Designing with Fire-Rated Glass: Integrating Life Safety, Transparency, and Aesthetics

Technology creates new glazing options
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Jerry Razwick

Deluge sprinklers were developed with the assumption that they can keep the temperature of non-rated glass low enough to prevent shattering. However, if water coverage is not entirely even and does not completely cover the surface of the glass, it is possible for sprinklers to cause the glass to vacate the opening during a fire, leaving a breach for flames and smoke to spread throughout a space or building.

This is a crucial issue, because when sprinklers suppress a fire, they can generate large volumes of deadly smoke. If glass windows have shattered, the smoke will escape into other areas of the building.


Standard time and temperature curve in fire tests.
Source: Technical Glass Products

The combination of deluge sprinklers and glass was analyzed in the "Hospital for Sick Children" test conducted at Canada's National Fire Laboratory in Toronto in 1984. A propane burner was ignited across the room from the tempered glass, 7 feet (roughly 2.13 m) from the glass surface. Deluge sprinklers were installed on the burn side of the glass, and were carefully positioned to ensure uniform water coverage. The non-rated glass was able to survive the test under these conditions.

However, the tests results raised questions. For instance, officials adjusted the water flow rate from the sprinklers during the test when dry spots appeared on the fire-exposed face of the glass. In a building fire, officials are not present to monitor water flow. Any dry spots on the surface of hot glass cause heat stress and can be a primary cause of glass fracture. Manually adjusting water flow during the test artificially allowed the glass to remain intact.


Thermal property comparison for different types of glass. Source: Technical Glass Products

A second important factor in the test was the sprinkler activation time. The sprinkler system used was a quick response type that activated two to three times faster than standard sprinklers. These test limitations were noted in an article by J.K. Richardson and D.J. Boehmer entitled "Fire Resistant Wall Assemblies with Glazing" in the Society of Fire Protection Engineers Bulletin (July 1987), as follows: "The location and response time of the sprinkler must be such that activation will occur before the glazing reaches critical temperature levels…Should sprinkler activation be delayed so that the temperature of tempered glass is in the range of 250 degrees Centigrade (approximately 482 degrees Fahrenheit), glass failure could possibly occur."

In 1995, Underwriters' Laboratories tested one type of deluge type system that combined specially designed sprinklers and non-fire-rated glass (File Ex683). Recognizing the importance of heat source location, UL conducted four tests in which the heat source was placed close to the glass. The glass failed three out of four times. In two of the tests, the sprinklers activated early, but the thermal shock proved too much for the glass, causing it to break. The UL test report states that in all three cases, "…large pieces of glass fell to the floor" after an average of just four minutes.

The Canadian Construction Materials Center reviewed UL's test results. They concluded that the window sprinkler system would work as long as flammable materials could be kept away from the glass surface. To accomplish this, they recommended construction of a 36-inch-high pony wall, or half wall, in addition to restrictions on curtains and blinds.

However, a pony wall would not prevent flammable objects, such as desks, file drawers, and coat racks, from being placed near the glass. The ledge created by a pony wall frequently becomes a convenient place to stack flammable books and papers. To prevent this condition, the National Evaluation Report No. NER-516 requires that when using this system, "all combustible materials shall be kept 2 inches (50.8mm) from the front face of the glass." Once a facility is occupied, however, enforcement of such instructions is not always possible.

 

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Originally published in Architectural Record.
Originally published in December 2005

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