Building Code Tip: Gain 33% More Exit Stair Capacity
Have you ever struggled to maximize the occupant load while avoiding increasing the size of the exit stairs and/or providing a third or fourth exit stair? If so, the Building Code provides a solution.
With the installation of an emergency voice/alarm communication (EVAC) system throughout a sprinkler protected building, the Building Code (IBC/CBC Section 1005.3.1, Exception 1) allows the capacity, in inches, of exit stairs to be calculated by multiplying the occupant load served by such stairways by an exit capacity factor of 0.20 inch per occupants in lieu of using an exit capacity factor of 0.3 inch per occupant.
A conventional 44-inch wide stair in a building not provided with an EVAC system has a capacity of 146 occupants. With an EVAC system this same stair has an increased capacity of 220 occupants. The advantages of applying this exception is best shown by this example:
These are a few instances that we are aware of where the 33% percent exit stair capacity increase cannot be taken:
In Los Angeles and Long Beach, CA where the emergency helicopter landing facility is not provided on an alternate means and methods basis for high-rises more than 420-feet tall, the 0.2 inch per occupant capacity factor for stairs is not permitted to be used even though the high-rise building is provided with an EVAC system.
For public colleges and universities where the California Office of the State Fire Marshal is the approving agency, the reduced exit stair capacity factor is not permitted in buildings otherwise required to have an EVAC system by other Sections of the CBC (e.g., high-rise buildings or assembly use areas with an occupant load of 1,000 or more).
What is an EVAC System?
Wikipedia provides a good description of an EVAC System:
Some fire alarm systems utilize emergency voice alarm communication systems (EVAC) to provide pre-recorded and manual voice messages. Voice alarm systems are typically used in high-rise buildings, arenas and other large "defend-in-place" occupancies such as hospitals and detention facilities where total evacuation is difficult to achieve.
Voice-based systems provide response personnel with the ability to conduct orderly evacuation and notify building occupants of changing event circumstances.
In high rise buildings, different evacuation messages may be played on each floor, depending on the location of the fire. The floor the fire is on along with ones above it may be told to evacuate while floors much lower may simply be asked to stand by.