This article details what to consider in specifying fire rated glazing. Some of the topics covered include fire protective vs fire resistive glazing, testings and standards and common misunderstandings in applications of fire rated glass.
by William O’Keeffe
Design professionals and building owners have embraced glazing for its ability to offer views and provide daylighting. However, Seeking increased open, transparent spaces means the design team must also face the challenge of maintaining energy requirement, occupant and property protection, safety, and fire performance. This article focuses on the last point, examining innovations in the fire-rated glazing products.
Although it may resemble and provide the benefits of clear, ordinary glass, new fire rated glazing products are different in their ability to help successfully protect occupants from fire, smoke, and radiant heat. They can also be customized to defend against hurricanes, ballistic attacks, and bomb blasts. Additionally, they can be specially madeto reduce ultraviolet (UV) ray transmission and offer noise abatement benefits. Virtually unlimited design make-ups are also possible, including products that are laminated, insulated, tinted, decorative, mirrored, or curved. These new fire rated glazing products still meet other building requirements such as energy performance, enhance the project’s aesthetic appeal, and maintain its fire performance.
Until the 1980s, traditional wire glass was the only available fire-rated glazing in the United States. A multitude of other materials have become available over the last 20 years, but with the advent of so many technologies in such a short span, there is confusion as to which products are appropriate for specific applications. Even when the use has been determined, it can be difficult to sort out performane features (and their related costs) best suit project requirements and budget limitations.
Fire-rated glazing falls into two basic categories-fire-protective and fire-resistive (the latter meeting code requirements that are more restrictive). Each of these application categories has different performance features . Consequently, fire-protective and fire-resistive glazing product typesare quite different based on how they meet these requirements during testing.
Choosing the appropriate glazing means the specifier must fully understand the features required of these applications. Contrary to the belief of some , code approval is not solely based on how long a product can endure a fire test (i.e. endurance rating of 20,30, 45,60,90,120 and 180 minutes ), But rather the overall performance desired of the product for its application—protective or resistive—as determied by testing.
In fire protecttive applications (i.e. 45 minutes or less) , the glazing maintains its integrity and contains smoke and flame, but does not prevent heat transmission. The treat of this radiant heat (and the potential of spontaneous combustion) is minimized in these <45-minute areas because the glazing is used in windows, sidelites, and transoms where the area of the openings are less than 25 percent of the wall.
There are also higher endurance applications for which fire-protective glazing is allowed, even up to 180 minutes. However, these would be 0.06-m² (100-si) applications where radiant heat is not a concern due to the small size.
Test standards for fire-protective glazing include:
Examples of fire-protective glazing products include wired glass, highly tempered, special edge-treated products, laminated glass, and ceramics.
Traditional wired glass (non-safety)
Traditional wire glass relies on fine, embedded wires to hold annealed glass in place during a fire test (and the subsequent hose stream portion, explained later in this article). These wires provide no protection against impact-in fact, they weaken the glass and substantially reduce its impact resistance. Traditional wired glass is unable to pass the Consumer Product Safety Commission (CPSC) 16 CFR 1201 safety standards for Categories I or II, and will break with as little as 50 ft-lb of force (i.e. a small child’s impact).
Established by the U.S federal government in 1977, 16 CFR 1201 is designed to protect people from injuries due to accidental impact with the glazing. Cat. I products survive the impact of a small child, set ft-lb. Glazing with this rating is limited to less than 0.84 m² (1296 si) per opening on the basis it is less likely to experience the greater impact received by larger areas. Cat II is for products that can survive the impact of an adult, set at 400 ft-lb. Glazing with a Cat II rating is not limited in size.
Traditional wired glass failed to meet this safety standard and was given a temporary exemption to meet a lower standard American National Standards Institute (ANSI) Z 97.1, Safety Performance Specifications and Methods of Tests for Safety Glazing Materials Used in Buildings, set at 100 ft-lb. The building coeds also limited its use to 1296 is and for solely in fire-rated areas. Today, the code has removed the exemption and requires all glazing ti meet the higher CPSC standards. Therefore, traditional wired glass cannot be used in doors or any hazardous locations for all occupancies under the 2006 International Building Code (IBC).
Safety wired glass
Safety wired glass has the same appearance as its traditional counterpart, but is either laminated or filmed to achieve a safety impact rating complaint with the latest building codes. Although more costly than traditional wired glass, it is still one of the most expensive options for this application. Laminated safety wired glass is impact-rated to only the lower Cat. I, while filmed fire-rated safety wired glass meets both Cat. I and the higher Cat. II standard. As such, the latter can be used in larger sizes and in doors without the 1296-si limitation.
Highly tempered, special edge-treated products
Widely used in Europe and Asia for 20-minute applications, this clear, highly tempered, special edge-treated product is made from annealed glass through a thermal and edging process that improves thermal shock qualities. It is able to withstand a much greater thermal differentiation without fear of shattering. These 20-minute, fire-rated products are safety-impact-rated to both Cats. I and II and can be used in an insulated glass unite (IGU) for energy savings. These materials are relatively low in cost and are approved by UBC without the hose stream portion of the fire test.
Highly tempered, reflective, special edge-treated products.
Highly tempered, reflective, special edge-treated products are fire rated from 20 to 60 minutes and provide additional defense against radiant heat transfer by reflecting the heat back toward the fire source. They are impact-safety-rated to both Cat. I and II for use in protective applications to doors, transoms, sidelites, and openings. In comparison to other safety-rated, fire-protective products, this material is approximately twice the cost of other non-ceramic materials (though still less expensive than ceramic glazing). However, it offers the benefit of being clear while providing greater safety.
Reflective tempered products in applications over 20 minutes are tested without the hose stream portion of the fire tests. While accepted by the General Services Administration (GSA) and others, some areas may require the approval of the authority having jurisdiction (AHI) for their use.
Special laminated glass
Non-wired, imported glazing is rated to 20 minutes and safety-impact-rated to Cat. I; it provides some portions from radiant heat transfer. Both the special laminated and the highly tempered, reflective, special edge-treated protective glazing do no meet the stringent fire-resistive ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials, which limits the temperature rise on the non-fire-side to 139 C (250F) over ambient conditions.
In addition to its low cost, an advantage of special laminated glass is that it can be cut and stocked. However, it is laminated (because of its Cat. I impact safety rating) to 1296-si applications and doors.
Ceramic is similar to the materials used in stove tops that allow heat transfer while tolerating high temperatures without cracking (thanks to a low thermal expansion and a high melting point). Introduced to the U.S market as a non-wired alternative, ceramic products have a fire rating of 20 minutes to three hours, but should not be confused with fire-resistive products. In fact, ceramics `pump’ radiant heat through to the non-fire side, rather than blocking its transfer.
Since ceramic are brittle, they must be laminated or filmed to achieved a Cat. I or Cat. II safety rating for hazardous locations, such as doors. Further, due to its manufacturing process, achieving optical clarity can be difficult. While polishing improves surface quality, most ceramics have a slight tint and some surface irregularity. Of all the fire-protective glazing products, ceramics are the most expensive.
Fire resistive applications call for products that can block radiant heat, usually for all uses where ratings of over 45 minutes are required. These transparent products are designed to contain flames and smokes and block the passage of radiant heat. Fire-resistive products are listed by recognized testing agencies as “transparent walls” and are found in the wall component section of UL’s and Internek/Warnock-Hersey’s (ITS/WHI’s) listing books. When tested as a “wall”, glazing does not have the area limits of fire-protective glazing—wall-to-wall and floor-to-ceiling expanses of glass can be used with appropriately rated framing. Test Standards include ASTM E 119 and;
Fire-resistive products include retardant-filled units and multi-laminates.
These products are composed of two tempered glass lites with sandwiched cavity filled with a clear, semi-solid fire retardant. (The thickness of the fire retardant determines the fire rating.) During a fire, the lite facing the fire breaks away and the exposed fire retardant material becomes an opaque, insulting, fire-resistive barrier that prevents the trannissionof flames , smoke and radiant heat.
Multiple sheets of annealed glass can be laminated together using special intumescent interlayers that swell during a fire to prevent the transfer of smoke, flames, and radiant heat. Adding multiple layers of laminate increases the material’s thickness, providing for higher ratings.
Multi-laminates can be factory-cut by a special diamond blade, whereas fire-retardant units cannot be trimmed to size. On the other hand, multi-laminate units can be thicker, weigh more, and may also be more susceptible to breaking or edge damage because they use annealed glass.
Most fire-rated glazing products receive listings and follow-up services from only two testing agencies—UL and ITS/WHI. Both test products to certain standards depending on the application for which they will be used. For example, fire-protective glazing for use in windows and doors is tested to NFPA 257/252, while fire-resistive glazing for use in doors or walls is tested to ASTM E 119. (As previously mentioned, they are therefore considered transparent walls.) As stated earlier, the distinction between the performance of fire-protective and fire-resistive glazing products is not evident in the tiome ratings they earn. Fire endurance (i.e. the time a product is tested) does not encompass the full performance that will requires of the product for use in an application.
Fire endurance and hose stream test
Due to the stress caused by uneven heating, normal glass breaks during a fire and allows smoke to travel to surrounding areas. The endurance test is used only to determine the time a glazing product can withstand fire and extreme heat, with temperature reaching up to more than 982 C (1800F). If the glass remains in its frame foe the duration of the test, it is certified with the appropriate endurance rating, ranging from 20 minutes to three hours.
The hose stream test was developed in the 1800s, when there were concerns cast iron structural elements (e.g. posts or beams) could shatter or collapse when thermally shocked by a firefighter’s hose. It was applied on it to show structural ability after an endurance test. During the procedure, the specimen is subjected to a high-pressure water stream at a fixed distance and pressure (greater than that of a standard fire hose) immediately after the fire endurance test. For the assembly to pass, it must sustain the minimum amount of breakage allowed by the test standards.
It is important to not the hose stream is only use in the United States and Canada—it is no longer considered appropriate for fire-rated glazing in any other country. If the laboratory does not conduct the test, the product’s listing states that the AHJ may be required to review and accept its use in applications longer than 20 minutes.
Impact safety test
Safety testing is required for any glazing to be placed in locations where accidental impact may occur. (The human impact safety standards are mandated not only by the federal CPSC [as previously described], but also by local building codes.)
Radiant heat test
Glazing products tested to ASTM E 119 for the fire-resistive applications must have the ability to block the transfer of radiant heat. During this test, the prime source of heat—produced by large gas or oil burners—follows a fixed time and temperature curve designed to stimulate a fire (i.e. it rises quickly, then gradually continues to increase). The heat allowed to be transmitted through the assembly is measured at different points on the unexposed surface. The average temperature calculated from these readings cannot exceed 193 C (250 F) above the initial temperature, and usually cannot go beyond 177 C (350 F) even when the opposing has, at two hours, reached temperature over 982 C (1800F).
Since many of the new fire-rated glazing products are highly technical, there have been several misunderstandings that require clarification with regard to material testing, performance, and application.
Hose stream test and fire protective glazing
In sections 4-2.1 of NFPA 251 and 7.108.2 of the Uniform Building Code, the hose stream test is specifically excluded for fire-rated construction of less than one hour. However, in the section of the code related to glazing, 45 minute fire-protective windows, transoms, and sidelites must be tested to the hose stream. The GSA and many AHJs recognize this inconsistency and acknowledge the host stream test should not be requirement for protective glazing used in nonstructural applications.
This 19th century hose stream test was never intended to evaluate glazing performance or gauge nonstructural elements such as windows and doors. As previously mentioned, it was created to measure the structural integrity of a building’s wall and supportive components to preclude a collapse on those fighting a fire.
Some believe the hose stream method is valid to test for thermal shock-resistance when fire-rated glazing is used with sprinklers. However, this is erroneous because the hose stream wa not designed as a thermal shock test for glazing and, in fact, is irrelevant for this purpose because the test is applied after the glazing has been exposed to a 982-C (1800-F) fire for 45 minutes or more. In a real fire, sprinklers activate at relatively low temperatures within the first few minutes.
Many European and Asian countries have long abandoned the hose stream test; British standards have not included it for more than 45 years. As previously noted, the United States and Canada are the only countries using this method, making it more difficult for design teams to specify economical, readily available, fire-protective glazing products. Non-hose-stream-testes products, including doors and windows, can cost a third less than products tested to the hose stream.
Approved alternates and AHJ approval
The authority having jurisdiction (usually a code official) has the power to enforce the minimum life safety and property protection requirements as expressed in the building codes. Sections 104.11 of IBC and 104.9 of the International Fire Code (IFC) give the AHJ the latitude to allow alternative materials.
When asking to use an alternative material, relevant product information from the manufacturer (including independent testing data and various performance features) is usually submitted to the AHJ for consideration.
Radiant heat and fire protective glazing
While ASTM E 119 is only required of fire-resistive glazing, the dangers of radiant heat should also be taken into consideration when specifying fire-protective applications. With 45-minute openings on one-hour exit corridors increasingly specified, radiant heat can play a significant role in blocking safe egress. Computer rooms or stocks of valuable goods (e.g. hard drive, pharmaceuticals) could also be in need pf protection by a radiant-heat-reducing product.
Radiant heat travels by invisible and intense electromagnetic waves with little resistance from the air. When these waves strikes an object, they absorbed with a heat value 45 1000-w bulbs directed at a 10-sf areas. This becomes dangerous to building occupants facing emergencies, as it would cause unbearable pain at levels as low as 5000 W/m². in effect, radiant heat blocks escape routes.
If a combustible product (e.g. paper, fabric, or wood) is subjected to this high amount of radiant heat, a fire will quickly start once the material’s ignition temperature is reached. To illustrate this point, Intertek/WHI conducted a standard time temperature curve test to three protective glazings of the same size—wired glass, ceramic glazing, and highly tempered, reflective, special edge-treated products.
Mannequins were placed in front of the glass samples to simulate people passing through an exit corridor, and radiant heat measurements were taken 0.9 m (3 ft) from the glazing. Five minutes into the test, the measurement of the mannequin in front of the wired glass sample was approximately 6590 W/m², well below the afore mentioned level for unbearable pain.
In the same test, the mannequin behind the wired glass ignited in approximately nine minutes, the one behind the ceramic in 12, and the third behind the special edge-treated product in about 17 minutes.
The 60-minute window
Another common misunderstanding in fire-rated glazing is the misnomer of calling out a “60-minute window”. Although 60- and 90-minute windows are often casually referenced in the building industry, they do not exist within the parameters of existing codes. The reason is these applications require the blocking of radiant heat. However, in 60-, 90-, and 120-minute assemblies, the glazing must be fire-resistive and thus tested to ASTM E 119, the wall standard limiting the temperature rise on the non-fire side to 121 C (250 F).