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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Advertorial course provided by Safti First

C.C. Sullivan

Radiant Heat: A Matter of Life and Death

So far, we have discussed that fire-rated glass can be classified in two categories:

Fire-Protective. This glazing can contain flames and flammable gases for short periods, but it will not block heat transfer to the other side of the glass. This category includes wire glass and ceramics. There are fire-protective glazing materials, such as tempered products and laminated products, which can reduce the transfer of radiant heat, but do not meet ASTM E119 and therefore still classified as fire protective. This type of glazing is limited to 25 percent of the wall area due to radiant heat concerns and have fire ratings under 60 minutes (except for wire glass and ceramics, which are rated up to 90 minutes and 3 hours respectively, but limited to 100 square inches in size).
Fire-Resistive. This type of glazing generally contains flames and flammable gas for longer periods of time and also blocks the transmission of heat through the glass. This type of glazing is rated over 60 minutes and is not subject to wall limitations when combined with an equally rated fire resistive framing system.

Fire-resistive products protect against radiant heat, which is a significant effect of fire events. It is also a poorly understood but critical consideration in specifying glazing. The reason is that fire can pass enough heat through glass and other materials to cause combustion on the non-fire side. This effect is called "non-piloted auto iginition"-or, less technically, spontaneous combustion. The mechanism of heat transfer may be called flux, measured in kilowatts at a standard distance from the non-fire side of the wall or separation.

Limitations of the Hose Stream Test for
Fire-Protective Glazing

As a way to evaluate fire-resistive materials and assemblies, the hose-stream test offers a long history and some useful results. As a means to test the fire performance of 45-minute openings, transoms and sidelites, however, it is questionable. There are several convincing reasons to reach this conclusion:

It's a structural impact test. Reading NFPA 252, it's clear that the determination of thermal shock performance has not been an intended result of the hose stream test. The section describes the test's use for "structural capability," and mentions the original use of weights to apply impact. The use of a water stream is related to its "uniformity and accuracy" of impact by means of weights.

The hose-stream test was not developed to study thermal stress in glazing materials, but rather structural impact performance.

It doesn't indicate field performance.For more than two decades, there have not been any field reports of thermal shock to glazing products tested without hose stream. That evidence means that millions of square feet of glazing products tested without hose stream do withstand thermal shock from fire sprinklers during fire events.

This is not surprising, because the hose stream test was never intended to measure thermal stresses. So the hose-test performance of a glass product will not describe its field capabilities for thermal shock.

For ratings under 1 hour, it's not needed. The NFPA 251 Section 4-2.1 and UBC Section 7.108 exempts from the hose-stream test materials and assemblies rated less than 1 hour. Similarly, 20-minute assemblies are also exempted from the hose-stream test in U.S. fire performance standards and building codes.

Other world markets have no hose-stream test.Outside of the United States and Canada, there's no requirement for the hose-stream test. International fire standards and codes reference other test protocols for structural integrity in fire-rated assemblies.

The hose-stream doesn't fully measure fire performance. The hose-stream test won't provide information on radiant heat transfer. This measurement indicates a glazing product's ability to prevent the spontaneous ignition of combustibles on the non-fire side of the glass. And it's only measured in a separate standard.

The transmission of radiant heat occurs through infrared radiation. These electromagnetic waves only carry energy-not temperature-and move in a straight line. When radiant energy is absorbed by matter and converted into heat, the result is fire.

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Originally published in December 2006

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