Torsion Box Shelf Ideas

      Concepts and examples for strong lightweight torsion-box book shelves. February 25, 2009

Can anyone shed any light on the idea of using a "core box" structure to fabricate long span wooden shelves? My specific application is: paint-grade, birch plywood shelves measuring 52" x 16", and preferably no thicker than 1" (1/2" core with 1/4" faces). Will this configuration hold up with little or no deflection under load? Any insight will be helpful.

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
Try searching using the term "torsion box." A core box is something completely different, and not what you're looking for.

From the original questioner:
Thanks for clarifying that. I'd still be curious to know any thoughts on rigidity: how much deflection can I expect to see on a 14" wide x 52" long torsion box shelf held on either side by shelf pins and loaded with books. The reading material I've found typically describes the torsion box structure as highly rigid, but I'd still like to hear of some actual experiences.

From contributor C:
We make 1" basswood lumber core for such stuff. 52" is no problem. They are not cheap but neither is doing it twice.

From the original questioner:
I'm going to go ahead and make a sample (1/2" MDF with 1/4" ply skins). I only need 12 shelves, so I think I'll go ahead and make them in-house.

From contributor E:
The torsion box shelves I've made were a bit thicker than what you're making. Using strips of wood for core and ply skins. This one was for a bronze sculpture display cabinet - 1" poplar core strips with 3/4 ply on one side 1/4" on other.



From contributor E:
This is the drawing for the shelf above - with tubing chases for wiring for puck lights.



From Gene Wengert, forum technical advisor:
The outside of a beam or shelf underload is the most critical for controlling deflection. Consider an I-beam, for example. Or consider wood trusses where there is a great deal of open space on the inside, but the outside is premium quality in all respects. This is why a torsion box works (in a few words). You might figure that if the wood was left in and it was solid through and through, it would work better but you need to consider that the interior when air weighs much less and when wood weighs much more. The extra weight detracts from the load.

Also, with solid wood, the interior, if wood, can result in warping in some cases unless the interior is made of several pieces of edge-glued pieces of wood. I have seen some cases where we use a composite (and weak) core material to hold the high quality veneer and this also provides a stiff product. The thickness of the skins is critical, as a little increase in thickness has a great impact on overall stiffness (2x thicknesses is 4x stiffness). I have seen some discussion about making one face thicker than another, but the compression and tension strengths of wood and the stiffness in tension and compression are essentially the same, so equal thicknesses are ok.

As an added comment, there was some interest years ago (1960s) when wood was plentiful in making house wall panels in this same manner as a torsion box, so that the entire panel was structurally sound (instead of the conventional 2x4 16" oc with sheathing). There was a big savings in wood without loss of performance, but wood was so cheap and tradition was so ingrained that this approach was abandoned.

From contributor A:
I know this is off topic but in the early 80's I worked in manufactured housing plant. We built stud-less walls with 2 x 8 plates and EPS foam core, drywall one face and OSB the other face. We set the perimeter framing on a vacuum table (8' x 30') and pressed the faces to the core wall .They were incredibly strong and had a very high R value. I now see firms in Japan and elsewhere doing this, we got the original idea from temporary buildings built during WWII.

From Gene Wengert, forum technical advisor:
Certainly from a wood usage and efficiency standpoint this is good. I hope we see more.

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