Volume 37, Issue 4, April 2002
Rethinking Sprinkler and Glass Systems
by Jerry Razwick
Recent studies suggest both fire-rated glazing and sprinkler systems are necessary to provide the necessary protection against fire in new construction projects.
The relationship between glass and sprinkler systems is unique. Understanding how the
two work together is critical for deciding whether to install fire-rated glass, or
non-rated products such as tempered or heat-strengthened glass. Ultimately, the choices
that are made can affect life safety drastically.
Currently, some building codes allow what are known as trade-offs. With a trade-off, an architect can circumvent fire-rating requirements by specifying sprinklers instead of traditional fire-rated building materials, including glass. The assumption is that tempered glass with sprinklers will provide an equivalent level of fire protection to fire-rated glass. However, a number of tests have been performed over the past two decades to determine how sprinkler systems and window glass interact during a fire. The results have shown that relying strictly on sprinklers for fire safety may be a dangerous choice.
Are Sprinklers Infallible?
Because sprinkler systems have been so effective in reducing fire fatalities and property damage, it is often taken for granted that they work 100 percent of the time.
Classified as active protection systems, sprinklers require a sequence of events to occur in order for them to activate properly. So even when they are mechanically sound, sprinklers are still subject to various forms of human error. There have been numerous incidents recorded where something interfered with that activation process and the sprinklers did not perform as needed.
Usually these problems have been due to improper maintenance of the sprinkler system. For example, tenants have accidentally painted over sprinkler heads. Water supply valves have been shut off, and water pressure has sometimes been inadequate to support the sprinklers. These are all preventable, yet at the same time they represent the unknown factor that is always present when systems are dependent upon people.
Occasionally, difficulties with sprinklers arise during the fire itselfin spite of good maintenance. In some cases a fire has overwhelmed and melted nonmetallic piping for sprinklers. Other times, firefighters have diverted water to their trucks too soon, leaving sprinkler systems with no supply.
The point is simply that installing a sprinkler system does not guarantee performance when it is needed. While complications may be the exception rather than the rule, any number of factors can inhibit the way sprinklers function in a real life emergency.
When sprinklers fail to suppress a fire, for any reason, other means of protection are needed to keep the flames and deadly smoke from spreading to other areas of a building. Fire-rated materials such as glass, doors, walls and ceilings are designed to act as fire barriers around the clockwhether or not the sprinklers are functioning. If those passive protection elements have been eliminated from the plans in lieu of sprinklers, then there is no failsafe when a breakdown occurs in the sprinkler system.
Glass and Sprinklers
What if the sprinkler system does work properly? If sprinklers seem to be reliable the majority of the time, what can be expected in terms of glass performance?
It is no secret that most glass does not do well in fire conditions. Typical window glass will break at 250 F, and tempered glass will only last until about 500 F. When temperatures soar beyond that (as is typical within the first five minutes of a fire), different products are required. Fire-rated glass products (such as ceramics, wired glass, transparent firewalls, etc.) have been furnace tested in excess of 1,600 F, and they are still able to remain in the frame and provide protection.
When you add sprinklers to the mix, the situation becomes more complicated. On the one hand, sprinklers do have a cooling effect. In theory, glass of any kind kept adequately cool should be able to survive a fire indefinitely. On the other hand, if water from sprinklers impacts non-rated glass that is already hot, the water creates thermal stresses in the glass that cause it to shatter. This means sprinklers can actually cause more harm than good if used in combination with the wrong type of glass.
Many types of fire-rated glass are on the market today that are compatible with sprinklers, regardless of glass temperature. To achieve a rating of 45 minutes or greater, it is mandatory (according to UBC 7-2, UBC 7-4, ASTM E-163, NFPA 252,NFPA 257, UL 9 and UL 10B) for any glazing material to endure a hose stream test immediately after the fire test. The hot glass and framing system is removed from the test furnace and blasted with water from a fire hose. The glass does not receive a passing grade if it falls out of the frame. As a result, fire-rated glass is well prepared to handle water from sprinklers, fire extinguishers, or other sources.
In spite of this fact, some people have pushed alternatives that would allow the use of non-rated glass in fire-rated areas. Manufacturers have developed special window sprinkler systems designed to activate early and bathe the entire glass surface with water.
Independent laboratories such as UL have tested such systems in combination with tempered glass. Their test results have shown that when fires start several feet away from the glass and sprinklers come on quickly, tempered glass is able to remain intact. However, if a fire starts close to the glass surface, it can create enough heat to break the glass before the sprinklers activate. Or, as mentioned above, if the glass gets hot and then the water hits it, the glass will shatter and fall out of the frame.
To compensate for this, some manufacturers recommend a 36-inch high ponywall to keep flammable objects away from the glass, reducing the potential for fires close to the glass surface. Yet ponywalls often turn into convenient storage areas for papers, picture frames, etc. Those items can easily ignite and negate the value of the ponywall.
If anything, this glass performance issue becomes more critical the better the sprinklers are working. Sprinklered fires generate tremendous amounts of smoke, since they usually continue to smolder for some time. Smoke inhalation is by far the leading cause of deaths in a fire. If windows have been blown out because of the sprinklers, smoke will spread throughout the building, threatening more lives.
If sprinklers are nearby, it is best to install fire-rated glass that can pass the hose stream test. This provides the greatest level of safety and ensures fire-protection that is independent from the sprinkler system. The stakes are too high to settle for products that only work under certain conditions.
Jerry Razwick is president of Technical Glass Products, based in Kirkland, Wash.
© Copyright Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.