Calculating the Maximum Allowable Building Area

Select buildings can have unlimited area. However, most building areas cannot exceed the limits specified in Table 503 of the International Building Code (IBC-2009). These limits are based on a building’s construction type and use group(s). When a building contains multiple use groups, the most stringent value applies. The tabular building area is permitted to be increased due to frontage and automatic sprinkler protection in accordance with Equation 1.

A_a = [A_t+(A_t × I_f )+(A_t × I_s )]                                                       Equation 1: Allowable Building Area

A_a = Allowable building area per story [sf]
A_t = Tabular building area per story per Table 503 in IBC-2009 [sf]
I_f = Area increase factor due to frontage per Section 506.2 in IBC-2009 [-]
I_s = Area increase factor due to sprinkler protection per Section 506.3 in IBC-2009 [-]

Where a building has more than 25 percent of its perimeter on a public way, or open space having a minimum width of 20 feet, the frontage increase (I_f) is calculated in accordance with Equation 2.

I_f = (F/P – 0.25) × W/30                                                                                   Equation 2: Frontage Increase

F = Building perimeter that fronts a public way or open space at least 20 feet wide [ft]
P = Perimeter of entire building [ft]
W = Width of public way or open space, 30-foot maximum per Section 506.2.1 in IBC-2009 [ft]

Best case scenario, a building has 100 percent open perimeter with at least 30-foot-wide public ways/ open spaces. In this scenario, the maximum frontage increase is calculated to be 75 percent (I_f = 0.75) of the tabular building area.

Where a building is equipped with an automatic sprinkler system, the tabular building area is increased 200 percent (I_s = 2). Unless the sprinklered building is only one story tall, in which the tabular building area is increased 300 percent (I_s = 3). Best case scenario, an automatic sprinkler system protects a one-story building. In this scenario, the maximum sprinkler increase is 300 percent (I_s = 3) of the tabular building area.

Substituting the best-case scenarios (I_f = 0.75, I_s = 3), Equation 3 is obtained.

A_am = [A_t+(A_t × .75)+(A_t × 3)]                                     Equation 3: Maximum Allowable Building Area

A_am = Maximum allowable 000: building area per story [sf]

If a building’s area exceeds the maximum allowable building Shotfarm area per story, it must be made smaller or separated into multiple buildings. Creating multiple buildings is accomplished by installing a fire wall to comply with Chapter 7 of IBC-2009 and the Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls (NFPA 221-2009). There are three basic styles of firewalls: cantilevered (freestanding), tied and double. Each are appropriate for different installations. Quality Fire Protection and Structural Engineers are vital in designing a fire wall for Free a specific project. The Fire Protection Engineer determines the wall’s fire characteristics while the Structural Engineer ensures it remains stable during a fire scenario.

Written By: Mark R. Richards, PE

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