Trusses for a torsion table

[Cliff Rohrabacher]

I wanted a big torsion table that I could use for steam bending as well as assembly.
I made these trusses and notched them the table is built as of today but I haven't got pix of it yet.

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I wanted these notched cause I planned the have the internal pieced dictate the parallel and flat plane. It worked just fine.

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[Joe Mioux]

You know, that is a great idea. what did you joint?

what about cross-bracing?

Joe

[Jim Becker]

Geepers...you could build a small house with those little I-joists!

[Cliff Rohrabacher]

You know, that is a great idea. what did you joint?

what about cross-bracing?

Joe

Well, I notched 'em so they can go together in a cartiesan X-Y pattern. That way there NO short little pieces because each individual component spans the width or length of the run it describes.
This way when I glue and screw the first skin on that skin pulls 'em all up to it nice and tight. The structural members all straight and the same height having been run through the J/P. So the net effect on attaching the first skin is: dead flat and true. the second skin m takes the rest of the wobble out of it by transferring all the stress to the beams.

The weakness introduced by the notches is eliminated by the 1/2" birch ply skin that is screwed and glued to all the members.

It's a bit of work but if you haven't got a reference surface and you really want a beast of a torsion box it seems a fine approach. This torsion box is not light weight.

I have attached 1.5" diameter steel tubing handles to it on three sides and detachable 6" square legs.
I figure that when I don't want it consuming all 4 * 8 square feet I'll stand it on edge somewhere with the legs stacked away.

[Chris Fierro]

Very nice. I am thinking of making some similar trusses for a different project. What type of glue did you use to adhere the OSB into the dados? I was going to use Titebond I, but I have read that you need to use construction paste or polyester glues.

[Cliff Rohrabacher]

Very nice. I am thinking of making some similar trusses for a different project. What type of glue did you use to adhere the OSB into the dados? I was going to use Titebond I, but I have read that you need to use construction paste or polyester glues.

I went looking for the perfect glue and ended up doing some glue block tests on OSB & pine in one square inch patches. I used Epoxy, a little sample of Resoursinol, and all 3 of the titebond products.

The winner was the fail point of the OSB glue~!!

It failed fairly early leaving flakes of OSB in the glue ups with just about every glue I tried so - - I went with Titebond III which seemed to be about as good as anything.
The Titebond III managed to weep into the flakes of the OSB making it a tad stronger (or so it seemed). I am guessing it's because of it's low viscosity. The weakest link in every case was the OSB.

However, since I was working significant lengths, the square inch numbers were substantial so probably any glue would be just fine.

[Chris Friesen]

I'm not sure that using I-beams inside a torsion box makes much sense. The whole point of an I-beam is that the compression/tension load is taken up by the top and bottom members, and the web separates the members.

In a torsion box the top and bottom skins handle the compression/tension load, making the top and bottom members of the I-beam basically irrelevent. As long as the web grid spacing is close enough that the web members don't buckle, you won't gain much strength by beefing up the individual web members.

In this particular case it might make some sense because it allows the web to be mostly OSB while still adhering strongly to the skins. This might save some money over plywood web members, while still being lighter than an MDF web.

[Cliff Rohrabacher]

I'm not sure that using I-beams inside a torsion box makes much sense. The whole point of an I-beam is that the compression/tension load is taken up by the top and bottom members, and the web separates the members.

That's one of the early questions I had to address.

The notch induces a weak point which is eliminated by affixing the 1/2" birch ply skin over all the notches. ERGO: the screwed and glued skin eliminates that weakness upon application.



In a torsion box the top and bottom skins handle the compression/tension load, making the top and bottom members of the I-beam basically irrelevent.This is true - and - not true depending entirely on how you build the thing and how you intend to load it. I made the webbing material to be a larger component of that torsion & compression handling system than a lot of folks do when building torsion boxes. Remember, one of the issues most folks are seeking to address is weight. I tossed that issue out the window. I wanted a BEAST of a box~!! This because (inter alia) I want to do steam bending on it.


As long as the web grid spacing is close enough that the web members don't buckle, you won't gain much strength by beefing up the individual web members.See above.




In this particular case it might make some sense because it allows the web to be mostly OSB while still adhering strongly to the skins. This might save some money over plywood web members, while still being lighter than an MDF web.AND the most important thing and the principle reason I went with the 1/2" OSB in this engineered truss format was to accomplish a couple things:
1.) I wanted to glue & screw the skin on placing the screws at regular close intervals and plywood is not the best material for driving screws into the edges nor is it the best material for gluing along the edges. Solid wood is (bar none) the best suited for those things. ERGO: I wanted to figure a way to use solid wood without the cost & weight of solid wood.

and of course:

2.) Using 1/2" OSB was cheaper than any other material but it's wobbly unless it is reinforced. ERGO: The engineered truss format.

So I got all the benefits of solid wood and the reduced cost & weight of a thinner web of OSB.

It was a win/win and I got to play at making trusses which itself was kind of fun~!!

[Bart Leetch]

WOW you sure went to a lot of work to make something that could have been made just a little thicker then a common door & done everything you need making cabinets & furniture. I know a commercial shop in Texas that has torsion box panels of various sizes that they set up to build items on & if you could see his portfolio it would make a believer out of you.

I built a torsion box base for a cabinet out of 3/4 plywood notched with 1/2" skins about 2' x 7' I think it was 3" web we put 400 pounds in the middle of it with it mounted & setting on 4 casters. It just laughed at us & said give me more.

[Cliff Rohrabacher]

WOW you sure went to a lot of work to make something that could have been made just a little thicker then a common door & done everything you need making cabinets & furniture.

I intend to do thick-cross-section steam bending on this. Because of that it has to be a BEAST of a table. And I only want to build one. This one will be around for ever.

[Cliff Rohrabacher]

These pics were taken a while ago I was just lazy about posting 'em.

During assembly
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I have no pix of the finished thing. It's in use now.
The skins are 1/2 birch ply, the working side is treated with white formica the rest is painted white and the legs are 6"*6" that bolt on and can be removed for storing this beast of a table on it's side.

[Greg Cole]

Cliff,
That's got to be the beefiest monster I've seen to date.
I believe you accomplished the goal of making sure you don't have to make another. Guess on weight?

Greg

[Jim Becker]

Hey, they look "store bought", Cliff! Nice job! And that's gonna be some table, too...wow...