Frontage increase methods from 2018 IBC to 2021 IBC

[Sifu]

Where am I going wrong, or am I? Given: Type IIB, 2 story, A4

For allowable area calculations with frontage using the 2018 IBC:

First is frontage using 506.3.3, equation 5-5
If= [ F/P-.25 ] W/30
If= [ (604/7980)-.25 ]30/30
If= [ .756 - .25 ] 1
If= .506 * 1
If= .506

Next is allowable area using 506.2.3, equation 5-2
Aa= [At + (NS * If) * Sa
Aa= [28,500 + (9,500 * .506)] *2
Aa= [28,500 + 4,807] *2
Aa= 33,307 * 2
Aa= 66,614 (total of floors) or 33,307 per floor

Now, same building using 2021 IBC:

First is frontage using Table 506.3.3,
604/798=75.6 corresponds to an allowable increase of 75%

Next is allowable area using 506.2.1, equation 5-2
Aa= [At + (NS * If) * Sa
Aa= [28,500 + (9,500 * .75)] *2
Aa= [28,500 + 7,125) *2
Aa= 35,625 * 2
Aa= 71,250 (total of floors) or 35,625 per floor

My first question is with frontage. Where are we directed by the 2021 code that in t506.3.3, the "percentage of building perimeter" column is the result of the 2018 F/P? Seems like it should be there. It may be obvious to some, but not all. Or am I misunderstanding it?

Next, comparing the 2018 with the 2021 methods for determining the frontage, there would appear to be a 25% increase in frontage by virtue of the fact that the old formula subtracted .25 from the F/P. I ask because I thought there was not supposed to be an actual change in the results, but just a new (easier) way of doing it, but maybe I am wrong about that.

 

[steveray]

It seemed more giving to me, but I never had the time to figure out the reality...

 

[Paul Sweet]

It looks like the table gives each bin the maximum increase, instead of doing a linear interpolation. If you had just under 75% open perimeter the increase would drop from 75% to 50%.

 

[Sifu]

It looks like the table gives each bin the maximum increase, instead of doing a linear interpolation. If you had just under 75% open perimeter the increase would drop from 75% to 50%.

Why didn't I think of that? Makes sense. (Maybe that's why.)

 

[walker.t]

Where am I going wrong, or am I?

No, you’re doing it right, Paul Sweet was correct when he mentioned the “bins” and what happens when you just cross the line.

Here’s a graph drawn to scale showing how if you interpolated (as the footnote in Table 506.3.3 allows) you’d get pretty much the same answer: