Glass considerations for safer schools

by Katie Daniel | September 7, 2017 11:13 am

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by Patricia Hernandez
Safety has always played a key role in the design of schools, but the necessity is more heightened than ever today. High-profile issues like school shootings add to long-held safety concerns, the most notable being fire. The National Fire Protection Association (NFPA) estimates there were roughly 4000 K–12 school building fires annually between 2007 and 2011. (For more, consult National Fire Protection Association’s [NFPA’s] September 2013 publication,“Structure Fires in Educational Properties Fact Sheet.”) Yet, at the same time, barricading children in solid-walled, stuffy environments is not an ideal solution. Not only is daylighting essential for reducing energy costs, but several studies[2] have also shown it improves occupant health and performance.

For schools in rural areas, the concern is compounded by the fact emergency response times may be lengthier, putting pressure on educators to protect children during fires or acts of violence. Glass products are keeping pace with these dangers to ensure security does not sacrifice design.

Exterior and interior glass, both as walls and in interior doors, is a key design component for creating flexible learning spaces and allowing ample natural light penetration into and throughout the building. Fortunately, there are a handful of glass solutions offering various levels of protection against fire and, in some cases, intruders, without sacrificing design, views, or performance.

Fire-rated glass options
Fire-rated glass offers architects the opportunity to meet design goals while still ensuring certain levels of safety and meeting building and fire code requirements. As technologies have advanced, opportunities to use glass in a wider variety of sizes and applications have also expanded.

The highly flexible product range allows nearly unlimited multifunctional and aesthetic possibilities, perfectly blending with adjacent non-fire glass types. Fire-resistant glass can be used for overhead glazing, windows, sidelites, butt-joint solutions, fire-rated doors, partition walls, glass floors, or curtain walls.

Several glass types fall into the fire-rated safety glass category.

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Specialty tempered glass
Specialty tempered glass is processed by controlled thermal treatments to increase its strength compared to normal (annealed) glass. Tempering creates balanced internal stresses that cause the glass, when broken upon impact, to crumble into small granular chunks instead of splintering into jagged shards. These products are made of thermally tempered safety glass to withstand fire exposure without the transmission of fire, and prevent the passage of smoke, flames, and hot gases. Typical 6-mm (¼-in.) thick specialty tempered glass has a fire rating of 20 minutes, is listed for a 542-J (400 ft-lb) impact rating by the Consumer Product Safety Commission (CPSC) 16 CFR 1201 Category II, Safety Standard for Architectural Glazing Materials, and can be used as a safety glazing material specifically in door applications per International Building Code (IBC) 2012.

Ceramic glass
Ceramic glass is a transparent sheet of ceramic material in a glass matrix.
It has a significantly higher softening temperature than regular glass and is therefore capable of withstanding a much longer exposure to fire conditions without slumping and creating unwanted openings. The ceramic has a high resistance to thermal shock because of an extremely low coefficient of thermal expansion. Unlike glass, this product cannot be tempered, so it is unable to meet impact resistance requirements on its own, restricting its use in some areas.
It can, however, be modified with a surface-applied impact safety film or laminated to meet the highest impact rating requirements of 542 J per CPSC 16 CFR 1201 Category II, which represents the vast majority of its modern use
in buildings.

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Insulating glass
The non-insulating fire-protective-rated glazing products mentioned above will not reduce heat radiation emitted by a fire or satisfy fire wall insulation criteria for more than a few minutes. If the glass has to comply with the full fire wall criteria of integrity and insulation, as it may if it is installed in a compartment wall, then a clear multi-ply insulating glass—either intumescent laminated or gel glass—is required.

Intumescent laminated glazing consists of layers of float glass interleaved with clear hydrated sodium silicate intumescent material. Increasing the number of intumescent layers within the glass increases the performance to a higher fire rating, resulting in overall glass thicknesses from 19 to 51 mm (¾ to 2 in.) and greater. When heated, the exposed layer of glass cracks, activating the first layer of intumescent. With time, the exposed intumescent is degraded by the hot gases in the fire, and the heat is conducted through the opaque layer, causing the next layer of glass to crack and the second layer of intumescent to be activated.

This procedure continues until there are no more layers of intumescent to activate. One by one, the intumescent interlayers turn opaque and expand to form a fully insulating heat shield, thus reducing the transmission of radiated and conducted heat through the glass for as much as 180 minutes in some cases. Another benefit of the opaque insulating interlayer is it blocks the view of the fire, which minimizes panic as occupants exit the building.

Intumescent gel glazings consist of two or more sheets of toughened safety glass. The cavity between the glass sheets is filled with a transparent gel that crystallizes into an opaque heat-absorbing char when exposed to fire. The protective interlayer is based on nanotechnology; it also offers high light transmission and can meet the demands of ‘impact-safe materials’ required for areas where there are people.

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Intumescent gaskets
When specifying fire-rated glazing materials, one must consider not only the glass itself, but also the entire glazing system of frame and gaskets as tested by the manufacturer. Gaskets and seals introduced to restrict the passage of noise and odors could generate significant volumes of smoke if incorrectly specified and could also compromise both the noncombustibility and zero surface spread of flame characteristics of the glass. As the fire temperature rises and the glazing system is put under stress, the hot gases will readily exploit a poorly designed glazing detail. An intumescent-based glazing seal can ensure the natural attributes of the glass are maintained by preserving the integrity of the glazing pocket. Intumescent seals can expand to accommodate any deterioration in the glazing pocket and, therefore, play a significant role in achieving fire safety.

The case against wired glass
Wired glass[6] gives the illusion of safety, but in reality, it is weaker than nonwired glass due to the incursions of the wire into the crystalline structure of the glass. Wire mesh glass also can be dangerous—if a person strikes the glass with enough force to puncture through it, the wire amplifies the irregularity of the fractures, making injuries more severe. In fact, wired glass has been banned in many jurisdictions in the United States, and recently lost its safety designation
in Canada.

Wired glass is still well-rated to withstand both heat and hose stream tests. The wire prevents the glass from falling out of the frame even if it cracks under heat stress, impeding flames and smoke from passing through and spreading throughout the building quickly. However, unless modified to achieve a higher impact safety rating required by building code regulations, wired glass is limited in size and to areas and applications away from people in any type of building.

Innovations in glazing technology, such as ceramic and intumescent laminated glazing, have dramatically changed the industry and brought numerous benefits over wired glass, including an expanded field of use with longer fire ratings and larger openings, application-adapted performance, and safer alternatives in the case of accidental human impact.

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Fire-protective versus fire-resistive
Along with the general categories outlined above, it is important to understand the differences between the two classifications of fire-rated glass—fire-protective and fire-resistive—to ensure each system meets codes and properly addresses the unique safety aspects of each area of the building.

Fire-protective glass
Also known as ‘filmed ceramics,’ fire-protective glass is designed to block (or compartmentalize) smoke and flames, but not radiant heat. This means the glass will help keep the fire from spreading, but will not prevent heat-related injury to occupants. Examples of fire-protective-rated products are polished wired glass, specialty tempered glass, some intumescent laminates, and glass ceramic.

While there are fire-protective-rated products available from 60 to 180 minutes, their application is limited to door vision panels, and size limitations may apply. For 180-minute doors, fire-protective-rated products may be listed for 64,516 mm² (100 si), although IBC does not permit any vision panel in a three-hour rated fire door. For 60- and 90-minute doors in exit enclosures and passageways, fire-protective-rated products are limited to 64,516 mm², whether or not the building is fully sprinklered.

Additionally, fire-protective-rated glass has limited use in one-hour walls, and is not permitted altogether as sidelites, transoms, and windows in two-hour interior walls because it cannot block radiant heat. These limitations are recognized in IBC, and recent revisions to the 2012 IBC give end-users clear guidance in applying those limits.

Many fire-protective-rated glass types can provide a CPSC Category II high-impact safety rating as well.

Fire-resistive glass
Fire-resistive glass is made with multiple layers of laminated glass with an intumescent liquid in between, providing an insulative layer to not only block smoke and flames, but also provide a barrier to radiant heat. This allows building occupants to exit the building without being subjected to radiant heat burns, making it suitable for egress applications such as stairwells designed with glass walls to encourage more frequent use. The glass conforms to non-loadbearing wall requirements under ASTM E119, Standard Test Method for Fire Tests of Building Construction and Materials. Its limits in application and size only depend on the product limitations of each manufacturer.

Fire-resistive glass is required for glazed assembly openings, door vision panels, and sidelites and transoms comprising greater than 25 percent of the common wall; these areas are classified as a ‘transparent wall’ and must comply with ASTM E119. Fire-resistive glass is also required for one- and two-hour walls, 60- to 90-minute door vision panels, and 60- to 120-minute sidelites and transoms when not in exit enclosures and passageways. Examples of fire-resistive products are intumescent multilayer laminated and intumescent multilayer gel glass.

It is imperative only a tested and approved system of fire-rated glass and frame be used as a fire-resistive-rated assembly.

Additional safety features
Along with fire resistance, glass options are expanding to include protection against intrusion while maintaining the cost-effectiveness required for traditional schools and buildings. One challenge with bullet-resistant glass is it is too expensive for widespread use. One new fire-protective-rated ceramic glazing product combines protection against fire, accidental impact, ballistics, and intruders.

As an intrusion barrier, this type of material deters or delays an active shooter to help prevent an attack or slow it until first responders arrive. In bullet-resistance tests, such glazing should not shatter; in threatening situations, it should protect occupants from injuries that can result from shattered glass. More importantly, the fact the glass does not shatter will help prevent intruders from potentially gaining access through the opening. In the event of fire, the glass will also contain smoke and flames, allowing more time for occupants to escape.

In addition, the glass features a patent-pending surface polishing technology with advanced composition[8] to create sharper, more accurate color-viewing. It has a color-rendering index (CRI) of 97.1 (out of 100), a high R96a value, and a low haze value at 0.5 percent. It is comparable to float glass in quality and lacks the orange peel effect often seen in standard fire-rated glass.

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Balancing act
Like many aspects of building design, schools present architects with a balancing act of sorts: how to create a meaningful, functional design while meeting code requirements, saving energy, maintaining safety, and staying on budget.

Determining which type of glass is required varies greatly by application and local code regulations. Architects will need to consult building codes and manufacturers to determine which ratings are required in each area of the building and how those requirements dictate allowable sizes.

For example, architects used several types of fire-rated glass to achieve Pfluger Architects’ design goals for Tom Glenn High School in Leander, Texas. Most prominent is a large central atrium housing the cafeteria, with glass windows looking down from the second level. For this application, the architects specified fire-rated ceramic glass.

Glazing was also a priority with classrooms because the school sought a collaborative learning environment, in which learning spaces include glass walls made with fire-rated ceramic glass. Along with offering fire protection, the double-paned glass features an internal blind system for further protection during a lockdown.

“Everybody wants collaborative learning that’s much more transparent,” notes Jessica Molter, principal at Pfluger Architects, “but you also have to think about the safety aspects.”

Indeed, safety has become just as important in school design as the creation of productive learning environments. Thankfully, architects have more options than ever to create spaces that are conducive to learning, collaborative in form, efficient in operation, and healthy—without sacrificing peace of mind.

Patricia Hernandez is the northwest regional sales manager for Vetrotech Saint-Gobain North America. Her territory includes Washington, Oregon, Montana, Northern Idaho, Alaska, and Western Canada. Hernandez works with the architectural and glazing communities to educate the industry on the unique aspects of fire-rated glass. She has more than 25 years of experience in the building materials industry and is active in numerous architectural and professional organizations. Hernandez is a current board member for the Washington Glass Association and is a member of CSI’s Portland Chapter. She can be reached via e-mail by writing to patricia.hernandez2@saint-gobain.com[10].

Source URL: https://www.constructionspecifier.com/glass-considerations-for-safer-schools/

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