What architects can learn from firefighters about radiant heat

Photo © Mike Schwartz Photography

By Diana San Diego
In any firefighting operation, the primary objective is to rescue building occupants. However, fire confinement, where firefighters use water or another extinguishing agent to limit the fire to one area, is also crucial. Stopping the fire’s forward progress must be done first, before it can be extinguished. (For more, consult 2004’s “The Firefighter’s Handbook: Essentials of Firefighting and Emergency Response [2nd Edition],” from National Fire Academy Alumni Association [NFAAA] and Delmar Thompson Learning.)

In carrying out both objectives, radiant heat is a major concern, because not only does it prevent firefighters from getting to the building that is on fire, but it also contributes to the flames spreading. Structures near a burning building (i.e. exterior exposures) and parts of the building not yet involved (i.e. interior exposures) must be protected to minimize danger to occupants, as well as to contain the fire. (This information is derived from “Engine Company Fireground Operations [2nd Edition],” by Harold Richman and National Fire Protection Association [NFPA] staff.)

Radiant heat is also a major concern for architects because the buildings they design are occupied by people, and usually surrounded by nearby structures that can catch on fire. In the event of a fire, a building’s design and the materials chosen to build it can directly affect occupants’ ability to safely exit. This can also impact how effectively firefighters can achieve the two aforementioned goals (rescuing building occupants and confining, then extinguishing a fire).

What is radiant heat?
Smoke and flames, the visual components of a fire, draw the most attention. However, there is also a third and invisible component called radiant heat, which is just as dangerous to building occupants and as effective in promoting fire spread. If you have ever stood in front of a fire to keep yourself warm, then you have experienced radiant heat first-hand. In small doses, it can be comforting. However, in large doses—such as the heat generated by a building fire—it can sometimes be fatal.

Radiant heat is composed of extremely intense, invisible electromagnetic waves that travel at the speed of light with little or no resistance from air. When these waves strike an object, they are absorbed, and their energy is converted to heat. If the object is a combustible material (e.g. paper, fabric, or wood), a fire will start when the material’s ignition temperature is reached.

To ensure protection against smoke and flames and limit the transmission of dangerous radiant heat, building materials must pass rigorous fire-resistive test standards. In Canada, they must meet CAN/ULC-S101, Standard Methods of Fire Endurance Tests of Building Construction and Materials. In the United States, the relevant standard is ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, or UL 263, Standard for Fire Tests of Building Construction and Materials. This means the assembly must be able to block smoke and flames and limit the transmission of radiant heat to no more than 139 C (250 F) above ambient for the duration of the fire endurance test (despite temperatures in the furnace reaching 982 C [1800 F]).

Both architects and firefighters must consider how best to combat radiant heat.
Photo © Nejc Vesel/Shuttershock

Gypsum wallboard, masonry, and concrete are probably the most common building materials that meet CAN/ULC-S101. However, when vision and transparency are desired, designers can use clear fire-resistive glass that also meets these standards. This glass can be used in any one- or two-hour wall or floor application up to the maximum size tested. (The maximum glass size varies by manufacturer, depending on the size for which it tested and passed during the lab test.)

Fire-resistive glass is a significant improvement on wired glass, which was the sole fire-rated glass option up until the 1970s, and only offered protection from smoke and flames. (For more information on wired glass, visit
www.constructioncanada.net/cgsb-releases-updated-safety-glazing-standard-excluding-wired-glass.) Ceramics was then introduced as a ‘wire-free’ fire-rated glass option, but it still only protected against smoke and flames, even if it was rated up to 180 minutes. As wired and ceramic glazing offer no radiant heat protection and cannot meet CAN/ULC-S101, they are classified as fire protective only, and typically limited by National Building Code of Canada (NBC) to applications rated for less than one hour and to 25 per cent of the wall area. Ceramics and wired glass can sometimes be used in 60- to 90-minute temperature rise doors, but are still limited to 64,516 mm2 (100 si) in the door vision area due to radiant heat concerns.

There are two types of fire-resistive glass: fire-resistive tempered units and fire-resistive annealed multilaminates.

Fire-resistive tempered units comprise two layers of tempered glass with a clear, fire-resistive intumescent interlayer between. The higher the fire rating, the thicker the interlayer gets. Using tempered glass enables the glazing to meet impact safety requirements. This type of fire-resistive glazing tends to be lighter and thinner than fire-resistive annealed multilaminates.

Fire-resistive annealed multilaminates involve multiple sheets of annealed glass layered together with a clear, fire-resistive intumescent interlayer. More layers of annealed glass and fire-resistive intumescent are needed to meet higher fire ratings, and using multiple layers also allows the glass to meet impact safety requirements.

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