Allowable building area per IBC table 506.2

Hi,

During last Sunday’s (November 16th) PA workshop with coach Elise Renwick we worked on an exercise using the Zoning and IBC codes. One of the questions looked like this:

  1. Assuming the building is Mixed use, non-separated, construction type II-B and fully sprinklered, what is the allowable area of the building?

The answer was – 28,500 sq ft per IBC table 506.2 (using A-3 occupancy as the most restrictive among the given).

My question is – by “allowable area of the building” do we mean the building footprint area or the total area of all floors?

If it is former, then the above answer seems to be correct as per 506.2.3, which says “No individual story shall exceed the allowable area (Aa) as determined by Equation 5-2…” (However, that refers to single-occupancy, multistory buildings. I could not find a similar clause for the mixed-occupancy, multistory buildings).

But if we actually mean the total area of all floors, then our answer should be at least the triple of 28,500 i.e. about 85,500 sq ft as per 506.2.4, which says “Each story of a mixed-occupancy building with more than one story above grade plane shall individually comply with the applicable requirements of Section 508.1. For buildings with more than three stories above the grade plane, the total building area shall be such that the aggregate sum of the ratios of all the actual area of each story divided by allowable area of such stories, …, shall not exceed three.”

If we calculate the Frontage it will add even more square footage to our allowable area.

I know all above sound pretty convoluted, but then again, what is the right answer – 28,500 or 85,500?

Andrew

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@coacheliserenwick do you mind helping Andrew?

@coachdarionziegler do you mind helping Andrew?

Hi @andrew,

Trying to post this again. Messed up the format of my original response

Great question!

28,500 square feet is the correct answer.

My question is – by “allowable area of the building” do we mean the building footprint area or the total area of all floors?
-This refers to the total area of the building, including all floors.

But if we actually mean the total area of all floors, then our answer should be at least the triple of 28,500 i.e. about 85,500 sq ft as per 506.2.4, which says “Each story of a mixed-occupancy building with more than one story above grade plane shall individually comply with the applicable requirements of Section 508.1. For buildings with more than three stories above the grade plane, the total building area shall be such that the aggregate sum of the ratios of all the actual area of each story divided by allowable area of such stories, …, shall not exceed three.”

-The portion of 506.2.4 you have quoted above refers to how you confirm the building remains compliant with a frontage increase. I know it says “stories”, which can be confusing when table 506.2 refers to whole building areas. It says stories because you are calculating a value for each individual story based on the allowable building area for the most restrictive use in that story, getting a ratio of that value vs. the total allowable area per the most restrictive use for the building, and adding those ratios for each individual story together. Your building is compliant if this number is under 3 for non-separated stories and 4 for separated stories.

I think an example probably helps more than my paragraph above. Check out this section from Upcodes at the link. Near the end of the page they run through an example of this:

If you have a copy of the code commentary I highly suggest reading through it for this section!

This is an area of the code that has really been developing over the years. For example, in the 2021 IBC (note that you use 2018 for the exam, I’m just mentioning the new one for context) they have eliminated the distinction between allowable areas with frontage for multi-story and single story buildings all together. Instead, these sections are only divided by “Single Use Buildings” and “Multiple use building”, simplifying this section and cleaning up the language somewhat. You can read this updated section at the link below if you’re interested.

Hope this helps! I know this can be tricky, so please feel free to tag me in any follow up questions and I’ll do my best to clear it up.

Thanks,
Darion

Hi Darion,

Thank you for the clarification and for the link.

I have figured out that the frontage increase calculation involves areas of individual stories.

Yet, while reading 506.2.3 I came across another question in this article:

“Sa = Actual number of building stories above grade plane, not to exceed three. For buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.2, use the actual number of building stories above grade plane, not to exceed four.”

903.3.1.2 – refers to NFPA R13 sprinklers, which are used in residential occupancies only.

Does it mean that Sa=4 can be used only for Group R, and any other occupancies (like B or M) can be calculated using Sa=3 as maximum, regardless whether they are equipped with sprinkles or not?

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Hi Andrew,

No worries, glad I can help! :slight_smile:

You’re exactly right! Sa=4 may only be used for Group R occupancies “up to and including four stories in height in building not exceeding 60 feet” equipped with an automatic sprinkler system installed throughout with NFPA 13R per the tenants of section 903.3.1.2.

You can use a max of Sa=3 in all other incidents and occupancies.

Let me know if you have any other questions!

Thanks,
Darion

Thank you Darion!

Andrew

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