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Course Information: ARCH 2614–5614
Department of Architecture, Cornell University

Fire Safety and Building Code Methods

Jonathan Ochshorn

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Based on International Building Code 2021 and 2024: The 2021 International Building Code and Commentary can be found online by entering MADCAD in the title field of a library search (Cornell students only), or one can use the free, online 2021 IBC or 2024 IBC provided by the International Code Council. While there are some differences between the 2021 and 2024 versions of the IBC, none affect the basic considerations outlined below.

I. Occupancy (what the building is used for)

II. Construction type

III. Other questions:

Frontage is, loosely speaking, the open space, yards, or courts outside a building. It is valuable (i.e., more is better) for two reasons: 1) it protects the building from fires originating in adjacent buildings; and 2) it facilitates fire department vehicular access. This open-space distance is measured perpendicular to the perimeter edges of the building — from the building's exterior surface to the property line, or, in cases where the building surface faces a street (right-of-way, or R.O.W.), to the far end of the R.O.W. In prior iterations of the IBC (i.e., in 2018 and earlier), the open-space width was taken as a weighted average of all distances around the building, only counting distances of 20 feet or more and counting any distances greater than 30 feet as 30 feet. Starting with the 2021 IBC, this somewhat cumbersome calculation has been simplified; it is now only necessary to find the minimum distance or width, still satisfying the two criteria in the older versions (not counted if less than 20 feet, and taken as 30 feet for any widths greater than 30 feet). The other parameter that enters into the frontage calculation is the ratio of the portion of the building perimeter for which the open-space width is 20 feet or more (F) to the total perimeter length (P). If this ratio, F/P, is less than 0.25, the bonus for frontage is not allowed. Then, taking W as the minimum open-space width, the so-called area increase factor (for frontage), If, = (F/P - 0.25) x (W/30). One can see that the maximum possible value for the frontage factor, with F/P = 1.0 and W = 30, is 0.75.

Interestingly, this equation (which appears in iterations of the code prior to 2021) is not actually provided in the 2021 or 2024 IBC. Instead, values for the area increase factor are provided in Table 506.3.3, whose values derive from this equation, at least if you interpolate between the given choices (something rather convoluted, but permitted — it's really much easier to just use the old equation!).

There is one more small point concerning frontage. Section 507 of the 2021 or 2024 IBC defines criteria for buildings that can have unlimited area (generally, but not always, requiring that the buildings be fully-sprinklered and 1- or 2-stories in height, with a minimum open-space (frontage) distance of 60 feet. Well, if the Section 507 criteria are met, except for the 60-foot minimum width, you are permitted to use a minimum width, W, no greater than 60 feet (instead of 30 feet). In this case, one can use Table 506.3.3.1, or the same equation as before (but with the upper limit for W being 60 instead of 30). This effectively doubles the maximum possible frontage factor from 0.75 to 1.5.

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 increase factor due to frontage as defined above.

For single-occupancy buildings of 1–3 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 4 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 NFPA 13 sprinklers (not these special "13R" residential-only sprinklers) must be used. Certain residential occupancies (R-3 and R-4) are also permitted to use the simpler and less expensive sprinkler type specified in NFPA 13D.

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."

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.

Area separation walls create separate buildings for calculations: need fire-rated walls; and limited openings.


Example: A 6-story office building (occupancy group = B) with Type II-A construction, sprinklers, and at least 30 feet of separation on each side could have a total allowable building area of:

3[112,500 + (37,500 x 0.75)] = 3(140,625) = 421,875 sq. ft.

If the same building is unsprinklered, the total allowable building area is:

3[37,500 + (37,500 x 0.75)] = 3(65,625) = 196,875 sq. ft.

Note: 37,500 sq.ft. is the tabular NS (no sprinklers) "per floor" area factor found in Table 506.2; 112,500 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 6 stories. The frontage increase coefficient is 0.75 where the width of all sides is at least 30 ft. since:

The equation for frontage, before the 2021 IBC, was If = (F/P - 0.25)(W/30) = (1.0 - 0.25)(30/30) = 0.75. Now, there are tables that replace the equation, although the tables are derived from the same equation, with one disclaimer: the new area increase factors for frontage no longer use a "weighted average" for the width of open space perpendicular to each edge along the building perimeter; instead, the minimum open space (at least, the minimum length no less than 20 feet) is used. Interpolation is permitted, which gets closer to the original equation.

IV. Calculator: See my allowable area calculator based on the 2021 or 2024 IBC:

IBC 2021–2024 Allowable Area Calculator.