Torsion box help

A buddy of mine "found" an old set of indoor stairs leading from his basement to his living floor. Only problem is, the stairs are topped by a floor introduced during a later reno rendering the stairs useless. He wants to re-commission them.
Intuition says it's simple....cut out the floor, build a "landing" for the framing and hinge the thing (trap door style) so it opens exposing the staircase when it's needed. Sure, this could be done but it ends up being ugly with a mess of sharp edges and framing lumber etc. He wants a "neat" way of doing it...
I got to thinking about torsion boxes. Specifically, I was wondering if instead of the structure offered by a typical 2/4" plywood floor over 2/8's on 16" centers, if a replacement panel could be made using torsion box construction. My thinking is that instead of a composite thickness of 10 (ish) inches we may be able to cut this in half and instead of an ugly "underneath" exposed when the panel (trap door) is up it would be a finished sheet of ????? On top of that, I think it would just be plain cool to have this...
Construction thoughts to date...far from finalized and no, I've not done any load calc's (yet): 3/4 cabinet grade ply on top, frame work of 1x4's (on edge) on ?? inch centers half-lapped and a 1/4" cabinet ply bottom skin all stapled and glued built in a makeshift vacuum press (can you say shop vac?)
Concerns. Total width span is relatively small...say 34" wide. Total length is about 72". I'd be concerned that even though the floor may BE strong, it would FEEL or sound weak. It would be pretty easy to incorporate a stiff leg but still, I can't help wondering if people would notice and be wary....
any thoughts?
Rob snipped-for-privacy@imajis.com
http://www.robswoodworking.com
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It isn't completely clear from your description what is going on here, however, these are my random comments.
1) If you cut the existing floor, you will need to make sure that IT is still structurally sound. This means headers where ever you have cut any joists and likely doubling up the joists on either side of the hole that you make.
2) The span is short, so you could get by with what you propose and it would still be plenty stiff. Even so, I'd vote for 1/2" ply on the bottom You are putting a ledger around the hole for this "door" to rest on right? Make the stringers go the short way. and make use something slightly thicker at the ends of them. No need to half lap them as they are only going one direction. I think if you put them every 12" you will find it to be plenty stiff.
3) Why bother with the vacuum press? glue and staples will be plenty. Since there are enclosed spaces inside the door it seems that inducing vacuum on one side could create more problems.
4) As for being neat, well that is what trim is for right?
5) Quietness. Attach the stringers to one piece of plywood. Fill with fiberglass insulation. Glue on other piece of plywood.
-Jack
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Why would you need 2x8's 16" o.c. to span 34"? 3/4" Plywood and some 2x4 framing would be sufficient. Lag some L channel for a frame and it should be undetectable from the surrounding structure.
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The framing I described is existing and no, I'd not do it that way either. I do like the L channel idea though, hadn't thought of that.
Thanks Rob

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Greetings,
Building codes have some specific things about stairs. Run your design by the inspectors before you start. They might ask you to get an engineer's evaluation before they will accept it.
If you build it, post some pictures.
Sincerely, Bill Thomas
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34 x 72? Sounds like one used stave core door would be overkill, would not sound or feel springy, could be installed with piano hinge and hidden latch or recessed D-handle pull. Could even be installed with gas strut to self open a bit like your car trunk lid.
#################### Keep the whole world singing. . . Dan G (remove the 7)

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