Torsion box loft
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'?