Frontage

[Sifu]

So that means you’re using 2021 IBC, but the applicant has submitted a plan using the 2018 IBC method - are you in a transition period that allows the designer to comply with either code? Table 506.3.3 says interpolation is permitted, it doesn’t say calculation per the 2018 IBC method is permitted.

Correct, they appear to want to have it both ways. But to be fair, so would I. However code has no provision for this. There is no transition period, this AHJ has been on the '21 for years.

I think it precludes anything on the far side of the reduced width from being considered frontage:

1. 2021 IBC 506.3.1 requires perimeter to be accessed from a street or fire line. Implied in my opinion is that the entire access path be open space per 506.3.2.
2. 2021 IBC 506.3.2 says minimum width of the open space is 20’.

Agree, though I am questioning the viability of all of it.

Nice catch, shows you’re doing a thorough review.

Thanks. I was really hoping this project would be better, but the more I dig, the lass confident I become...so the more I dig.

 

[Tim Mailloux]

Because the interior perimeter of the doughnut is not available for firefighter operations..Because that is the intent…

Read it like this a little…

506.3.1​

To qualify for an area factor increase based on frontage, a building shall have not less than 25 percent of its perimeter on a public way or open space.

If the space doesn’t have direct access from a street or fire lane it doesn’t count…IMO

I always always include courtyards into the total perimeter number, and not count any of that courtyard perimeter as frontage. My approach is actually more conservative. Same with fire walls, I count the fire walls as building perimeter with zero frontage.

 

[Sifu]

I always always include courtyards into the total perimeter number, and not count any of that courtyard perimeter as frontage. My approach is actually more conservative. Same with fire walls, I count the fire walls as building perimeter with zero frontage.

That is the way I have always done it using 2018. But I've been wrong before. In this case, steveray's way would seem to penalize them for leaving a lilttle section open over enclosing it entirely.

2021 IBC with the new table instead of the old equation is somewhat less descriptive since it doesn't use "P= Perimeter of the entire building (feet)" as the 2018 did.

As skeptical and distrustful of "AI" as I am.... when I ask the question it says to use the outer P, and add the inner courtyard wall P. Where it comes up with this is a mystery so I can't pick and choose my skepticism and just accept what it says to suit my own perception.

 

[Sifu]

I'll let y'all digest this before I offer my thoughts.

 

[walker.t]

I'll let y'all digest this

Interesting, thanks for sharing.

 

[steveray]

Amazing that they use the same firefighter operations/ availability language I do...

 

[Sifu]

Amazing that they use the same firefighter operations/ availability language I do...

But only when "absolutely necessary"?

 

[steveray]

But only when "absolutely necessary"?

Well that part is just dumb...

 

[Sifu]

I am discounting all the courtyard perimeter walls on the basis they are not accessed by a street or fire lane without turning a corner and not accessed by spaces with a min. 20' open and usable space even where they MIGHT be. I am also discounting the use of a weighted average and requiring the smallest space. This will place the building above the allowable area, but even though it is absolutely necessary to use it I am not accepting it. I am sure it will end up with the CBO/CFO, which I guess is where they want it.

 

[Sifu]

Well that part is just dumb...

That is my most favorite part. What if it is only somewhat necessary? Maybe only slightly necessary? What if I was just trying to learn what the heck it meant so I could accurately and consistently administer the code? What if it was only absolutely necessary in the designer's eye, but not mine or the CBO? JMHO, but that part is "absolutely ridiculous".

Why would anyone go to the trouble to figure this stuff out if the idea of a minimum code wasn't already "absolutely necessary"?

 

[steveray]

That is my most favorite part. What if it is only somewhat necessary? Maybe only slightly necessary? What if I was just trying to learn what the heck it meant so I could accurately and consistently administer the code? What if it was only absolutely necessary in the designer's eye, but not mine or the CBO? JMHO, but that part is "absolutely ridiculous".

Why would anyone go to the trouble to figure this stuff out if the idea of a minimum code wasn't already "absolutely necessary"?

Pretty sure they are just keeping the door open for someone that wants to.....

 

[Yankee Chronicler]

I'll let y'all digest this before I offer my thoughts.

That is one of the weakest, most wishy-washy staff opinions I have ever seen from the ICC (or from BOCA before that). It's a three-page word salad that, IMHO, is totally worthless. Fortunately, it's a staff opinion, which is advisory only and non-binding.

 

[Sifu]

That is one of the weakest, most wishy-washy staff opinions I have ever seen from the ICC (or from BOCA before that). It's a three-page word salad that, IMHO, is totally worthless. Fortunately, it's a staff opinion, which is advisory only and non-binding.

Yes. Two of the three questions are "up to the BO", one is only if absolutely necessary, but at least one is definitive, which was a gimme anyway.

 

[Sifu]

Yet another take on 2021 Frontage: The following building has 1173'6" total perimeter with 1019'5" 30' or greater and 154'1" at zero (firewall).



The way I would do it they would have 86.8% of 30' or greater, allowing a 75% increase. The way they did it (I think somehow as a weighted average based on each line of the table but frankly not sure what math they are showing) they only get 65%.

 

[walker.t]

Yet another take on 2021 Frontage

I also get 75% for the frontage increase factor in Table 506.3.3, not sure why they did it the way they did it.

2021 IBC 506.3.3 says that the area factor increase is determined using Table 506.3.3, as you know the calculation they used doesn’t use Table 506.3.3.

 

[Sifu]

I also get 75% for the frontage increase factor in Table 506.3.3, not sure why they did it the way they did it.

2021 IBC 506.3.3 says that the area factor increase is determined using Table 506.3.3, as you know the calculation they used doesn’t use Table 506.3.3.

I am befuddled. I thought I had a handle on it. IMO, though I am not accustomed to it yet, the '21 is an easier way, which is what the code change was aiming for. Using the smallest frontage greater than 20' may make it a little more conservative, but that doesn't explain this.

I have been wondering about the "interpolation" footnote and how that applies and if possibly that is what they are trying to demonstrate. Checking the 2024 commentary it appears they realized they needed to explain the "interpolation", so they offer guidance saying the interpolation as being used for specific frontage distances when in between the 20' to 25' & 25' to 30' columns, not to calculate each individual face of the building which I think they are attempting.

Forgetting for a moment what these people are trying to do, I do see an interesting issue with the smallest distance above 20' thing. Say a building has 30' on 3 sides, and 20' on the 4th side which is very small. If you calculated frontage using 20' as the smallest distance from the table the percent frontage is lower than if you simply negated the side all together and removed it from the F calculation. So if if instead of calculating the one side at 20' it would make more sense to ignore it as if it were 19'. Previous editions accounted for this with the weighted average, but 2021 and beyond penalizes you. Or I don't understand it at all.

 

[walker.t]

…I do see an interesting issue with the smallest distance above 20' thing.…Previous editions accounted for this with the weighted average, but 2021 and beyond penalizes you.

The 2021 IBC commentary on 506.3.3 Amount of Increase says, “This section takes the equation used in IBC editions prior to 2021 for frontage area increase and puts it into a table for easier calculation.” They sacrifice the precision of the weighted average calculation in exchange for “easier calculation” - I think most people would rather bear with the inconvenience of the weighted average calculation so they can maximize allowable building area when they qualify for the frontage increase. So your “interesting issue” could be considered a bug - a good intention with the unfortunate result of yielding a lower (penalized) area factor increase.

I have been wondering about the "interpolation" footnote and how that applies and if possibly that is what they are trying to demonstrate.

Me, too, actually. The open space columns in Table 506.3.3 are asking for the minimum open space width, not a weighted average, so the calculation you showed in #38 would not apply. Here’s a graph showing how I am thinking interpolation of the open space width would work:

Or I don't understand it at all.

In my opinion, that you noticed this and ran your test calculation indicates you have a good understanding of the matter.

 

[Sifu]

I read the code change proposal and it seems the idea was that nobody ever used the weighted average, and too many people didn't know how to correctly use frontage calculations. And I can't disagree. Any time I have received frontage increases and ran them I have found a lot of anomalies. The frontage examples use simple square buildings at right angles to the fire lanes. Unfortunately most buildings are never that simple. The one I have now has one building at 1100' of P, and any way I run it they actually have 1500+ of P, so I have no idea how they are getting their numbers. My only option is to ask for a graphic representation of where they get their P and F data. I like the new table, but I have had two submittals in a row where it still may be a problem.

I think the graph you use to show interpolation is what they describe in the 2024 commentary. I think they missed the mark if that is what they consider simpler.

I still can't really decipher what they are trying to show with their calculation in post #38.

 

[walker.t]

and it seems the idea was

I think a benefit of the new method is that it should make plan review a bit faster and easier - if the drawings follow the method and show where they are getting their numbers. But under the old method someone could have (and should have) showed their weighted average calculations on the drawings to support their calculations, so maybe it’s not such an advantage in plan review.

Any time I have received frontage increases and ran them I have found a lot of anomalies.

That’s awesome, that you’re looking that closely and confirming the calculations.

The frontage examples use simple square buildings at right angles to the fire lanes.

I noticed that, too, more complicated examples would be helpful.

The one I have now has one building at 1100' of P, and any way I run it they actually have 1500+ of P

That’s weird, maybe it was a typo that slipped by.

My only option is to ask for a graphic representation of where they get their P and F data.

As a plans submitter, I would not consider that to be an unreasonable request. I’d probably feel a little stupid for not having thought to include that information the first time.

I still can't really decipher what they are trying to show with their calculation in post #38.

You shouldn’t have to spend too much time deciphering the drawings, there is an expectation they will provide clear information or provide additional information to give you the clarity you need to do your job.