Fire-Protective vs. Fire-Resistive Glazing: Radiant Heat, Tests and Ratings
Case histories have shown the benefits of using glass that limits radiant heat transfer to contain fires within interior spaces as well as fires in adjacent structures, brush and trees.
Window Codes: 60 Minutes = Fire-Resistive |
The 60-minute window is a misnomer. All 60-minute glazing must be rated as fire-resistive assemblies. Key citations on this subject include the longstanding UBC - ICBO code Interpretations, Building Standards, January 1991, with an updated interpretation referencing UBC 1997 in November 2000. Referring to UBC section 4306 (g): "Section 43.301 of Uniform Building Code Standard No.43-4 limits the rating of light transmitting (or window) assemblies to 45 minutes."
Another useful reference is the International Building Code (IBC), which does not specify any applications over 45 minutes for windows, sidelights or transoms. Section 714.3 limits glazing in opening protectives tested to NFPA 257 to 45 minutes. The IBC also references NFPA 80, which in section 13-2.2 does not recognize windows rated over 45 minutes. Also, in section 2-3.2.2, glazing in sidelights and transoms is clearly described as permitted in frames rated 45 minutes or less. |
To illustrate this point, independent radiant heat flux testing was conducted on three 45-60 minute fire-protective glazing materials-wire glass, ceramics, and specialty tempered. Using ASTM E119 test standards, mannequins simulating people passing through an exit corridor were placed 3 feet from the glass samples. To get a feel for radiant heat measurements, lower levels of about 5 kilowatts per square meter, are enough to cause serious injury to humans, and spontaneous combustion of wood caused by radiated heat happens at 12 kilowatts per square meter. At 5 minutes, wire glass was at 6.59 kW/square meter, ceramics was at 5.37 kW/square meter and the specialty tempered was at 3.91 kW/square meter and did not reach 5 kW/square meter until approximately 9 minutes.
What the test showed was in a situation where building occupants would have to pass through the non-fire side of an exit corridor glazed with fire protective glass, specialty tempered products can provide up to 5 minutes of additional egress time before it reaches levels of unbearable human pain, making the corridor impassable.
The same ASTM E119 test was conducted on fire resistive glazing materials rated 60 minutes and over. The multilayer laminates and fire retardant-filled intumescents effectively limited the temperature rise to 250 degrees F (as required by ASTM E119) and radiant heat flux to 0 kW/square meter, making these materials safe for use in wall applications.
Where the aim is to protect lives and property, or where combustible materials or contents are located near walls with glazed areas, the architect must be mindful of the risks of radiant heat. When left uncontrolled due to wall design, it can leave egress paths impassable during fire events. For those reasons, architects must carefully consider such interior layout parameters as: corridor width; how quickly occupants will pass by glazed areas; and the amount and type of flammable materials located outside of the spaces with glazing.
In these critical life-safety locations, specifying the safe use of fire-rated glazing requires an understanding of how much radiated energy passes through specific glass products.