Glass: The Invisible Fire Fighter
By Jerry Razwick, Technical Glass Products
Many different building components have a part to play in the defense against fire damage. Smoke alarms help detect fire at the earliest stages. Extinguishers offer a way to douse small fires. Sprinklers can be placed strategically to suppress any bigger flames that appear. But the important job of containing a fire to a limited area is carried out by one of the least noticeable fire fighters of all: Glass.
Nearly every commercial building design includes windows and/or glass doors for lighting, aesthetics or security reasons. Yet all those glass-filled openings raise a question: How will the glass react if there�s a fire? When the glass gets hot, will it shatter and leave the way open for flames and smoke to spread? Or will it act as a barricade, providing protection for people and other areas of the building?
As might be expected, the answer to those concerns depends on the type of glass in question. Ordinary window glass cannot withstand the high temperatures associated with a structure fire, and it will break and fall out of its frame at about 250� F. On the other hand, glass that is classified as "fire-rated" can tolerate heat in excess of 1600� F. Clearly, selecting the right product is essential to ensuring life safety.
For decades, fire-rated glazing was limited to one product: polished wired glass. It was the only glass able to survive the rigorous testing process. Even today, it is one of the most commonly used products in fire-rated locations. Code officials and fire fighters can quickly recognize it by the characteristic criss-cross wire pattern.
Yet wired glass has its drawbacks. Many people mistakenly assume that the wires make the glass stronger and more impact resistant. In reality, wired glass is only � as strong as tempered glass and provides only a minimal amount of protection against impact. With such low impact resistance, building codes are beginning to consider the number of locations where wired glass will be allowed in the future.
When wired glass was the only fire-rated glass option available, it posed a real dilemma: Which priority takes precedence -- fire safety or impact safety? One or the other had to be compromised in many locations, simply because no product existed that could fully satisfy both needs.
Fortunately, the situation has changed dramatically. A number of newer, "wire-free" products have emerged that are greatly expanding choices. Diverse in make-up and characteristics, these new invisible fire fighters have been able to substantially surpass wired glass in terms of fire and impact safety performance.
One category of fire-rated glass that has emerged isn�t technically a glass at all. In fact, it is ceramic. Ceramic has long been known for its outstanding heat tolerance, and it is now used in such "hot spots" as cooktops, car engines and gas fireplaces. Utilizing state-of-the-art technology, manufacturers have developed the ability to create transparent sheets of ceramic that look like ordinary window glass. Glass ceramic (such as the FireLite family of products) has earned fire ratings up to 3 hours. It also is available with high impact safety ratings, making it an ideal option for high traffic areas such as busy corridors and lobbies in schools and hospitals. Ceramic can also be specified in insulated units to meet energy codes for exterior applications.
Another category of wireless fire-rated glass found in the market today is specially tempered glass. This group (including such products as Fireglass20, SuperLite I and Pyroedge) offers limited fire protection, because specially tempered glass cannot withstand what is known as "thermal shock." When glass is tested for a fire rating, it is blasted with water from a fire hose immediately after being heated in a furnace fire. This important test makes sure the glazing product, hot from the fire, will stay in place if sprayed with water from sprinklers or other sources. Specially tempered glass cannot survive this portion of the test, and so the codes dictate that it can only be given a 20 minute rating. The limitations of this product category are important to keep in mind, because 20-minute rated specially tempered glass is frequently specified inappropriately in locations such as sidelites and windows that may require a higher rating.
Transparent wall units are a final classification of fire-rated glass. These products, such as Pyrostop, Contraflam and others, are actually tested as "walls", with ratings up to 2 hours. In addition to stopping flames and smoke, transparent wall units block the transfer of heat, similar to a fire-rated masonry wall. If desired, transparent wall units can be installed from wall to wall and floor to ceiling, and include doors if desired. Designers then can divide space without the use of solid walls that diminish visibility, security and light.
Of course, all these glass options need to be installed in frames. Until recently, framing options lagged behind fire-rated glazing in terms of new developments. In most cases, designers were forced to resort to traditional hollow metal steel frames. However, new narrow profile European style doors and framing have changed that. Products, such as Fireframes, now offer new opportunities for architects seeking alternatives to traditional wrap-around framing.
The new developments in fire-rated glazing and framing continue to raise the standard for both performance and design. When properly specified and installed, fire-rated glazing can be a powerful ally against the devastating effects of fire, while offering expanded aesthetic opportunities.
FIRE-RATED GLASS & FRAMING SPECIFICATION TIPS
Before choosing which fire-rated glass and framing system to use, keep in mind these questions:
- What is the required fire rating (in minutes)?
- Is there an impact safety requirement (doors, sidelites, etc.)?
- Will sprinklers be located near the glass? (Is "thermal shock" a potential problem?)
- Is heat transfer through the glass an issue? (Does the glass need to act like a "wall"?)
- Has the product been tested and listed by an independent laboratory? (Such as UL?)
- Can your supplier offer both glass and frames? (Does your "system" have a rating?)
- What style framing best matches your project? (Traditional or the newer narrow profiles?).
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