Stiffening Long-Span Shelves

      Woodworkers discuss torsion boxes and steel-angle reinforcement for long-span shelf situations. November 11, 2005

Question
I have a shelf unit to build which will have two fixed shelves 12"D X 60" long, with no center support. Would a torsion box with 1/4" ply top and bottom, 1" X 3/4" frame work? What should spacing be on frame members?

Forum Responses
(Cabinetmaking Forum)
From contributor S:
I've built torsion boxes for several projects. On each project, I wished I had some method to figure out if it would work before I built it. Fortunately, they've always been strong enough. Based on my experience, and nothing else, I would say your box would work for a normal shelf. If I was concerned about overloading the shelf, and thickness was limited to 1 1/2", I would consider using 1/8 ply and increasing the depth of the strips by the 1/4". I would use a 4X4 grid. I believe the strength increase by increasing the height of the strips is far greater than increasing the thickness of the plywood. I built a platform for a local community band director last year out of two sheets of 1/4" Russian birch (5'X5'). The strips were 1-1/8" high, 3/4 wide in a 4"X4" grid. It sets on 4 legs. It can hold three adults and doesn't seem to flex. Does anyone have something other than experience as a starting point for us non-engineers?



From contributor W:
Is there anything in your experience that might help predict what a torsion box would look like if it was asked to span 12' and support a solid granite countertop 1 1/4" thick by 24" deep? That would be about 800 lbs, according to the granite shop.

The architect seems to think a 2x4 ladder frame would do the trick. I kind of don't think so, the way he's designed it anyway. Been thinking about torsion box... also steel square tube. Or maybe I just build to his design and let it fail (it's a small part of a much larger job).



From contributor H:
Insane that an architect would spec that. He should know better. Using a ladder frame to support that weight and material you would need 2x6 doubled rim with joists (if you will) spaced 12" on center. A 2x8 frame with 16" centers would be sufficient.


From contributor C:
Why don't you go to the hardware store and buy a piece of 1" steel 90 degree angle and screw it behind hardwood under the shelf? Or you could buy 1.25" tall x 1/4" thick steel and screw inside your torsion box. That stuff is cheap and easy to work with.


From contributor J:
Steel is the best solution. It should be no problem for you cabinet engineers. Our local building codes have values for all different sizes and shapes of steel lintels, I-beams, etc. Should be no problem to predict deflection and build to suit the load. A store fixture company I once worked for used welded steel parts inside cabinet components to produce long, seemingly unsupported spans with good results.


From contributor T:
I have used steel in my cabinets going back to the 1980's and have had good results. As stated, the steel comes in many configurations off the shelf and is easily and cheaply available. This makes for worry free design and installs. I have found that adding wood supports and braces only adds to the weight and increases the points of attachment (joints) that can fail. Of course, I am speaking of cabinets, not construction woodworking.


From contributor S:
It really wouldn't surprise me if a torsion box built of 2X4's on about a 6" grid with 3/4 top and bottom ply would support this slab. I say that not knowing if it will or not, but I'm always amazed at the strength of the torsion boxes I've built. I wouldn't build and sell one this way without an engineer. I'm going to guess the biggest concern would be how much flex granite can stand - I doubt much. The granite guys are always pretty careful not to give it an opportunity to flex during installs. The folks suggesting steel are probably the best bet.


From the original questioner:
Thanks for all of the responses. Question on using bar stock - I usually biscuit and glue on a 3/4" nosing on a built-up shelf. How would you attach the nosing over the steel? Dado back of nosing, drill, screw, plug? The shelf is stain grade.


From contributor T:
In your specific case, angle iron no thicker than 1/4" would be all that is necessary. You screw one angle to the back of the nosing and the other angle to the shelf. This would be all that is needed and no additional steel or bracing would be required. You just might want to consider whether or not there is any reflection from mirrors or shining horizontal finishes that might reflect the hardware in the finish. If so, you can easily leave a recess at the bottom to attach a 1/4" finished bottom.

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