Spiral vs "regular"

[Yankee]

Does anyone have inside information on why the code allows a higher rise and shorter tread depth for a spiral stair than it does for a straight run or winder stair?

 

[ICE]

I have been inside only once. Trust me, it's a scary place not for the faint hearted. I'll never go there again. While I was there, I was privileged to find out how the number 50 was chosen as a trigger for two exits. A number was needed and fifty sounded about right. Not a shred of empirical evidence resulted in going to the hat. It was as simple as that and here we thought a lot of powerful thinking goes into this stuff.

 

[Francis Vineyard]

"Spiral stairs have the disadvantage of being very steep. Unless the central column is very large, the circumference of the circle at the walk line will be small enough that it will be impossible to maintain a normal tread depth and a normal rise height without compromising headroom before reaching the upper floor. To maintain headroom most spiral stairs have very high rises and a very short going."

http://en.wikipedia.org/wiki/Stairway#Spiral_and_helical_stairs

 

[Yankee]

Someone better come up with a better answer than that or the man upstairs is going to give a "waiver" on a (non-compliant) straight run stair with a (non compliant) winder at both top and bottom because a spiral stair complies.

 

[imhotep]

Someone better come up with a better answer than that or the man upstairs is going to give a "waiver" on a (non-compliant) straight run stair with a (non compliant) winder at both top and bottom because a spiral stair complies.

Spiral,straight run or winder? Pick one and apply the relevant criteria. A straight run stair with winders at a turn, top or bottom is not a spiral stair.

 

[Yankee]

Spiral,straight run or winder? Pick one and apply the relevant criteria. A straight run stair with winders at a turn, top or bottom is not a spiral stair.

I DO know that, how do I explain to someone else that the criteria for a spiral stair cannot be cherry-picked for other types of stairs? WHY are the dimensional requirements of the spiral stair considered safe to traverse where those same dimensional criteria are not safe to traverse on other types of stairs?

 

[ICE]

When people traverse stairs they pay attention to the first step or two and then they have established a gait. Once we have established a gait, we quit paying attention to the stairs and up or down we go. That is why stairs are required to be within 3/8" of each other. With spirals, we don't do that. We are not comfortable on spirals and we pay attention all the way. We don't establish a gait on spirals and we watch every step.

In as much as spirals could not exist otherwise and we are extra careful on spirals, it seems to work. Now change a step on a conventional stair and that's a recipe for disaster. Let people establish a gait but change the path and accidents will happen. Stairs are probably the most dangerous component of any building and it would be foolhardy to allow less than the minimum code.

I didn't dream this stuff up. There is science behind this explanation. Where you will find it escapes me but it is out there.

I apologize for not telling you this yesterday but I was certain that an architect would step up to the plate.

Tiger

 

[imhotep]

I DO know that, how do I explain to someone else that the criteria for a spiral stair cannot be cherry-picked for other types of stairs? WHY are the dimensional requirements of the spiral stair considered safe to traverse where those same dimensional criteria are not safe to traverse on other types of stairs?

I presume that you are reviewing a residence and dealing with a relatively inexperienced designer or homeowner.

2009 IRC

R201.3 Terms defined in other codes. Where terms are not defined in this code such terms shall have meanings ascribed to them as in other code publications of the International Code Council.

2009 IBC

SECTION 1002 DEFINITIONS

STAIRWAY, SPIRAL. 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.

It is either a spiral stairway or it is not. If it is a spiral then the maximum rise is 9 1/2". If not then then maximum rise is 7 3/4" per IRC. Spirals are special stairs closer akin to a ladder than a straight run flight. Ladders are never allowed in a means of egress path. Ships ladders are allowed only in limited circumstances. Spiral stairs use is also addressed in the IBC.

1009.9 Spiral stairways. 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 galleries, catwalks and gridirons in accordance with Section 1015.6.

Tiger addressed the human factors that boil down to the fact that the user never establishes a gait.

 

[Yankee]

Thank you Tiger, that is exactly how I understand it from my design training, just needed a confirmation from an "unbiased" source!

 

[brudgers]

When people traverse stairs they pay attention to the first step or two and then they have established a gait. Once we have established a gait, we quit paying attention to the stairs and up or down we go. That is why stairs are required to be within 3/8" of each other. With spirals, we don't do that. We are not comfortable on spirals and we pay attention all the way. We don't establish a gait on spirals and we watch every step. In as much as spirals could not exist otherwise and we are extra careful on spirals, it seems to work. Now change a step on a conventional stair and that's a recipe for disaster. Let people establish a gait but change the path and accidents will happen. Stairs are probably the most dangerous component of any building and it would be foolhardy to allow less than the minimum code. I didn't dream this stuff up. There is science behind this explanation. Where you will find it escapes me but it is out there. I apologize for not telling you this yesterday but I was certain that an architect would step up to the plate. Tiger

Some of that is true, on the other hand having climbed a spiral many times a day for the better part of the last six years, you do develop a gait over time. The real hazard is the bottom step in the dark - because of the higher rise and variable tread depth that one can cause some serious hurt.