as in circular?

blah blah blah ... Winder treads shall have a minimum tread depth of 11 inches measured at a right angle to the tread's leading edge at a point 12 inches from the side where the treads are narrower and aminimum tread depth of 10 inches.

Exceptions:

4. In group R-2, R-3 occupancies ... the minimum winder tread depth at the walk line shall be 10 inches (that's the line 12 inches in from the narrower treads); and the minimum winder tread depth shall be 6 inches.

1009.7 Curved stairways Curved stairways with winder treads shall have treads and risers in accordance with Section 1009.3 and the smallest radius shall not be less than twice the required width of the stairway.

Exception: The radius restriction shall not apply to curved stairways for occupancies in Group R-3 and within individual dwelling units in occupancies in Group R-2.

so, mleitner, what's the occupancy and what do you need clarified?

FRaC, it's an R-3 occupancy.
The exception states out that the minimum winder tread depth shall be 6 inches. Most stairs incorporating winding transitions have treads that taper to ~0 inches on the narrow side(see images). Is this not permissible under IBC? Where do you measure the 6 in.?

yes, you cannot go to 0" per that image. that's not allowed per the IBC or the previous UBC.

so you need 6" minimum where the treads wind around and then 12" in from there the treads have to be at least 10". you'll end up with a little radius on the inside and not that straight vertical line you used to get back in the day.

if you think about it when you're walking up the stairs your feet are about 12" in from the handrail so the code wants you to have that minimum tread depth so you don't fall down.

oh yeah !! (of course that interior winding part doesn't have to be curved ..)

So the "walk line" is not measured at the center of the tread?

Thank you FRaC, your explanations are very helpful.

yeah the IBC isn't very clear on the definition of what a 'walk line' is under 1009.3 exception 4. but if you go to '1009.3.2 dimensional uniformity' it states 'The greatest winder tread depth at the 12-inch walk line within any flight of stairs shall not exceed the smallest by more than 0.375 inch measured at a right angle to the tread's leading edge.'

so that's where i get that the walk line is 12 inches in from the edge of the narrower (winding) treads.

The Patterson Handbook concurs with that interpretation. I don't know why they didn't just repeat the language if they were trying to say the same as above.