammering or clamps) due to their thickness, flexibility and web spacing, but it will support a considerable weight- like an airplane wing with its thin aluminum skins. In the photo it is loaded with over 300 lbs of bricks yet is deflecting only 1/2" at mid span.

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If weight is not a key factor you can buy sheets of 1 1/8" MDF at larger lumber yards.

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Not to waffle, but the risk with torsion box/monocoque construction is always that while the bending stiffness as Jamie says goes up with the cube of the thickness/depth of the section, you eventually reach a limit where failure ill happen due to buckling/crumpling/rippling, loss of flatness or the low ding resistance of the relatively thin skin. So care is needed to make sure that the skin is thick enough to handle the intended use.

In engineering terms there's a point beyond which the beam calculation (I = b.d3/12) ceases to predict accurately because the skin has got so thin that it'll buckle/crumple on the compression side of the bend before it reaches the maximum load the formula mistakenly predicts it can handle. There's a whole separate set of calculations based on buckling theory that are used to predict this....

10. I'm not seeing the plywood ribs being and asset. Since fiberboard egg crate is frequently used it looks like thinness, height and glue-ability are the main criteria. Light, defect free, construction lumber has been my choice. The flanges of optimal I beams are about 10 times as wide as they are thick. The load never reaches past that width. So if you think of a torsion box made of repeated I beams the rib spacing should be 10 X t. The ribs can be perforated or hour glass shaped in section if it going in a Lear Jet or space shot.

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To add to what Gary said, too many people get hung up making the webs too hefty (thick and heavy) which defeats one of the main attributes of a torsion box- light weight. While the webs need to handle some stresses of their own, their main purpose is to maintain separation of the skins. Likewise, there is no need to dado the skins to accept the web. The web needs to resist compression on the edges and needs to be bonded to the skins fairly well, however. Also, there is no need to cut the transverse web into pieces and brad nail the joints (like David Marks or the Wood Whisperer did). It is easier and quicker to do it with glued halved joints cut on a table saw with a simple indexing jig shown below. (my article shows how)

For a flat and durable torsion box that is relatively inexpensive and uses all the same material for economy and simplicity, I usually recommend everything be made from 1/2" MDF. 1/2" web is overkill, but it provides a good glue edge and can be cut from the same stock. MDF is flat, dimensionally stable, with no internal stresses.

Again, depending on intended use and desired attributes- strength, durability, resistances to point loads, weight, etc. etc. you will need to make certain tradeoffs in material and construction, and there is no simple plug-and-chug equation to help you do that.

By the way, here is a photo of the hardboard torsion box from my previous post. Glue has been applied to the edges of the web and I am about to add the second skin. If you didn't realize it already, to end up with a flat torsion box, you must start with a flat construction platform or table!! This torsion box is sitting on my adjustable height assembly table that is also a torsion box. On the far right and in the following photo you can see another method- a construction platform built with 2 X 4s that have been jointed and ripped to ensure they are flat and the same width, then mounted on saw horses attached to the the floor- the thin black and with boards are winding sticks used to test the platform for flatness. (my article shows all this stuff)

Last edited by Alan Schaffter; 10-18-2011 at 10:55 AM.

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