Torsion box capacity

[Roger Los]

Is there somewhere online that has general guidelines for torsion box load bearing capacity? I need to make some 12" wide x 8' long beams that will only be supported at the ends. Light weight is the key if this is going to be viable, but it's not expected to support 8' of books, either.

From what I understand, the depth of the box is the key to its loading, and the skin and grid materials are less relevant. I haven't been able to find any real data, though.

I suppose I could just glue up some samples and see which fails first. :eek:

[Stu Ablett in Tokyo Japan]

I'm sure someone who has actual knowledge on this subject will chime in, but I just wanted to say that the variables are huge.

The strength of the materials, grade of plywood, for example, or the glue used, and such.

My eperiances with torsion boxes is limited to a few times, but every time I was just simply amazed at how strong a correctly built torsion box is.

Good luck!

[Mike Cutler]

It all depends on the material that the torsion box is made from. The modulus of elasticity, and physical/compressive load properties for each species of wood are different.

It sounds as if you are making some form of an engineered beam? If light weight, and strength are the key attributes, I would use Sitka Spruce. Fortunately for you, the area of the country that you live in should have many sources for it.

[Roger Los]

Actually, this is meant to be a balance beam for dog agility in the backyard. We like to move the equipment around so only modest patches of grass are killed at a time...and my wife does the moving. The beam would carry about 50 lbs of live weight, but I would also like to be prepared for the idiot like me who has to prove his balancing abilities (or decides he needs to sit down), which would be nigh on 250lbs. I was thinking as thin a plywood as I could get away using with perhaps red cedar edging, all throughly sealed with tough exterior paint.

If this all paints me as a rank amateur, guilty as charged! :D

[Stu Ablett in Tokyo Japan]

Roger, for that application, you could get away with fairly thin stuff for sure.

I've seen doors that are thin plywood with cardboard as the on edge stuff get used as workbenches and such, they seem mighty strong.

Cheers!

[Julio Navarro]

Wow Stu!! You quoted Michelle Malkin!

[Al Willits]

Remembering I'm new at this....
I just built the torison table that's on the web site below, its made out of 1/2" and 3/4" MDF and is 5 foot x 4 foot and I have to say that I doubt it'd flex with major amounts of weight on it.
For outside I'd think plywood would work quite well and down sizing the DIY plan to what you need should work well.

I'd think even 1/2" plywood in a grid system would be incredibly strong and hold a dog running over it with no problem.
btw, we tried ultility for a bit, but this fat old guy just can't move fast enough....have fun...:)

Al


http://www.diynet.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278181,00.html

[Jim O'Dell]

Roger, let me know what you come up with and how it works. I'll eventually have to build one for my wife to use with our Irish Setters. I've thought a little about the design, but haven't decided on anything I thought would work best. I hate to buy one, as they are pretty expensive, and difficult to transport. Keep us informed! Jim.

[Peter Mc Mahon]

This does not answer your question but...I built my own shop doors each one is 4x8'. The exterior is 1/2" ext. grade plywood and the interior is 1/8" masonite panel. The box work inbetween is steel studs from the borg. I framed it just like a wall on 16" centers with a top and bottom plate. The doors are incredibly stiff, and have showed no movement over the past 8 years. Torsion box is really amazing. Peter

[Julio Navarro]

Remembering I'm new at this....
I just built the torison table that's on the web site below,

What was your biggest challenge building this torsion box? It seems to me that getting the 2x4's level would be a bit challenging.

I am thinking of making one.

[Stu Ablett in Tokyo Japan]

I wonder if you could make the sides more of an "A" shape, if it would make it stronger in the middle and lighter as well?

[tod evans]

What was your biggest challenge building this torsion box? It seems to me that getting the 2x4's level would be a bit challenging.

I am thinking of making one.

julio, gluing up torsion boxes in a vacuum bag makes for good glue joints and a straight platform...02 tod

[John Stankus]

On the APA website they have a document on Design and Fabrication of Plywood Stressed-Skin Panels (aka torsion box)

It is Plywood Design Specification, Supplement #3 "Design and Fabrication of Plywood Stressed-Skin Panels (Form U813 - 28 pages)supplement #3." The pdf is free but it looks like they require registration now.

Link to publications list
http://www.apawood.org/level_c.cfm?content=pub_ply_libmain


Regards

John

stankus@ix.netcom.com

[Brian Evans]

What was your biggest challenge building this torsion box? It seems to me that getting the 2x4's level would be a bit challenging.

I am thinking of making one.

That DIY guide is really good for a table if that's what your intention is.
Here is another. The first part talks about the torsion box.
this is what I followed for my mobile cabinet base.
http://www.bt3central.com/articles/layoutpage.asp?ArticleId=83

[Roger Los]

Thanks for the link to the guide, John! I'll check it out.

[Jim O'Dell]

I wonder if you could make the sides more of an "A" shape, if it would make it stronger in the middle and lighter as well?

Stu, that's the way the commercial units are made. Usually steel with cross bars every 8 to 12". the base of the "A" is in the middle where the pivot point is. Hey, just a thought. How is a teeter-totter made? Same principle. Or what about using some strong type of wood 2 X 3's for the upper and lower section, then cut a piece of 3/4" plywood for the webbing glued into a daddo. Like a wooden I-beam, just narrower on the ends? That should be plenty strong, one on each side, then 5/8" plywood across the top for the running surface. One thing I'm thinking about when I build mine is to have the textured sprayon bedliner on the running surface. (But then I know some guys that do the liners and can probably get a better price than most.) It should help it be more skid proof, especially since they don't want the wooden "rungs" on the surface anymore. Just some thoughts. Jim.

[Cliff Rohrabacher]

After buzzing 6through this threat I see one thing I think hasn't been touched on.
You plan to put directly it on the ground. Water in all forms is going to be a permanent issue.

I submit that a penetrating epoxy such as those Abatron makes used for restoring old rotted wood might go a very long way to sealing the moisture out of the wood.
They have the stuff in small volumes.
They may also have two part uncatalized urethanes. They are super and penetrate supremely well.

Both epoxies and uncatalized urethanes will penetrate and catalize in and around the wood fibers at the molecular level creating a bond that nothing else match.

Another good source is Epoxy Systems but I don't think they don't sell as small a volume package as Abatron.

[Al Willits]

""""""""""
Originally Posted by Julio Navarro
What was your biggest challenge building this torsion box? It seems to me that getting the 2x4's level would be a bit challenging.

I am thinking of making one.
"""""""""""""""""



The leveling was a bit rough, the first saw horses I used developed a slight bow to them when 2 sheets of 1/2" MDF, and the 2x6's I used to level were all loaded on them, so I have to make heavier saw horses, then I spent probably a hour plus getting everything level after that.
I also laid a sheet of 3/4" MDF down as a base first, and that seemed to help.

After that it went fairly well untill I had to brad nail the short cross pieces in, the design shows 5 or 6 pieces that run end to end and shorter pieces installed by gluing and nailing running at a 90 degree to the long pieces.

Trying to toe nail some of the smaller pieces in, the boards would move slightly.
If I were to do it again, I wouldn't use short pieces for the one direction, I'd notch all boards to interlock and glue and nail them that way.
Have to try it on a smaller version and see how it works though.

I made mine 4 foot by 5 foot (same as the one on DIY) and it came out pretty decent and more importantly, level.
I used some tung oil and tupentine to finish the table because I'm on a tung oil kick, and then waxed both sides with Johnson's paste wax, seems glue doesn't stick to wax very well.

Word of caution, I downloaded the plans off the DIY website and I'd make sure I read them a couple of times (remember I'm getting old...) he likes to throw them helpful hits in after you've done whatever step your on...
Like telling you to wax the 1/2" MDF after you've got the darn stuff leveled and ready to start gluing.

I'd also make sure you have help or long strong arms, this thing starts to get heavy and akward the closer to done you get.

I'm attaching some recessed eye bolts to hang this thing out of the way when I'm not using it, if you have pleanty of space should be no problem though.
I figure if I can do it, any of you guys should have no problem.

Hope this helps.

Al

[Charlie Plesums]

The idea of the torsion box is that the top is in compression, and the bottom is in tension. The farther they are apart, the stronger - if I remember 40+ years back, it is the 4th power of the distance apart (twice as far apart is 16 times as strong). You need enough grid to keep the top and bottom the same distance apart, acting like a single beam, so the bottom (being stretched) doesn't take a shortcut, and so the top (compressed) doesn't buckle. So that it doesn't act like two thin boards that happen to be a ways apart, the best bet is a fine grid of smaller boards, rather than just a few large boards, thoroughly glued into a single unit.

I have made several "Murphy" desks that fold up... originally I used 1/4 inch plywood on top and bottom, total 2 inches thick, and I bet that would have held my weight, but had to go to a thicker top when something heavy fell between the grid and punched a hole in the 1/4 inch plywood. Barrint the need to handle the point blow, I bet the overall strength of 2 layers of 1/4 inch plywood (or even masonite), held a couple inches apart, would be adequate.

[Roger Los]

Charlie, that's what my seat-of-the-pants thought was. I also appreciate the expoxy advice, though I've also considered fiberglassing the entire thing. I think I'll make up a sample and do some real world testing.

[Frank Chaffee]

Roger,
A hollow core bi-fold door section would serve you well here as it is a consummate torsion box design. I have used them in 6’-8” lengths as bookshelves, and when supported along the backside they have remained level.

Likely you will need to reduce the width of what you will be able to find (16”?),so you could rip the door and evacuate the core spacing with a router to the depth required to for a jointed and planed edge, which you could glue in.

Frank

[Daniel Fisher]

FWIW, I think that a torsion box is overkill. The dog walk that we have (made by a friend) is just a 1 by with 2 battens underneath. A torsion box would be stronger, but it might not be any lighter. Also, if your dog is going to compete, so far as I know, no competition walks use torsion box construction. Some do use aluminum. If your dog is used to the feel of a torsion box, the difference might be a problem.

Have you done a google search for construction plans? It's been a while since I looked, but I recall several sites had plans.