Torsion Box Shelf Question

[Danny Thompson]

Here's hoping someone has a quick and dirty way of calculating torsion box capacity.

I want to put a shelf above my one-car garage door to store ladders and plywood. I have 21" of vertical clearance for a 10' span from wall to wall.

To maximize storage capacity, I am thinking a thin torsion box would be better than 2x4's. The question, of course, is, "How thin can I go?"

Another post points to the definitive plywood torsion box sizing article (http://www.sawmillcreek.org/showpost.php?p=373561&postcount=13). I downloaded and attempted to read the article, but about page 17 I felt a small aneurism forming in my cerebral cortex. With that "in mind" I thought I should try a different tack.

What if I built a 3" x 24" x 10' panel with 1/2" plywood skins and three 1.5 x 1.5" x 10' stringers and 1.5" x 1.5" x 9.75" headers and blocking spaced 12" on center. The ends would be monted to 2x4 cleats attached to the wall joists. Would that do the trick? What if I halved it--3"x12"x10'? What if I used 1/4" ply?

Thanks.

[Mike Henderson]

A torsion box is essentially the same as an I-beam. If you can find an equation for the strength of an I-beam you'll come pretty close to the strength of a torsion box.

A problem is that you need to define the requirements. That is, what load must it carry, with what deflection?

Note that the strength of a torsion box is in the skins. The webs are only used to space the skins apart and to keep them from moving relative to each other. The webs can be quite thin and you won't lose much strength, but gain is weight savings.

If you want to figure it out empirically, make yourself an I-Beam of the length you want and see what the deflection is with the flange and web sizes you chose, and the load you need to support. The results won't be exact but it'll give you an idea of what to use for your torsion box.

Mike

[Let me add one thing. A torsion box will be stronger - all other things being equal - if the webs are "higher". That is, the further apart you space the flanges/skins (the thicker the box), the stronger the box (everything else being the same).]

[Joe Jensen]

I think in that kind of application, the limter will be how strong the edge of the torsion box is where you bolt it to the wall. I'm not saying this clearly, but what I am saying is how it bolts to the wall is the limit. There will be a lot of leverage and I could easily see drywall crushing where it bolts to the wall.

[Jamie Buxton]

...To maximize storage capacity, I am thinking a thin torsion box would be better than 2x4's....


I'm a big fan of torsion boxes. They're almost as stiff as solid lumber of the same thickness, but they weigh a great deal less. However, notice that they're not as stiff as solid lumber. If you think a 2x4 deck would not support your load, a torsion box which is 1 1/2" thick would also not support the load.

[Peter Quadarella]

If limiting the width of the shelf is the goal, I would look towards steel. Some steel bars bolted together across the length could be very thin and hold a lot of weight.

[Danny Thompson]

Thanks, all. You gave me exactly the information I needed.

I'm going to abandon the shelf option and go with a pulley system for the ladders and stick with my current lumber cart for the sheet goods for now.