Is a torsion box the best solution?

[iublue]

A friend is adding on to his house.  At present they have a door that covers up the stairs to the basement.  Think old style, lift the door and go into the unfinished basement.
The addition will make the door to the basement more in the middle of the room.  They did not want to pay the considerable amount just to move the stairs since the basement is only use as a mechanical room.  He has asked me to build a door strong enough to support being walked on but light enough to use gas actuators to help in the lifting and to keep the door open.

I am thinking torsion box.  Maybe use 3/8" or 1/2" ply for each side of the box and a frame work inside of oak, ash or locust.  Maybe use poplar for weight?  I was thinking 3/8" thick by 2 or 2 1/2" wide.  The door will be about 3' x 7' and I can make it rest on solid wood around the perimeter when closed.  I am afraid that the 1/2" ply may make the door too heavy.  1/4" may not be strong enough .  3/8" seems like the Goldilocks spot to me.  I would need to use good ply since, even though I use it, I have had some problems with ply at Menards delaminating when stressed.  I think that with 3/8" ply, I could do an 8" x 8" grid pattern inside.  I will also have to allow for flooring on the door to try and make it as invisible as possible.

I am also wondering about the kind of hinges I should be used.  Obviously, I don't want knuckles showing but there has to be enough operating space to be able to open the door.

One last item, the door will be opened from the end.

A challenging project to be able to make it work and make it look good!

Lay your knowledge and insight one me!

[abernat]

I think you're overthinking this


I have a similar basement access. It's 5' by 3' and hinged on the 3' side. The hatch itself is just a sheet of 3/4" plywood with 3/4" maple on top and has no flex at all. It's supported on all four sides by a 1" lip, so you've basically got a 3' span. The area isn't tremendously high traffic, but I haven't seen any need to brace it.

For hinges I went with three Soss hinges - specifically three 1 in. x 4-5/8 in. sized ones. Drilling them into the existing floor was a pain (lot of chisel work) but they're rock solid. I also used the longest screws I could find (5", I think?) and flooded the cavities with epoxy. 

Gas struts are fantastic, especially when you're opening long ways. It turned the hatch from something I had to muscle open to something my kids could easily operate. I used a supplier that also ran the calculations for me ... and I can't find a receipt so I don't remember who. I entered the length and weight of the hatch and they worked out everything. I think my hatch was fifty pounds. You've got an extra 50% of size, but that's still 75 pounds - the struts might take the weight, especially if you use two.

[fredhargis]

Do you think that is someone dropped a heavy-ish object on the 3/8" ply and it landed between the webs it would penetrate the plywood. That was the only thought I had.

[abernat]

I take it back - I found a note that said my hatch was 75 pounds, so you'd be in the 100 range. 

I think a 3/8" skin would work fine if you've got a decent floor on top (see Fred's comment about damage). Heck, at that point 1/4" would probably be fine since you're talking tension and compression. What's the flooring?

[iublue]

I take it back - I found a note that said my hatch was 75 pounds, so you'd be in the 100 range. 

I think a 3/8" skin would work fine if you've got a decent floor on top (see Fred's comment about damage). Heck, at that point 1/4" would probably be fine since you're talking tension and compression. What's the flooring?

If I remember correctly, it was vinyl plank flooring that I would have to glue down.

After the addition, this access will be almost be in the middle between the kitchen and the dining room so there will be a lot of traffic across it.  It will really be in a main traffic area.  Since it will be so out in the open, I need to make it as invisible as possible which won't be easy!

[barryvabeach]

Torsion box is a great solution.  Remember that the strength comes from the distance separating the top and bottom layers -  the further they are apart from one another, the stronger.  I would go with 1/4 ply, and work with a grid spacing that makes you comfortable. I used 1/2 sandeply for the grid.   I made a ramp for a brother who was in an electric scooter, so it probably had to support 500 lbs ( him plus scooter plus another person ) and had to ship it to his location and put it together there, so I was really focused on weight.   I just played with a small square of 1/4 inch  ply, and used blocks to experiment on grid spacing.  IIRC,  I took a one inch square piece and put it on top of the sample ply and stood on it, ( to simulate a concentrated load)  then moved the blocks so they were a little further apart, and repeated it through different grid spacing.  Note that once you glue it to the grid, it will be much stiffer than the test i was doing.    If someone dropped a hammer, or something sharp, though could damage the floor, though most floor loading is not any where near that concentrated.

[abernat]

Vinyl's not going to add any stiffness, so yeah - just a sheet of plywood won't do it. 

In that case, I'd lean towards a 1/2" top skin and a 1/4" bottom skin. You really only need 1/4" to actually provide stiffness, but imagine someone tromping in high heels or dropping something sharp - 1/4" won't prevent penetration and 3/8" is iffy. And like Barry said, the thickness is what provides strength, so as thick as you can.

Then just make sure you have solid points for mounting the struts. Good luck!

[NickNC]

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.

[Cooler]

I just went to the sagulator site: https://woodbin.com/calcs/sagulator/

Using 3/4" thick fir plywood and 1-1/2" x 3/4" edging on a 36" x 36" opening, and asking the door to support 300 pounds (33.3 pounds per square foot), it lists the sag as "boarderline".

The total calculated sag per foot = 0.034 in per foot

When I figure glue + screw two layers of 3/4" ply (1.5" total thickness), the sag is 0.005 in per foot and is listed as "acceptable. 

I think I would use a generous amount of glue and screw 6" on center.  I would make it slightly oversize and trim after laminating the boards.  Total weight would be 54 pounds.  

These soft close hinges are rated for 60 pounds (per pair).

https://www.amazon.com/Springs-Support-P...XUM1&psc=1

[barryvabeach]

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.