# IBC Allowable Building Area Calculation



## MKolchins (Apr 30, 2021)

I'm an architect who unfortunately doesn't get a lot of new commercial construction projects so it's been a while since I had to calculate allowable area. I use the NJ Edition of the 2018 IBC. Table 506.2 does not state whether the allowable areas are per floor or total building. In the old days it was clearly indicated as per floor; however, references to "building area" in the text suggest maybe it's total building now.

I am working on an addition to an existing 3 story mixed use A-2 and R-1 structure. Section 506.2.4 only references the open perimeter calculation of equation 5-3 in relation to a mixed use structure greater than 3 stories. In the old days open perimeter was an increase over the base allowable area and applied to all buildings. Does equation 5-3 and the equivalent equations in other portions of section 506 apply only to structure greater than 3 stories or to all structures?


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## cda (Apr 30, 2021)

Welcome

Not into math, check this out and see if it helps any


You can check out an allowable area calculator based on the 2015 IBC here: https://www.buildingcode.blog/allow...ulator---non-separated-mixed-occupancy.html#/









						Allowable Area
					

I am doing the old allowable area calculation for a new building.  The tabular area has me confused (since changes to the code) I know the basics but the most basic thing is bothering me.  Is the tabular or basic allowable number (in this case Type III B) for a single floor or used for the...



					www.thebuildingcodeforum.com


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## cda (Apr 30, 2021)

One more might help::







						Allowable Area 508.4.2, how to do this math?
					

In 2015 IBC, 508.4.2, when they say "divided by the allowable building area of each separated occupancy", is this the allowable area from the table, or from 506.2.4? (or elsewhere else???)   For example: I've got a 2-story sprinkled VB, there's A2 downstairs and R2 upstairs (Just to make math...



					www.thebuildingcodeforum.com


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## MKolchins (Apr 30, 2021)

cda said:


> Welcome
> 
> Not into math, check this out and see if it helps any
> 
> ...


I've done the math and would like to avoid trying to learn to use the online calculator. I just need to know if the results of equation 506.2 open perimeter increase apply to all buildings or just building greater than 3 stories and also if the allowable area in table 506.2 is per floor or the sum of all floors.


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## SH225 (Apr 30, 2021)

The open perimeter (frontage) increase applies to any building. Apply the frontage increase formula to the NS table value for your occupancy and type. Add it to the SM table value.  That gives you the floor allowable. The building allowable is the floor allowable times 3.


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## MKolchins (Apr 30, 2021)

SH225 said:


> The open perimeter (frontage) increase applies to any building. Apply the frontage increase formula to the NS table value for your occupancy and type. Add it to the SM table value.  That gives you the floor allowable. The building allowable is the floor allowable times 3.


Based on the preponderance of evidence I can find I think you are right. Hopefully future editions of the IBC will indicate table 506.2 is PER FLOOR and the allowable increases for open perimeter apply to all building not just building over three stories.


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## SH225 (Apr 30, 2021)

MKolchins said:


> Based on the preponderance of evidence I can find I think you are right. Hopefully future editions of the IBC will indicate table 506.2 is PER FLOOR and the allowable increases for open perimeter apply to all building not just building over three stories.


The per story aspect of it is in the sections that contain Equations 5-2 and 5-3. The table value is technically only a factor in those equations. It does not stand alone as the floor allowable area. It is a value for the equation.


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## MKolchins (Apr 30, 2021)

Equation 5-3 Aa= Allowable area (square feet). It does not state this is per floor and implies it's total building. 

The exception states: ..."the total building area shall be such that the aggregate sum of the ratios of the actual area of each story divided by the allowable area of such story determined in accordance with Equation 5-3 based on the applicable provisions of section 508.1, shall not exceed four." This implies table 506.2 might be intended to be per floor but it's not clear. If anyone from ICC follows these threads please add the words "per floor" to the heading on table 506.2 as was the case in the old days to make it clear it's per floor and not total building. And also indicate the open perimeter increases apply to all buildings and not just building greater than three stories as the current text suggests.


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## SH225 (Apr 30, 2021)

MKolchins said:


> Equation 5-3 Aa= Allowable area (square feet). It does not state this is per floor and implies it's total building.
> 
> The exception states: ..."the total building area shall be such that the aggregate sum of the ratios of the actual area of each story divided by the allowable area of such story determined in accordance with Equation 5-3 based on the applicable provisions of section 508.1, shall not exceed four." This implies table 506.2 might be intended to be per floor but it's not clear. If anyone from ICC follows these threads please add the words "per floor" to the heading on table 506.2 as was the case in the old days to make it clear it's per floor and not total building. And also indicate the open perimeter increases apply to all buildings and not just building greater than three stories as the current text suggests.


I’ll agree it could be presented more clearly, but first two words of the section are “Each story.”


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## MKolchins (Apr 30, 2021)

SH225 said:


> I’ll agree it could be presented more clearly, but first two words of the section are “Each story.”


What section are you referring to. I checked pretty thoroughly for any indication that area were per floor and couldn't find it. I use the NJ edition of the 2018 IBC but I doubt NJ would edit out the per story reference.


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## MKolchins (Apr 30, 2021)

SH225 said:


> I’ll agree it could be presented more clearly, but first two words of the section are “Each story.”


I just noticed your link to section 506.2.4 which does include the words "each story" but it's in reference to section 508.1 and not table 506.2. It's another one that indirectly hints that table 506.2 might be intended to be applied per floor but is not clear.


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## SH225 (Apr 30, 2021)

MKolchins said:


> I just noticed your link to section 506.2.4 which does include the words "each story" but it's in reference to section 508.1 and not table 506.2. It's another one that indirectly hints that table 506.2 might be intended to be applied per floor but is not clear.


I see why it’s not clear, when I really look closely. In the first (2000) edition of the IBC it said per floor at the top of the table.


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## MKolchins (Apr 30, 2021)

SH225 said:


> I see why it’s not clear, when I really look closely. In the first (2000) edition of the IBC it said per floor at the top of the table.


I go back to the old BOCA Code which also indicated it was per floor in the Title to the table. ICC needs to have someone read through these sections from the viewpoint of a user who doesn't already know what they intend the section to mean. I've already spent a half day trying to figure out the correct allowable area for a project where it's all academic anyway as the actual area is way under allowable if it were the whole building and without the open perimeter increase. Thank god I found a forum where I can get some informed opinions.


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## cda (Apr 30, 2021)

MKolchins said:


> I go back to the old BOCA Code which also indicated it was per floor in the Title to the table. ICC needs to have someone read through these sections from the viewpoint of a user who doesn't already know what they intend the section to mean. I've already spent a half day trying to figure out the correct allowable area for a project where it's all academic anyway as the actual area is way under allowable if it were the whole building and without the open perimeter increase. Thank god I found a forum where I can get some informed opinions.





ICC always welcomes code change input!!!!!!


WOW:::  someone likes us!!!   """Thank god I found a forum where I can get some informed opinions."""


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## redeyedfly (Apr 30, 2021)

MKolchins said:


> I go back to the old BOCA Code which also indicated it was per floor in the Title to the table. ICC needs to have someone read through these sections from the viewpoint of a user who doesn't already know what they intend the section to mean. I've already spent a half day trying to figure out the correct allowable area for a project where it's all academic anyway as the actual area is way under allowable if it were the whole building and without the open perimeter increase. Thank god I found a forum where I can get some informed opinions.


If I don't need the frontage increase, I don't calculate it.  It isn't required and can be a cumbersome calc depending on the lot/building shape.


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## Paul Sweet (Apr 30, 2021)

I just love the way the IBC changed one simple table into 3 complicated tables.

The top number (NS) in 2015 IBC Table 506.2 is what used to be the tabular area (area per floor).  This number is used for non-sprinklered buildings and to calculate frontage (perimeter) increase (506.3).

S1, S13R, & SM are areas per floor for sprinklered buildings.  They already include the sprinkler increase (300% for single story, 200% for multi-story), which used to be calculated separately.  You have to add the frontage increase (based on NS) to get the allowable area Aa (506.2.1 - 4).

Buildings over 3 stories get a little more complicated.  You have to multiply the allowable area by 3 for most buildings (or 4 if it has a NFPA 13R sprinkler system) to get the allowable area of the entire building.  No individual story can exceed the allowable area for a 1-story building. 

As far as I know, the 2018 & 2021 codes are basically the same, except for table or paragraph numbers.


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## ShowMeTheCode (Sep 15, 2021)

Paul Sweet said:


> I just love the way the IBC changed one simple table into 3 complicated tables.
> 
> The top number (NS) in 2015 IBC Table 506.2 is what used to be the tabular area (area per floor).  This number is used for non-sprinklered buildings and to calculate frontage (perimeter) increase (506.3).
> 
> ...



I'm using Ohio Code code:
*506.2.4 Mixed-Occupancy, Multistory Buildings*

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 grade plane, the total building area shall be such that the aggregate sum of the ratios of the actual area of each story divided by the allowable area of such stories, determined in accordance with Equation 5-3 based on the applicable provisions of Section 508.1, shall not exceed three.







 (Equation 5-3)

If someone can tell me where in the code it specifies how to determine he allowable area for a 2-story mixed occupancy building, I'd appreciate it.  This is the only section that mentions it, and this section does not specify the use of any equations, the use of table 506.2, or anything else.  It says each story shall comply with the requirements of 508.1.

I realize the INTENT is to use equation 5-3 for each story to check area compliance, and because it is not more that 3 stories above grade plane the total building area doesn't really matter because it can't mathematically exceed 3x the maximum allowable if each of the 2 floors comply, but I'd like to understand where this is spelled out.


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## cda (Sep 15, 2021)

ShowMeTheCode said:


> I'm using Ohio Code code:
> *506.2.4 Mixed-Occupancy, Multistory Buildings*
> 
> 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 grade plane, the total building area shall be such that the aggregate sum of the ratios of the actual area of each story divided by the allowable area of such stories, determined in accordance with Equation 5-3 based on the applicable provisions of Section 508.1, shall not exceed three.
> ...




Welcome,

Give it a couple of days for answers


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