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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 x If, where:

Excerpt from Table 506.2 Allowable area factor from 2015 IBC

For single-occupancy buildings of 1 or 2 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 3 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.

graphic description of maximum areas for buildings

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

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 x more for 1-story buildings (S1) and 3 x 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):
OccupanciesSprinklers required?Story limitIBC 2015 ref.Notes
F-2, S-2No1Sec. 507.31
A-4Yes1Sec. 507.41, 2
B, F, M, SYes2Sec. 507.4 and 5-7.51
A-3, Type IIYes1Sec. 507.61, 3
A-3, Type III and IVYes1Sec. 507.71, 4
HSee Code for detailsSec. 507.8 – 507.101
EYes1Sec. 507.111, 5
Motion picture theatersYes1Sec. 507.121, 6
Covered or open malls and anchor buildingsYes3Sec. 507.131, 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-B 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).

5. Example: Design with limited site area.

What does "design" mean in this context?

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

Table 506.2, 2015 IBC, for this example
Aa is computed above for a sprinklered multi-story building with a frontage factor = 0.42.

Choices:

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

See my handout on Fire Safety and Building Code Methods.