Fire separations and compartments

Danielle Roy comes back for the sixth week with a roll of drawings under her arm: the fire separation plans for a hospital wing she has just finished reviewing. Every wall on the plans is drawn in one of six line types, each meaning a different rating, and every door is tagged with a number that matches a schedule of fire-rated doors. 'This is what a building looks like to a fire engineer,' she says. 'Not rooms. Boxes. Each box is meant to hold a fire long enough for people to get out of the next box, and for firefighters to come in. Your job as architects is to draw the boxes so they are continuous, with no holes in them, and to make them disappear into a building people want to be in.'

The idea of compartments

Compartmentation is the oldest form of fire protection. If a fire is kept within the room or suite where it starts, it threatens only the people in that space, and they are usually the first to know about it. The rest of the building has time to evacuate, and firefighters have a contained fire to put out. A compartment is a space enclosed by floors, walls and ceilings that resist the passage of fire and smoke for a known time, with every opening in them protected by a door, a shutter, a damper or a firestop that resists fire for a comparable time.

Codes build compartments at several scales. At the largest scale, fire walls divide a development into separate buildings. Within a building, rated floors divide it story by story, so that a fire on the second floor does not spread to the third. Within a floor, rated walls separate different occupancies, separate each apartment or tenant suite from its neighbors and from the corridor, enclose the exit stairs and shafts, and surround rooms with particular hazards such as furnace rooms, storage rooms and waste rooms. The smallest compartments are the rooms themselves, whose ordinary walls and doors slow a fire even when they are not rated.

The International Building Code uses several distinct terms, each with its own section and its own rules. A fire wall, in Section 706, divides a building into separate buildings. A fire barrier, in Section 707, is a rated wall that encloses exit stairways, shafts and incidental uses and separates mixed occupancies and fire areas; it must be continuous from floor to floor or roof. A fire partition, in Section 708, is a rated wall of lower demands that separates dwelling units and guest rooms and encloses corridors; it may stop at a rated ceiling. A horizontal assembly, in Section 711, is a rated floor or roof. Smoke barriers and smoke partitions, in Sections 709 and 710, resist the passage of smoke. Openings in each must be protected by opening protectives with the ratings set in Table 716.1(2).

Ratings for walls, doors and glazing

The rating required for each wall depends on what it separates. In an R-2 building like Greenway Homes, dwelling units must be separated from each other and from the corridor by fire partitions with a one-hour rating, reduced to half an hour in Types II-B, III-B and V-B construction when the building is sprinklered. Corridors serving more than ten occupants in a sprinklered R building need a half-hour rating. Interior exit stairways must be enclosed by fire barriers with a two-hour rating where they connect four stories or more, and one hour where they connect fewer, and never less than the rating of the floors they pass through. Shafts follow the same rule. Incidental uses such as large furnace rooms, laundry rooms over 100 square feet and waste and linen collection rooms need a one-hour separation or sprinklers, as Table 509.1 sets out.

Opening protectives need ratings that are usually lower than the rating of the wall, because a door is not expected to carry heat as a wall must. A two-hour fire barrier around an exit stair needs a one-and-a-half-hour door; a one-hour fire barrier around a stair needs a one-hour door, but around other uses a three-quarter-hour door; a one-hour fire partition at a corridor needs a twenty-minute door; a dwelling unit entrance door from a corridor needs a twenty-minute door. Fire doors must be self-closing or automatic-closing and positive-latching, and may be held open only by devices that release them when the fire alarm sounds or smoke is detected.

Glazing in rated assemblies is a special problem for architects who want light in corridors and stairs. Traditional wired glass is limited in area and is no longer permitted in many locations because it breaks dangerously under impact. Modern fire-rated glazing comes in two families. Fire-protective glazing, such as ceramic glass, stops flame and smoke but lets radiant heat through, so it is limited in area and in where it can be used. Fire-resistive glazing, made of laminated layers with intumescent interlayers, blocks heat as well and can be used as part of a rated wall, at a much higher cost. Ingrid's glazed corridor along the greenway side of Greenway Homes is not a fire separation at all, since it is an exterior wall, but the glass beside the doors from the corridor into each stair must be rated.

Spatial separation and exposure

Fire can spread from one building to another across a lot line, carried by radiant heat from windows and burning walls, and by flying embers. Codes protect neighboring buildings by limiting the openings and combustible cladding in walls that face a lot line closely. The closer a wall is to the lot line, the fewer unprotected openings, such as ordinary windows, it may have, and the higher the fire-resistance rating its wall must have.

The International Building Code sets limits by the fire separation distance, measured at right angles from the building face to the closest interior lot line, to the centerline of a street, alley or public way, or to an imaginary line between two buildings on the same lot. Table 705.8 sets the maximum share of each story's exterior wall that may be openings, by fire separation distance and by whether the openings are protected and the building sprinklered. Table 602 sets the fire-resistance rating of exterior walls by their fire separation distance, construction type and occupancy, regardless of the rating the construction type itself would need.

The students test the faces of Greenway Homes. The west face of the townhouses looks across the back lane at the small houses beyond; the east face of the east bar looks across Marlow Street; the north face of the north bar looks across the greenway, which is city land.

# Exterior wall openings check for Greenway Homes (IBC Table 705.8, sprinklered, unprotected)
rows = [(3, 0.15), (5, 0.25), (10, 0.45), (15, 0.75)]   # lower bound of FSD band (ft), share allowed

def allowed(fsd):
    if fsd < 3:
        return 0.0
    share = 0.0
    for lo, s in rows:
        if fsd >= lo:
            share = s
    return share if fsd < 20 else 1.0

faces = [  # face, fire separation distance (ft), wall area per story (ft2), opening area (ft2)
    ("townhouses, west to the alley centerline", 15 + 10, 720, 196),
    ("east bar, east to the Marlow St centerline", 10 + 33, 1030, 330),
    ("north bar, north to the greenway lot line", 10, 1620, 570),
    ("east bar, south to Aldous St centerline", 10 + 30, 575, 150),
]
for name, fsd, wall, op in faces:
    a = allowed(fsd); share = op / wall
    print(f"{name:<43} FSD {fsd:3d} ft  openings {share:4.0%}  allowed {a:4.0%}  -> {'OK' if share <= a else 'reduce or protect'}")

townhouses, west to the alley centerline    FSD  25 ft  openings  27%  allowed 100%  -> OK
east bar, east to the Marlow St centerline  FSD  43 ft  openings  32%  allowed 100%  -> OK
north bar, north to the greenway lot line   FSD  10 ft  openings  35%  allowed  45%  -> OK
east bar, south to Aldous St centerline     FSD  40 ft  openings  26%  allowed 100%  -> OK

Every face passes, but the north bar is a close call. The greenway is city land, not a street, so the fire separation distance is measured to its lot line, not to its center, and it equals the 10-foot setback exactly. That puts the face in the band from 10 to 15 feet, which allows 45% of unprotected openings, and the north windows at 35% pass. Had the bar crept a few inches north during design development, the face would have dropped into the band from 5 to 10 feet, where only 25% is allowed, and a window in every room would have had to shrink. Wren dimensions the setback as a minimum on her drawings, notes the code reason beside it, and asks the surveyor to confirm the lot line before the foundations are laid out.

Interconnected spaces and atriums

Some of the most memorable spaces in architecture break compartments on purpose: an open stair between floors of a library, a double-height commons, an atrium in an office building. Smoke rises through such spaces and can fill every floor connected to them. Codes allow them under conditions designed to keep smoke away from people and exits.

The International Building Code allows openings between floors in several ways, set out in Section 712. An opening connecting only two stories that is not part of the required means of egress and does not connect a basement may be left open in many occupancies if the building is sprinklered. Larger openings, connecting three or more stories, are atriums under Section 404 and need sprinklers, a smoke control system, fire barriers or approved glass with sprinklers between the atrium and the surrounding spaces, and limits on travel distance through the atrium. The hub's commons, two stories high with a mezzanine to the library, falls within the two-story provision.

Callum's double-height commons therefore survives, but Kostyshyn warns that it has used up the easy allowance. If Callum had wanted the library's mezzanine to continue up to a third level, the commons would have become an atrium with all the requirements that go with it.

Penetrations, joints and continuity

Roy spends the last part of the class on the details that make or break a fire separation in practice. A fire separation is only effective if it is continuous. A rated wall must meet the rated floor or roof above it with a joint that is itself protected. Every pipe, duct, cable and conduit that passes through must be sealed with a firestop system tested for that type of penetration and that assembly. Ducts passing through rated walls and floors need fire dampers, which close when heat melts a fusible link, or combination fire and smoke dampers. Electrical boxes in rated walls must be limited in size and spaced apart on opposite faces. Concealed spaces above ceilings and within walls must be divided by fire blocking so that fire cannot run through them unseen.

Roy has seen fires spread through all of these gaps. She asks the students to mark on their Greenway floor plan every place where a fire separation is broken by something: the sprinkler mains passing through corridor walls, the dryer vents leaving each apartment, the kitchen exhaust ducts, the plumbing stacks, the cable trays above the corridor ceiling. The list runs to more than forty items on a single floor. Every one needs a detail or a specification, and on site, every one needs an inspection.

Exercise 6: fire separations

• Prepare a fire separation plan of one floor of your Design Studio IV project, with the rating of every wall and the rating of every door.
• Check the spatial separation of every exterior face of your building against the code in force.
• Draw one penetration of a rated floor and one head-of-wall joint at a rated wall, at full size, naming the tested systems.
• Identify every opening between floors in your project and state how the code treats it.

Self-check

• Is every fire separation continuous from floor to floor or to the rated ceiling, with every opening protected?
• Have I measured limiting distances correctly to lot lines, street centers and imaginary lines?
• Have I counted the penetrations, and shown how each is sealed?