Curved Stair flight Inner Radius

[tbz]

Good afternoon everyone,

Working on a set of drawings for a client and I have a question that seems to allude me as to why it exists and I don't see it commonly enforced.

A little background information; brand new small private office building for a single user, the building is only 2 stories, 1st fl 2,800 sqft, 2nd fl 1,900 sqft, 13ft fl to fl total rise.

Inside the first first floor there is an elevator that is used to go from 1st floor to 2nd floor.

Outside the building under cover is the MOE compliant stair flight for the 2nd floor.

However, inside the building in the corner is an interior NON-MOE designated stair flight, a convenience stair, again not the designated MOE. The stair flight is curved in one direction and no other treads, now for my question.

The Stair flight has an inside tread radius of 40.75" and an outside tread radius of 84.75". The treads are slightly over sized with 6.875" risers, thus they exceed the tread depth requirements at the required walk line.

Project is in Florida by the way: 2017 FL IBC

Now Section 1011.9 Curved Stairways. "Curved stairways with winder treads shall have treads and risers in accordance with Section 1011.5 and the smallest radius shall be not less than twice the minimum width or required capacity of the stairway."

Now to me winder treads are treads that go in different directions, like making an "S" turn or straight treads and then curved treads and the definition of "Winder" in chapter 2 of the 2017 Florida IBC reconfirms part of this with "A tread with NON-Parallel edges."

Well on a curved stair the radius edges are parallel but the front and back edges are not, as they go out in a fan tail pattern.

Now the current design and flow has the inner tread surface over the minimum required on a large radius stair with the same requirements and thus I am trying to verify how 1011.9 does or does not affect this stair flight.

For the record, I looked at a spiral stair definition and it notes it has a center column, so whether you can have a tighter radius is solely based on column being present?

I am looking for insight to see if I am over thinking this or is my question just in that a spiral stair can be real tight, but a curved stair can't be less than twice the width at the inner radius even with larger stair treads to offset a tighter radius at the walk line.

How do others read section 1011.9 and apply it to a single direction curved stair with no straight treads, same direction and uniform size treads for the complete stairlfight, top to bottom.

Look forward to your responses - Regards Tom

 

[Builder Bob]

It will make the spiral stair case wider than residential style spiral stairs.

 

[tbz]

It will make the spiral stair case wider than residential style spiral stairs.

Bob, not following you, the stair flight is a curved stair with a inside radius less than 2 times the 36" tread width. However the treads surface area for foot placement is large than the minimum required for a stair with a 6ft inside radius. Which is why I am questioning the radius restriction rather than minimum tread area size?

 

[Builder Bob]

The commercial application creates a wider circular stair than residential.

 

[jboggs]

tbz, did you get any further on this by chance? had a colleague contact me today about the same issue. just trying to understand the logic regarding the 2x inner radius especially when all other factors are met. almost identical to your original post.

 

[tbz]

Jboggs,

I have traced the wording back, prior to the 2000 IBC publication into other legacy codes, however, I have not been able to find where the science comes into play for the requirement.

I was going to present a code change in the Part A cycle but did not have any ducks in a row to do that. As thus, I have plans to dive deeper and possibly present a proposal if I can't find any justification other than wanting to make sure not a tight radius. The question becomes, IMO if you can't have a curved winder stair as part of a MOE, then if you meet all the requirements except for the minimum radius of 2x the min width, why?

 

[jboggs]

Thanks, tbz. Exactly. I can't wrap my head around it unless there's just some formula or threshold that was generally true for dimensional tread safety factors stemming from either a 36" min or 44" min egress width. It shouldn't matter as long as the 11" min tread depth is maintained per code. In our case, we have the 11" tread depth starting at the inner stringer for our winder stairs, so we're more than covered. I'll be interested to learn more. I'll post if I do!

 

[Paul Sweet]

It might have been to reduce the difference between the inside and outside depth of a tread. A more logical approach would be similar to winders - a minimum tread depth at the inside and a minimum depth at a certain distance from the guard.

 

[tbz]

I am not sure about why the double the width, and not a fixed minimum radius to the required minimum only, as the or allows, double the minimum required width, should that not be it period?

I will venture a guess when established it was based on providing a fluent flow while descending, as a curved stair is allowed in an MOE.

I have been wondering if the requirement could not be relaxed for flights that are not part of the MOE and situated in a location with minimum occupancy.