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How long can I make a torsion box desk without it drooping?

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Forum topic by Tony1212 posted 01-07-2019 03:19 PM 787 views 0 times favorited 10 replies Add to Favorites Watch
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Tony1212

346 posts in 2215 days


01-07-2019 03:19 PM

Topic tags/keywords: torsion box desk design length

I’m working out a design for a torsion box desk. I’m thinking of 1/2” plywood for the top and bottom with 3/4” pine as the framing inside – 2ft wide by as long as I can make it. I’d like it to span the entire length of my office with as few supports as possible – ideally only one at each end. I’d like to roll my chair from one end to the other without banging my knees on supports.

Does anyone have an idea of how long it can be before it starts to droop in the middle? Does it matter how thick the inner framing is, possibly a 1×3? Or are there any online calculators that anyone knows about?

I know there are online calculators for shelf length, but that is for a single board, not a torsion box.

-- Tony, SW Chicago Suburbs


10 replies so far

View shipwright's profile

shipwright

8357 posts in 3279 days


#1 posted 01-07-2019 10:36 PM

When I was building boats I used to have torsion boxes that I used for staging to walk on. They were 8’ long and 2 1/2” (maybe 3”) thick, entirely made of 1/4” plywood to keep them light. All joints were epoxy fillets. Supported on the ends on horses, they had very little deflection when I stood on them in the middle. I weigh ~220 lbs.
At 100 lbs I doubt there would have been noticeable deflection at all.

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese! http://thecanadianschooloffrenchmarquetry.com/

View AlaskaGuy's profile

AlaskaGuy

5338 posts in 2790 days


#2 posted 01-07-2019 11:11 PM

I’m guessing the you office it 50 feet wide. Is that right? And I guess it’ll be attached to a wall?

-- Alaskan's for Global warming!

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AlaskaGuy

5338 posts in 2790 days


#3 posted 01-07-2019 11:23 PM

When I worked for the school district I used a ton of these and 1 1/8 thick plywood or particle board to make counter down long walls. At the end of the day they prove to be kid proof and you don’t really notice the support on the wall.

Not what you asked but it’s an good alternative IMHO

https://www.fastcap.com/product/speedbrace

-- Alaskan's for Global warming!

View bondogaposis's profile

bondogaposis

5518 posts in 2832 days


#4 posted 01-07-2019 11:31 PM

It all depends on the width of the inner framing, it is like an I beam, the wider the web the stiffer the beam.

-- Bondo Gaposis

View clin's profile

clin

1057 posts in 1477 days


#5 posted 01-07-2019 11:45 PM

The width of the internal webs is critical to how much a torsion box will sag for a given load. Unfortunately you have not said anything about how long this needs to be and what loads will be on it.

But, keep in mind that a torsion box is similar to having a solid piece of material with much of the internal material removed. A torsion box IS NOT stiffer than a solid structure made with the same materials. The nature of structures is the material in the center contributes almost nothing to the strength while the surface or skins handle most of the load. All the internal structure (the webs) is doing is keeping the skins held apart and fixed relative to each other.

The point is you could use the sagulator online calculator to estimate the droop of a solid structure of similar overall dimensions. As you play with this you will see that the thickness makes a huge difference.

While this is not a torsion box estimator it will be in the ball park. The torsion box will sag slightly more, all things being equal. But since the torsion box is much lighter, there may be less sag due to the weight of the shelf itself.

https://www.woodbin.com/calcs/sagulator/

I did a quick calculation of a 3” thick plywood, 24” deep and 20 feet long. Floating at the ends and 100 lbs in the middle. It sags 0.44” in the middle. I think a torsion box of similar dimensions as you described 1/2” skins and 2” webs (total 3”) would be similar.

If you made the webs 3” thick (total 4” thick), the sagulator estimate is less than half as much sag for the same 100 lb centered load (0.19”). This highlights how thickness is the most important thing aside from using say wood versus jello as the material.

If this is along a wall (be weird if it wasn’t), then you should certainly put a ledger board along the length of the wall and support the back edge the entire length of the shelf. Then it really isn’t going to sag much and the most in the front edge in the middle of the span. If it really is not against the wall, then perhaps you could still put a piece under it much like a modesty panel on a desk. A 10” wide modesty panel firmly attached to the top would add a great deal of stiffness.

-- Clin

View shipwright's profile

shipwright

8357 posts in 3279 days


#6 posted 01-08-2019 03:08 AM

Correction: my staging pieces were 16’ long (2×8’ plywood). It was a long time ago. Sorry

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese! http://thecanadianschooloffrenchmarquetry.com/

View Tony1212's profile

Tony1212

346 posts in 2215 days


#7 posted 01-08-2019 02:03 PM



I m guessing the you office it 50 feet wide. Is that right?

- AlaskaGuy

Almost. The north and south walls are 50ft, but I want to put it along the east wall that’s only 10ft. :)

But seriously, I was hoping to NOT attach it to the wall. Not a deal breaker if I have to, but at $30 for the 15×18 fastcap supports, I’d rather not.


Unfortunately you have not said anything about how long this needs to be and what loads will be on it.

—Clin

I’m trying to figure out how long I can make it. That is what will determine the placement in the office. 11ft is about the maximum distance between any of my walls. But, if it will sag at that distance, I’ll have to come up with an office layout that gets me everything I want but with the smaller desk.

As far as load, it will mainly be office stuff. Computer monitors, speakers and such. But that’s also where I’d be doing a lot of my electronics work. So maybe a guitar amp (approx 40lbs on average) with an oscilloscope, frequency generator and other assorted equipment.


This highlights how thickness is the most important thing aside from using say wood versus jello as the material.

—Clin

I don’t think Jello would make a good desk. Too much wiggling and jiggling. Although, it would be cool and fruity.

-- Tony, SW Chicago Suburbs

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Tony1212

346 posts in 2215 days


#8 posted 01-08-2019 02:11 PM

NEW IDEA:

I was thinking about leaving the last (outside) webbing piece off. Then I could use the voids to hold small drawers. For instance, if the plywood is 2ft wide by (let’s say) 8ft long, I’d put 3 webs (ribs?) lengthwise. One at the back, another centered 6 inches in, then one more 6 inches from the second. That would leave about 5 5/8” left on the front.

The widthwise ribs would extend from the back, all the way to the front and would be dividers for the drawers.

Using the sagulator, I would say it is only 1.5ft in width, correct?

-- Tony, SW Chicago Suburbs

View BlueRidgeDog's profile

BlueRidgeDog

499 posts in 260 days


#9 posted 01-08-2019 02:40 PM


Using the sagulator, I would say it is only 1.5ft in width, correct?

- Tony1212

Right, but the the front space now becomes weight that the span is supporting.

Making the webs out of 4” nominal stock (3.5”), with three of them, would easily span 10’. What the tool fails to calculate in this instance is that one of the webs will be rigidly held to the wall. Three webs, spanning only 10’ with one attached at 16” centers to wall studs would hold a significant weight, especially if epoxied into a torsion box setup that mimics and Ibeam.

Build it and if you encounter an issue due to static load (monitor or other heavy item, then put in a 45 degree brace from the lip to the wall.

View Phil32's profile

Phil32

598 posts in 384 days


#10 posted 01-08-2019 07:57 PM

It may depend on whether or not you rest your elbows on the top while pondering your work.

-- Phil Allin - There are mountain climbers and people who talk about climbing mountains. The climbers have "selfies" at the summit!

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