How the Maximum Occupancy of a Building is Calculated
Generally speaking, the maximum occupancy of a room or building is primarily determined by the available exits, with each exit accommodating only a certain number of people before bottlenecking occurs. The other key component in determining the max occupancy of a building or room is the intended use of the space, whether it’s, for instance, a restaurant with tables and chairs or a more open event space.
More specifically, the International Building Code (IBC) provides an international standard for calculating the maximum occupancy for an area. The IBC defines an exit, or a means of egress, as “A continuous and unobstructed path of vertical or horizontal egress travel from any occupied portion of the building or structure to a public way.” (2009 International Building Code, p 218)
In the United States, the National Fire Prevention Association (NFPA) also forbids an exit from leading occupants through a kitchen or storage room on their way out of the building.
In order to determine the maximum number of people who are able to safely be in a room or building, the IBC recommends a certain number of inches of doorway per occupant. Exits that consist of a stairway need to have 0.3 inches of doorway per person, and all other exits need 0.2 inches of doorway per person. So a function hall that has a maximum occupancy of 1,000 people and an exit that consisted of a corridor without stairs will need 200 inches of doorway. With doorways at around 36 inches in width—wider than the Americans with Disabilities Act requires in the United States—that function hall would need approximately six such doors.
As mentioned, another important consideration for determining the occupancy of a space is the intended use of the space. A restaurant with chairs and tables will have a smaller maximum occupancy than a bar with an open dance floor or layout. The IBC recommends for spaces with unconcentrated use of chairs and tables, such as a restaurant, that 15 square feet on that floor of the building be dedicated to each occupant. That means a 500 square foot restaurant might have a maximum occupancy of 33 people. However the IBC recommends areas with concentrated use of chairs, such as a bar with a dance floor, have 7 square feet of floor space on that floor of the building per person. The same 500 square foot space in a bar with a dance floor would have a tentative maximum occupancy of 71 people. That said, those numbers are all dependent on whether or not enough exits exist to allow all of those occupants to safely leave the building during an emergency.
The impetus for these regulations in the United States dates back to the worst hotel fire in the country’s history. The Winecoff Hotel Fire occurred in the early hours of the morning on December 7, 1946 in Atlanta, Georgia. The 15-story hotel considered “fireproof” due to its brick exterior had a single staircase that allowed guests to move from the upper floors to the first floor. That staircase had turned into a chimney for the smoke and fire by the time that the first of the fire fighters arrived at 3:45 AM. A number of trapped guests tried to escape through their windows, but only a small portion trapped managed to survive their escape while the rest either died inside or fell to their deaths. The fire resulted in 119 fatalities which amounted to 41 percent of the hotel’s guests that night. Of the remaining, 65 were injured and the rest made it to safety whether because they were below the third floor fire level or were rescued by the 385 fire fighters who ultimately responded to the fire. This fire occurred just six months after the La Salle Hotel fire which killed 61 people. After these fires, President Truman ordered a national conference on fire prevention to revise fire codes for buildings.
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- One of the deadliest nightclub fires in U.S. History took place at the ‘Cocoanut Grove’ in Boston, killing 492 people. (Legally the club was only authorized to have a maximum of 460 people in it.) There was only one other such event fire, at Chicago’s ‘Iroquois Theater’ in 1903, that had a higher death toll- 602 people. Like the Winecoff Hotel Fire, the Cocoanut Grove nightclub fire brought about some major changes in safety standards and building codes. Most notably, all exterior doors to commercial buildings had to open outwards. This was not the case at the Cocoanut Grove club, and when the fire started the crowd of people rushed to the doors to get out. The surge of people pushing against the doors prevented the inward opening doors from opening, trapping those inside.
- The Station Nightclub Fire, one of the most recent deadly nightclub fires, holds the rank of 4th deadliest nightclub fire in the U.S. after 100 people died in the 2003 blaze caused by pyrotechnics.
- The Americans with Disabilities Act requires single doors to be 32 inches wide and double doors to be 48 inches wide.
- International Building Code 2009. Country Club Hills, Ill.: International Code Council, 2009.
- NFPA 1: Fire Code. 2012 ed. Quincy, Mass.: National Fire Protection Association, 2011.
- Square Footage Definitions: GSF, ASF, Non ASF, SSF
- Occupancy Calculations
- Merseyside Fire & Rescue Service
- Calculating Occupant Load
- The Cocoanut Grove Fire
- Historic Loss of Life: The Winecoff Hotel Fire
- The Station Nightclub Fire
- ADA Standards for Accessible Design
- ADA Door Regulations
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