Area modification with multiple occupancies

[YongMNLad]

Hello all,

I am re-looking at the design of a H-3, fully sprinklered, Type II-B building with 60+ feet of public yard. With the products stored in the building, it seems possible to split the building into H-4 and H-3 occupancies using a 3 hour fire barrier per 508.4.

I am not quite sure on how to calculate the area modifications. The building is 120' wide by 400' long. The fire barrier will split the 400' length into 287' and 113' long areas (pending the area modification calcs).

The way I'm currently seeing it, there are 3 different ways to calculate the allowable area:
1. Use If = 0.75 for the entire building:
Total Area Modification - Aa = {At + [At x If] + [At x Is]}
Area 1 - Group H-3:
Aa = {14,000 + [10,500] + [0]}
Aa = 24,500 SF
Area 2 - Group H-4:
Aa = {17,500 + [13,125] + [52,500]}
Aa = 83,125 SF
Sum of Ratios
Area 1 - Group H-3: 13,560 / 24,500 = 0.553
Area 2 - Group H-4: 34,440 / 83,125 = 0.414
0.967 < 1

2. Do not use a frontage increase:
Total Area Modification - Aa = {At + [At x If] + [At x Is]}
Area 1 - Group H-3:
Aa = {14,000 + [0] + [0]}
Aa = 14,000 SF
Area 2 - Group H-4:
Aa = {17,500 + [0] + [52,500]}
Aa = 70,000 SF
Sum of Ratios
Area 1 - Group H-3: 13,560 / 14,000 = 0.969
Area 2 - Group H-4: 34,440 / 70,000 = 0.492
1.461 > 1

3. Figure the frontage increase separately for the two areas as if they were separate buildings:
Area Modifications (IBC 506)
Frontage Increase (IBC 506.2)
Area 1 - Group H-3:
If = [F/P-0.25]W/30
If = [346/466-0.25]30/30
If = [0.492]1
If = 0.492
Area 2 - Group H-4:
If = [F/P-0.25]W/30
If = [694/814-0.25]30/30
If = [0.60]1
If = 0.60

Total Area Modification - Aa = {At + [At x If] + [At x Is]}
Area 1 - Group H-3:
Aa = {14,000 + [6,888] + [0]}
Aa = 20,888 SF
Area 2 - Group H-4:
Aa = {17,500 + [10,500] + [52,500]}
Aa = 80,500 SF
Sum of Ratios
Area 1 - Group H-3: 13,560 / 20,888 = 0.649
Area 2 - Group H-4: 34,440 / 80,500 = 0.428
1.077 > 1


If methods 2 or 3 are used, I will obviously have to change the location of the fire barrier to get the sum of ratios less than 1.

Any help is greatly appreciated!

 

[YongMNLad]

I forgot to mention that IBC 2012 is applicable.

I think I may have found my answer in a copy of 2006 IBC with Commentary: "For determination of the allowable perimeter increase, use the entire building perimeter-not the occupancy perimeters."