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Allowable Area 508.4.2, how to do this math?

K

kasa

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Joined
Apr 13, 2012
Messages
46
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 simple, say each floor is 8000 SF). I figured their frontage increase would be .38. So, for 1st floor I take 8000 SF divided by.... ???...

....would it be 18000 because it's a sprinkled multi-story building?

....or would I do the math found in 506.2.4 to get that 'allowable area, which I think would be [18000+(6000x.38)]=20280 SF.

I think I'm asking then, if the sum of the ratios is supposed to factor in frontage increase?

Secondary question: What is the separation requirement between the 1st and 2nd floor, would this be per table 508.4, 1hr?
 
kasa said:
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 simple, say each floor is 8000 SF). I figured their frontage increase would be .38. So, for 1st floor I take 8000 SF divided by.... ???...

....would it be 18000 because it's a sprinkled multi-story building?

....or would I do the math found in 506.2.4 to get that 'allowable area, which I think would be [18000+(6000x.38)]=20280 SF.

I think I'm asking then, if the sum of the ratios is supposed to factor in frontage increase?

Secondary question: What is the separation requirement between the 1st and 2nd floor, would this be per table 508.4, 1hr?
Click to expand...
It is the total allowable area permitted, which includes the frontage increase.

The separation between floors would be per Table 508.4.
 
RLGA said:
It is the total allowable area permitted, which includes the frontage increase.

The separation between floors would be per Table 508.4.
Click to expand...
My state went from 2009 to 2015, was there a big shift in allowable areas between those two? I did not have occasions to use 2009 much at the time, and now just trying to get a handle on 2015 first....
 
Not really, they just incorporated the sprinkler increases into the tables and removed them from the calculation.
 
One other note, you are within the allowable height, area and stories for both occupancies, so you could take a non-separated, mixed occupancy approach. With this, there is no need to do the sum of the ratios that you described, you can just show on your code sheet that you are below the max area/height for the most restrictive occupancy (Group A-2). With Group R2, the dwelling units and corridor walls would still require rated separations per IBC 420. See image below, which I just input your data into this calculator: https://www.buildingcode.blog/allowable-height--area-calculator---separated-mixed-occupancy.html#/




Screenshot 2021-03-22 083552.png
 
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