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Fire Safety and Building Code Methods

Jonathan Ochshorn

© 2009 Jonathan Ochshorn.

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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)

• A. Refer to Chapter 3 (Use and Occupancy Classification)
  1. A = assembly - lecture rooms, auditoriums, stadia, etc.
  2. B = buisness - 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: Note that sprinklers may substitute for 1-hr construction (note "d", Table 601) if they are (1) not elsewhere required, or (2) used for an area or height increase - see below.
• 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
  • noncombustible exterior walls of masonry or concrete with certain combustible materials inside;
  • minimum dimensions on wood structure: 8" min. for columns; 6x10 minimum for beams;
  • no concealed ceiling spaces, etc.
  • was known as "heavy timber" construction.
  • Can be used where 1-hour fire-resistance (or less) is required.
• 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 602) depends on:
  1. Occupancy
  2. Construction type
  3. Distance to property line (fire separation distance)
  4. Whether walls are bearing or nonbearing (for bearing walls, see also 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 (705.8.2)
• C. Required separation in buildings of mixed occupancy (Table 508.4) depends on:
  1. Occupancies
• D. Basic allowable building heights and (single floor) floor areas (Table 503) depend on:
  1. Occupancy
  2. Construction type
  3. Sprinklers: Note that height may be increased 20 ft. and maximum number of stories increased by 1 story when sprinklers conforming to NFPA 13 are used (see IBC sec. 903.3.1.1).
• E. Basic building area can be increased for frontage, sprinklers, and numbers of stories as follows:

  Allowable area per floor, A_a = A_t(1.0 + I_f + I_s) where A_t is the "tabular" value found in Table 503, and I_f and I_s are as defined below. See Section 506, Equation 5-1.

  For sprinklered buildings, the total allowable building area = the allowable area per floor times the number of floors. For unsprinklered buildings, the same equation is used for one-, two-, and three-story buildings only. For unsprinklered buildings with more than 3 stories, the maximum allowable total building area = the allowable area per floor times 3. Note that in all cases, any floor cannot exceed the allowable floor area computed above.

  Where there is no frontage increase, or a building is unsprinklered, the values of I_f or I_s are taken as zero.

  1. I_f = Frontage increase coefficient = (F/P - 0.25)(W/30) where

  • F = portion of building perimeter fronting on public way or open space with 20 ft. minimum width; note that if F is less than 25% of the total building perimeter, I_f is taken as zero.
  • P = total building perimeter;
  • W = Weighted average of open space/public way width in front of building:
    • Only count widths that are greater or equal to 20 ft;
    • Use 30 ft. for all widths that are greater than 30 ft, except as described below under "Unlimited Area Buildings";
    • Open space must be on building lot or reserved for public use; also must be connected to a street or "fire lane,"
    • Widths fronting right-of-ways (ROWs) can be measured to the far side of the ROW rather than to the near side (i.e, rather than to the property line).
  • Sample calculation of frontage increase coefficient (see site plan below):

    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.)

    I_f = (220/320 - 0.25)(28.64/30) = 0.42.

    

  2. I_s = Sprinkler increase coefficient = 3 for 1-story buildings and 2 for multi-story buildings, when sprinklers conform to NFPA 13.
  • Note that height and story increases can still be taken with sprinklers in addition to this area increase;
  • There are exceptions for H occupancies: see details in IBC Section 506.3;
  • Also, this increase cannot be taken when sprinklers are used per note "e" of Table 601 (i.e., to substitute for 1-hr. fire-resistiver construction).

  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 2006 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 the sprinklered building "per floor" allowable area is multiplied by the number of stories (4), whereas the unsprinklered building "per floor" allowable area is multiplied by the maximum value of 3. The frontage increase coefficient is 0.75 where the width of all sides is at least 30 ft. since:

  I_f = (F/P - 0.25)(W/30) = (1.0 - 0.25)(30/30) = 0.75.

IV. Calculator: See my IBC 2006 Allowable Area Calculator.

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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 July 31, 2009