Maximum stair nosing projection discrepancy- IBC vs ANSI?

[tatami]

I cannot find much documentation on the apparent differences between ANSI's 1.5" maximum stair nosing projection and IBC's 1.25" maximum projection- has anyone found any articles or commentary on this issue?

More specifically, 2009 and 2017 versions of ANSI A117.1 Section 504.5 Nosings notes "The permitted projection of the nosing shall be 1 1/2" maximum over the tread or floor below." The 2015 and 2018 versions of the IBC Section 1011.5.5.1 notes "Nosing Projection size. The leading edge (nosings) of treads shall project not more than 1 1/4 inches beyond the tread below."

Obviously the more restrictive 1 1/4" would rule in a building under both codes, but most diagrams found online with a quick google search show the 1 1/2" dimension. Since there is a good deal of referencing and coordination between the two codes, I'm surprised they haven't agreed upon a unified dimension.... or am I missing something?

 

[Paul Sweet]

My guess is that one of the model codes had a 1 1/4" limit before ADA, and it just got carried over into the IBC.

 

[mtlogcabin]

I did not find a nosing requirement in the UBC
This is from the 1986 CABO One & Two Family Dwelling Code

 

[tbz]

1st off there really isn't a discrepancy between the 2. The standard followed is as you noted the more restrictive model in the IBC.

The 2010 ADA is based on the Access Boards 2004 model, based off of the 1998 ANSI A117.1, and limited 2000 model IBC.

The current cycle of the A117.1 under review has a proposal being reviewed, submitted by the SMA to bring the A117.1 in line with the IBC model codes for nosing projections, but that will take more than a few years to filter out.

But the reality of it, is that vast majority of stair manufactures know and follow the building code adopted at the local AHJ, and the A117.1 id overlooked,

Thus the 1.5" is more of a known item never followed by stair fabricators who have educated themselves.

It is second only to the 2-1/4" handrail minimum clearance that is in NFPA 101 that many AHJ modify out when adopting and change to align with the IBC, A117.1 and the 2010 ADA.

 

[Jay Smith]

For those wondering which code might have been the source, 1996 BOCA has a limit of 1 1/4 inches.