Means of egress: occupant load, exits and travel distance

Getting people out of a building in an emergency is the oldest life safety purpose of building codes, and the egress provisions are among the most detailed. The code asks how many people may be in each space, how far they may have to walk to reach safety, how many separate ways out they need, how wide those ways must be and how they must be built, lit, signed and protected. The answers come from the occupancy and occupant load, with adjustments for sprinklers and building height. This lesson works through the main egress calculations a construction manager should be able to follow and check: occupant load, number of exits, exit width, travel distance, separation of exits and the protection of exit routes.

Construction managers deal with egress every day. Stair dimensions, door swings, hardware, exit signs, emergency lighting, corridor widths and the absence of obstructions are all inspected at occupancy. Egress must also be maintained during construction and renovation, often while part of the building is occupied, which requires temporary exits and planning.

The egress system

The IBC divides the means of egress into three parts. The exit access is the portion of a means of egress system that leads from any occupied portion of a building to an exit: rooms, aisles, corridors and unenclosed stairs. The exit is the portion separated from other interior spaces by fire-resistance-rated construction and opening protectives, providing a protected path between the exit access and the exit discharge: exterior exit doors, interior exit stairways and ramps, exit passageways, horizontal exits and exterior exit stairways. The exit discharge is the portion between the termination of an exit and a public way: an exterior path or, in limited cases, a lobby at the level of exit discharge.

Occupant load

Lesson 4 introduced occupant load as floor area divided by an occupant load factor. For egress, the occupant load is calculated for each room, each floor area or story and the building as a whole, and it is the starting point for every other egress calculation. Where a space has fixed seats, the occupant load is the number of seats. Where a space serves more than one use at different times, such as a hall used for dining and dances, the design uses the more demanding load. The occupant load of a floor also includes people from upper floors who will pass through it on the way out, at the level of exit discharge.

The IBC requires the occupant load to be calculated from Table 1004.5 unless a higher actual load is proposed and the egress is designed for it. A lower load than the table may be approved in limited cases. Assembly spaces must have the occupant load posted near the main exit, and maximum floor area allowances per occupant are a design minimum, not a limit on actual use.

Number of exits

IBC Table 1006.3.3 requires at least two exits from each story with an occupant load of 1 to 500, three exits for 501 to 1,000 occupants and four exits for more than 1,000. Spaces within a story need two exit access doorways when the occupant load exceeds the limit in Table 1006.2.1, such as 49 for most business, assembly and mercantile spaces, or when the common path of egress travel exceeds its limit. Single-exit stories are permitted only for small buildings within the limits of Table 1006.3.4, for example a first story of Group B with up to 49 occupants and 75 ft of travel.

Two exits are required because one fire, or the smoke from it, may block an exit. For the second exit to be a real alternative, the exits must be far enough apart. Codes therefore set a minimum separation between exits.

The IBC requires exits or exit access doorways, where two are required, to be placed a distance apart equal to at least one half of the length of the maximum overall diagonal dimension of the building or area served, measured in a straight line between them. In buildings with an automatic sprinkler system throughout, the separation may be reduced to one third of the diagonal.

Travel distance

Exit access travel distance is measured from the most remote point within a story along the natural and unobstructed path of egress travel to the entrance of an exit. IBC Table 1017.2 sets maximums by occupancy and sprinklering, for example 200 ft unsprinklered and 300 ft sprinklered for Group B; 200 ft and 250 ft for Groups A, E, F-1, M, R and S-1; and 300 ft and 400 ft for Groups F-2, S-2 and U. Common path of egress travel, the distance before occupants have a choice of two paths, has its own limits in Table 1006.2.1, such as 100 ft for Group B where sprinklered.

Exit width and capacity

The IBC sets the capacity of egress components in Section 1005.3. The required width is the occupant load times 0.3 in per occupant for stairways and 0.2 in per occupant for other egress components, such as doors, ramps and corridors. In buildings with sprinklers and an emergency voice/alarm communication system, the factors may be reduced to 0.2 in for stairways and 0.15 in for other components (not in Groups H and I-2). Minimum widths also apply: 44 in for stairways serving 50 or more occupants (36 in for fewer), 44 in for corridors serving 50 or more, and 32 in clear for doors. Where a stair serves several stories, its capacity is based on the largest occupant load of any one story it serves, not the sum.

Exit capacity must also be balanced. If 60 percent of the occupants are likely to head for the main entrance, codes in both countries limit how much capacity can be lost if one exit is blocked, so that the remaining exits can still serve a reasonable share of the load. Assembly occupancies often require the main entrance to handle a stated fraction of the load.

Building the exits

The calculations fix how many exits and how wide; detailed rules fix how they are built. These are the requirements inspectors check at occupancy and that construction managers must coordinate across trades.

• Stairs: riser height, tread depth and uniformity limits, headroom, landings at each door, handrails on both sides and guards at open edges. Uniformity matters: a single riser that differs from the rest causes falls.
• Doors: swing in the direction of travel where the occupant load exceeds a threshold, hardware operable without keys or special knowledge, panic or fire exit hardware in assembly and other occupancies, and limits on the force needed to open.
• Exit signs: illuminated signs at each exit door and along routes where the exit is not visible.
• Emergency lighting: lighting of exits and principal routes, powered by batteries or a generator when normal power fails, typically for 30 minutes to 2 hours depending on the building.
• Protection: exit enclosures built as rated fire separations or fire barriers, with no storage, no unrelated services and limited openings.
• Exit discharge: a clear path from the exit to a public thoroughfare or safe open space.

The IBC requires exit signs with letters at least 6 in high, internally or externally illuminated, and emergency lighting averaging at least 1 footcandle at floor level, with a minimum of 0.1 footcandle at any point, for 90 minutes. Stairs in most buildings must have risers between 4 and 7 in and treads at least 11 in deep, with handrails 34 to 38 in high and guards 42 in high.

Egress during construction

Existing buildings under renovation, and new buildings partly occupied before completion, must keep their exits working. Blocking an exit with materials, locking a door to secure a work area or removing an exit stair for repair can leave occupants with no way out. Fire codes in both countries require a fire safety plan for construction and demolition sites, and the plan should show how exits are maintained, where temporary exits and signage are provided and how workers and occupants are kept apart.

IBC Chapter 33 and IFC Chapter 33 require that required means of egress be maintained during construction, alterations and demolition, and that stairways be provided in buildings under construction once they reach a stated height. OSHA's construction standards also require exit routes for workers.