Do Torsion Boxes still sag?

Torsion boxes resist twisting and warping - and are great for provding a flat surface, I believe. But would they resist sagging better than a similar sized and supported piece of MDF or Baltic Birch plywood?
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You can easily(and cheaply) find out for yourself some of the properties of tosion boxes by making some like I did.
http://home.mchsi.com/%7Elarrylhote/torsionbox/torsionbox.htm
Larry
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They do look functional, however, how do you get them to stick to your cement ceiling? (some of the pictures are upside down on your website)
I have made a couple of torsion shelves and a rolling work bench with a torsion frame, all are rock solid.
Mike Coonrod
Lawrence L'Hote wrote:

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You better clean your monitor! The pictures are right side up! Greg
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By gosh you're right, I took a quick look and sure enough. They are right, my apologies Larry.
The funny thing is, in a foot in mouth sort of way, I have the same thing in my garage and didn't place it, maybe I do need shinny new flat screen monitor, or maybe just a better glasses.
Mike Coonrod
Greg O wrote:

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In rec.woodworking

What do you mean by similar sized? Are you talking 4" thick MDF? The short answer is yes, a torstion box of any significant thickness will be MUCH more resistant to sagging than 2 layers of the same material stacked.
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Torsion boxes are used to form a light weight structure that has good torsional [twisting]stiffness. as far as sagging goes they perform just like any other beam of similar construction .
Aircraft wings are are probably the ultimate refinement of torsion boxes, any aircraft structural design book will provide structural analysis [strength] details
-- mike hide

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One thing about this fact is that a torsion box weighs less, and will therefore sag less than a solid plate with the same stiffness. Of course, if the load is significant, then the weight savings will negligably decrease sag.
-Mike

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Agreed. After all, any other beam of similar construction is also a torsion box. };-)
On a unit weight basis a box beam will resist normal loading better than a solid beam. It will resist torsional loads (twisting) MUCH better. I think that is where Mike was headed, 'closing the section' stiffens a member in both torsion and normal laoding, but the improvement in torsion is greater.
--

FF

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'mttt':
Not as one would notice! As Mr. L'Hote suggests (some time back, I built some, more or less to his 'recipe' quite a while before reading his piece on them), I second this recommendation to build some yourself - they're cheap enough - and see.
Mine are of 2x8's @ 24"oc (4'-0"wide) x 18'-6", supported @ ends only. I built in a 3" crown, Faced with 1/2" ac/both sides, glue with 'Liquid Nails' (full bead) and 1-1/4" ss screws @ 8" oc all the way around. 'Have loaded them with a (sloppily distributed) array of less than daily use equipment totaling 850lbs. (+/-). within approx. 1 year, the crown was reduced to 2" (under load) and I have not bothered to unload it to see yet. All that equipment, all that work, mere curiosity...too much 'time' wasted! Well THAT'S MY STORY...and I'm stickin' to it!
Seriously, a (very) good idea. Use it!
I sincerely hope this helps you out...
Warmly, Griz
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"Torsion boxes resist twisting and warping - and are great for provding a "flat surface, I believe. But would they resist sagging better than a similar "sized and supported piece of MDF or Baltic Birch plywood? "
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'Torsion box' is a three sylable term for a 'box beam'.
Unless very badly designed, a box beam will resist sagging much better than a solid beam of the same weight made from the same material.
If you twist it instead of or in adition to normal loading, it will resist torsion much better too.
Box beams are a construct that concentrates the material where the stress is maximum. I-beams do the same for normal loads but do not resist torsional loads significantly better than a solid rectangular beam of the same weight. A box beam that weighs the same as the I-beam will resist torsion much better.
Here's something you can try if you can find a matchbox with a sliding cover. Take the drawer, and twist it. Observe how easily it twists. Now slide the cover on and try again. THen try twisting the cover without the drawer inside. The cover for a matchbox typically is made of thinner material than the tray so that the tray has about the same weight of material. But the cover is much stiffer in torsion. IT is stiffer in normal loading too, the only problem is stopping it from buckling, for both cases.
--

FF

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So here's a related question: I've seen where some designs for mobile tool cabinets and whatnot have torsion boxes that make up the bottom section out of a torsion box. What concerns me is that some of these might be 5-8' long w/ the supports (casters) just in the corners. On the one hand, these look like they would be prime candidates for sagging, especially since some of these are built out of 4-6 sheets of MDF, not counting the tools and are quite heavy. On the other hand, I keep seeing these designs so if they sagged people would come up w/ something else? Yes/No/Maybe...?
TIA,
nuk
--
I know more than enough *nix to do some very destructive things,
and not nearly enough to do very many useful things.
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Yes.

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It is difficult to say without doing some calculation and knowing what the torsion box is made of. If one assumes that the box is made of the same material that you are comparing to in solid form, AND you neglect the weight of the material, then the solid WILL resist sagging more than the hollow box.
From that point you need to determine if the increase in load (due to having more material) has a greater effect on the capacity than the increase in bending resistance that it adds. The calculations are fairly simple and have been posted in this group before.
The point of the torsion box is not because is is stiffer than an equivalent solid thickness of the same material, but rather that it is lighter and uses less material to achieve a similar stiffness. It does this by being THICKER than the solid material.
-Jack

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JackD wrote:

This is what many people forget, the idea of a torsion box is to greatly reduce the weight of the box without substantially reducing it's stiffness. The weight saved by reducing the materials used in the box directly translates into weight that can be placed on top of the torsion box as a payload.
One other slight advantage to a torsion box over solid material is that one can choose the placement of the load carrying members. They can choose a material that is very strong in tension for the underside skin, and a material that is good in compression for the top skin. The internal members can have their grain oriented (if wood) such that forces act along the length of the grain instead of across it.
-Bruce
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All true. It is all about getting the most out of the materials.
-Jack
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