Torsion box loft

greenbankApril 15, 2007

Hi, I have a 12' wide by 30' deep stall that is crying out for a loft at one end to get some extra storage--I have 20' ceilings. Unfortunately, this loft needs to be freestanding, not attached to the structure, as the space isn't mine. There is a very solid concrete slab as a floor. The loft doesn't need to be very high--5' or 6' will give me room to tuck the noses of a couple of future restoration project cars underneath. I also need to build the components of this loft off-site.

My thoughts are to construct three bents, 12' long by 5' high, using 4' - 4 x 4 uprights on six foot centers with a 12' - 4" x 8" beam created from two 2 x 8's glued and screwed together resting on top of the posts. I would use triangular plywood gussets to attach them to the posts and provide resistance to racking.

I would place these bents 4' apart, and diagonally brace them to each other using a couple of bolted 2 x 4s per post.

For the actual loft floor, I would construct 3 torsion boxes, 4' x 8', out of 1/2" plywood skins with 2 x 4 outer edge pieces and 1 x 4s as the webbing, placed on 1' centers. These would be laid across the bents and bolted to each other, as well as attached to the outside bents.

The expected load on top of the loft might be about 2000# I think this plan is very over-engineered for that load (heck, you could probably park a car on it), but better safe than sorry.

What do you think of the plan--am I right in thinking this is plenty strong enough? What about using two bents instead of three? Will the torsion boxes be strong enough to span 8'?

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"heck, you could probably park a car on it"

I think you could park a small house on it.

Spread out over an area that large, 2000# isn't much, and I do think the deck seems over-engineered. I'd be more inclined to build the deck in place like a conventional floor, with no bottom skin.

I'm trying to make sense of the dimensions you've mentioned. If those bents are only 12' long and the deck is 8' from front to back, what sorts of cars do you hope to fit under there? Will the cross-bracing and close parking make it impossible to open the doors and access the gear shifters?

To what extent do you need to "build the components of this loft off-site?" Is it a dust issue? Noise?

    Bookmark   April 15, 2007 at 6:37AM
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There is no power on-site, and I think for liability reasons the owner of the property would probably have to say "no" to building this on-site conventionally. As it is, I can argue that these are "shelves." Plus I can potentially re-use them in the future elsewhere or sell them to future users of the space.

I don't need to fully get cars underneath, just the noses. The cars will be on wheel castors (they go under each wheel if you haven't seen them, allowing movement in any direction).

This is me trying to buy some extra space by putting a bunch of boxes which can be stacked five or six deep but which are taking up valuable floor space up there. I have to fit three small cars, two motorcycles and some vintage woodworking floor tools into the 30' x 12.75' floor. The cars are small foreign machines that are about 12' long and 4.5' wide.

Doing some calculations, I probably have 60 boxes which have an average weight of 50-60#, so the loading on the entire structure might be 5000#, so perhaps the "engineering" is along the right lines.

I can put the torsion boxes in my truck, along with the beams and posts, and have the whole thing assembled in an hour or two on-site by myself. I would bolt the torsion boxes to each other and the beams in a few places to add extra rigidity.

Lifting 5000# 6' might take me a bit longer...

    Bookmark   April 15, 2007 at 7:40AM
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Does it have to be wood? The big box stores use the type of shelves you described made out of steel and carry a few tons of weight and leave much more room underneath.

    Bookmark   April 18, 2007 at 10:35PM
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Years ago I built a sleeping loft for my daughter's roomates in college. I used pine 1 x 8's , laid on the diagonal with 1/2 cuts so the thing went together eggcrate fashion within a huge? 1 x 8 box. Other than finishing nails as temp holders, the basic adhesive was gobs of LiquidNails. (I was thinking of an airplane wing.)Then up on 4 x 4's @ ea corner, bolted in, stain the whole thing dark brown, add a ladder. It was strong enuf to sleep 2/3 young women. It was a little bouncy. but it wasn't going anywhere. Engineering/drafting design was a little complicated, but it was cheap, if plain. Serviceable.

    Bookmark   April 23, 2007 at 4:15PM
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Torsion box construction puts an absolute premium on the glue used.
If you need any type of inspection you will have a very hard time getting an AHJ to allow this.

Without detailed engineering calculations the design you have proposed may or may not be suitable.
The 1 foot spacing of the 1xs sounds a little large.
You also may encounter shear limits in the plywood.

Why not just frame up as you prposed and then use appropriate 2x joist and plywood?
You will probably need more than just plywood gussets also.
Like a torsion box, gussets produce shear loads, but on the fasteners in this case.
You need to determine what the loads will be and then what the limit is for the fasteners in the material, and use enough fasteners in an arrangement that does not produce a splitting failure in the wood.
In many cases you need a LOT of fasteners (think about all the 'teeth' on a truss plate).

    Bookmark   April 26, 2007 at 3:56PM
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