# ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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# Area Calculations and Examples

## Determination of Allowable Building Area

Basic building area can be increased for frontage, sprinklers, and numbers of stories as follows:

Allowable area for single-occupancy buildings, per floor, Aa = At + NS × If, where:

• At is the "tabular" area factor found in Table 506.2 (see excerpt from table below, or see 2015 IBC, NY State version)
• 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 – 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 (if unsprinklered) or four (if sprinklered). These two cases (unsprinklered and sprinklered) are illustrated in the diagrams below. 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.

For mixed occupancies: See next lecture.

1. If = 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, If 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.)

If = (220/320 − 0.25)(28.64/30) = 0.42.

2. Sprinklers: Tables in Chapter 5 allow increased building height — 1 extra story and 20 extra feet — and also allow significantly more floor area — 4 times more for 1-story buildings (S1) and 3 times more for multiple-story buildings (SM).

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 (based on selected information from Sec. 507):
 Occupancies Sprinklers required? Story limit IBC 2015 ref. Notes F-2, S-2 No 1 Sec. 507.3 1 A-4 Yes 1 Sec. 507.4 1, 2 B, F, M, S Yes 2 Sec. 507.4 and 5-7.5 1 A-3, Type II Yes 1 Sec. 507.6 1, 3 A-3, Type III and IV Yes 1 Sec. 507.7 1, 4 H See Code for details Sec. 507.8 – 507.10 1 E Yes 1 Sec. 507.11 1, 5 Motion picture theaters Yes 1 Sec. 507.12 1, 6 Covered or open malls and anchor buildings Yes 3 Sec. 507.13 1, 7

Notes:

1. See IBC Code for other details and exceptions.
2. Not Type V construction.
3. Type III and IV construction, and used for place of worship, community hall, dance hall, exhibition hall, gym, lecture hall, indoor pool, tennis court; no stage except platform.
4. Type II construction, and used for place of worship, community hall, dance hall, exhibition hall, gym, lecture hall, indoor pool, tennis court; no stage except platform. Assembly floor within 21 inches of street or grade level; all exits have ramps.
5. Type II, IIIA or IV construction; each classroom has at least 2 means of egress, one directly outside.
6. Type II construction. Motion picture theater must be on 1st story; building call be higher.
7. Must comply with Sec. 402.

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. Example: Find the allowable area for a sprinklered Type II-A office building on a big enough site so that the area of the building is not limited by frontage or property lines in any way (i.e., If = 0.75).

• From various tables in Chapter 5 (2015 IBC), the "tabular" limits are 6 stories and 112,500 s.f.

• Allowable per-story area:
Aa = At + [NS × If] = 112,500 + 37,500(0.75) = 140,625 s.f.

Allowable total area:

• 6 stories permitted with sprinklers
• Allowable total area = 140,625(4) = 562,500 s.f.
• Equal area per floor = 562,500/6 = 93,750 s.f.
• The footprint of such a building could be, for example, 468' × 200', as shown in the image below.

Of course, since the maximum total floor area is 4 times the maximum allowable per-floor area, it's possible to configure a 4-story building with the same total area, with 140,625 square feet per floor. Such a building would have a larger footprint, for example, 703' × 200'.

## If the building is unsprinklered, here's what changes:

• From Table 504.4 and 506.2 (2015 IBC), the tabular limits are 5 stories and 37,500 s.f.
• Allowable per-story area:
Aa = At + [NS × If] = 37,500 + 37,500(0.75) = 65,625 s.f.

Allowable total area:

• 5 stories permitted without sprinklers
• Allowable total area = 65,625(3) = 198,875 s.f.
• Equal area per floor = 198,875/5 = 39,375 s.f.

Biggest Type II-A Unsprinklered Office Building

There are other configurations for an unsprinklered building with maximum total floor area; most obviously, a 3-story building with 65,625 square feet per floor. Such a building would have a larger footprint than the 5-story variant, for example, 328' × 200'.

5. Example: Design with limited site area.

What does "design" mean in this context?

• Find the least expensive construction type that works
• Confirm that a particular construction type works
• Find the biggest building compatible with required site parking (from zoning)
• Etc.

Assume that building size has been determined by zoning setbacks and parking requirements (see site plan above with 100 ft. × 60 ft. building and If = 0.42. Find "least expensive" construction type.

Aa is computed above for a sprinklered multi-story building with a frontage factor = 0.42.

Choices:

• All construction types "work" for the per-story area of 6,000 s.f., but the maximum number of stories allowed varies.
• For a 1–3-story sprinklered building: Type V-B.
• For a 4-story sprinklered building: Type V-A, III-B, or II-B
• For a 5-story sprinklered building: Type III-A (or II-A or IV)
• Unsprinklered 5-story building: Type III-A (or II-A or IV)

Calculator: See my IBC 2006–2015 Allowable Area Calculator.

See my handout on Fire Safety and Building Code Methods.