Load on a torsion box shelf


Looking to build a torsion box style shelf in the workshop. It will store some heavy items, but before I do it, curious if anyone has a good idea how much weight it will hold. I have some leeway as to the number of shelves and length.
The shelf will be about 9" wide and about 36" long and the box depth - don't have a clue yet, depends on what I hear from this request.
I have the OLD FineWoodworking article by Ian Kirby about torsion boxes but there's no idea about loads, tho it does have a picture of him sitting on the one he built.
I did a "Google" and ran into nothing about this.
Curious if anyone has actually had some experience torsion boxes and loads.
Thanks much.
MJ Wallace
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That would require some serious engineering and even then, I'm not sure anybody would bet the farm on how much a particular torsion shelf might hold.(I doubt there is a "formula")
(1) Define "heavy" (2) Define your "planned" shelf size and span
I have built several torsion boxes for various things and they will hold a "metric assload" of weight, which is certainly not a very scientific measurement.
My outfeed and side table on my table saw are both "torsion boxes" and I have placed several hundred pounds of weight on both with little or no flex.
Here is a picture:
http://home.att.net/~mboceanside/wsb/html/view.cgi-photo.html--SiteID-639331.html
They are both made from 1/2" plywood framework and covered with 1/2" MDF and then laminated on both sides. They are VERY strong.
snipped-for-privacy@onebox.com wrote:

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Nice work, Pat and thanks for publishing the pictures.
When will you start using your shop? Its way too clean to have been used. :-)
Bob
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Pat Barber wrote:

Using '*' for multiplication, '/' for division, and '^' for exponentiattion, the moment of inertia of a solid rectangular beam is:
I = (b*h^3)/12, b = width, h = height(thickness)
The section modulus of a solid rectangular beam:
S = (b*h^2)/6, b = with, h = height(thickness)
For a hollow (box) beam I and S may be calculated by substituing in the values for the outside dimensions and then subtracting from that value the result obtained by substituting the values of the inside dimensions.
For a beam of length L that is simply supported at the ends and carries a load per unit length, w, so that the total load is W = w*L the deflection at the center is:
d = -(w*L^4)/128*E*I)
(see http://www.engineering.ucsb.edu/~me15web/6_beams/text_6-0.htm and please check my algebra) The fact that the example beam is solid and circular in section is immaterial, just calculate the moment of inertia properly for your cross section.
So you can decide, how much deflection you can tolerate for a given load and work backwards. This assumes the top and bottom plates of the box beam do not buckle, meaning you have enough core material, like a honeycomb to prevent that.
As a practical matter it will probably be the case that increasing the load will cause an objectionably high deflection long befor the shelf breaks.
That said, the maximum tensile stess in the shelf will be at the center of the top suface and will be:
s = M/S, where M is the maximum bending moment in the shelf. For a point load at the middle of the shelf that would be W*L/2, or (w*L^2)/8 for the distributed load.
So long as that is less than the tensile strength of the material (probably plywood) the shelf won't beak. It is possible for a shelf to fail in pure transverse shear or in shear due to bending, or in compression of the lower surface but unlikely absent very peculiar design or loading.
As noted above, for a wooden shelf deformation will probably be a problem long before rupture.
--

FF


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How heavy is heavy?
My garage shelves are 19" x 48" and made of 5/8ply with 1x2's edge glued & screwed every 8" or so under the long edges of the shelf. These are supported at the ends and I can sit mid span with negligible deflection. I weigh ~190lbs.
Art

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: How heavy is heavy?
: My garage shelves are 19" x 48" and made of 5/8ply : with 1x2's edge glued & screwed every 8" or so under : the long edges of the shelf. : These are supported at the ends and I can sit mid : span with negligible deflection. I weigh ~190lbs.
How is the shelf mounted to the wall?
    -- Andy Barss
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Each end is supported by a pair of vertical 2x4's with horizontal 1x2 "rungs", kind of like a ladder, which the ends of the shelves rest on. The 2x4's are secured to the wall studs with hurricane ties. The load is transferred to the floor via the 2x4's and the walls only provide lateral support.
I can post some pics on abpw if you want me to.
Art
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Bending strength is proportional to the square of the depth. The strength can be calculated fairly easily given the materials and dimensions used. You need to define what you mean by heavy first.
j
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It will also be strongly dependent on how well put together it is. If the join between the surface and the torsion frame is not sufficient in shear, you won't be able to generate the full strength that you might expect from a simple analysis based on the wood properties.
Mike
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snipped-for-privacy@onebox.com wrote in

At 9" by 36", I'd be at least as concerned with the means of supporting this shelf, assuming wall mounting and connection.
My Gorilla shelves from the BORG support hundred of pounds per shelf, at 18" depth, and 36" width. They are maybe 5/8" MDF, supported on all sides.
And since you too are in the seismic zones of the West, you'll likely want to give that some consideration, too.
The 'how' on torsion boxes was covered pretty well in a TV episode by that semi-famous woodworker from Santa Rosa. Check the website, maybe?
Patriarch, who belives that organization in the shop is highly over-rated...
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Thanks to all who responded to my request for help.
The load in question are boxes of woodworking magazines that I plan to keep in plastic storage boxes. They are quite heavy and until I go thru them or convert them to digital format, they got to go somewhere.
One of our local (homegrown) home centers has a shelving unit on sale that might work instead. For one, it will already be built!
But interested in the question for other reasons as well.
Again, thanks so much for responding!
MJ Wallace
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