What architects can learn from firefighters about radiant heat

by Katie Daniel | October 6, 2017 3:19 pm

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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 [2^nd 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 [2^nd 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]).

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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[3].) 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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How radiant heat impacts rescue
Radiant heat is extremely dangerous to building occupants, since it can quickly reach a level that causes unbearable pain, followed rapidly by second-degree burns. The pain and burns sustained by building occupants from the uncontrolled passage of heat can be so intense the occupants are unable to exit the building safely. This is why NBC includes provisions that require use of building materials meeting CAN/ULC-S101 in critical areas such as exit enclosures, exit passageways, and fire-rated barrier walls and floors.

Depending on the building type or occupancy, these areas must be protected from smoke, flames, and radiant heat from one to four hours. This is intended to give building occupants a path of safe egress. They can also provide an area of refuge where occupants can await rescue, which can be necessary in hospitals, urgent care, long-term care, and other healthcare facilities where immobile patients make evacuation difficult or impossible.

How radiant heat can impact fire confinement
As mentioned, the firefighters’ secondary objective after rescue is fire confinement. Firefighters cannot effectively extinguish the fire if they cannot control how far it spreads. The heat generated from a building fire can be so intense it can ignite adjacent areas and surrounding buildings, making fire confinement much more difficult. As one firefighting training guide describes it:

Radiant heat moves away from the fire building in all directions; it is not affected by winds. Thus, fire may spread by radiation to any building near enough to the fire building to absorb sufficient heat… Radiant heat will also pass through transparent glass and ignite materials within a building. If the outside surface of a building is in danger of ignition from radiant heat, the areas within its windows constitute an equal hazard.

Two of the most effective ways to prevent radiant heat transmission from one building to the next are providing significant distance between the two structures and having unpierced fire walls. However, this is not always possible. In densely populated areas such as Toronto and Vancouver, buildings are constructed close together—either a few metres apart or right at the property line. While building codes allow some openings in exterior fire walls, there are restrictions to such openings because, as stated above, radiant heat will pass through the glass and ignite the combustible materials behind it.

NBC has provisions on the amount and size of protected and unprotected openings allowed in exterior walls. As fire-protective glass, such as ceramics and wired glass, does not protect against radiant heat, it is either limited in size or prohibited altogether, depending on the fire separation distance. However, these limitations do not apply to fire-resistive glazing that meets CAN/ULC-S101, because these materials are considered in the code as a wall capable of limiting radiant heat, even if it is transparent.

In other words, if an exterior fire wall incorporates fire-resistive glazing for vision or transparency, it is still considered a fire wall under NBC. As mentioned earlier, having unpierced fire walls is one of the best ways to prevent radiant heat from spreading from one building to the next—especially when there is very little distance between them. This is very important because, again, only when a fire is confined can firefighters effectively extinguish it.

‘Unpierced’ is not a defined term under NBC, but is used in this context as a fire wall that does not have openings. Fire walls with openings are essentially weaker compared to those without, because they contain areas where radiant heat can pass through. NBC limits the amount of openings in exterior fire walls depending on fire separation distance, occupancy type, and other factors, but using fire-resistive glazing in exterior fire walls can still allow designers to have vision and transparency without compromising the fire wall. In other words, fire-resistive glazed openings function in CAN/ULC-S101 like a fire wall that prevents the passage of smoke, flames, and radiant heat. (It is important for design and/or construction professionals to consult local codes and regulations for project-specific guidance.)

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A word of caution about sprinkler tradeoffs
Today, active fire protection systems such as sprinklers have proven to be helpful in containing the fire to its room of origin. However, an over-reliance on sprinklers can also be dangerous, especially when used as a tradeoff to eliminate fire-resistive glazing in one- to two-hour rated exit enclosures and fire-rated walls.

Sprinklers and active fire protection systems are designed to suppress fire, and require an outside trigger in order to operate. To be effective in the event of a fire, regular maintenance must also be performed on the sprinkler system to ensure it works and has sufficient water supply and pressure. On the contrary, passive or built-in fire protection systems such as fire-resistive glazing contain fire, smoke, and heat to the point of origin without the need for any outside triggers or maintenance.

One is not more important than the other—both active and passive fire protection systems are equally important in containing and suppressing the fire. This approach, known as balanced fire protection or safety layering, is more important than ever when sprinklers fail—which, as the U.S. National Fire Protection Association (NFPA) has reported, can and has happened. (John R. Hall’s report, “U.S. Experience with Sprinklers,” was published by NFPA in 2013.)

In the same NFPA report, it was noted most sprinkler failures occurred because the system was shut off, and the majority of sprinkler ineffectiveness was caused because water did not reach the fire or not enough water was released. Damage to the sprinkler system or decreased water pressure is especially a concern during natural disasters such as hurricanes or earthquakes, where the threat of fire is increased.

Having passive or built-in fire-resistive building materials that perform 24/7 without any triggers gives building occupants the best chance to exit the building safely, and minimizes the danger to firefighters responding to the scene. As is stated in a white paper from Fire Safe North America (FSNA):

The concern is not so much that a properly maintained automatic sprinkler system will fail, but that a natural disaster, human error or lack of maintenance could disable the system to the point where additional layers of protection may be the only measures preventing or delaying a building or an entire block from being destroyed by fire. When those safety layers do not exist, the building will not be able to withstand as big of a fire and will fail sooner, putting occupants and especially firefighters at great risk. (To read it, visit firesafenorthamerica.org/about/about/white-paper[6].)

Conclusion
Architects can gain some perspective from learning about the devastating effects of radiant heat on people and property from the firefighters who have experienced it first-hand. In a way, the building materials architects specify are the first line of defense when it comes to limiting radiant heat, and offer significant assistance to firefighters in their rescue and fire confinement objectives. Advanced life-safety building products, such as fire-resistive glazing systems meeting CAN/ULC-S101 criteria, enable architects to combine maximum fire protection with vision, transparency, and all the esthetic benefits glass has to offer.

Diana San Diego has more than 11 years of experience in the architectural glazing industry, and more than 13 years of experience in public relations and marketing. As the vice-president of marketing at SAFTI FIRST[7], a manufacturer of fire-rated glass and framing systems, she oversees the advertising, content management, media relations, promotional activities, and communication initiatives for the company. San Diego has contributed articles to various publications and is involved in creating and promoting educational programs. She can be reached at dianas@safti.com[8].

Source URL: https://www.constructioncanada.net/architects-can-learn-firefighters-radiant-heat/

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