Jonathan Ochshorn

© 2009 Jonathan Ochshorn.

contact | homepage | ARCH 2614/5614 homepageBased 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)- A = assembly - lecture rooms, auditoriums, stadia, etc.
- B = buisness - office buildings, banks, outpatient clinics, colleges and universities, etc.
- E = educational - schools (grade 12 or less), daycare
- F = factory and industrial
- H = high hazard - buildings containing hazardous material
- I = institutional - nurseries, health care, jails
- M = mercantile - merchandise sales (department stores, markets, service stations, etc.)
- R = residential - dwellings, hotels, apartment houses
- S = storage - storage, including storage of hazardous materials
- U = utility and miscellaneous - accessory types of structures like garages, towers, tanks

- A. Refer to
**Table 601**(Fire-Resistance Rating Requirements); two basic categories are:- Types I and II: basically non-combustible, with some exceptions
- Types III, IV and V: allow combustible material (i.e., wood)
- 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 in roof construction 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.

- A. Exterior wall fire resistance (
**Table 602**) depends on:- Occupancy
- Construction type
- Distance to property line (fire separation distance)
- Whether walls are bearing or nonbearing (for bearing walls, see also
**Table 601**)

- B. Openings in exterior walls (
**Table 705.8**) depend on:- Distance to property line (fire separation distance)
- Classification of opening (protected or unprotected)
- Whether building is sprinklered (
**705.8.2**)

- C. Required separation in buildings of mixed occupancy (
**Table 508.4**) depends on:- Occupancies

- D. Basic allowable building heights and (single floor) floor areas (
**Table 503**) depend on:- Occupancy
- Construction type
- 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**where A_{a}= A_{t}(1.0 + I_{f}+ I_{s})_{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 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, I_{s}, 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 I_{s}.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/P - 0.25)(W/30) where_{f}= Frontage increase coefficient- 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**= 3 for 1-story buildings and 2 for multi-story buildings, when sprinklers conform to NFPA 13._{s}= Sprinkler increase coefficient- 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 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:

I

_{f}= (F/P - 0.25)(W/30) = (1.0 - 0.25)(30/30) =**0.75**. - 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

**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