Fire Sprinklers & Fire-Rated Glass Systems
By Jerry Razwick, Technical Glass Products
Reprinted from The Construction Specifier
If they gave an award for the most instantly recognizable fire safety feature in a building, sprinklers would win hands down. Most people on the street could identify whether or not sprinklers are present in a facility, and popular opinion would hold that a good fire protection plan should include sprinklers.
Sprinklers have made invaluable contributions to building safety. They have saved countless lives and extinguished many fires. No one would argue with the assertion that sprinklers should be mandatory in non-residential construction, and there is a strong case for expanding sprinkler usage in residential construction as well.
However, sprinklers have frequently been portrayed as the only form of fire protection needed - to the exclusion of other important elements in a comprehensive plan. When sprinklers are installed without additional fire safety systems, there can be a false sense of security that exposes a building and its occupants to unnecessary risk.
Fire industry professionals refer to two types of fire safety measures in a building: active systems and passive systems. Active systems include sprinklers, smoke detectors and extinguishers. These systems are referred to as active because they require something to happen before they respond to a fire. Sometimes this is an automated process and sometimes it requires human intervention. In other words, active systems must be "activated" before they begin offering protection. Conversely, they cease to offer protection if they are for some reason "deactivated" -- water supplies may be shut off, sprinkler heads may be tampered with, etc. It should be noted that active systems are most directly concerned with fire detection (smoke and fire alarms) and suppression (sprinklers and extinguishers).
Passive systems, as implied by the name, require no activation. Fire-rated building materials such as walls, ceilings, sealants, framing and glass fall into this category. They act as a barrier to fire around the clock, because they are inherently capable of stopping or slowing the spread of a fire and smoke. Passive materials do not address issues of detection and suppression. Instead, they are designed to help contain or compartmentalize a fire and restrict it to its point of origin.
Currently, there is a great deal of debate as to how active and passive systems should interact. Is one method preferable to another? Should they be used together or separately? Can well-placed sprinklers eliminate the need for fire-rated building materials?
For example, the relationship between sprinklers and glass is frequently misunderstood. Some fire sprinkler manufacturers have proposed that it is unnecessary to install fire-rated glazing if a sprinkler system is in place. Their opinion is based on the idea that if a fire starts, sprinklers will activate and bathe the glass surface with water using a deluge system. Thus the glass will remain in the frame and continue to compartmentalize the fire. Based upon this theory, sprinkler manufacturers have recommended the use of ordinary tempered glass or heat-strengthened glass and non-rated framing in conjunction with their sprinklers.
Is this a wise suggestion? Can a sprinkler trade-off of this nature offer equal protection to standard fire-rated materials?
Fire-Rated Glass: The Difference
Part of the problem is that fire-rated glass looks deceptively similar to standard window glass. But there is no comparison when it comes to performance. Very few glazing materials can endure the rigorous testing process necessary to achieve fire-rated status.
The test begins with glass being installed in a door or window frame and mounted in a laboratory furnace. The intent is to simulate conditions in an actual structural fire to see how the glass and framing will perform. Just five minutes into the test, temperatures near the surface of the glass are raised to approximately 1,000° F. By the time the test has been conducted for 45 minutes (a standard rating level for glass), the surface temperature exceeds 1,600° F. By way of comparison, ordinary window glass will fail at approximately 250° F, and tempered glass will fail at around 500° F. Because fire-rated glass is intended to help contain a fire, any smoke or flames that appear on the non-fire side of the glass indicate the glass has failed.
If the glass passes that portion of the test and is to earn more than a 20 minute rating, it then must undergo a fire hose stream test. While the glass system is still hot from the furnace, it is doused with a high-pressure blast of water from a fire hose. Some glass that makes it through the furnace intact is unable to survive the thermal shock of water and ends up shattering and falling out of the frame. For a 45 minute rating or greater, the glazed assembly must be able to withstand the fire hose stream test in a manner that does not create openings for fire and smoke to pass through.
Sprinklers & Non-Fire-Rated Glass
Still, it seems logical that if sprinklers were able to bathe the glass with a deluge system and prevent it from experiencing those high temperatures, there would be no need to insist that the glass and other materials be fire-rated. Why go to the trouble and expense of a precaution if it is redundant?
Several different experiments have been conducted over the last 15 years to test that very idea. In 1995, Factory Mutual Research Corp. observed tests to determine whether a non-fire-rated window assembly and sprinkler "system" could provide protection equivalent to that of a fire-rated assembly. The test used specially designed sprinklers, and researchers discovered that tempered or heat-strengthened glass systems could in fact survive the test if the fire started far away from the glazing (approximately 8 feet) and if the sprinklers activated very soon after the start of the fire.
However, when the heat source was moved close to the non-fire-rated glass system in subsequent tests, the glass failed in less than 5 minutes. As a result, Factory Mutual declined to offer a passing grade to the system for use in general fire-rated locations.
Similarly, tests were conducted at Lawrence Livermore National Laboratory (LLNL) in 1986 to examine how well non-fire-rated (tempered) glass and sprinklers would perform when the fire source was close to the glass surface. A large fire (250 kw) and a smaller fire (40 kW) were used in separate tests. In the test using the large fire, the sprinklers activated early and the glazing remained intact. But the two tests performed with the smaller fire resulted in glass fracturing and falling from the test assemblies in less than 4 minutes. In other words, the glass failed before the sprinklers even activated.
What do such findings indicate? It appears that tempered glass may perform adequately when exposed to relatively large fires that cause the room temperature to rise fast enough to activate sprinklers before high stresses can be generated in the glass. However, when a fire is concentrated in an area near the glass surface and is not large enough to activate sprinklers at an early stage, sufficient stresses can be generated to shatter the glass.
More recently, Underwriters Laboratories (UL) conducted tests on behalf of a sprinkler manufacturer that wanted to test its deluge type "system" of specially designed sprinklers and non-fire-rated glazing. When the heat source was placed close to the glass, the glass failed in three of the four tests. In two of those tests, the sprinklers activated first, but the glass could not withstand the "thermal shock" of water hitting the hot surface. The test report states that in all three tests, "...large pieces of glass fell to the floor" after an average of only 4 minutes. UL did not give the manufacturer a listing for the sprinkler and glass wall system.
Some creative solutions have been proposed to make the system workable. One idea is to keep fire away from the surface of the glass by requiring construction of a 36-inch-high pony wall (Canadian Construction Materials Centre Evaluation Report, CCMC 12752-R, June 24, 1996). The CCMC reviewed UL test results regarding the Central Sprinkler's Model WS Sprinkler System. It concluded that this particular sprinkler model complies with CCMC's Technical Guide for Sprinkler-Protected Glazing Systems, Masterformat 15335, dated April 12, 1994, if used in accordance with the limitations and conditions stated in the report.
Those limitations and conditions are significant. A pony wall does not prevent flammable objects such as desks, file drawers, coat racks, etc., from being placed near the glass. Also, the window "sill" created by the pony wall can become an attractive location for storage of flammable materials such as books and papers. Some sprinkler installation instructions state "all combustible materials shall be kept 2" (50.8mm) from the face of the glass." After occupants move into a building, it is difficult to enforce such a provision.
Curtains, blinds or other window coverings can also affect the performance of sprinklers. When these materials are placed between the glass and sprinkler, the water is unable to cool the glass, thus causing the glass to fail early on during a fire. Recognizing this fact, one sprinkler manufacturer states in its literature: "Blinds or curtains must not be between sprinkler and glass."
Yet sprinkler heads can be located up to 12 inches away from the windows, while blinds and curtains in offices, schools, hospitals and other commercial facilities are generally located very near the surface of the glass. And who is to ensure that subsequent building tenants will be aware of this critical warning?
Sprinkler Limitations
Another concern with a "sprinklers only" approach to fire protection is that it makes life safety dependent on a single system operating correctly. If the sprinklers don't function as intended and non-rated glass has been used in fire-rated locations, fire protection literally goes out the window.
Sprinklers are generally reliable, but they are not infallible. The massive sprinkler recalls in recent years make that point abundantly clear. Even when they are in perfect working order when they leave the factory, sprinklers are dependent upon regular maintenance in order to function effectively. Human error can quickly negate any fire safety an active system offers.
Before specifying both sprinklers and glass in a project, it is helpful to ask these important questions:
- What carries the listing--the combined sprinkler and glass "system," or only the sprinkler head?
- Can the risk of a fire close to the glass surface be eliminated?
- If the sprinklers fail, will the glazing offer any type of fire protection?
- Can the glass withstand thermal shock if the sprinklers come on after a fire starts?
Too much is at stake in fire and life safety to rely exclusively on a single method of fire protection. A better solution is to incorporate both sprinklers and fire-rated glass systems that can pass the fire and hose stream tests.
Jerry Razwick is president of Technical Glass Products, Kirkland, Wash., factory agents for a variety of fire-rated glazing and framing products.
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