Allowable area confusion

[Sifu]

2018 IBC. I am trying to figure out an allowable area. I will try to articulate it as best I can. I don't see it working as non-separated. But using separated I am not so sure. When using equation 5-1, for either separated or non-separated we use the multiplier for frontage increase. Is the same frontage increase multiplier used for both separated and non-separated calculations? In other words, using non-separated, (F) is the entire building perimeter that fronts on the public way, but using separated, is (F) now going to be only the perimeter of the space that fronts on the public way. Commentary to 508.4.2 tells us to use the entire building perimeter but I'm unsure if that means use the entire building perimeter (P) and then only the space (F), or if we are supposed to use the entire building perimeter (F) the fronts on the public way.

Working the problem both ways, using the entire building perimeter and the entire building perimeter that fronts on the public way I get a sum of ratios less than 1. Using the entire building perimeter and the only space perimeter that fronts on the public way I get a sum of ratios more than 1. Big difference.

A normal non-separated formula for frontage would look like this
If=[F/P - .25)W/30

Or would the formula for a separated strategy look something like this:
If=[space F/ building P -.25]W/30

Or, something else?

 

[walker.t]

Is the same frontage increase multiplier used for both separated and non-separated calculations?

Yes, the same frontage increase factor I(f) applies to the entire building. 2018 IBC 506.3.3. says that F is “building perimeter” and P is “perimeter of entire building.”

If you had a fire wall you would have two buildings and therefore different frontage increase factors for each side of the fire wall.

 

[RLGA]

Yes, use the same frontage increase factor for all area calculations.

 

[Sifu]

Very good. I knew if we had a fire wall it would change...thanks to this forum and discussions (specifically with RLGA!) I just wasn't sure if the same held for separation by a fire barrier.

To clarify though, we would then use the "space" (F) & (P) where a fire wall is the separation?

 

[walker.t]

To clarify though, we would then use the "space" (F) & (P) where a fire wall is the separation?

The “space” would be the entire area on one side of the fire wall. Then you consider your open space relative to that side of the building, so on the fire wall side of the building there is no open space.