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Todd Burch
01-14-2004, 10:47 PM
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
01-15-2004, 12:03 AM
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
01-15-2004, 7:22 AM
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
01-15-2004, 7:36 AM
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 :rolleyes: , 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
01-15-2004, 8:21 AM
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
01-15-2004, 9:04 AM
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
01-15-2004, 9:20 AM
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
01-15-2004, 9:29 AM
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
01-15-2004, 9:44 AM
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/index.html?story_id=105159&big=1051593

Its during the day so the light is transmitting
Mark

John Scarpa
01-15-2004, 12:21 PM
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
01-15-2004, 1:58 PM
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
01-15-2004, 3:56 PM
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
01-15-2004, 6:41 PM
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
01-15-2004, 7:34 PM
http://members.roadfly.com/agent99/bench.jpg

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/catalog/groups.asp?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
01-15-2004, 8:31 PM
Glenn,
The coffee, we both have that too!
Mark

Charles McKinley
01-15-2004, 8:41 PM
Hi Todd

Great thread, one of the current ww magazines has a table top for cutting sheet goods using this technique. Sorry I can't remember the magazine. I'll look the next time I'm out and post it if I find it.

Mark Singer
01-15-2004, 9:28 PM
The torsion box acts very much like a steel "I" beam does in construction. The large section modulus is what creates the strength. The large flanges seperated by a web is a strong section. The web only needs to be strong enough to keep the flanges apart.When bending occurs from load the bottom flange is in tension and the upper flange is in compression. There is actually no bending stess along the neutral axis. A torsion box is like a series of "I" beams in both directions. The deeper the section the stronger. The thicker the skins..the stronger. The web or honeycomb needs to be strong enough not to buckle from the forces transfer from the flanges.
Like "I" beams torsion boxes will be most effective if the elements are properly proportioned to achive maximium strength with a minium of weight.

Robert Ducharme
01-18-2004, 1:21 AM
I am building a torsion assembly table (based on David Marks description) and not real happy about it yet. My problem is the grid is not acceptably "flat". Even trying to make a flat reference was difficult because I did not have anything large enough to do the work. Saw horses did not work and I finally used scaffolding. It probably sagged as the wood weight was added. Maybe I should have used the floor as a starting reference!

I have the bottom skin on the grid and am considering taking it to a shop with a large sander to make the top and bottom parallel. An alternative is to keep the structure as is and put a 4' x 5' x 3/4" marble slab on top with the gaps filled with silicon. That probaby would not warp but not certain it is worth the expense.

Any other ideas how to get the beast totally flat at this point?

Todd Burch
01-18-2004, 9:18 AM
Three quick ideas.

First, for the final glue up, cut a couple 2" x 12"s, and make a 1/2 lap big "X" on the floor (flat as you can find). Put the ends of the "X" at the corners of the box. Drill a big hole near the ends of each leg of the "X" to insert a clamp head into to hold the box down as the glue for the top skin dries. You'll have to level the "X" good too, even when on the floor. Position the bottom skin/grid so that the corners can be pulled down. You can start with an oversized top skin, that way you can trim to fit after the glue dries and alignment isn't such a pain while the glue is wet.

Second, there are some floor-leveling compounds on the market, that find their own level you might could use. Build a temporary fence around the top so the fluid doesn't run over the edge.

Third, just glue it up, and instead of marble (pretty costly), just use another skin of 3/4" ply or MDF, and wedge it underneath where it needs it to bring it flat.

I've never done any of these personally.

Here's some drawings describing what I mean visually. (I'm a visual kinda guy)

Todd

Dave Avery
01-18-2004, 10:08 AM
http://members.roadfly.com/agent99/bench.jpg

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/catalog/groups.asp?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.


Chris,

Is that folded LAUNDRY on your drill press table!!!! :)

Glenn Clabo
01-18-2004, 10:29 AM
I'm sure it's just towels for wiping the sweat off his brow...but what would you use the Clorox for?

Chris Padilla
01-19-2004, 12:45 PM
Chris,

Is that folded LAUNDRY on your drill press table!!!! :)

Man, I have gotten more PMs about that laundry on my DP table!!

Those are my shop/garage towels and they come in handy for many, many things. I found a deal somewhere on the web for cheap towels and picked up a bunch. My wife had just washed a bunch for me and piled them there.

Man, I gotta be careful about what shows in future pictures I guess! :D

Robert Ducharme
01-19-2004, 7:54 PM
I got my torsion box straightened out via a large sander.

After having put this together, I realized that there may be an easier construction technique than what I used (David Marks Instructions). These instructions say to cut long stringers and then short separators to keep the stringers apart. This resulted in 9 - 5' boards and 90 - 4.5 inch boards. Gluing and nailing was a pain.

Instead, what if the 9 long stringers were notched half way through the board where every cross piece goes (9 notches per board) and the cross pieces were cut to about 42.5 inches and also notched where the long boards would cross. This would allow them to be assembled very easily with minimal need to nail and no change of strength or stability.

Does that make sense or am I wrong someplace.

I attempted a simple drawing (very rough) showing a long stringer and one cross piece.

Todd Burch
01-19-2004, 8:03 PM
Perfect sense. That's how I usually make them. They stay more square too, and are much faster to assemble.

Good idea! Todd.

Tom Scott
01-20-2004, 1:27 AM
The torsion box acts very much like a steel "I" beam does in construction. The large section modulus is what creates the strength. The large flanges seperated by a web is a strong section. The web only needs to be strong enough to keep the flanges apart.When bending occurs from load the bottom flange is in tension and the upper flange is in compression. There is actually no bending stess along the neutral axis. A torsion box is like a series of "I" beams in both directions. The deeper the section the stronger. The thicker the skins..the stronger. The web or honeycomb needs to be strong enough not to buckle from the forces transfer from the flanges.
Like "I" beams torsion boxes will be most effective if the elements are properly proportioned to achive maximium strength with a minium of weight.

Hey Mark,
That's a pretty good explanation of the principals behind a wide flange beam...for an architect (sorry, I couldn't resist).

Tom

Mark Singer
01-20-2004, 7:57 AM
Thank you! I have a degree in engineering also. I have not done any engineering on buildings I design for years...with lateral here in Zone 7 its pretty tough. Was my analogy of a wide flange to a torsion box correct? I checked your profile and I see your a structural engineer. It is nice to have one on SMC. I was helping another member with questions on his patio cover just the other day...no I know where to send him.
Mark

Chris Padilla
01-20-2004, 10:24 AM
Instead, what if the 9 long stringers were notched half way through the board where every cross piece goes (9 notches per board) and the cross pieces were cut to about 42.5 inches and also notched where the long boards would cross. This would allow them to be assembled very easily with minimal need to nail and no change of strength or stability.

Does that make sense or am I wrong someplace.
Robert,

That was how the torsion box for my tool stand was built as well. In fact, I had the notches so dead-on at 3/4" to match the MDF, that once I put two cross-pieces together, it was very difficult to remove them.

These are the times when you need to plan your cuts such that all the pieces are EXACTLY the same size and all the notches are in the EXACT same place on every piece.

Since the MDF fit so tightly together dry, I left it that way and just glued both skins (of more 3/4" MDF) to the grid.

This is also a plug for using MDF for a torsion box (or at least for the gridwork inside) since you can count on it being unifromly 3/4". I don't know the tolerance for MDF but it is pretty tight, me thinks. :)

Tom Scott
01-20-2004, 12:21 PM
Mark,
Yes. You're anaolgy of a wide flange beam to the torsion box was correct...put the material where you need it...on the outsides. In fact, I was going to use that as an example until you beat me to it.

I've done several buildings in CA, and one since the new Code. PITA structurally. SE's make a lot more money in CA compared to the rest of the country, but they earn it.

Tom

Todd Burch
10-05-2004, 8:58 PM
Keith, please make into an article. Todd

Bob Winkler
10-06-2004, 7:56 AM
The Workbench 2003 issue has a lot of good info on torsion "panels", which is a thinner version of a torsion "box". It's a stronger and lighter substitute for a plywood panel. The inner core is made of either 1x or 2x material.

When I made my closet panels and shelves, I used 1x4 doug fir material, pocket screwed at the joints. The joinery is not too important, but the result must be a flat core. For some reason, my Lowes has 1x4 doug fir which I could pick through to get the straightest boards. I was impressed how inexpensive they were.

As others have said, the glued connection to the skins is where the strength comes from. Even 1/4" plywood is wicked strong in tension.

The overall assembly totaled 1.25", which I think looked great, especiallly after edging the panels with solid cherry.

It was this project that I tried the Burgess Edge system (based on Todd's comments) to apply the hardwood edge, which I ultimately stopped using and returned because I couldn't keep the edge consistent on my router table. I'm not saying that it doesn't work for others, but I thought it was too much trouble for me.

In summary, the torsion panels were a little more time consuming to make, but I was happy with the end result.

Bob

Keith Starosta
10-06-2004, 11:19 AM
Keith, please make into an article. Todd


?? Make what into an article? :confused:

Keith

Todd Burch
10-06-2004, 12:22 PM
Keith Outten.

Chris Padilla
10-06-2004, 1:18 PM
Hey Bob,

Interesting to hear about your experience with the Burgess Edge...I love my bits! The key is to use featherboards to get consistent cuts...this is true for any joint cut on a router table. One little bobble can cost you a lot of time, money, and grief.

However, when I use the BE bits, I normally free-hand it on the router table (only way to do it for curves)...you just have to keep your hand pressed down tighly at the cutting point on the plywood. Yes, I've also chewed my fair number of plywood edges with inconsistent cuts using the BE system. I guess it just takes a few time to learn correctly.

Bob Winkler
10-06-2004, 1:40 PM
[QUOTE=Chris Padilla]Hey Bob,

Interesting to hear about your experience with the Burgess Edge...I love my bits! The key is to use featherboards to get consistent cuts...this is true for any joint cut on a router table. One little bobble can cost you a lot of time, money, and grief.

Chris,

I eventually sent the bits back to Burgess. He gave me a portion of what I paid in refund. Last thing I needed around the shop was more tools I don't use;) .

I tried to make it work. I was using cherry plywood and loved the concept for centering solid edging. But, after removing the center section, the cherry veneer was so fragile that if I looked at it wrong, it broke off. I did spend a couple of hours trying to tune it, but in the end gave up and decided it wasn't worth it to me.

Maybe if I was a production shop and spent more time in setup, it would be a different story. But for me, it added stress and time spent away from actually working with wood.

Everyone certainly has to decide for themselves what works for them.

Take care, Bob

Chris Padilla
10-06-2004, 2:24 PM
You don't really want/need a "knife" edge for the top/bottom veneers of the plywood--you are correct, way too fragile. You should aim to set up the bits to get the full thickness of the veneer on the top/bottom of the ply. That extra thickness will last nicely until you get the edging glued it.