Torsion Boxes

[Todd Burch]

I got asked a question earlier today about a torsion box - what is it, why is it, why would you want one. So, I figured I would start a thread about them to share my limited knowledge, and hope that others would join in to elaborate on the engineering principles behind it and any other information that would be pertinent to the concept, application or execution of a torsion box. So, I'll start, and you others chime in with additional info. Thanks.

I describe a torsion box as a couple of "skins" of material with a gridword of some sorts sandwiched between the skins. They provide for a very rigid structure that is not likely to bend or sag.

Hollow core doors are made this way, with 1/8" plywood skins, a meager pine (typically) frame running the perimeter of the door, and cardboard for the inards of the door. You've probably never seen a door sag (the door itself - unless it became delaminated), so it works pretty good. Airplane wings are a similar concept with their honeycombed interiors.

Here's a simple drawing showing a typical grid-style box. I did not draw the usual framing surrounding the entire gridword, but I think you can get the idea from it. (I used a transparency fill to show the whole drawing better.)

Anyways, let's hear what you have to say about these types of boxes. Todd.

[Mark Stutz]

Thanks for starting this thread. I am designing some basic bookshelves on cabinets for the family room. They will be painted--UGHH!--but will match the millwork and crown. Since some of the shelves will be for hardbound books and some for audio equipment, I was thinking of using torsion box construction for the shelves instead of plywood with a hardwood facing. I've read about but never built a torsion box. How thin can they be? Would a total thickness of 1 inch work? 1/4" birch ply with i/2" gridwork? Thanks. I've already learned a lot from all your posts.

[Mark Patoka]

David Marks built a torsion box table for his workshop on his DIY Network show Woodworks. I hadn't heard of the torsion box until then and he went through the whole process of getting it completely level (apparently it was staying in one place) and the construction of it. Seems like a very sturdy but relatively lightweight construction technique.

[Keith Starosta]

Todd,
I did some research yesterday afternoon, and found a couple of sites that gave pretty much the same description that you gave in your post. The strength of the box apparently comes from the gluelines, which are very thin individually but add up to a lot of glue surface overall. I'm not sure how many of us have a good supply of resin-impregnated cardboard honeycomb lying around , so the thin strips forming a lattice seems to be the best alternative.

Mark,
Take a look at your standard hollow-core interior door. That can't be much more than 1.5" thick right? Would that hold the amount of weight that you need to it to? If so, I'd say you're good to go.

Keith

[Todd Burch]

My grandfather, on several occasions, would rip flat panel cheap hollow core doors in half (longways), fill the hollow edge (cardboard edge) with a strip of pine, and use them for longer shelf spans.

[Tyler Howell]

Todd,

Your grandfather sounds like quite a guy.
Vast source of info. Must have beein interesting growing up with a mentore like that

[Todd Burch]

Tyler, you CAN NOT imagine how blessed I am to have had him in my life!

Here are a couple more shots of the upper shelf that I made recently that incorporated torsion box principles. The greenish color shows the strips of wood that I used for the ribbing (anything I had within grabbing distance - walnut plywood, birch plywood and poplar). While they are not in a grid format, they get the job done to make the assembly very rigid.

This shelf was built from the inside-out, bascially. I started with the bottom 3/4" ply and the top 3/4" ply. I glued and nailed the ribbing strips to the back-side of each. Then, I assembled the box itself (added the two sides and the partition). Then, added top and bottom skins (glue and staples), then the khaki-colored end cleats. Face frame got nailed on after the shelf was in place. I used pocket screws for the face frame - only 1 screw per joint. It just had to be held togher long enough to hold its shape during transport and installation.

The last shot is an isometric view of the end of the shelf, showing that you could see all the way through the ribbing and that there was no perpendicular gridwork.

Contruction time, about 3 hours I guess - designing some as I went. Next will be about 1/2 that time, or less.

Todd.

[Jim Becker]

Great thread, Todd. I've been planning on building an assembly surface using this technique for some time now. It makes for a light-in-weight solution that is easy to store. Support would be several sizes of knock down half-lapped "X"s to provide different heights as needed for a project. But the key is the surface...stong and flat without making it impossible to move.

[Mark Singer]

Todd,
Excellent construction technique. I have used this structural concept in the buildings I have designed for years. Sometimes its called a "sandwich" panel. The ceiling/ roof of my kitchen is such a product called KalWall. It is an aluminum torsion box with fiberglass skins on top and bottom surfaces. It transmits light and there is a center core of thermal insulation . It is strong enough that you can walk on it! I do get up there once in a while. To the center beam it spans 11'-6" (23'total width) 37' long.
Here is a link to an article that shows the ceiling and the room:

http://www.trendsarticles.com/story/...59&big=1051593

Its during the day so the light is transmitting
Mark

[John Scarpa]

Todd,
Starting this thread (with illustrations no less) in the design forum was an excellent idea. This kind of information just keeps making SMC a better resource.

Your further description on the details of the shelf construction that showed how you fab'd assembled and hung the unit is knowledge that many of us can put to use in our projects.

Hey your Grandfather was ingenious. from hollow core door to a long span set of shelves in much less time and money than many other solutions. Keep em coming

In my business, in addition to aircraft wing assemblies you will also find the concept in rotary wings (helicopter blades) and some aircraft cargo decks etc. An unbieveably lightweight, strong design and construction technique.

Mark,
Wow I had no idea that material even existed. besides providing the structural span and light, it looks great. All of these unique uses of this concept makes me wonder where the original design idea originated.

John

[Keith Starosta]

Todd,
Did you use a french cleat, sliding the shelf onto its locking cleat, or were they just as you drew, butted up to each other?

Keith

[Todd Burch]

Keith, they are merely butted up to each other. With walls not being flat or square, designing something with a more complicated joint would have been futile and taken way too long.

After the shelf was slid into place, I put about 5 nails each side, from the interior of the cabinet, into the cleat mounted on the wall.

Also, the shelf fit loosely (planned looseness) side-to-side, a little over 1/4" play, because I didn't want anything to bind on the way in. I used shims in front on both sides, wedged in place, and then nailed through to hold them there. Plus, with these type of "nooks", the narrowest measure is usally right at the front corners, as they is where the corner beads are for sheetrock and all the mud used for floating. I dinged up the drywall corners a bit from the tight fitting back panels, but I knew the painter was coming in right behond me.

The face frames for the shelf and cabinets were applied after the casework was fixed in place.

Todd.

[Glenn Clabo]

Hey Mark...we have two things in common in our kitchens...the faucet and the dishwasher. After that...well we do own a little CapeCod in little Rhody.

[Chris Padilla]

This is a picture of my bench built directly from the plans that showed up in American Woodworker from 2000, I think. This bench is on the front cover and has a red formica top...you can't miss it.

The bottom where the casters attach is a torsion box made from 3/4" MDF. Within the squares that reside directly above the casters, I put a bunch of 3/4" plywood material so that I would have something solid to secure the casters to. Being made of MDF, the torsion box was solid and heavy.

If one is careful cutting the grid work and criss-cross joints, you should end up with a decently flat torsion box. However, the torsion box will only be as flat as the surface you glue it up on.

In the article to build the workbench, they recommend you build the carcass very carefully first and then build the torsion box using the carcass as a base to get it as flat as possible. I used Gorilla Glue for the torsion box innard glue and when I ran out of that, finished up with yellow.

I have a nice Starrett steel straight-edge (flat within 0.0002" per foot) and it looked great from all angles. BTW, if you don't have a precision straight-edge, I'd highly recommend you get one (http://catalog.starrett.com/catalog/...sp?GroupID=396). Starrett makes the best.

You know how when you have a nice flat board and you gently drop in on another flat surface that you get that whoosh of air and the nice low-frequency "thud" sound when it falls. You know that it is so flat and perfect that it squeezes all the air out as it meets the surface and it can even be hard to pick back up. That is how my torsion box turned out. Pretty cool...heavy as hell due to MDF...but pretty cool!

Clamping it up they recommend that you pile as much crap on top of it as possible while it dries. I piled all my sheets of MDF plus about 8-10 concrete cinder blocks. Cinder blocks come quite in handy for these type of glue-ups.

For those of you considering this kind of construction, I'd say it is a bit of effort to get it going right. Someone posted about a 1" thick shelf. Why not laminate two sheets of 1/2" ply together? The reason plywood is quite strong is due to the glues and laminations. 1" plywood is very strong but it all depends on the support spacing and the breadth of support. Still, some 1/4" ply for the torsion section and 1/4" skins would work quite well, too.

I recently built a floating TV shelf for a ~150 lb. TV out of 3 sheets of 3/4" laminated plywood. My favorite glue for this kind of operation is Gorilla Glue. It has yet to let me down.

[Mark Singer]

Glenn,
The coffee, we both have that too!
Mark