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
I did a "Google" and ran into nothing about this.
Curious if anyone has actually had some experience
torsion boxes and loads.
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:
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.
Using '*' for multiplication, '/' for division, and '^' for
exponentiattion, the moment of inertia of a solid rectangular
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
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)
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.
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 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
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.
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.
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?
who belives that organization in the shop is highly over-rated...
Thanks to all who responded to my request
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!
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