Torsion box question. Now progress reports.

Jim O'Dell

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Between Aledo and Fort Worth, TX
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.
 
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[size=+1]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.[/size]
 
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.
 
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.
 
... 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.
 
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, 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.
 
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.
 

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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.
 
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.
 
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.

I think my next version will be for my outfeed table. Just trying to decide if I'm going to incorporate the router table into it or not. If I do, then I may make it as a more traditional woodworking bench instead of the T-box, with vices and dog holes, and have a place on one end for the router table. Jim.
 
I'm no expert on T-Boxes, but I stayed at Holiday Express the other day......

So, head over to your local bookstore and pick up the June 2007 issue of WoodWorker''s Journal. The main article is all about T-boxes. Should answer any questions you have. I still need to find time to sit down and read the article.

Hmmm.... might be time to spend the night at Holiday Express again.

Matt is right, there is a good article in there on building T-boxes.
The August issue of WWJ just came out. If you can't find the June one, check a local WW store. I know the Rockler near me always has stacks of the previous month's issues sitting around.
 
jim, given your measurements and strength requirements i`d suggest webbing of 1/2" baltic with two layers of 1/4" baltic glued together for the top-n-bottom, effectively giving you 1/2" skins.....i have a 15" wide x 10` long walkboard made in this manner 2-1/2" thick that will support two grown men and tools...
 
Thanks for the suggestion, Todd. Didn't get on to FWF yesterday to see your note. I went ahead and bought 2 sheets of 1/2" MDF to try. Under 35 bucks, so if it goes south, I haven't lost too much. And it should work great for the laminate to adhere to. Also got 4 of the HF 12" [FONT=ARIAL, HELVETICA]Hand Screw [/FONT]wood clamps. They aren't [FONT=ARIAL, HELVETICA]Jorgensen[/FONT]s, but for 5 smackers each, they aren't bad. wood clamp.gif
I'd go out and start ripping MDF, but LOML and guest are going to be back in an hour, and guest is taking us out to eat at our favorite Mexican food restaurant...Uncle Julio's. I can taste the shrimp now!!! Jim.
 
new question 7/6

OK, after waddling back home after supper, and letting the stomach bulge calm down a little, I've plotted out the cuts on graph paper. Need to try a cutlist program, but alas, I still don't have access to my computer (guest bedroom) yet. So I can get everything out of the 2 sheets of MDF with the exception of the 2 stiffening plates over the joints on the top and bottom pieces. Instead of purchasing more MDF, I'm thnking about using some of my left over 1/2" OSB. Any problems with doing that? TIA!! Jim.
 
OSB is bad:bang: (sorry Steve)

I made a workbench out of two layers laminated together... figured I would use MDF for the top, for smoothness, and plywood for the bottom for strength. The workbench is plenty strong, but the different materials have different response to the humidity, so sometimes my bench is concave, sometimes convex. Therefore I will never again use dissimilar wood products laminated face-to-face.

On the other hand, the webbing is only to keep the top and bottom layer a constant distance apart, so I wouldn't hesitate to use OSB for part of the webbing, hopefully saving enough MDF for the splice. When I make beds with a torsion box foundation, the outside pieces and pieces where I will have a joint in the faces are good material, but the rest of the web is strips of plywood (finally, a use for that scrap plywood :D )
 
OSB is bad:bang: (sorry Steve)

I made a workbench out of two layers laminated together... figured I would use MDF for the top, for smoothness, and plywood for the bottom for strength. The workbench is plenty strong, but the different materials have different response to the humidity, so sometimes my bench is concave, sometimes convex. Therefore I will never again use dissimilar wood products laminated face-to-face.

On the other hand, the webbing is only to keep the top and bottom layer a constant distance apart, so I wouldn't hesitate to use OSB for part of the webbing, hopefully saving enough MDF for the splice. When I make beds with a torsion box foundation, the outside pieces and pieces where I will have a joint in the faces are good material, but the rest of the web is strips of plywood (finally, a use for that scrap plywood :D )

Charlie, good information. That would still keep me from having to buy more wood. In fact, I might do all the webbing out of OSB except for the outer edges. Save the MDF for something more useful, and get rid of the pile of OSB scraps standing up in the corner. I can use the MDF part for the front of the built in fence. I will still need some 5/8 or 3/4" for the top so that the t-track material won't sit above the surface. Now to go back and see if I will have enough of the MDF to do that without having to buy more. If not, I'll wait and do the front and top of the built in fence all out of one 3/4" sheet. Thanks again for the heads up Charlie. Jim.

PS: On second thought, carrying the dis-similar types of wood problem forward some, I think I might be better off staying with all MDF for this thing. What if I used some of the left over strips that will be 1 to 2" wide, cut them 11" long and glue/secure them across the joint so that the strips are running perpendicular to the joint? Would this be strong enough? That way, all of the wood inside is the same, and will react the same to any changes in temp/humidity. I'll be sure to glue all edges very well so that it becomes one piece. Jim.
 
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snipPS: On second thought, carrying the dis-similar types of wood problem forward some, I think I might be better off staying with all MDF for this thing. What if I used some of the left over strips that will be 1 to 2" wide, cut them 11" long and glue/secure them across the joint so that the strips are running perpendicular to the joint? Would this be strong enough? That way, all of the wood inside is the same, and will react the same to any changes in temp/humidity. I'll be sure to glue all edges very well so that it becomes one piece. Jim.

I wouldn't worry about mixed wood except when laminating dissimilar flat materials together.

The strength requirements of the grid are minimal. Many interior doors are just a VERY thin plys separated by corrugated cardboard as the grid, and how many college students use them as desks and bookshelves.

I don't know the engineering of the joints, but personally I would probably use the 2 inch boards the length of the joint, glued carefully making a continuous joint, rather than 2 inch wide boards 11 inches long crossing the joint. You are just adding strength to the end-glued joints, which are relatively weak.

Remember that the torsion box changes the primarily bending forces into a stretching force on the bottom layer, and a squeezing force on the top. Think poured concrete for a moment... cement is tremendously strong when compressed, but weak for stretching. A metal wire or bar is very strong for stretching but weak for pushing/compressing. Therefore rebar is added and needs to be at the bottom of the slab where it helps with the stretching, and does very little good at the top of the slab where the cement handles the compression. Now back to the torsion box... the top will be compressing, so almost no glue required as long as the pieces stay in alignment. The bottom will be stretched, so get as much face glue area as you can (since face glue is stronger), and be sure to include the whole joint, not just a few connectors.
 
Charlie, I can tell you've never had the pleasure of watching me glue something together.:rofl: Especially when the drips won't show. :rolleyes: Thanks for the further lesson in Torsion Box Theory 101. Hadn't thought specifically about the compression and stretching, but it makes perfect sense. Jim. (who miscounted and is going to have to get one more sheet of MDF:eek:)
 
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