School of PE Blog

The main goal of safety in a built environment is to get occupants out of a building as quickly as possible in the event of an emergency. Fire protection engineers work to arrange exits in such a way that all occupants can leave safely before they are overcome by heat or smoke.

The way to leave a building is defined as the means of egress, which is a continuous and unobstructed path from inside a building to the public way. The means of egress is split into three distinct parts: exit access, exit, and exit discharge. The exit access is the pathway leading to an exit. An example would be the path between cubicles to get to an exit corridor. The exit provides a protected path to the exit discharge. Corridors and stairwells are common examples of an exit and are often protected by fire walls. The exit discharge connects the exit to the public way. This can be an exterior path leading from the building to the public sidewalk. All three aspects are required for a means of egress to be code compliant.

Intuitively, the more people there are in a building, the more exits are required for egress. If there are 49 people or less in a room, only one exit is required. A space containing between 50 and 499 people requires two exits, 500 to 999 occupants require three exits, and an area with over 1000 occupants would require a minimum of four exits. The number of occupants that are allowed to be in a particular area is determined by the occupant load calculation. The occupant load calculation is performed by dividing the area by the occupant load factor, which can be found in the applicable life safety code (NFPA 101, IBC, local codes, etc.).

Using a business occupancy as an example, the occupant load factor according to the IBC is 150 square feet per person, so a 10,000 square foot office space can have a maximum of 66 people (10,000 square feet divided by 150 square feet per person). This also means there must be a minimum of two exits from the space. The square footage per person varies by occupancy type. For instance, a warehouse could have an occupant load factor of 500 square feet per person while an event center could have an occupant load factor of 15 square feet per person. Changes in occupancy are important to recognize because that same 10,000 square foot office space could be converted to an event center, which would increase the occupant load to 666 people (10,000 square feet divided by 15 square feet per person), and an additional exit would be required.

The locations of the exits must also be considered. Exits cannot be too close together; the exits in a sprinklered building must be separated by at least 1/3 the diagonal distance of a room, and that distance increases to 1/2 diagonal distance in an unsprinklered building. Exits cannot be obstructed or otherwise impeded by furniture or obstacles.

The width of the egress component must also be considered. For instance, two 32-inch doors would not be adequate to serve a concert hall with 400 occupants. Generally speaking, the width of the stairs and ramps can be found by multiplying the number of occupants by 0.3 and the width of the doors and other level components is found using a factor of 0.2. Therefore, the total width of doors required for the 400-person concert hall is 80 inches (400 times 0.2). This can be split between the two doors required, and one of the doors must be a minimum of 32 inches, which allows for passage of wheelchairs.