# 2006 IBC Area determination changed in 2009 IBC

whoops, we lost something here.

section 506.4 allowed the total allowable building area to increase depending on the number of stories. this logic was consistent with the 1997 ubc, section 504.2.

but in the 2009 ibc, this is only allowed for "single occupancy buildings". huh? i haven't seen a single occupancy building (other than residential) in years. what happened? so potentially any building we did under the 2006 ibc may be three times larger than what is allowed in the 2009 ibc? or did i miss something?

thanks for any help.
Original Post
i read the section, but it does not provide doubling or tripling total allowable area for multiple story buildings. it only exempts basements from the total building allowable area (with limitations). the rest is an expanded version of the 2006 ibc section 506.4.1.
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You're not understanding the requirements.

Single occupancy buildings (or mixed occupancy buildings using nonseparated uses) have the allowable area of buildings determined by the number of floors times the allowable area up to three floors. This is covered in 506.4.1. No problem there.

With mixed occupancy buildings using the separated uses option, the allowable area per floor is determined by the sum of ratios of actual areas to allowable areas for each occupancy group. As long as the sum is less than 1, then the floor area is okay. This is applicable for each floor up to 3 floors. In other words, the first story can have a sum that does not exceed 1, the second story can have a sum that does not exceed 1, and the third story can have a sum that does not exceed 1. When the building exceeds three stories, then the sum of ratios of ALL stories cannot exceed 3, with no single story having a sum exceeding 1.

For example, let's assume there are three buildings.

Building #1: Two stories with sum of ratios for each story as follows:
Story 1 = 0.95 < 1 (Okay)
Story 2 = 0.85 < 1 (Okay)

Building is acceptable.

Building #2: Three stories with sum of ratios for each story as follows:
Story 1 = 0.95 < 1 (Okay)
Story 2 = 0.85 < 1 (Okay)
Story 3 = 0.75 < 1 (Okay)

Building is acceptable.

Building #3: Four stories with sum of ratios for each story as follows:
Story 1 = 0.95 < 1 (Okay)
Story 2 = 0.85 < 1 (Okay)
Story 3 = 0.75 < 1 (Okay)
Story 4 = 0.75 < 1 (Okay)

Sum of ALL ratios = 0.95 + 0.85 + 0.75 + 0.75 = 3.3 > 3 (Not Okay)

Building is NOT acceptable.

I hope this helps.
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got it. it is the doubling or tripling of the ratio sum that allows the additional area. thanks!
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Hey Brent,

Do I need to update my alowable area calculator?

I am getting ready to teach this fall and having to change my entire approach due to the New CRC coming online 1/12011.

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The method of determining allowable building area for separated, mixed-use occupancy buildings has not changed from the 2006 IBC to the 2009 IBC--only the way it is described. Supposedly, it is clearer now.

Your calculator only calculates allowable area for a single occupancy group. It can be used, however, to calculate the allowable areas for each occupancy group in a separated, mixed-use occupancy building. Then the ratios can be determined by dividing the actual floor areas by the allowable areas determined by your calculator.
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Brent,
All floors are within the allowable limits (sum of ratios for separated occupancies).
When checking the building per Sec. 506.4.1 with 16,000 sq. ft. at 4th floor sum of ratios for entire building is over 3 (even if for the 4th floor itself the allowable area is 24,000 sq. ft), and message appears on the upper right corner “Building Area is Over limit per Sec. 506.4.1”.
Use the calculator for mixed separated uses and you will see the difference. Try to input 12,000 sq. ft for the 4th floor…
This is the website:

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thanks riga and ara, appreciate the help.

steve, riga is right, you may want to add an additional section for multi-occupancy buildings. also, you may want to look at some of your formulas (a simple "if/then" formula works), so that if the user enters distances less than "20" in the side yard fields, they don't get an error message or negative result for If (which incorrectly reduces the Allowable area).
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