Fire-Protective vs. Fire-Resistive Glazing: Radiant Heat, Tests and Ratings

Improving building safety through the use of fire-rated glazing
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Fire-Protective Glazing

Fire-protective glazing offer fire ratings of 20 minutes and up to 3 hours. The glass types are typically in thicknesses of 3/16" inch to 3/4" inch, and include such types as polished wired glass, specialty tempered glass, ceramic glass, and specialty laminated or filmed glass, either with wire or without and insulated glass units.

Traditional wired glass (non-safety) is limited in terms of its use to fire-rated and non-hazardous locations not requiring human impact safety. Wired glass fire ratings range from 20 to 90 minutes. Filmed or laminated ceramics offer fire ratings from 20 minutes to 3 hours, and when filmed or laminated, can meet the safety requirements for use in hazardous locations. Both unfilmed or unlaminated wired glass and ceramics have important limitations in terms of their use in locations that require protection for hazards associated with human impact. Also, wired glass and ceramics rated 60 minutes and over are restricted to 100 sq. inches in size due to radiant heat concerns.

Other fire protective glazing that carry a 20-minute rating include monolithic tempered glass impact safety rated to Cat. I and II and laminated glass impact safety rated to Cat. I with added radiant-heat protection. These 20-minute rated products may be used as vision panels in doors up to 9 square feet. Another type of fire protective glazing are radiant heat reducing specialty tempered monolithics fire rated from 45 to 60 minutes and impact safety rated to Cat. I and II. This product is used in doors, transoms, sidelites and openings under 60 minutes. Currently, this product requires the approval of authorities having jurisdiction (AHJ) because it is fire-tested without hose stream.

In general, fire-protective glazing is limited by code to 25 percent of a wall area due to radiant heat concerns. Other restrictions and exemptions on their use also apply.

Fire-Resistant Glazing

Glazing in the category fire-resistant or fire-resistive-the terms are used interchangeably -includes intumescent multiple-laminate and fire retardant-filled transparent units. The listings for the products classify them as "transparent walls" that are not restricted to 25 percent of a wall area when the framing system has a rating equal to the glazing. The fire-resistive class includes products also rated for acoustical performance, blast and bullet resistance, and hurricane protection.

Intumescent products are sandwiches of annealed glass containing special interlayers that expand and turn dark when heated. This quality helps contain heat and smoke from fire. Fire retardant-filled units have a clear semi-solid material in their cavities that crystallizes during a fire, helping to contain smoke and heat.

Protective vs. Resistive: The Big Picture

To summarize, fire-protective glass is not designed to block or stop radiant heat; fire-resistive glazing does. Fire-resistive glass is considered as a "wall," tested just like a brick, masonry, or gypsum board assembly. All glazing rated at 1 hour and over is fire-resistive, except wired glass and ceramic, which have ratings of up to 90 minutes and 3 hours, respectively. But because wired glass and ceramic products do not protect against radiant heat, they are defined as "fire protective."

The ratings required by the U.S. building codes are determined based on applications for fire-protective and fire-resistive constructions. The application requirements are based on time-the time to allow for safe egress and to maintain the structural integrity of the building.

So why does the distinction between fire-protective and fire-resistive matter? Because it's an important factor in ensuring occupant life safety.

Standards, Testing and Ratings

Glazing products receive fire ratings and listings from groups like Underwriters Laboratories and Intertek/Warnock-Hersey. The ratings are measured and awarded in terms of time, based on the how long an installed glass panel can reliably perform in an actual fire. Glass ratings may range from 20 minutes to 3 hours. Directly heating glass panel specimens in frames is one of the main tests. A second protocol called the "hose-stream test" (discussed in detail below) evaluates structural behavior at high temperature by means of pressurized water.

However clear it may seem, the distinction between fire-protective and fire-resistive glazing is not described in such a straightforward way by the subsequent ratings the products earn. And fire ratings do not always and fully describe the fire performance of a glass product.

 

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Originally published in Architectural Record.
Originally published in December 2006

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