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

Fire Safety and Building Code Methods

Jonathan Ochshorn

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)

A. Refer to Chapter 3 (Use and Occupancy Classification)
1. A = assembly - lecture rooms, auditoriums, stadia, etc.
2. B = business - office buildings, banks, outpatient clinics, colleges and universities, etc.
3. E = educational - schools (grade 12 or less), daycare
4. F = factory and industrial
5. H = high hazard - buildings containing hazardous material
6. I = institutional - nurseries, health care, jails
7. M = mercantile - merchandise sales (department stores, markets, service stations, etc.)
8. R = residential - dwellings, hotels, apartment houses
9. S = storage - storage, including storage of hazardous materials
10. U = utility and miscellaneous - accessory types of structures like garages, towers, tanks

II. Construction type

A. Refer to Table 601 (Fire-Resistance Rating Requirements); two basic categories are:
1. Types I and II: basically non-combustible, with some exceptions
2. Types III, IV and V: allow combustible material (i.e., wood)
3. Sprinklers: Come in different "flavors": the default building sprinklers are based on NFPA 13; but there are two less expensive types permitted for certain Group R occupancies, based on NFPA 13R and NFPA 13D.
B. Types I and II fire-resistive
- noncombustible materials (concrete, masonry, steel)
- substantial fire-resistance ratings (up to 3hrs)
- differences:
  - type I — fewer height or area limits, but 2-3 hr. fire-resistance for structure;
  - type II — substantially lower height limits; other limits on floor area and number of stories;
  - less fire resistance required for type II structures means less cost.
C. Type II A, B [Type B is "unprotected"]
- noncombustible materials;
- less substantial or no [type II B] fire-resistance ratings;
- smaller floor areas, heights, number of stories allowed (mostly 4 stories allowed).
D. Type III A, B
- noncombustible exterior walls of masonry or concrete with anything else inside;
- similar limits as Type II A, B (since exterior walls have such a high f.r.-rating);
- was known as "ordinary" construction.
E. Type IV
- The 2021 IBC expanded this construction type from what was originally known as "heavy timber" (IV-HT); there are now three additional sub-types for mass-timber buildings.
F. Type V A, B
- basically light wood framing;
- greatest limitations on area, heights, etc. where permitted at all.

III. Other questions:

A. Exterior wall fire resistance (Table 705.5) depends on:
1. Occupancy
2. Construction type
3. Distance to property line (fire separation distance)
4. Whether walls are bearing or nonbearing (load-bearing walls must also comply with Table 601)
B. Openings in exterior walls (Table 705.8) depend on:
1. Distance to property line (fire separation distance)
2. Classification of opening (protected or unprotected)
3. Whether building is sprinklered.
C. Basic allowable building heights and (single floor) floor areas depend on:
1. Occupancy
2. Construction type
3. Sprinklers: Note that the 2021 and 2024 IBC tables include height increases (Table 504.3), maximum-number-of-stories increases (Table 504.4), and area increases (Table 506.2) when sprinklers conforming to NFPA 13 are used; NFPA 13 sprinklers are referenced in IBC Sec. 903.3.1.1).

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), I_f, = (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 = A_a = A_t + (NS x I_f)

where A_t 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 I_f 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 I_f 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 Group "A" and a Group "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.

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 I_f = (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.

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 March 11, 2024