Spiral Stair Walkline Radius

[VLADIMIR LEVIN]

In R311.7.10.1 what does a maximum walkline radius of 24 1/2" mean or have to do with anything? Is it saying that at the 24 1/2" walkline radius the min tread depth is 6 3/4"?

 

[Inspector Gift]

R311.7.10.1 Spiral Stairways

Spiral stairways are permitted, provided that the clear width at and below the handrail is not less than 26 inches (660 mm) and the walkline radius is not greater than 241/2 inches (622 mm). Each tread shall have a depth of not less than 63/4 inches (171 mm) at the walkline. All treads shall be identical, and the rise shall be not more than 91/2 inches (241 mm). Headroom shall be not less than 6 feet 6 inches (1982 mm).

The above code section could have been worded better - simply and clearly. Defined terms are necessary to understanding the code section.
SPIRAL = Circular in pattern
WALKLINE = optimum line of travel used in design of the stairs -- usually measured 12 inches from the end of the tread.
RADIUS = 1/2 of the diameter of the circle - measured from the center to the outer edge.

 

[e hilton]

Actually, that section reads pretty clearly. I looked up spiral and winder in my IBC, and the winder definition is very confusing.

One question ... why is there an upper limit on the walk radius? The radius is essentially a function of the diameter of the “column” that the stairs wrap around. Suppose the column was a 10 ft dia round room ... that exceeds the 24-1/2” radius.

 

[Mr. Inspector]

One question ... why is there an upper limit on the walk radius? The radius is essentially a function of the diameter of the “column” that the stairs wrap around. Suppose the column was a 10 ft dia round room ... that exceeds the 24-1/2” radius.

I guess that would be a winder tread per R311.7.5.2

[RB] WINDER. A tread with nonparallel edges.

 

[Enri Code]

In R311.7.10.1 what does a maximum walkline radius of 24 1/2" mean or have to do with anything? Is it saying that at the 24 1/2" walkline radius the min tread depth is 6 3/4"?

Yes, it means that at the 24 1/2" maximum radius of where the walkline would be, the minimum tread depth is 6 3/4".

Walkline is the arc that is located 12" from the inside stringer.

The significance of the 24 1/2" measurement is that it sets a limit to the dimension of where the inside stringer is placed which is at maximum (24 1/2" minus 12"... ) 12 1/2" from center of the spiral stair. This limit then removes the ambiguity of what would technically be considered a spiral stair by code.

This is valuable information to us because spiral staircases cannot be used as the primary means to access an upper floor due to its limitations and would still necessitate another more conventional stair to be paired with it for example.

So exceeding the 24 1/2" radius will technically move you away from the stair being a spiral staircase and more as a curved staircase which opens you up to using the staircase as your primary stair and not just a decorative element.

Personally, I like a 72" minimum radius to a walkline if space and budget permits. I find that very practical and usable.

 

[e hilton]

[RB] WINDER. A tread with nonparallel edges.

You are correct sir, the definition in chapter 2 is very clear and simple.
Its para 1011.4 Walkline that confuses me, particularly the second sentence:

The walkline across winder treads shall be concentric to the direction of travel through the turn and located 12 inches from the side where the winders are narrower. The 12 inch dimension shall be measured from the widest point of the clear stair width at the walking surface of the winder. Where winders are adjacent within the flight, the point of the widest clear stair width of the adjacent winders shall be used.

 

[VLADIMIR LEVIN]

What is meant by "closed circular form" in spiral stair definition?

 

[Enri Code]

What is meant by "closed circular form" in spiral stair definition?

It doesn't mean much unless you mean "closed circular form" in the context of being viewed from above or what we would call a plan view.

Since it is a stair that primarily winds or "wraps" around a central column/ post or axis, if we look at it from above, it looks like a closed circular form.

If we are thinking 3 dimensions, it is really more of a spiral or a helix.

 

[VLADIMIR LEVIN]

It doesn't mean much unless you mean "closed circular form" in the context of being viewed from above or what we would call a plan view.

Since it is a stair that primarily winds or "wraps" around a central column/ post or axis, if we look at it from above, it looks like a closed circular form.

If we are thinking 3 dimensions, it is really more of a spiral or a helix.

In plan view does it have to form a complete (closed) circle or can it be 3/4 of a circle (where the top riser is not over the bottom riser)?

 

[Enri Code]

In plan view does it have to form a complete (closed) circle or can it be 3/4 of a circle (where the top riser is not over the bottom riser)?

The short answer to your question is "yes".

If it conforms to the configuration of what a spiral stairway is as defined in the applicable International Building Code, then that "3/4 of a circle (where the top riser is not over the bottom riser) stair is a spiral stairway.

Long answer:

I suggest you don't just focus on one part of the definition and look at the whole definition. Here it is:

SPIRAL STAIRWAY: A stairway having a closed circular form in its plan view with uniform section-shaped treads attached to and radiating from a minimum-diameter supporting column.

I would beware in saying or thinking that it means that it is a "complete circle form" because that just changes the meaning entirely.

"Closed circular form" in plan view is a helix and a "complete circle form in plan view" is a disc.

I understand how this may be confusing but it's one of those technical things. If you actually look up "closed circular form" in and of itself out of the context of a stairway... chances are you'll see scientific papers on helical shapes such as DNA or other helical molecular structures.

So if your "3/4 of a circle" stairway has a closed circular form in its plan view with uniform section-shaped treads attached to and radiating from a minimum-diameter supporting column... then it is a spiral stairway even if it is not a "complete (closed) circle".

 

[VLADIMIR LEVIN]

Thanks Enri Code.
FYI: IRC definition says "...minimum diameter circle"

 

[Enri Code]

Thanks Enri Code.
FYI: IRC definition says "...minimum diameter circle"

You are correct.

I was using the 2015 IRC definition which most people still tend to use especially here in this forum.

The 2018 IRC definition had been updated to "... minimum diameter circle" and rightfully so because there are spiral staircases that can be supported in other ways other than being attached to a central column.

Good call by the ICC on that.

They also rearranged some of the words to try to be more clear.

[2015 IRC] SPIRAL STAIRWAY: A stairway having a closed circular form in its plan view with uniform section-shaped treads attached to and radiating from a minimum-diameter supporting column.

[2018 IRC] SPIRAL STAIRWAY: A stairway with a plan view of a closed circular form with uniform section-shaped treads radiating from a minimum-diameter circle.

 

[Enri Code]

 

[ADAguy]

Yes, it means that at the 24 1/2" maximum radius of where the walkline would be, the minimum tread depth is 6 3/4".

Walkline is the arc that is located 12" from the inside stringer.

The significance of the 24 1/2" measurement is that it sets a limit to the dimension of where the inside stringer is placed which is at maximum (24 1/2" minus 12"... ) 12 1/2" from center of the spiral stair. This limit then removes the ambiguity of what would technically be considered a spiral stair by code.

This is valuable information to us because spiral staircases cannot be used as the primary means to access an upper floor due to its limitations and would still necessitate another more conventional stair to be paired with it for example.

So exceeding the 24 1/2" radius will technically move you away from the stair being a spiral staircase and more as a curved staircase which opens you up to using the staircase as your primary stair and not just a decorative element.

Personally, I like a 72" minimum radius to a walkline if space and budget permits. I find that very practical and usable.


View attachment 6662

Excellent info, now; why are residential stairs allowed to have only one handrail?

 

[Enri Code]

Excellent info, now; why are residential stairs allowed to have only one handrail?

To be honest, I hadn't really put too much thought on why residential stairs are allowed to have only one handrail. At least for the types of residences that are under the IRC, I've taken it for granted that those types of structures are primarily of lower volume occupancy than what the IBC would cover hence the logic.

Now you got me thinking so pardon my rambling...

Just to clarify my thought process, I don't generalize residential buildings in one category since both IRC and IBC cover residential buildings but just not the same types.

I still think that IRC residential structures are okay with the one handrail because of lower volume occupancy versus IBC residential structures (e.g. 4 story townhouse) that would have a higher volume occupancy and more height hence needing two handrails minimum. This assumption of lower volume occupancy I believe is also supported by the width of allowable IRC stair being narrower than allowable IBC stairs.

As a matter of managing risks, the IRC has similar required ranges of height for both the handrail and guardrail which enables guardrails to double as handrails in most cases so a stair that has both sides open - a riskier situation - would be required to have each side have a guardrail and effectively a handrail on both sides in effect.

I have no idea if these are truly the reasoning behind allowing just one handrail for IRC by people who put it together but personally, I can see those being my reasons if I was part of that group.

From a design perspective, if I am only putting in one handrail, I tend to place it on the descent side of a right handed person. Just playing with statistics and trying to cast the widest net.

This thought exercise can be a rabbit hole though. Now I'm thinking about all the lefties and maybe even moral obligation to maybe always consider 2 handrails for IRC residential structures regardless of allowed minimum...

 

[ADAguy]

Aha! got you thinking. As you get older you need more support in either direction. Could it have anything to do with swords?
Better practice to provide on both sides. FLW wasn't big on them, bathrooms or kitchens either.

 

[Enri Code]

Aha! got you thinking. As you get older you need more support in either direction. Could it have anything to do with swords?
Better practice to provide on both sides. FLW wasn't big on them, bathrooms or kitchens either.

Looks like I need to brush up on my history. It does make sense to have one hand free to wield a sword to defend oneself in the olden times. Thanks for the brain exercise. Now I need to do some more reading. Thanks.