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

Developments in glazing techniques and technology are providing new design opportunities for use of fire-rated glass. Traditional wired glass, as opposed to newer laminated wired glass, has long been the only glazing material permitted in fire-rated areas, as set forth by local building codes. Complying with a fire rating often meant giving up clear visibility through doors and windows, due to the wires, and sacrificing impact safety, since wired glass is easy to break. Design choices often consisted of creating a solid wall without windows, or an opening with institutional-looking wired glass, with its inherent performance limitations.

As a result, glass manufacturers have created new solutions, thereby providing more options and a sophisticated range of materials for a variety of applications. These choices offer many levels of performance that directly influence project design and construction requirements.

Project: Tommy Hilfiger in New York. Architect: Bridges & Lavin. Photographer: Stephen and Gil Amiaga.

Fire-rated glass has two primary functions: to protect life and property in the event of a fire, and to allow visibility. A solid barrier wall can often give adequate fire protection, but it blocks light and vision from one space to another. At the other end of the spectrum, ordinary window glass lets the light in but does nothing to stop the spread of a fire. Fire-rated glass combines both functions in a single product, maintaining a barrier to flames and smoke while at the same time opening up a room visually.

Glass must undergo rigorous testing to earn a fire rating. Several pieces of different sizes are installed in a test furnace and then subjected to a blaze that exceeds 1600° F. To successfully pass the test, glass must remain in the frame for the duration of the test. The longer it can withstand the heat, the higher the fire rating it can be given, from 20 minutes to 3 hours.

After the fire test, if the glass is to be given anything above a 20-minute rating, it must also endure a hose stream test. While the glass is still hot, it is doused with a blast from a fire hose. Most glass cannot tolerate the difference in temperatures and will shatter. This can be a critical factor in a real-world fire, where hot glass may be exposed to water from hoses, sprinklers or extinguishers. The rare types of glass that are able to survive these grueling conditions earn the right to be considered "fire-rated."

Once thought of as a design limitation, fire-rated glass has become a resource for innovative design concepts. Meeting life safety and building code requirements can be integrated with creative design alternatives, rather than purely functional solutions.

Building codes are beginning to reflect the changes in the industry. Revisions to the 2003 International Building Code (IBC) have eliminated the use of traditional wired glass for hazardous locations in schools, daycare centers, and athletic facilities. These changes will apply to all types of construction in 2006.

This marks a significant shift, because wired glass, a product once mandatory in all glazed fire-rated openings, is no longer considered adequate for many building types.

Larger Size Glass

The vast majority of the newer wireless fire-rated glass products are listed with independent testing laboratories for use in larger sizes than polished wired glass. Larger sizes offer more flexibility for design concepts where ratings must be provided. The maximum dimensions for glass in fire doors and windows have been greatly increased.

For example, when wired glass is used in a 45-minute opening (a typical requirement in fire-rated corridors), codes have historically limited the size to 1,296 square inches (9 square feet), the maximum size successfully fire tested by Underwriters' Laboratories (UL), or other independent labs. Since wired glass was such a dominant product for years, many building codes established 1,296 square inches as the maximum allowable size for any type of fire-rated glass.

Newer fire-rated glazing materials carry high ratings and may be used in larger sizes than wired glass. Mary & Leigh Block Museum of Art, Northwestern University, Evanston, IL Architect: Dirk Lohan Photo: Steinkamp/Ballogg

As new products became available, they were capable of exceeding the existing code limitations. Standards had to be rewritten accordingly. Today, depending on the product and application, a window requiring a 45-minute rated product may have a single piece of glass over 40 square feet. These increases in allowable glass size have given tremendous design flexibility to architects and designers, by minimizing the amount of required framing, while maximizing transparency and aesthetic appeal.

Overall allowable glass size in doors has also increased. In the past, a typical steel fire door with a 90-minute fire rating would often have used a 100-square-inch vision panel. Whether the door lite was 10-inch by 10-inch or a narrow 4-inch by 25-inch strip of wired glass, vision through the door was minimal. When combined with the industrial look of a steel fire door, the overall aesthetics were limited. Fire-rated ceramics have greatly increased the allowable glass size in steel or wood fire doors. Instead of 100 square inches, 90-minute steel doors may now incorporate ceramics exceeding 9 square feet per glass lite. For lower fire ratings, such as 45- or 60-minute doors, allowable glass sizes are even larger.

Increased allowable glass sizes provide architects with greater design flexibility. By using ceramics that allow larger sizes and wireless vision panels, doors, windows, and entryways can blend seamlessly with non-rated systems used throughout a building. However, with increases in glass sizes, other important aspects of fire-rated glass must also be considered, including the ability to withstand human impact and overall performance during a fire.

Impact Resistance

Fire is a major concern in all buildings. Yet in public facilities with high traffic volumes, injuries from glass breakage are often much more of a risk. Although wired glass offers excellent fire ratings, it cannot withstand much in terms of human impact. In fact, once broken, wired glass can be more dangerous than ordinary window glass, since the broken wires can create sharp snags.

For years, this posed a dilemma as to which need was more critical, since no glazing material could provide both fire and impact protection. The codes came down on the side of fire, determining that the threat of damage from fire was greater than the threat of injury from glass breakage. So in the 1970s, the Consumer Product Safety Commission (CPSC) granted a temporary exemption to wired glass, allowing its use despite its inability to earn high impact ratings. With limited options that could meet fire codes, there was little choice.

Ohio State University Fire-rated glazing offers high impact resistance. Project: Ohio State University, Columbus, OH Photo: OSU Photo Services

As time has gone by, the demand for safer facilities has continued to fuel increasingly stringent code requirements, making it more difficult to focus on a single safety need to the exclusion of another. Seeing the opportunity for innovation, manufacturers began developing products that could better serve dual roles of fire and impact protection.

As a result, most fire-rated glass products available today satisfy CPSC 16CFR1201 (Category II), the highest standard of impact safety for window glass. Whereas wired glass can only withstand approximately 100 foot-pounds (ft.-lbs) of impact, materials satisfying the Category II requirements are subjected to an impact approximating a full-grown adult running into the glass, or 400 ft.-lbs of impact. This means many of today's fire-rated glazing products exceed the safety impact resistance of traditional wired glass by four times, or more.

Performance improvement has led to increased impact safety. The newer fire-rated glazing materials surpass traditional wired glass in terms of fire safety as well. Some of the new glass ceramic products highlight this point. Ceramic materials are well known for being able to withstand heat. From cooktops to car engines, manufacturers have taken advantage of the fact that ceramic has a very low coefficient of expansion when heated. Fire-rated glass composed of transparent ceramic has been able to earn fire ratings as high as 3 hours, making it an outstanding thin and wireless alternative to wired glass. In fact, the use of ceramic to hold the flames in the 2002 Olympic Cauldron during the Winter Games in Salt Lake City shows that containing fire for days, rather than hours, is not a problem. Such performance is not possible with wired glass.

Fire-rated ceramic contained the continuous flame of the 2002 Salt Lake City Winter Olympic Games. Photo: Courtesy Technical Glass Products

When determining allowable ratings of windows in a fire-rated corridor, design professionals often note that windows are required to provide 45 minutes of fire protection, when the corridor is rated for 1 hour. This condition relates to the historic performance limitations of wired glass. Like its size limitation of 9 square feet, the 45-minute rating was established because that was the longest duration wired glass could perform during a fire. Any longer, and the panel of wired glass would tend to slump from the opening, allowing passage of smoke and flames. As a result, building codes were developed in consideration of this performance limitation. While wired glass couldn't provide the optimal result by matching the 60-minute performance of the surrounding wall, some level of protection was considered to be better than none at all.

Transparent ceramics, however, have no such performance limitation and can provide a fire protection rating that matches the surrounding wall. By using ceramics, corridors rated for 60 minutes can now have openings that also provide a 60-minute rating. The weak link of allowing 45-minute windows in a 1-hour corridor is no longer necessary when architects and specifiers use ceramics that provide a full 60 minutes of fire protection.

Occasionally, the need for higher fire ratings is accompanied by the requirement to block the passage of heat, in addition to flames and smoke. Heat can build up quickly in stairwells and other areas where building occupants could become trapped for long periods of time. Extremely high temperatures transferring through the glass could be just as devastating as the fire itself. For this reason, certain types of fire-rated walls do not allow openings of any kind. For many years, this requirement put severe design limitations on architects who desired transparency in these areas. Be it a three-story exit stairwell or two-hour occupancy separation, solid cinder block or sheetrock walls were the primary means of meeting code requirements. With developments in fire-rated glass and framing, however, these design limitations no longer exist.

Glass fire walls (sometimes called transparent wall units) address this issue directly. They are composed of multiple layers of glass with a layer of an intumescent material in between, which turns to foam during a fire. Glass fire walls are tested to the same standards as barrier walls. This means that while allowing full vision for aesthetic and security reasons, they perform in a manner similar to cement block walls during a fire.

These products greatly reduce heat transfer and therefore can be used in applications where other types of fire-rated glass would not be sufficient. And because they're not considered an opening, glass fire walls allow an unlimited amount of glazing in a wall, making it possible to design with floor-to-ceiling glass, while maintaining a 2-hour fire rating. Full-lite doors can also be utilized within these transparent fire walls.

Multiple Functions of Fire-Rated Glass

Product and technology improvements have resulted in a degree of hybridization, combining multiple characteristics in fire-rated glass. Often, building materials must address several design issues on a project, such as energy conservation, acoustic control, hurricane resistance or security needs. In each case, fire-rated glass products are available to provide additional benefits.

One way this has been accomplished has been through the introduction of fire-rated insulated glass units (IGUs). This enables a tremendous amount of flexibility, since the second piece of glass in the IGU can be virtually any type of product: tinted, coated, or mirrored. The IGUs offer energy code compliance and sound control, which expands the capacity of fire-rated glass beyond containing flames and smoke.

Glass fire walls block the transfer of heat. Photo: Courtesy Technical Glass Products

Greater attention is being paid to security risks as well. Some fire-rated glazing materials have earned a Level 3 bullet resistance classification, meaning they can stop a .44 Magnum bullet, and greater resistance levels are possible when combined with other glazing products.

Framing Developments

Until recently, fire-rated framing was not keeping pace with innovation in glass products. Using traditional hollow metal steel framing was as predictable a choice as using wired glass. Despite the track record of proven performance, hollow metal doors and frames have limitations.

Hollow metal frames and doors exhibit a bulky appearance due to the method of construction. Rather then being extruded, hollow metal frames are formed from flat sheets of steel, cut to size, then bent into the desired shape. Due to this manufacturing process, hollow metal steel doors and frames lack the crisp edges provided by extruded aluminum systems. In addition, window glass is held in place using glass stops with exposed fasteners. With exposed screws placed approximately every 12 inches to 16 inches, aesthetics are noticeably compromised. The design of hollow metal steel frames makes it difficult to match the appearance of aluminum systems as well. Unlike aluminum frames that sit within a finished opening, hollow metal frames wrap entirely around the surrounding wall.

In the last few years, however, framing options have emerged to allow more aesthetic applications of fire-rated glazing. New types of steel framing systems have been introduced in North America that were previously available only in Europe. Similar in appearance to aluminum storefront framing, these narrow profile frames are nearly extruded from steel tubes, providing aesthetics and performance unmatched by ordinary steel frames. This modern manufacturing process provides a narrower, more streamlined appearance.

For example, 45- to 90-minute rated window frames are often available with a width and depth of less than 3 inches by 2 inches. In addition, door stiles and rails need only be 3 inches wide, rather than 6 inches to 8 inches typically required with hollow metal doors. Further, glass stops use hidden fasteners and the frames install into the wall assembly like a typical aluminum storefront. When finish-painted at the factory, these steel framing systems are difficult to distinguish from ordinary aluminum storefronts. Despite the sleek appearance, tremendous strength is retained in these new systems, because the profiles are formed, rather than bent steel.

New narrow profile "storefront", or floor-slab-to-floor-slab, door and frame systems provide additional design options, and are available with fire ratings of 20 to 90 minutes. As with hollow metal steel, these frames do not provide a barrier to heat transfer. They are most commonly used with thin fire-rated glazing products, like ceramics. Due to their visual appeal, narrow profile doors and frames are sometimes used throughout a building, in fire-rated and non-fire-rated areas, to achieve a consistent appearance.

Some fire-rated steel framing systems utilize an insulated steel profile, and have achieved ratings up to 90 minutes in doors, and up to 2 hours for other applications. Acting as a barrier to heat transfer, these frames and transparent panels allow an unrestricted amount of glass in walls and doors, and are compatible with glass fire wall products.

Such systems allow for full-lite doors and large expanses of glass, creating new possibilities for designers. For example, in a 4' x 8' door, it is now possible to have a single piece of glass providing a full-lite, narrow-stile door.

Fire-rated steel framing offers narrower profiles than traditional hollow metal steel frames. Project: L'Anse Creuse High School North, L'Anse Creuse, Michigan. Architect: Wakely Associates, Inc. Photo: Courtesy Technical Glass Products

As with other recent developments in steel framing, solutions now address curtainwall applications as well, including a fire-rated framing system spanning several stories, like an aluminum curtainwall. Whether interior stairwells or exterior property line applications, curtainwalls have been developed with up to 2-hour ratings. Similar to the fire-rated storefront systems, these steel curtainwalls are difficult to distinguish from aluminum systems.

Although steel remains the most widely used material for fire-rated framing, significant advancements have also been made using wood. For many interior applications, nothing can compare to the warmth and beauty of natural wood. With recently developed systems, wood doors and frames can be used in fire-rated openings. Available with fire ratings from 20 to 60 minutes, these hardwood framing systems can incorporate a wide range of fire-rated glazing materials with glass sizes that surpass traditional systems. Hardwood frames are typically available in a wide variety of species, from red oak or cherry, to mahogany and maple.

Glass and Sprinklers

A comprehensive fire protection program should address three basic needs: detection, suppression, and compartmentation. Components in the first two categories generally require some type of activation, while components of the third category work without any type of trigger. For instance, smoke alarms, which provide detection, and sprinklers, which provide suppression, both require a signal to switch into active mode. Smoke or heat sets them off. In contrast, fire walls, doors, and ceilings compartmentalize and contain smoke and flames without any activation process. They offer passive, round-the-clock protection by acting as physical barriers to fire and smoke.

With active systems, there is always the danger of mechanical failure, human error, or poor maintenance interfering with the way the systems function. A sudden loss of power or an unexpected drop in water pressure can render smoke alarms or sprinklers ineffective. Manufacturing defects can further interfere with product performance.

The National Fire Protection Association (NFPA) Journal has cited numerous additional causes that have resulted in inoperable sprinklers in building fires, stemming from valves painted over, systems shut down during construction, fire burning through PVC supply pipe, and fire fighters diverting water.

Like any fire protection system, sprinklers have limitations. Relying solely on a single method of fire protection, especially one requiring activation, may not be the best design solution. Examining the relationship between sprinklers and fire-rated building materials, such as glass, allows a greater understanding of how these systems work.

Deluge sprinklers bathe glass surfaces entirely to keep them cool. Photo: Courtesy Technical Glass Products

Deluge sprinkler systems are a case in point. Occasionally, in lieu of fire-rated glass, code officials approve the use of deluge sprinkler systems with non-rated glass. Deluge sprinklers are highly specialized, and function in a dramatically different way than regular sprinklers, by producing a directed spray that bathes window glass with water during a fire.

This can pose a significant problem, due to an issue known as thermal shock. Most glass cannot tolerate drastic variations in temperature on the surface. If one area is hot and another is cool, the glass doesn't expand or contract, but instead typically shatters and falls from the opening. This principle is visible when water is sprayed on the glass doors of a fireplace when a fire is going.

Even when water isn't present, ordinary float glass breaks at about 250 degrees Fahrenheit, and tempered glass at about 500 degrees Fahrenheit. In contrast, fire-rated glass is often capable of withstanding temperatures above 1600 degrees Fahrenheit. With temperatures in a building fire often exceeding 1000 degrees Fahrenheit during the first 5 minutes, these figures indicate that window glass cannot provide significant fire protection.

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.

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. Some sprinkler manufacturers advise that blinds or curtains must not be placed between sprinklers and glass.

Locating sprinkler heads up to 12 inches (30 cm) away from the windows presents an awkward challenge. Once tenant improvements are made to a building, landlords and owners may find that initial advisories regarding window coverings may be difficult to enforce.

Two additional conditions are worth mentioning. Since the source of potential fires cannot always be identified ahead of time, the sprinkler and tempered glass systems require deluge sprinklers on both sides of the glass. Also, window designs with intermediate horizontal mullions are not allowed, since the mullions would interfere with the water's ability to evenly bathe the glass surface.

Other laboratory tests conducted over the last several years demonstrate the complexity of the sprinkler and glass relationship. In 1995, officials at Factory Mutual Research Corporation observed a test to determine if a non-fire-rated window assembly and sprinkler system could provide equal protection to that of a fire-rated assembly. The system (which combined specially designed sprinklers and tempered or heat-strengthened glass) was exposed to fire, with the hope that the glass would be able to stay intact.

The researchers discovered that the glass could survive the test if two conditions were met. First, the fire had to start far away from the glazing assembly, in this case approximately 8 feet (2.44 m). Second, the sprinkler needed to activate soon after the fire started.

When the heat source was brought closer to the non-fire-rated glass, the glass fell out of the frame in less than five minutes. Apparently, the close proximity of the flames caused the temperature of the glass to rise too quickly, outpacing the sprinklers' ability to cool the glass surface.

Nearly a decade prior to that test, Lawrence Livermore National Laboratory (LLNL) conducted a similar experiment to find out what would happen when a fire started near the surface of non-fire-rated, tempered glass (see Fire Journal, Vol. 80, No. 4). They used two different sizes of fire, 250 kiloWatt (kW) and 40 kW, with sprinklers installed for the test. When the larger fire was started, the sprinklers activated early and the glass remained intact. But in both tests conducted with the smaller fire, the glass fractured and fell out of the test assemblies in less than four minutes, even before the sprinklers activated.

Looking at the results of both tests, it appears that when fires are relatively large and distant, non-rated, tempered glass may perform as needed. The overall room temperature rises rapidly enough to activate sprinklers before the glass becomes stressed. However, when a smaller fire is concentrated close to the glass surface, it may not activate sprinklers early enough, generating sufficient stress to shatter the non-rated glass.

Clearly, sprinklers have improved the standard of fire safety. Yet, as the NFPA has acknowledged in their evaluation of U.S. experience with sprinklers, sprinklers are not adequate alone; they must be part of a comprehensive fire protection program. If, for any reason, a sprinkler system should fail, there must be some form of compartmentation in place to prevent the fire from spreading further.

There are many glass products available that offer outstanding fire ratings and the ability to withstand thermal shock. From a life safety standpoint, there is no reason to compromise on either issue. Combining sprinklers with fire-rated glazing offers the best of both worlds.

Summary

The benefits of fire-rated glazing include safety, performance, flexibility, and aesthetics. Developments in fire-rated glass, related technology, and framing systems offer architects many planning and design options that simultaneously meet design goals and life safety code requirements.

Jerry Razwick is president of Technical Glass Products. He frequently writes and speaks nationwide on fire-rated glass and framing.