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Based on International Building Code 2009: IBC (latest version) can be found online, Cornell students only, by entering MADCAD in the title field of a library search.
I. Occupancy (what the building is used for)
Allowable area per floor, Aa = At(1.0 + If + Is) where At is the "tabular" value found in Table 503, and If and Is are as defined below. See Section 506, Equation 5-1.
For unsprinklered buildings up to 3 stories, and for sprinklered buildings of with any number of stories, the maximum total allowable building area is the allowable area for a single floor times the number of stories (but note that the sprinkler coefficient must correspond to the total number of stories). For unsprinklered buildings with more than 3 stories, the maximum allowable total building area = the allowable area per floor times three. Note that in all cases, any floor cannot exceed the allowable floor area computed above. There is an exception for Group R buildings with so-called NFPA 13R sprinklers: for such buildings, limited to 4 stories, the maximum total building area is the allowable area for a single floor times the number of stories (but note that no more than 4 stories are allowed, and that the sprinkler increase, Is, is not permitted when the sprinklers are designed according to NFPA 13R. If the sprinklers were to be designed according to NFPA 13, instead of 13R, the maximum allowable building area would be limited to 3 times the allowable single floor area (even for a 4-story building), but the allowable single floor area would be increased according to the coefficient Is.
Where there is no frontage increase, or a building is unsprinklered, the values of If or Is are taken as zero.
1. If = Frontage increase coefficient = (F/P - 0.25)(W/30) where
F = 60 + 100 + 60 = 220 ft. (Note that only sides A, B, and C are counted because the width of side D is less than 20 ft. Also note that the width of side C is measured to the centerline of the right-of-way.)
P = 60 + 100 + 60 + 100 = 320 ft.
W = [25(60) + 30(100) + 30(60)] / (60 + 100 + 60) = 28.64 ft. (Note that actual widths greater than 30 feet are counted as 30 ft. in the weighted average, and that only widths of 20 ft. or more are included.)
If = (220/320 - 0.25)(28.64/30) = 0.42.
3. Unlimited area buildings are permitted where the minimum yard distance on all sides is 60 feet, and occupancy, sprinklers, and number of stories meet the criteria in the following table:
|Occupancies||Sprinklers required?||Story limit||IBC 2009 ref.|
|F-2, S-2||No||1||Sec. 507.2|
|B, F, M, S||Yes||2||Sec. 507.3,4|
|A-4 (not Type V)||Yes||1||Sec. 507.3|
Note that for occupancies that meet all criteria for unlimited area except for the 60-foot yard requirement, the parameter, W, used in the frontage increase calculation may be taken as no greater than 60 feet (rather than limited to 30 feet).
4. Area separation walls create separate buildings for calculations: need fire-rated walls; and limited openings.
5. Mixed occupancy buildings: sum of fractions of actual / allowable areas cannot exceed 1.
6. Example: A 4-story office building (occupancy group = B) with Type II-B construction, sprinklers, and at least 30 feet of separation on each side could have a total allowable building area of:
4[23,000(1.0 + 0.75 + 2)] = 345,000 sq. ft.
If the same building is unsprinklered, the total allowable building area is:
3[23,000(1.0 + 0.75 + 0)] = 120,750 sq. ft.
Note: 23,000 sq.ft. is the tabular "per floor" value found in Table 503; also note that both the sprinklered and unsprinklered building's "per floor" allowable areas are multiplied by 3, even though there are 4 stories. The frontage increase coefficient is 0.75 where the width of all sides is at least 30 ft. since:
If = (F/P - 0.25)(W/30) = (1.0 - 0.25)(30/30) = 0.75.
Disclaimer: Building codes (including the International Building Code) are typically structured as a maze of basic statements and qualifying assertions. Often, the qualifiers — sometimes found in other sections, or in footnotes to tables — are more important than the basic statements they modify. For this reason, it is important to scrutinize all relevant sections of the code before drawing any conclusions. The material contained on this page does not contain all qualifying assertions, and provides only a basic overview of fire-safety issues related to occupancy and construction type. These methods are not intended to be used for the design of actual structures, but only for schematic (preliminary) understanding of building code principles. For the design of an actual structure, a competent professional should be consulted.
Last updated September 22, 2010