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Torsion box with voids

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Forum topic by jtrz posted 03-11-2015 01:13 AM 656 views 0 times favorited 5 replies Add to Favorites Watch
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jtrz

7 posts in 633 days


03-11-2015 01:13 AM

So I’m working on a design for a desk. I have decided to use a torsion box for the top because I want it to be able to have a decently long span with out aprons or other supports. I like having the underside of the table as open as possible so can put rolling cabinets, etc underneath. It also means the table top won’t be too heavy.

Because it will mainly be used as a computer desk, I would like small voids/holes that go through the whole thickness of the torsion box. These voids would be for cords to go through.

Since a torsion box is already kind of overkill for a desk that won’t be under major loads my thinking is that a hole or two won’t majorly effect it’s ability to withstand these loads.

So does anyone have any experience with a torsion box with holes or voids? I’ve done pretty extensive searches and haven’t found anything that talks about this particular issue.

FYI
I am thinking the desk with be 2’ x 6’. I’m hoping to uses 1/8” hardboard for the skins and some 3/4” HD plywood for the core. The core will have a spacing of around 4-5” and be 1 3/4” tall. So an overall box height of 2”

Thanks for any input you all might have.


5 replies so far

View altendky's profile

altendky

169 posts in 1670 days


#1 posted 03-11-2015 01:58 AM

My engineering judgement suggests that a couple 2” holes 1-2” off the back edge will be wholly irrelevant to the structural integrity. The skins are like a bunch of diagonal bracing, so much that it becomes solid. If you even removed an entire 4-5” block of skin it wouldn’t do much even at an edge.

That said, I might suggest making sure you are satisfied with the stiffness of 1/8” hardboard with 5” spacing. My gut says that will be a bit flexy when it comes to point loads like leaning on the desk. Maybe at least 1/4” hardboard on the top side if not 3/8” ply to be really sure? Shouldn’t be hard to test this aspect with a single 5” square and judge for yourself.

And finally…


a desk that won’t be under major loads

How many desks don’t get sat on? :]

View TheFridge's profile

TheFridge

5764 posts in 946 days


#2 posted 03-11-2015 02:07 AM

I don’t see why it would be a problem. Especially if it’s near a support.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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jtrz

7 posts in 633 days


#3 posted 03-11-2015 02:45 AM

Thanks for the replies. I definitely plan on doing a test as far as grid spacing and hdf’s deflection under a point load as I had wondered the same thing. Won’t have a shop to work in for another week or so so I’ve been relegated to researching, sketching, and doing CAD models.

I know the rule, according to maybe Kirby, is that the spacing should be no more than 12x the thickness of your skin total (which comes out to be 3” in my case) I was just trying to cut down on the amount of core members. And now that I think about it I can’t remember what my reasoning for doing that was so I’ll probably scrap it.

I’ve been trying to keep the skins thin just because each time you make them just a bit thicker the weight of the whole thing goes up considerably. Though I may be too worried about the weight of the thing. I still don’t want it super heavy but there is something to be said about having a top that feels solid. The problem is finding 1/4” mdf or 3/8” ply without going real far out my way.

Your point about someone sitting on it is something I hadn’t thought about. I don’t run into that problem with my desk now as it’s usually a mess but that is something that I’ll take into account going forward.

View levan's profile

levan

472 posts in 2439 days


#4 posted 03-11-2015 03:01 AM

Search for pipe organ wind chest. they are a torsion box, filled with holes, voids and slots. I think the biggest issue will be your choice of woods, joinery and glue.
Most wind chest are built with poplar for the grid and veneered (mdf) medium density fiber core for the top and bottom.
They are built to hold hundreds of pounds for years and stay perfectly flat.
I just wonder how good the adhesion, between the edge of the plywood and hardboard will be. For your purpose it may be fine. As far as holes going through, its a non issue. You might think about using 3/4 ” ply flat and 1/2” mdf or ply for the skins. It would be heavier, but I think it would also be stronger. That small of a top shouldn’t be to bad for weight.
best wishes
Lynn

-- "If you think you can do a thing or think you can't do a thing, you're right". Henry Ford

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jtrz

7 posts in 633 days


#5 posted 03-11-2015 03:41 AM

The wind chests are pretty impressive!

The only reason I’m using 3/4” ply is because I have some. In fact, it is screwed down to a hollow core door and under the laptop I’m typing on right now. I just built this real quick a while back because I needed a desk stat. It’s spanning a bit over 5’ and deflects in the middle about 3/16”. Hopefully, the bow it has won’t affect the core grid. I guess I’ll find out once I start cutting.

I’d love to use poplar for this table but I’m trying to refrain from the hour drive out to the lumberyard where I would get it until I’m ready to buy a larger quantity of wood.

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