Is a torsion box the best solution?

[Cooler]

There are fire code rules for laundry drops that require that the chute and "door" be fireproof.  The opening would act as a flue otherwise.  I wonder if that is true for these bunker hatchs.  A quick look online show that they range from $600.00 to $2,500.00.  So there must be something more than  a couple of sheets of plywood and some internal structure going on.

https://www.google.com/search?q=bunker+h...00&bih=775


This aluminum hatch is $868.00.
https://www.google.com/search?q=bunker+h...9427114820



This solid cast aluminum plate costs just $400.00 and is listed as "suitable for pedestrian traffic only".

https://drainageproducts.us/24-aluminum-solid-cover/

[iublue]

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.

This is to a basement that is nothing more than a mechanical area.  No one is going down there outside of a repairman for the furnace etc. or the homeowner to put salt in the softener.

[iublue]

There are fire code rules for laundry drops that require that the chute and "door" be fireproof.  The opening would act as a flue otherwise.  I wonder if that is true for these bunker hatchs.  A quick look online show that they range from $600.00 to $2,500.00.  So there must be something more than  a couple of sheets of plywood and some internal structure going on.

https://www.google.com/search?q=bunker+h...00&bih=775


This aluminum hatch is $868.00.
https://www.google.com/search?q=bunker+h...9427114820



This solid cast aluminum plate costs just $400.00 and is listed as "suitable for pedestrian traffic only".

https://drainageproducts.us/24-aluminum-solid-cover/

I would in no way, shape or form call what I am doing a bunker hatch.  That would need a ladder.

This is an old house where I am sure the stairs were originally outside and then an addition put the stairs inside and now another addition puts it basically in the middle of the room.

[iublue]

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.

I believe the problem with an 1 1/2" of plywood is the weight issue.  I will look at all options.

[iublue]

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.

What structural requirements would a torsion box not meet?

[TDKPE]

The purpose of a torsion box is for maintaining flatness and stability against warping or wracking, like for a RAS or DP table.  With the hatch closed, it will (or could) lay on a sill around its entire perimeter, so there is zero need for such stability.  What you do need is stiffness across its drum head like surface, which is easily framed the short way.  An open frame with a 3/4" top would be stiffer than the floor around it, and maybe either make the sill adjustable, or slightly lower than what keeps the top flush when closed, and shim it to fine tune it.

[NickNC]

What structural requirements would a torsion box not meet?

Well, first, you’re missing floor joists.  Plywood doesn’t apply unless it’s an engineered parallam or gluelam type beam (IBC 2308.4.3).  Floor joists account for a certain amount of deflection that wood joists and beams absorb and which a torsion box does not.  It is not “structural” by building standards.
Second, you can’t possibly follow the required fastening schedule required for floors.  Third, the weakest point will be any lip or hinge connection that holds the box in place. 16 screws and a piston do not qualify as “continuous support”
 
This is covered in section 2304 of the International Building Code in sections 2304 and 2308.  Table 2308.4.2.1 gives you minimum requirements (smallest I see is a 2x6) for floor framing by species for 10 psf floor dead load, and if this is a bedroom, a live load of 30 psf is required.
 
How often it gets used is irrelevant to the function it will serve and that is, bottom line, a floor.  Also, the framing required to hold such an opening will require support or footings of some sort.  It will need to be fire rated, and likely if the house is ever sold, it will likely need to be ripped out because it's simply not code compliant.
 
You’re designing an expensive solution to what amounts to a little excavation and a $500 metal bulkhead door.  Human lives are at stake, not just the occupant’s, but service techs as well.

[AHill]

This might have been mentioned before, but I would first consider all the things your friend might drag across that door. A fridge on a dolly? A large person jumps on the door or runs across it? Just considering the static (non-moving) weight of a heavy person wouldn't include those events. Consider what a normal floor is made from. For sure, the span of the floor joists has something to do with it, but so does the thickness of the underlayment and floor covering. Underlayments for typical second floor floors are 5/8" to 1" thick. Add another 1-2" for the finish part of the floor. Also consider what that door is resting on.

[iublue]

Well, first, you’re missing floor joists.  Plywood doesn’t apply unless it’s an engineered parallam or gluelam type beam (IBC 2308.4.3).  Floor joists account for a certain amount of deflection that wood joists and beams absorb and which a torsion box does not.  It is not “structural” by building standards.
Second, you can’t possibly follow the required fastening schedule required for floors.  Third, the weakest point will be any lip or hinge connection that holds the box in place. 16 screws and a piston do not qualify as “continuous support”
 
This is covered in section 2304 of the International Building Code in sections 2304 and 2308.  Table 2308.4.2.1 gives you minimum requirements (smallest I see is a 2x6) for floor framing by species for 10 psf floor dead load, and if this is a bedroom, a live load of 30 psf is required.
 
How often it gets used is irrelevant to the function it will serve and that is, bottom line, a floor.  Also, the framing required to hold such an opening will require support or footings of some sort.  It will need to be fire rated, and likely if the house is ever sold, it will likely need to be ripped out because it's simply not code compliant.
 
You’re designing an expensive solution to what amounts to a little excavation and a $500 metal bulkhead door.  Human lives are at stake, not just the occupant’s, but service techs as well.

The solution of a door would be MUCH more than what you state.  You would also have an ugly metal door in the middle of a room.

The overly melodramatic "human lives are at stake" is amusing.

[NickNC]

The solution of a door would be MUCH more than what you state.  You would also have an ugly metal door in the middle of a room.

The overly melodramatic "human lives are at stake" is amusing.

I'm suggesting relocating the basement entrance, not putting the metal door in the room.
It's only "overly melodramatic" until someone gets hurt.  Just trying to help.