Is a torsion box the best solution?

[Tapper]

I would re-think the design. If your friend is going to the trouble and expense of adding on to his home, I'd spend a little extra dough and design an access to the basement that didn't require a door in the floor. Any architect can probably help with the design work for a nominal fee.

As the old saying goes, it only hurts once.

Doug

[iublue]

I would re-think the design. If your friend is going to the trouble and expense of adding on to his home, I'd spend a little extra dough and design an access to the basement that didn't require a door in the floor. Any architect can probably help with the design work for a nominal fee.

As the old saying goes, it only hurts once.

Doug

I would totally agree and an alternate access was proposed but she didn't want to give up that space so I am tasked with making it work!

I forget exactly how much it was going to cost to put in another access but in was pretty involved and actually involved adding on to the basement also.

[iublue]

Not that I want to be “that guy”, but check your local codes.  If that is the only way in or out of that basement, I’m pretty sure most states require at least one exit directly to the exterior, and not through another room.  
Assuming you can get away with it (and likely would need to be fire rated) Bilco offers a few.  You’d need to prove engineering on anything permitable.

Obviously I would try and follow codes but there are no inspections here.  When I was building homes, the ONLY thing inspected was the septic system.  I now believe that out in the county there are some inspections for the construction of a new home but there are none in the small town where my friend lives.

[iublue]

Cooler,  a torsion box is a totally different animal than two pieces of plywood screwed together.  It is similar to an I joist https://www.apawood.org/i-joist   If you made a torsion box with the web or frame of 1/4 inch thick plywood, 6 inches wide, and spaced them appropriately, and glued 1/4 ply to the top and bottom ( meaning the top and bottom were now 6 inches apart from each other ) it would hold far more weight than two 3/4 inch plywood pieces screwed together.

For possible impacts, I would think that a thicker ply would be necessary for the top.  Maybe do 1/2" top and 1/4" bottom.  3/8" might be adequate for the top.

I am also concerned about the type of hinge to use.  I can recess the hinges but I would still have to hide the knuckles to make it visually more appealing and obviously it needs to open without anything rubbing!

[abernat]

Soss hinges - they're totally invisible once installed, no visible knuckle.

https://www.soss.com/invisible-hinge-product-overview/

[Wipedout]

As for making in invisibile - may I suggest a rug.  There is always a rug over the hidden trap door in every movie

[iublue]

Soss hinges - they're totally invisible once installed, no visible knuckle.

https://www.soss.com/invisible-hinge-product-overview/

I don't know if they make a hinge for the weight that this door will be.

[abernat]

I don't know if they make a hinge for the weight that this door will be.

I used three on a 75 pound door, no worries at all. The gas struts take the weight anyway. 

If you've got 2" of depth (I didn't) you can use the https://www.soss.com/product/model-220-invisible-hinge/ ones. I tossed in 100 pounds and they said four. 

Or the rug. Rug works

[Cooler]

Cooler,  a torsion box is a totally different animal than two pieces of plywood screwed together.  It is similar to an I joist https://www.apawood.org/i-joist   If you made a torsion box with the web or frame of 1/4 inch thick plywood, 6 inches wide, and spaced them appropriately, and glued 1/4 ply to the top and bottom ( meaning the top and bottom were now 6 inches apart from each other ) it would hold far more weight than two 3/4 inch plywood pieces screwed together.

I know what a torsion box is, but it is overkill.  You can achieve satisfactory strength by using doubled-up 3/4" plywood.  Why over-complicate this?

Yes, a torsion box can achieve higher strength.  It is highly dependent on the workmanship of the builder.  The doubled-up plywood depends almost entirely on the strength of the materials.  I believe it is more fool-proof.

[NickNC]

Obviously I would try and follow codes but there are no inspections here.  When I was building homes, the ONLY thing inspected was the septic system.  I now believe that out in the county there are some inspections for the construction of a new home but there are none in the small town where my friend lives.

Whether or not inspections get performed doesn’t mean codes shouldn’t be followed.  As an Architect, I can tell you a torsion box doesn’t meet the structural requirements for a floor.  What’s being proposed isn’t uncommon – I lived in several houses with “bunker hatches” built during the cold war and one in tornado alley.  That said, there’s reasons they aren’t allowed anymore.

If you can get away with this, at least look into something like a bunker hatch that’s “traffic rated” (i.e. you can walk on it).  The last thing I’m sure you want on your conscience is someone falling through the floor or getting banged on the head from a faulty strut.  Keep in mind the strut only slows the close and will be little help if someone elderly doesn’t have enough muscle to push a 70-pound door up.