Construction types

Kostyshyn arrives with two blocks of wood and a short length of steel channel. She sets the steel channel on a stand over a camping burner and, beside it, one block of glulam the size of a fist. After ten minutes the steel is glowing dull red and sags under a small weight; the glulam is black on the outside and still holding its load. When she saws it open, the inside is bright and pale, with a dark band only a finger's width deep. 'Which of these is combustible?' she asks. 'The wood, of course. Which one failed? The steel. That is why the code does not simply say: wood bad, steel good. It asks what the structure is made of, whether it burns, and how long it can carry its load in a fire. Construction types are the code's way of putting those questions together.'

What construction type means

Every building code sorts buildings by what their structure and enclosing elements are made of and how well they resist fire. The classification answers two questions. The first is whether the materials of the building contribute fuel to a fire: a building framed in wood adds to the fire load, while one framed in concrete or steel does not. The second is how long the structural elements, floors and walls can resist a standard fire before they fail: a floor with a two-hour fire-resistance rating can hold its load and stop fire passing through it for much longer than an unrated floor. Together these define the construction type, and the occupancy and size of a building determine which types are allowed.

A fire-resistance rating is the time, in minutes or hours, for which an assembly such as a wall, floor or column has resisted a standard fire test. In the test, a full-size sample is built in a furnace and exposed to a fire whose temperature follows a standard curve, rising to about 840 °C at thirty minutes, 925 °C at one hour and 1,010 °C at two hours. The assembly fails when it collapses under its load, when flame or hot gases pass through it, or when the unexposed side becomes hot enough to ignite things touching it. In Canada the test is CAN/ULC-S101. A rating says nothing about what happens in a real fire, which may be hotter or cooler, shorter or longer; it is a way of comparing assemblies on a common scale.

Construction types in the code

The National Building Code uses a small number of construction types. Combustible construction is construction that does not meet the requirements for noncombustible construction, such as wood frame. Noncombustible construction uses materials that pass the code's test for noncombustibility, such as concrete, masonry and steel, although the code allows many limited combustible components within it, such as wood blocking, plastic piping, roofing membranes and some interior finishes. Heavy timber construction is combustible construction with wood members of large minimum dimensions and no concealed spaces; because large timbers char slowly and keep their strength, the code accepts heavy timber in many places where a 45-minute rating would otherwise be required. Encapsulated mass timber construction, introduced in the 2020 edition, uses mass timber elements such as cross-laminated timber and glulam protected by encapsulation, usually gypsum board, so that the wood does not take part in a fire for a set period; it is permitted for some residential and office buildings up to twelve storeys.

The construction type is chosen by the designer within the limits the code allows for the occupancy and size. A building may always be built to a more fire-resistant type than the minimum: a two-storey library could be built of concrete, although there is no need. The design question is usually the opposite: what is the least demanding construction type that will allow the building the designer wants? The answer affects cost, structure, the materials the architect may expose, and the building's character.

Ratings from tests and from calculation

A designer can establish the rating of an assembly in several ways. The most common is to use an assembly that has been tested and listed by a testing laboratory, with a design number that specifies every component, from the size and spacing of the joists to the type and thickness of gypsum board and the spacing of screws. The second is to use the generic ratings in the code's appendix on fire performance, which give ratings for common walls, floors, columns and beams built of standard materials, and a component additive method for wood and steel assemblies. The third is to calculate the rating by an engineering method, such as the calculation of the fire resistance of timber members in the wood design standard CSA O86, which predicts how fast wood chars and how much strength remains.

A rated assembly is only as good as its weakest point. Kostyshyn shows photographs from site inspections: a rated wall with an unsealed hole for a pipe, a rated floor with a recessed light fixture punched into the gypsum ceiling, a rated corridor wall that stopped at the ceiling tiles instead of continuing to the floor above. Each penetration and joint in a rated assembly must be protected by a tested firestop system, and each rated wall must continue to the element above it, unless the code allows otherwise. The architect's details and specifications must show this, because the trades on site will not invent it.

Why wood can resist fire

The burner demonstration at the start of the lesson showed something every architect working with timber needs to understand. When a large wood member is exposed to fire, its surface chars. The char layer insulates the wood behind it, so the burning front moves inward slowly and steadily, and the core of the member stays cool and strong. Steel, by contrast, does not burn but conducts heat quickly and loses about half its strength at around 550 °C; an unprotected steel beam can fail in a fire within fifteen or twenty minutes. This is why heavy timber and mass timber can earn fire-resistance ratings of one or two hours without being covered, and why steel structures are usually protected with spray-applied fireproofing, intumescent paint, concrete or gypsum board.

CSA O86 predicts the fire resistance of a timber member by a reduced cross-section method. During a standard fire, glulam chars at a notional rate of about 0.7 mm per minute on each exposed face, and a further layer of about 7 mm behind the char line is assumed to have no strength. The member's resistance after a given time is then calculated on the remaining section, with the loads that act during a fire, which are lower than the full design loads.

The script below checks the glulam floor beam from the hub's wing, which Marta Szymańska sized in Structures for Architects. It is exposed to fire on three sides, with the CLT floor above protecting its top.

# Residual section of the hub's glulam floor beam after a fire (three-sided exposure)
b, d = 175.0, 570.0          # beam width and depth (mm)
rate, zero = 0.70, 7.0       # notional char rate (mm/min) and zero-strength layer (mm)
S0 = b * d ** 2 / 6
for minutes in (30, 45, 60, 90):
    x = rate * minutes + zero            # loss on each exposed face
    br, dr = b - 2 * x, d - x            # both sides and the bottom are exposed
    if br <= 0:
        print(f"{minutes:>3} min: section consumed"); continue
    Sr = br * dr ** 2 / 6
    print(f"{minutes:>3} min: residual {br:5.0f} x {dr:5.0f} mm, bending capacity {Sr / S0:5.1%} of the original")
print("Fire-case load is about 45% of the factored design load, so the beam needs about 45% of its capacity.")

 30 min: residual   119 x   542 mm, bending capacity 61.5% of the original
 45 min: residual    98 x   532 mm, bending capacity 48.7% of the original
 60 min: residual    77 x   521 mm, bending capacity 36.8% of the original
 90 min: residual    35 x   500 mm, bending capacity 15.4% of the original
Fire-case load is about 45% of the factored design load, so the beam needs about 45% of its capacity.

After 45 minutes the beam keeps about 49% of its bending capacity, just enough for the reduced fire-case load, so it would achieve a 45-minute rating on its own. After an hour it keeps only about 37%, so a one-hour rating would need a wider beam, or a beam protected by gypsum board. Szymańska's beam is 175 mm wide because of deflection, not fire; Callum now knows that a beam about 215 mm wide would give the wing a one-hour rating without any covering, should the classification ever require it.

Choosing the type for the projects

For Greenway Homes, the study article for Group C, up to four storeys, sprinklered, permits combustible construction, with floors built as fire separations rated at one hour and the walls, columns and arches that support them rated at one hour. That is ordinary light wood frame with two layers of Type X gypsum board on the ceilings and on loadbearing walls, and Wren's drawings show it. Had Greenway asked for five or six storeys, the next article would still permit combustible construction but with stricter conditions on exterior cladding, roofs, firefighting access and the use of the first storey. A seventh storey would push the building into noncombustible or encapsulated mass timber construction.

For the hub, the study article for Group A, Division 2, up to two storeys, sprinklered, permits combustible construction, with floor assemblies rated at 45 minutes and the elements supporting them rated at 45 minutes, and allows heavy timber construction in place of the 45-minute rating. Callum's glulam frame, CLT floors and glulam hall trusses can therefore be left exposed, provided every member meets the minimum dimensions the code sets for heavy timber, and the CLT floor panels meet the minimum thickness for heavy timber floors. The calculation above confirms that the floor beam would also reach 45 minutes on its own.

Kostyshyn uses the two projects to make a point about design. In each case the architects knew before they drew that they wanted to build in wood and to see the timber. Construction type was not a constraint discovered at the end; it was a choice made at the beginning, tested against the code, and used to shape the structure and finishes. 'The best code analysis,' she says, 'is the one that was done before the design, and then again after it.'

Exercise 5: construction types

• Determine the least demanding construction type permitted for your Design Studio IV project, and the ratings it requires for floors, roofs and supporting elements.
• Find a listed or generic assembly for one floor and one wall that meets those ratings, and draw it at a scale of 1:5 or larger.
• Calculate the residual section of one exposed timber member after the fire duration your building requires.
• Visit a building under construction and photograph three penetrations through rated assemblies; describe how each is firestopped.

Self-check

• Have I chosen the construction type, or just accepted it?
• Can I name the assembly, test or calculation behind every rating on my drawings?
• Have I shown firestopping at every penetration and joint in rated assemblies?