Torsion Box Plank

[jimran]

I'm thinking of building a torsion box for use as a plank on scaffolding. The box will need to be ten feet long and capable of supporting two men or around 500 pounds.
I considering a torsion box to reduce the weight of the plank.

Does anybody have any idea of the dimensions I should use? I would like to end op with a plank about 16 inches wide. Can I get by with 1/4 inch skins?

Any and all suggestions would be appreciate.

Thanks,

Jim

[LarryG]

Interesting idea. Can't think of any reason that wouldn't work.

For a 16" wide torsion box, I think I'd use three cells at 5.33" each, less the thicknesses of the dividers. One problem, though: TB eggcrates are often assembled by toe nailing with air-driven fasteners, and cells this small won't give you enough room to get the gun down into the cells. You might be able to use half-lapped joints and get around this. Or you could use two rows of 8" cells.

With either size, 1/4" skins won't be enough. When a man takes a step, his entire weight is borne on just a few square inches of one foot. A 200 lb man plus whatever tools he's wearing/holding would punch his foot right through 1/4" material. For three rows of 5.33" cells, I'd use 3/8" plywood as a minimum but would prefer 1/2". For two rows of 8" cells, I'd use 1/2".

Maybe what all this is suggesting is that two rows of 8" cells plus 1/2" skins will provide the lightest, strongest, and easiest-to-build assembly.

The depth of the ribs will depend on how long you want the TB to span without supports. This is just shadetree engineering, mind you, but for a 10' long TB I'd make the ribs at least 4" tall (5" overall, with 1/2" skins), and even taller if the supports will be all the way out at the ends. I've stood on my TB, which has roughly 8" square cells and 1/2" MDF skins, many times. It's plenty stout, but the leg assemblies are only about three feet apart.

[Tom Miller]

Do you need to go to all the trouble of a true torsion box? How about a 2x12 plank, with a 2x6 rib on edge running the length underneath? Or something similar....

Just thinking outside the (torsion) box -- keep or toss.

Regards,
Tom

[Dan Tipton]

Re: Torsion Box Scaffold Plank

Before making any sawdust, I think that I would check to see how much a torsion box assembly will end up weighing. By the time you get 2 pieces of MDF with the associated torsion box pieces put together, you might be apporaching or exceeding a standard scaffold plank....

[JeremyM]

Tom's right.

As a man from the construction world, I wouldn't waste my time on it that way. Much less, you wouldn't get me on an MDF one anyway, as I wouldn't know if there had been moisture in it at some point or not.

2x12's with a 2x6 on edge tacked together is mighty strong. You can even take cheap OSB ($7 a sheet now, thankfully!) and tack it to stuff instead of 3/8" or 1/4" making a torsion box of sorts with 2x6 or 8's with OSB. I've used a 4x8 sheet of OSB with 2x4's on 16"OC from basic framing construction techniques and not worried much about it. But with two men, I think I would definitely want some more beef there. But put you a peice of OSB on the back and you have got something tough.

But remember, if in doubt, don't do it. Make sure you are safe when suspended. And don't risk another person's life.

Btw, this is NOT OSHA approved. Those planks are expensive and heavy glue-lam boards.

[RayintheUK]

Absolutely agree with JeremyM - don't waste your time on a torsion box. Two men moving about on a ten-foot span will severely test anything, so I'd look at hiring a suitable piece of staging.

Ray.

[LarryG]

Who said anything about MDF? I mentioned my MDF torsion box assembly table as an example, but suggested plywood for this application.

A torsion box can be as strong as anything, built properly. Airplanes stay up in the air courtesy of torsion boxes (wings).

That said, the OSHA comment is a good one and a wrinkle I'd not considered.

[dbuche]

Qualcraft Aluminum I-beams

Qualcraft used to sell Aluminum I beams (the width of 2x stock). You screw two of them into 2x stock (one on each side) to make planks that do not flex.

[JeremyM]

I apologize.

I crossed my wires. I made the mistake of thinking you were using MDF on that project.

By wasting time, I mean that with a couple 2x6's and a 2x12 and you've got a super sturdy plank, or a couple 2x12's with some ply across them.

Just make sure it's something you'd send your most beloved out there walking on with out a second thought.

[lrogers]

If you're going to be using this in a commercial venture, you'd better pay a lot of attention to the OSHA aspect. If it failed and there was an injury........

[RickCarpenter]

Qualcraft used to sell Aluminum I beams (the width of 2x stock). You screw two of them into 2x stock (one on each side) to make planks that do not flex.

Why not buy a good quality garage sale extension ladder, take it apart, and skin one section with 1/2 ply?

[crokett]

Why not buy a good quality garage sale extension ladder, take it apart, and skin one section with 1/2 ply?

That's what I'd do. And exactly what I was thinking.

[vaking]

Many good correct responses:
properly built plywood TB should be lighter and stronger than something made out of 2X. However in matters like this OSHA approval may be worth more than money or weight saved. If you decide to build it yourself - test it on the ground with at least 3 times the expected load before you go on it yourself. Make sure that if it is ever going to fail - it is not when you are standing on it.