Torsion box question. Now progress reports.

[Jim O'Dell]

I've been hard pressed to get on here for a few days. We have a friend visiting from out of state, and my computer is in the guest bedroom!
Question: In building the tops for the multistation. posts over at SMC have me leaning toward using 1/2" MDF top and bottom instead of the 3/4" I had planned on originally. Thought is that the strength of the torsion box comes from the height of the material inside the box. What should I use for the inside? Stay with the 1/2" MDF, or use plywood? Which one would be more stable and less prone to movement?
These tops will be 14" wide by 10' long and 3" thick. They will be covered with laminate on the top and three sides, and sealed with poly or something else on the back and bottom. No tools will be mounted to them, so no requirements to hold screws or bolts.
TIA for any insight you can offer. Jim.

[Jim DeLaney]

Jim,
As long as it never gets wet, the MDF will work great for a torsion box - both for the faces, and the interior framing. However, that sucker is gonna be HEAVY! By the time you do both faces, and all the gridwork, you're going to have a lot of material in it - probably 75 or 80 pounds worth, at least.

Oh, BTW, do you have access to ten foot long MDF or ply? The skins, for rigidity, should be one solid piece, with no seams.

[Charlie Plesums]

As a demonstration, I made a torsion box out of scrap 1/4 inch plywood on top and bottom, about 4 feet long, and 4 inches wide, with internal spacers of 1/4 inch plywood about every 6-8 inches. It is extremely light, and carries my weight when I stand in the middle, with the box supported at the ends. A similar size 3/4 inch furniture plywood broke under my weight.

I bet 1/4 inch ply or masonite would be ample under laminate, even a 10 foot span since it is a relatively thick 3 inches (which is where the strength comes).

The top layer will be compressed sideways, and the bottom layer will be stretched. The ideal would be 10 foot long material, as Jim suggests. However, if you have to patch top and bottom pieces, be sure to brace the joint carefully, so the bottom layer supports the stretch factor, and the top layer cannot slide with one part above the other.

[Jim O'Dell]

No 10' material that I'm familiar with. I will stagger the seams top and bottom, and double the grid on the inside between both seams. Possibly half lap the seams an inch if I can. It won't have to sustain any appreciable weight. Just hope it will stay flat.
Thanks for the info! Jim.

[Charlie Plesums]

... I will stagger the seams top and bottom, .... Possibly half lap the seams an inch if I can. ...

Staggering the seams won't help in this case. It would improve "shear" strength, but that has nothing to do with the bending/flat issue. A patch on the inside - double the material - 6-8 inches of extra material glued over the joint - would work great. The magic of the torsion box is that the grid converts the lower layer to tension (stretching lengthwise) and the upper layer to compression, and practically removes the bending forces. If you were stretching a 14 inch wide sheet of plywood, it wouldn't have to be very thick before it could tow a truck (ignoring the connection points for that tow job). The laminate on the top will add strength, too. I would worry most about the bottom (like putting rebar closer to the bottom of a concrete slab)

...and double the grid on the inside between both seams. .... Jim.

As long as there is enough grid to keep the layers a constant distance apart, extra grid at the joint won't contribute anything.

I made 6 queen size beds (3 queen bunk beds) with 1/4 inch plywood top and bottom and a grid 1 1/2 inches thick. Since I can't buy 60 x 80 inch plywood, I had joints, which I just centered on the grid. The only risk is someone stepping on the plywood between the grid, and breaking through the 1/4 inch, so nobody is allowed on the bed until the mattress is in place.

I made a queen size handicap bed and put 1/2 inch plywood on the top to remove any restriction, but the strength is primarily from the 1/4 inch plywood on the bottom. Since the 1/2 inch plywood distributes the weight more evenly, I used far less wood in the grid.

[Jim O'Dell]

snip A patch on the inside - double the material - 6-8 inches of extra material glued over the joint - would work great. The magic of the torsion box is that the grid converts the lower layer to tension (stretching lengthwise) and the upper layer to compression, and practically removes the bending forces.
As long as there is enough grid to keep the layers a constant distance apart, extra grid at the joint won't contribute anything.
snip

Charlie, I had thought about doing that, but then read where the height of the grid added to the strength. I can easliy put in a second layer of 1/2" MDF at these points. I'll go ahead and stagger the joints so that I don't have these patches on top of each other, thus reducing the grid across them to 1". Thanks for the info!! Jim.

[Charlie Plesums]

Charlie, I had thought about doing that, but then read where the height of the grid added to the strength. ....

It is the distance that the upper and lower surfaces are apart that determines the bending strength. Most of the time, if you make the grid taller, that puts the outer surfaces farther apart, but if you have to shorten the grid in one area because the surface is thicker at that point, no problem. Like the center of an I-beam, the purpose of the grid is to keep the upper and lower part a constant distance apart, so you could make part solid if you wanted... it would lose the "light-weight" and "less material" advantages, but it wouldn't reduce the strength.

[Bart Leetch]

Jim as I remember my inside material was cut either 3" or 3 1/2" 3/4" plywood & wrapped in 3/4"plywood on the outside edges & 1/2" plywood skins. & was about 24" wide & just over 7' long. I put over 400 lbs in the center as a test & it just laughed it off.

The cabinet in the last shot is what sets on top of the T-box.

[Jim O'Dell]

Thanks for the update Charlie. Bart. Great pictures. Looks like you did the half lap joints instead of the individual pieces that David Marks and Marc Spagnuola show. That's how I did the grid in the doors I built for the shop, but my outside edges were 2X material so I could sink lags in for the hinges. I think I used 3/8" plywood for the skins and the grid.
OK, so I'll go get some 1/2" MDF after work tomorrow. Was going to do it today, but want to see if HF has the 12" wood screw clamps in stock, and they aren't answering the phone, so most likely are closed, as they should be for a holiday. I'll make it all in one trip to save gas. I want about 4 of them to help with some of the assembly. Plus I've wanted some for a while now. Thanks again guys! Jim.

[Bart Leetch]

[QUOTE=Jim O'Dell;47556]Thanks for the update Charlie. Bart. Great pictures. Looks like you did the half lap joints instead of the individual pieces that David Marks and Marc Spagnuola show.

1/2 lap joints took a bit longer to make but I think assembly is quicker & I think its stronger.

When I get into a bigger shop I want to make some T-boxes that can be set up as assembly platforms.