Threat of radiant heat should not be ignored
According to a United States Fire Administration’s congressional report, the rate of fire related deaths per capita in the United States is two to three times that of several European nations and is at least 20 percent higher than most developed countries. For a country that still has one of the worst fire safety records in the industrialized world, equating safety with a glazed assembly’s ability to endure a hose stream test after 45 minutes of exposure to temperatures exceeding 1,600 degrees Fahrenheit, while ignoring or downplaying the real threat of radiant heat to people and property, is a misinterpretation of critical factors. The United States and Canada are alone in applying this test standard to fire-protective assemblies. Proponents of the hose stream test in the United States continue to ignore the need for radiant heat resistance standards for glazing rated for less than 60 minutes. The inconsistent applications of the hose stream requirement within current codes from the International Code Council, Washington, D.C., regarding walls, windows and doors are glaring given that NFPA 257 hose stream test requirements allow for a 30 percent loss of glazing around the perimeter of the test unit and a 5 percent loss at the center. How does this acceptable void in coverage aid in the containment of flames and smoke? Proponents are yet to present verifiable evidence that in the case of 45-minute assemblies the test standard has saved lives and property.
Fire-rated glazing assemblies can perform at two functional levels: Fire-protection assemblies can withstand high temperatures for up to 45 minutes, and block the immediate spread of flames and smoke. Except for exterior applications along property lines, fire-protective glazing would commonly be used in doors and windows along interior paths of egress and is typically monolithic. In the United States, fixed fire-protection assemblies are limited to less than 25 percent of the total wall area, minimizing their influence on the wall’s load-bearing capabilities. Fire-resistance assemblies can remain in place for 60 minutes or longer in areas where they are an integral structural component and serve to block the rampant spread of radiant heat, insuring building access via corridors and enabling safe egress. U.S. standards include the hose stream test to evaluate performance at both levels. However, current ASTM, IBC and NFPA test standards specifically exclude fire-rated construction having a rating less than 60 minutes from the hose stream requirement. This begs the question: Why is a fire-protection glazing assembly rated less than 60 minutes treated differently?
Glazing that does not survive the hose stream can survive potential thermal-shock from sprinklers simply because these are two different scenarios. Proponents of the hose stream requirement who stage mock thermal-shock demonstrations using a garden hose and glass heated to more then 1,600 degrees are overlooking basic logic and reality. The hose stream was originally developed to test the structural integrity of a building’s wall and supportive components. It was not developed to test non-structural elements such as windows and doors where fire-protective glazing is commonly used. The test standard stipulates that a hose stream is applied after the glass has been exposed to a minimum of 1,638 degrees for 45 minutes. Sprinkler discharge would have occurred at about 165 degrees within a few minutes of the initial outbreak of the fire, virtually eliminating any threat of thermal-shock.
Despite the efforts of hose stream proponents to use recent code committee commentary to paint a black and white picture, voting on challenges to various hose stream test requirements within the code have never been entirely unanimous. The current debate in the U.S. regarding the inclusion of the hose stream test for fire-protection glazed assemblies centers on to what extent the test disqualifies constructions that actually serve their fire resistance purpose acceptably under contemporary fire and fire extinguishment conditions in buildings. Materials and methods of construction have advanced drastically since the test’s inception.
Although the hose stream test was never specifically intended for use in evaluating fire-rated glazing, it is still being used in the United States despite being dropped from similar test standards applied throughout the world. For instance, this test was discarded more than 40 years ago from fire-rating test standards used in the United Kingdom. According to the most recent statistics and commentary published by the Office of the Deputy Prime Minister of the U.K., deaths due to fire have continued to decline annually since reaching a quantitative peak in 1979. In contrast, deaths due to fire in the United States and Canada remain high. Ironically, these are the only remaining countries requiring the hose stream test as part of the performance criteria for fire-protection assemblies. The National Academy of Sciences reports that deaths-by-fire rates in the U.S. are typically two to four times that of European nations.
With the obvious exception of the United States, developed countries throughout the world add the need to limit heat transfer due to conduction, convection and radiation to fire-rated performance requirements for glazed assemblies at all levels. Radiant heat is generated and spread by fires through electromagnetic waves. The waves strike objects, heating them to the point of auto-ignition. Studies of critical radiation levels show that significant human pain occurs at 5 kilowatts per square meter. Spontaneous combustion of wood occurs at 12 to 13 kilowatts per square meter, and values as low 7.5 kilowatts have been reported.
Wolfgang Zernial, director of fire-protection glass with Pilkington Deutschland AG Germany, has written that based on in-house testing using the standard time-temperature curve, at the 30-minute mark for fire-rated monolithic glass “someone would have to stay 23 feet away from the glass surface in order to avoid being burned by the heat radiating from the glass. The implications are clear: Careful thought should be given to the use of non-barrier-to-heat glazing along exit routes and other areas where radiant heat could become a threat to life safety.”
As it was in Europe, hopefully the tug-of-war over the value of the hose stream test will one day be set aside by fire-rated glazing suppliers jockeying for market share in the U.S. Perhaps then the loudest and most vocal proponents of the hose stream test for fire-protection glazed assemblies will become just as zealous in supporting the call for significant radiant heat resistance levels being added to existing codes, now that the UL 9-5.1A test standard for radiant heat measurement has been adopted within the International Building Code.