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Based on International Building Code 2015: IBC (latest version) can be found online by entering MADCAD in the title field of a library search (Cornell students only), or by using the free online NY State versions.
I. Occupancy (what the building is used for)
Allowable area for single-occupancy buildings is found as follows:
Allowable area per floor = Aa = At + (NS x If)
where At is the "tabular" area factor found in Table 506.2, NS is the tabular area factor for a nonsprinklered building (even if the building actually is sprinklered) and If is the area factor due to frontage as defined below.
For single-occupancy buildings of 1 or 2 stories (above grade plane), the maximum total allowable building area is the allowable area for a single floor times the number of stories. For buildings with 3 or more stories (above grade plane), the maximum allowable total building area = the allowable area per floor times three. Note that in all cases, the area per floor cannot exceed the allowable per-floor area computed above.
There is an exception for Group R buildings with so-called NFPA 13R sprinklers: in such cases, the total allowable building area is the allowable per-floor area times 4 (and such residential buildings cannot be more than 4 stories above grade plane). Of course, residential buildings can be more than 4 stories high, but in those cases, regular sprinklers (not these special "13R" residential-only sprinklers) must be used.
Where there is no frontage increase, the value of If is taken as zero.
Allowable area for mixed-occupancy buildings: See Section 508 of the 2015 IBC for details. In general, the allowable area depends on whether the occupancies are classified as "accessory," "nonseparated," or "separated."
Accessory: Design as if the whole story consisted of only the main occupancy, as long as the accessory occupancies don't take up more than 10% of the floor area of a given story, and also satisfy the nonsprinklered area requirements for their actual occupancies.
Nonseparated: Design for allowable area and height as if the whole story consisted of only the most restrictive occupancy; other restrictions apply — see Code Section 508 for details — including the requirement that the most restrictive sprinkler requirements (in Chapter 9 of the IBC) and other requirements for height and area (in Section 403) for any of the occupancies apply to all the occupancies. That's the bad news. The good news is that no fire separations are required between the occupancies.
Separated: Well, first of all, you need to actually separate the occupancies with fire barriers or horizontal separations based on Table 508.4. For example, you would need a 1-hour separation between a class "A" and a class "B" occupancy in a sprinklered building (2 hours in a nonsprinklered building). Secondly, allowable areas are computed as follows: "In each story, the building area shall be such that the sum of the ratios of the actual building area of each separated occupancy divided by the allowable building area of each separated occupancy shall not exceed 1." And each occupancy still needs to meet its own height limits, per Section 504.
In addition to the requirement that each story meet the area limits based on the sum of the ratios of actual to allowable area for each occupancy, there is a limit for the total building area: the sum of all the sums for these ratios, for all floors of the building taken together, cannot exceed 3. In other words, this limit really only takes effect for buildings greater than 3 stories above grade plane.
Explanation of separated vs. nonseparated occupancies:
If you're wondering about the logic underlying this area calculation method, based on ratios of actual to allowable not exceeding one, here's a hypothetical example illustrating the same principle, but in the sphere of economic policy:
Let's imagine that a new socialist government wants to place limits on income. To encourage actual work, and to discourage more parasitical forms of income, two separate limits are established: $120,000 per year for income derived from actual labor; or $90,000 per year for income derived from interest, dividends, capital gains, or inheritance. The two income limits are easy to understand and easy to apply if you only have one source of income, but what happens if you have multiple sources of income? By analogy to the IBC requirements for mixed occupancies, you could calculate your income limit in one of three ways.
First, if one form of income was quite small compared to the other, say 10% or less, you could consider the smaller income "accessory" to the larger income and just include it in the larger ("main") amount. But what of the other two methods?
On the one hand, you could just take the most restrictive limit and apply it to everything, so that your total income limit would be $90,000. This is analogous to "nonseparated" occupancies in the IBC. On the other hand, you could earn, for example, half of the allowable income from work (i.e., $60,000) and half of the allowable income from dividends (i.e., $45,000) for a total allowable income of $105,000. In this case, the sum of the ratios of actual to allowable income would be 60,000/120,000 + 45,000/90,000 = 1/2 + 1/2 = 1.0. Or you could earn 2/3 of the allowable income from work (i.e., $80,000) and 1/3 of the allowable income from dividends (i.e., $30,000) for a total allowable income of $110,000. In this second case, the sum of the ratios of actual to allowable income would be 80,000/120,000 + 30,000/90,000 = 2/3 + 1/3 = 1.0. The general statement of this economic policy would be as follows: that the sum of the ratios of actual to allowable income for the two categories of income cannot exceed one. This would be analogous to "separated" occupancies in the IBC. Got it?
The question could also be raised: why would anyone calculate income based on "nonseparated" income streams, when it seems more beneficial to use the "separated" income method? Well, in this hypothetical example, the latter method requires more work, i.e., more effort making the calculation. (Or, to improve the analogy, one could impose a fee, say $10,000, for the right to use "separated" income streams.) The same is true with decisions about mixed occupancies: it takes more "work" (or costs more money), since one must construct fire barriers and horizontal separations, to meet the conditions for separated occupancies and, in some cases, it's not worth the extra cost, especially if the requirements for nonseparated occupancies are easily met, without any penalty.
Based on Table 508.4: Required separation of occupancies (hours) for nonsprinklered buildings
|Occupancy||A, E||I-1, I-3, I-4||I-2||R||F-2, S-2, U||B, F-1, M, S-1||H-1||H-2||H-3, H-4||H-5|
|I-1, I-3, I-4||—||N||NP||NP||2||2||NP||NP||NP||NP|
|F-2, S-2, U||—||—||—||—||N||2||NP||4||3||NP|
|B, F-1, M, S-1||—||—||—||—||—||N||NP||3||2||NP|
Based on Table 508.4: Required separation of occupancies (hours) for sprinklered buildings
|Occupancy||A, E||I-1, I-3, I-4||I-2||R||F-2, S-2, U||B, F-1, M, S-1||H-1||H-2||H-3, H-4||H-5|
|I-1, I-3, I-4||—||N||2||1||1||1||NP||3||2||2|
|F-2, S-2, U||—||—||—||—||N||1||NP||3||2||2|
|B, F-1, M, S-1||—||—||—||—||—||N||NP||2||1||1|
|Occupancies||Sprinklers required?||Story limit||IBC 2012 ref.|
|F-2, S-2*||No||1||Sec. 507.3|
|B, F, M, S*||Yes||2||Sec. 507.5|
|A-4 (not Type V*)||Yes||1||Sec. 507.4|
|A-3 (Type II-no stage*)||Yes||1||Sec. 507.6|
|A-3 (Type III/IV-no stage; ramps for egress within 21 in. of street*)||Yes||1||Sec. 507.7|
|E (Type II, IIIA or IV; each classroom has 2 exits, one directly outside*)||Yes||1||Sec. 507.11|
|Motion picture theaters on first story (Type II*)||Yes||1||Sec. 507.12|
|Mall and anchor buildings complying with Sec. 402*||Yes||3||Sec. 507.13|
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) — see the exception to Section 506.3.2 for details.
* See Code for detailed requirements.
Area separation walls create separate buildings for calculations: need fire-rated walls; and limited openings.
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:
3[69,000 + (23,000 x 0.75)] = 258,750 sq. ft.
If the same building is unsprinklered, the total allowable building area is:
3[23,000 + (23,000 x 0.75)] = 120,750 sq. ft.
Note: 23,000 sq.ft. is the tabular NS (no sprinklers) "per floor" area factor found in Table 506.2; 69,000 sq.ft. is the tabular SM (sprinklered, multiple stories) "per floor" area factor found in the same table; also note that the building's "per floor" allowable area is 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.IV. Calculator: See my IBC 2006–2015 Allowable Area Calculator.
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 May 31, 2017