Means of egress

Kostyshyn takes the class out of the school for the seventh week. They walk to the old downtown hotel near the river, now apartments, and she leads them up its main stair to the top floor. Then she asks them to find their way out as if the corridor were full of smoke, without using the stair they came up. Wren finds the second stair at the far end of the corridor. Callum finds a door marked as an exit that opens into a storage room. Ingrid times the walk from the farthest apartment to the nearest stair: 51 metres. Joon-ho counts the steps on the way down and finds that two flights have risers of different heights. Back in the classroom, Kostyshyn writes on the board: 'Every building is designed for the day it is on fire. That is the day the exits matter.'

The parts of a means of egress

A means of egress is a continuous path of travel from any point in a building to a safe place outside it. The National Building Code divides it into two parts. Access to exit is the part within a floor area that leads to an exit: the path through a room, along a corridor or across an open floor to the nearest exit door. An exit is the part that leads from a floor area to a separate building, an open public thoroughfare or an exterior open space protected from fire exposure: an exit stair enclosed by fire separations, an exterior door from the ground floor, a horizontal exit through a firewall into another building, or an exterior passageway. Once a person is inside an exit, they should be protected all the way to the outside, without having to pass back through a floor area.

The design of egress rests on a few simple ideas. There must be enough exits, so that one blocked by fire leaves another. The exits must be far enough apart that a single fire is unlikely to block them both. They must be close enough to every occupant that people reach them before conditions become untenable. They must be wide enough for the number of people who will use them. And every part of the path must be easy to find, easy to use in the dark and in smoke, and impossible to lock against the people inside.

Occupant load

Egress design begins by counting people. The occupant load of a space is the number of people for which its means of egress must be designed. For most spaces the code sets it by dividing the floor area by an occupant load factor, the area per person typical of that use. Assembly spaces with tables and chairs, chairs alone or standing room have small factors, because people are packed densely; offices, storage and residential spaces have large ones. Where a space has fixed seats, the load is the number of seats. Where the designer expects more people than the factor gives, the higher number must be used; where fewer, the code may allow a lower number to be posted, but only with the building official's agreement.

Number, arrangement and distance of exits

The National Building Code requires every floor area to be served by at least two exits, with exceptions for small floor areas where a single exit is acceptable within limits of area, occupant load and travel distance. Every room or suite whose occupant load or size exceeds certain limits needs at least two egress doorways. Exits must be located so that the distance between them is not less than one half of the greatest diagonal of the floor area, with permitted reductions in some cases. The travel distance from any point in a floor area to the nearest exit may generally be no more than 30 m, or 45 m in a sprinklered building, with stricter limits for high-hazard uses. Where a floor area is divided into suites opening onto a public corridor, the travel distance is measured from the suite door, and the corridor must not have a dead end longer than 6 m.

Kostyshyn explains the reasoning behind each of these. Two exits are required because one may be blocked by the fire itself. The separation by half the diagonal keeps them at opposite sides of the floor, so a single fire near the middle is unlikely to cut people off from both. The travel distance limit keeps people's walk short enough that they reach an exit before conditions become untenable; it is longer in sprinklered buildings because sprinklers slow the fire. The dead-end limit protects people who turn the wrong way in a smoky corridor: they must not walk far before discovering they must turn back.

Width and capacity

The width of each part of a means of egress must be enough for the number of people expected to use it. The National Building Code sets the required width by the occupant load served: 6.1 mm per person for doorways, corridors, ramps and other level components, and 8 mm per person for stairs with risers no higher than 180 mm. Minimum widths also apply regardless of occupant load: generally 1100 mm for exit corridors and stairs, and 800 mm of clear width at doorways. Where several exits serve a floor, the occupant load is shared among them, but the exits must be sized so that losing one does not leave the others hopelessly inadequate.

The hub's hall makes a good test. It is the largest room in either project and the one that would be most crowded on a busy night. The script below sets its occupant load for two kinds of event, checks the number and width of exits, and then checks Greenway's corridors.

# Egress check for the hub's hall and for a Greenway corridor
area = 520                       # net area of the hall (m2)
events = {"dinner, tables and chairs": 0.95, "meeting, chairs only": 0.75}
for event, factor in events.items():
    load = area / factor
    exits = 2                        # at least two; more if width or arrangement needs them
    door_width = load * 6.1        # total clear door width required (mm)
    print(f"{event:<28} load {load:4.0f} people, exits {exits}, total door width {door_width:6.0f} mm")

doors = [1830, 1830, 1830]           # clear width of each hall exit (mm)
load = area / 0.75
print(f"hall doors give {sum(doors):.0f} mm; required {load * 6.1:.0f} mm; "
      f"with the largest door blocked {sum(doors) - max(doors):.0f} mm")

# Greenway north bar: travel from the farthest suite door to the nearest stair
corridor = 40                   # corridor length between the two stairs (m)
far_door = corridor / 2             # worst suite door is midway between the stairs
print(f"Greenway corridor: worst travel to a stair {far_door:.0f} m (limit 45 m in a sprinklered building)")

dinner, tables and chairs    load  547 people, exits 2, total door width   3339 mm
meeting, chairs only         load  693 people, exits 2, total door width   4229 mm
hall doors give 5490 mm; required 4229 mm; with the largest door blocked 3660 mm
Greenway corridor: worst travel to a stair 20 m (limit 45 m in a sprinklered building)

For a meeting with chairs only, the hall holds about 690 people and needs about 4,230 mm of exit door width. Callum's three pairs of doors, each with about 1,830 mm of clear width, give 5,490 mm. With one pair blocked, the other two give 3,660 mm, a little less than the full requirement but well over half of it, which is the kind of margin the code's arrangement rules are meant to secure. The occupant count also triggers other requirements: an assembly space of this size needs doors that swing in the direction of travel, with panic hardware, and an occupant load sign posted near the main entrance.

Stairs, ramps, doors and handrails

Exits must be safe to use quickly by people in a hurry, some of them old, some carrying children, some unable to see well through smoke. Codes therefore control the geometry of stairs and ramps closely. Every riser in a flight must be the same height, within a very small tolerance, and every tread the same depth, because people descending a stair quickly set their stride by the first steps and stumble on any that differ. Handrails must be continuous, graspable and at a set height, and stairs above a certain width need handrails on both sides and sometimes in the middle. Guards, which prevent falls from the open sides of stairs, landings and balconies, must be tall enough and closely enough spaced that a small child cannot climb through.

Under the National Building Code, stairs in exits serving Part 3 buildings must have risers between 125 and 180 mm high and treads at least 280 mm deep. Handrails are set between 865 and 965 mm above the nosing line, guards are at least 1070 mm high in most locations, and openings in guards must not allow a 100 mm sphere to pass. Exit doors must swing in the direction of exit travel, and doors in assembly occupancies and other crowded spaces need hardware that releases the latch when a push bar is pressed. Exit signs show a green running figure, the pictogram used internationally since the 2010 edition of the code replaced the older red letters.

Egress must also work for people who cannot use stairs. The National Building Code requires that floor areas served by an accessible path be protected so that people who cannot use stairs have somewhere safe to wait, by providing areas of refuge, horizontal exits into another fire compartment, or elevators protected for use in a fire, depending on the building. Danielle Roy reminds the students that the residents of Greenway Homes include Ernest Kowalchuk, who uses a walker, and children in strollers. Every stair landing in the building must have room for a person to wait out of the path of those going down.

Locks, security and egress

The tension between security and egress has killed people since the Iroquois Theater. Owners want to keep strangers out and stop theft; the code requires that people inside can always get out. The National Building Code requires that every door in a means of egress be openable from the inside without keys, special devices or specialized knowledge, with limited exceptions for detention buildings, for some care occupancies and for electromagnetic locks that release on fire alarm, power failure or after a short delay when pushed. The hub's daycare, which must keep children in and strangers out, is the hardest case. Callum works with the fire department and the daycare operator on a system of door alarms, delayed-release locks on the yard gates and staff-controlled release, with everything unlocking when the fire alarm sounds.

Exercise 7: means of egress

• Prepare an egress plan for each floor of your Design Studio IV project, showing occupant loads, exits, travel distances, dead ends and the separation of exits.
• Check the width of every exit door and stair against the occupant load it serves.
• Draw your main exit stair in section at 1:20, with every riser, tread, handrail and guard dimensioned.
• Walk the exits of a building you use and note one thing that would slow people down in a fire.

Self-check

• Does every occupant have two ways out, far enough apart, within the travel distance limit?
• Is every exit wide enough for its share of the load, with margin if one is blocked?
• Can every door in the path be opened from inside without a key?