Spiral vs Curved vs Winder; "within individual dwelling unit"

[Mr.Mannerist]

Hello all, long time listener first time caller.
Posting this in commercial rather than residential because it's multifamily and I wanted answers from a more rigorous perspective.
I'm working on a fairly compact proposal for a condominium development subject to California Building Code and I'm making use of discrete means of egress to cut down on wasting space with corridors. The development is under four stories, so I've arranged things such that only four units share a stair and don't require double means of egress. The rest of the units are multi-story and will exit directly to the right of way at the level of discharge. I have one remaining unit that will get its own stair. I know this is all very zany as a narrative, but on paper it makes more sense.
My two questions are:
1. Can I have a stair serve one dwelling unit, and consider it "within an individual dwelling unit" even if it is exterior? Could this be accomplished by having a locked door/gate at the bottom? The principal access to these units would be by elevator from a mechanical parking lobby.
2. Where is the line between a spiral stair and a curved stair or winder? Is a spiral stair allowed to have straight run sections at the top and bottom and follow the typical spiral stair standards so long as it is constructed, on the whole, more like a spiral stair? This would be for the stair that would hopefully be considered "within an individual dwelling unit." The unit in question is a ~500sf studio. I got the idea to treat it this way from a Loos project with a similar arrangement.

 

[tbz]

I will take a shot here, seems no one wants to bite.

1.) I don't have a code section I can hang my hat on for your first question, but all the projects I have worked on over the last 35 plus years have always considered open to air as outside and on the property, and within has always been determined to be not outside, like is a balcony only accessible to a single condo unit allowed to take the 36" guard height, as the balcony is within correct, no 42" unless meets one of the other exceptions.

So I would say within means inside, not necessarily conditioned space, but not an open air environment.

2.) Difference between Curved and Spiral stairs are

2019 CBC Section "1011.9 Curved Stairways. Curved stairways with winder treads and risers in accordance with Section 1011.5 and the smallest radius shall not be less than twice the minimum width or required capacity of the stairway."
"Exception: The radius restriction shall not be apply to curved stairways in Group R-3 and within individual dwelling units in Group R-2"

Spirals: are section 1011.10 Spiral Stairways

So what are the major differences between a curved stair and a spiral stair for residential R-3 & R-2 inside individual units were the radius restriction is exempt.

The major difference is that curved stairs flights are required per exception 3 of section 1011.5.2 to have a minimum 6" tread depth on the smallest side and 10" on the walkline and a maximum of 7.75" of riser height.

A spiral stair per Section 1011.10 of the 2019 CBC is allowed to have a 6.75" Minimum tread depth at the walkline, 3.25" less, and 9.5" maximum riser heights, thus 1.75" higher risers.

Additionally, a spiral in a residential is a secondary stair, as it can only be used for a M.O.E if the area it serves is 250sqft or less per 1011.10.

The base dimensions and tread and riser layout will drop it in to one or the other, as a spiral can't be done without higher risers to meet the minimum headroom, and curved stairs with 7.75" risers or less will flop it into the winder/curved stair area.

 

[Mr.Mannerist]

1011.10 says "Spiral stairways are permitted to be used as a component in the means of egress only within dwelling units or from a space not more than 250 square feet (23 m2) in area and serving not more than five occupants, or from technical production areas in accordance with Section 410.5." Within dwelling units *or* from a space not more than 250sf. I can't think of any other way to read this than that the 250 limitation doesn't apply within a dwelling unit.

 

[Mr.Mannerist]

I will take a shot here, seems no one wants to bite.

1.) I don't have a code section I can hang my hat on for your first question, but all the projects I have worked on over the last 35 plus years have always considered open to air as outside and on the property, and within has always been determined to be not outside, like is a balcony only accessible to a single condo unit allowed to take the 36" guard height, as the balcony is within correct, no 42" unless meets one of the other exceptions.

So I would say within means inside, not necessarily conditioned space, but not an open air environment.

2.) Difference between Curved and Spiral stairs are

2019 CBC Section "1011.9 Curved Stairways. Curved stairways with winder treads and risers in accordance with Section 1011.5 and the smallest radius shall not be less than twice the minimum width or required capacity of the stairway."
"Exception: The radius restriction shall not be apply to curved stairways in Group R-3 and within individual dwelling units in Group R-2"

Spirals: are section 1011.10 Spiral Stairways

So what are the major differences between a curved stair and a spiral stair for residential R-3 & R-2 inside individual units were the radius restriction is exempt.

The major difference is that curved stairs flights are required per exception 3 of section 1011.5.2 to have a minimum 6" tread depth on the smallest side and 10" on the walkline and a maximum of 7.75" of riser height.

A spiral stair per Section 1011.10 of the 2019 CBC is allowed to have a 6.75" Minimum tread depth at the walkline, 3.25" less, and 9.5" maximum riser heights, thus 1.75" higher risers.

Additionally, a spiral in a residential is a secondary stair, as it can only be used for a M.O.E if the area it serves is 250sqft or less per 1011.10.

The base dimensions and tread and riser layout will drop it in to one or the other, as a spiral can't be done without higher risers to meet the minimum headroom, and curved stairs with 7.75" risers or less will flop it into the winder/curved stair area.

Also, I know this is beside the point, but since it's supplied as anecdotal anyway, I don't know of any situation in which a 36" railing is permissible, even within dwellings/the R-3 I usually do in the office.

 

[Mr.Mannerist]

I guess another test case would be 'what would the AHJ say about 7 1/2" riser heights between levels on a terrace'

 

[mark handler]

A locked door/gate at the bottom, presents a problem. Are you trapping the exiting occupants?

What is a mechanical parking lobby?

 

[mark handler]

A winding staircase completes a turn with angles and does so with angular steps without a center pole.
A spiral staircase is typically a tight radius stair that uses a diameter for sizing and wraps around a center pole.

 

[Mr.Mannerist]

A locked door/gate at the bottom, presents a problem. Are you trapping the exiting occupants?

What is a mechanical parking lobby?

No more than the locked door of your house is.

Mechanical parking systems have a lobby where you park your car, exit, and the mechanism then transports your car to the storage area.

 

[Yikes]

For a residential dwelling unit with an interior spiral stair serving a mezzanine, has anyone calculated what the minimum dimensions in CBC 1011.10 translate into as a minimum stair (plus handrail) diameter?
Also, I note that the 2019 CBC changed the minimum tread depth from the previous 7.5" down to 6.75".
any idea as to why the shallower tread was approved?

 

[Mr. Inspector]

I have a situation for a change of occupancy M to B. 2018 IEBC1005.1 Means of egress in portions of buildings undergoing a change of occupancy classification shall comply with Section 1011. The stairway going to the 2nd floor has a section that is a curved or a winder area in it. IBC1011.5.3 says winder treads are not permitted in a means of egress but curved stairways are OK. 1011.9 says curved stairways can have winder treads per 1011.5. which tells me how winder treads should be. I'm confused, can this be a means of egress or not?