Designing for Fire Protection

Expanding the possibilities of wood design
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Sponsored by Think Wood and the American Wood Council
Jeffrey B. Stone, Ph.D. Based on the Code Conforming Wood Design Series by the American Wood Council and International Code Council

New Height and Area Provisions

Chapter 5 of the 2015 IBC has been heavily revised. However, the revisions focus primarily on format and are not intended to be technical in nature. The increases for both height and area, when sprinklers are installed, are now integrated into the tables with separate tables used for height and area.

Open Perimeter Fire Protection

In general, there are two passive measures that decrease a building’s fire hazard: isolating the building from other structures and constructing the building with fire-resistive materials. IBC Chapter 5 provides for increases to the allowable area factor in Table 506.2 for the addition of open perimeter spacing and the use of fire-resistive assemblies.

Isolating a building from adjacent structures decreases the fire hazard of a building as well as that of adjacent structures. The allowable area of a building is determined in accordance with the applicable provisions of Sections 506.2.1 through 506.2.4 and Section 506.3.  In a single occupancy building with no more than one story above grade plane, allowable area is determined in accordance with Equation 5-1. 

Equation 5-1:
Aa = At + (NS x If
where: Aa = Allowable area (square feet)
A
t = Tabular allowable area factor (NS, S1, or S13R value, as applicable) in accordance with Table 506.2
NS = Tabular allowable area factor in accordance with Table 506.2 for nonsprinklered building (regardless of whether or not the building is sprinklered)
The area factor increase based on frontage is determined in accordance with the following Equation 5-5:
If = Area factor increase due to frontage (percent) as calculated in accordance with Section 506.3

Allowable Increases for Frontage Protection

Buildings located next to a public way or open space adjoining a public way, with the exterior wall a minimum of 20 feet from the public way for more than 25% of the building perimeter, may qualify for an area factor increase based on frontage distance in accordance with Equations 5-4 and 5-5.

Equation 5-4:
W = (L1 × w1 + L2 × w2 + L3 × w3…)/F (Equation 5-4) where:
W (Width: weighted average) = Calculated width of public way or open space (feet).
Ln = Length of a portion of the exterior perimeter wall.
wn = Width (≥ 20 feet) of a public way or open space
 associated with that portion of the exterior perimeter wall.

F = Building perimeter that fronts on a public way or open space having a width of 20 feet (6,096 mm) or more.

Equation 5-5:
The area factor increase based on frontage is determined in accordance with the following
If = [F/P - 0.25] W/30 where:
If = Area factor increase due to frontage
F = Building perimeter that fronts on a public way or open space having minimum distance of 20 feet (6,096 mm)
P = Perimeter of entire building (feet)
W = Width of public way or open space (feet) in accordance with Section 506.3.2

It should be noted that frontage widths (W) greater than 30 feet will only receive credit for a value of 30 feet. The maximum increase that can be obtained for frontage would occur when 100% of the perimeter has frontage of 30 feet or more and would result in a 75% floor area increase.

AUTOMATED SPRINKLER SYSTEMS

 

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Originally published in Engineering News-Record

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