Today's buildings are more transparent than ever. Advancements in glazing technology have allowed designers to open up both interior and exterior walls. Offices and hotels, for instance, are being designed with grand atria with large glass curtain wall exteriors.
Ensuring that these large, open lobby spaces meet building code for fire protection can be challenging for building teams.
"Fire protection in atrium spaces requires the entire gamut: detection, active and passive suppression, smoke control, maintenance and even means of egress," says Thomas Gardner, principal with Chantilly, Va.-based fire and life-safety engineer Gage-Babcock & Associates. "Although they're pretty well defined in the code, atriums can be difficult because they're so different in size."
A prime example is the 250,000-sq.-ft. Farragut Center in Washington, D.C. The office complex features a 10-story atrium with glass curtain wall on three sides. The building employs a comprehensive system that incorporates sprinklers along the glass curtain wall, smoke and fire detection, and smoke exhaust.
"Because the atrium ceiling is more than 100 feet high, we couldn't use sprinklers at the top of the atrium," says Mike Leavitt, project manager with New York City-based general contractor Bovis Lend Lease. "We instead designed a water curtain along the three curtain walls consisting of sprinklers spaced 6 feet apart and 6 to 10 inches from the glass. Standard fire protection allows for one head per every 225 square feet of space, or roughly 15 feet apart. With this approach, we intensified the coverage."
Leavitt says the space is also equipped with a smoke exhaust system that during a fire will automatically exhaust the smoke.
"Smoke control is perhaps the biggest factor in fire protection for atriums, and it has gone through some big changes in the last several years," says Gardner. "In the past, the code simply called for six air changes per hour, so we would figure the size of the space to calculate the size of fan needed."
It is much more stringent today under the Uniform Building Code. "Now we have to calculate how large the exhaust fan needs to be so that the smoke will never descend lower than 10 feet above the highest walking surface, so that occupants can exit safely," says Gardner. "The problem is that fan sizes are getting unbelievably large compared to what they used to be, proving that design for smoke control for atrium spaces in the past was woefully inadequate."
Wireless fire-rated glass
A recent advancement in fire protection for glass-enclosed structures is wireless fire-rated glazing. The marketplace now offers glazing that provides up to a three-hour fire rating, without the "institutional" look of the traditional polished-wired glass.
"Twenty years ago, only glass block and wired glass were rated for fire protection," says Gardner. "Today, there's some interesting fire-rated glass on the market, as well as other technologies, such as window sprinklers that are meant to work in conjunction with large glass systems to create a fire-rated wall."
"Several products are available that are tested and rated for different criteria," says Eric Rosenbaum, director of A & E Services for Hughes Associates, a Baltimore-based fire science and engineering firm. "Some are tested as a wall, others as a door or window."
Fire-rated glass can be organized into four primary categories:
Polished-wired glass is the most well known fire-rated glass product. "However, architects hate designing with it," says Gardner. Wired glass has a 45-minute fire rating in sizes up to 9 square feet and a 90-minute rating for fire doors up to 100 square inches.
It is a misconception, however, that wired glass has high impact resistance. In fact, it meets only the minimal 100 pounds per square foot impact standards. With liability issues in mind, some building teams are specifying wireless fire-rated glass instead of wired glass for schools and other high-traffic spaces to get a higher impact rating, even though the costs are greater.
Ceramic glass is a category of fire-rated glazing that has entered the market during the past 12 years. It's completely transparent and is available in a range of makeups that can provide many different characteristics, including fire ratings up to three hours, high-impact safety ratings and sound reduction.
Specially tempered glass has become a popular alternative for low-level fire safety. It is clear, wireless and has a fairly moderate initial cost and a high-impact rating, making it suitable for door applications.
It does have some drawbacks, however. Unlike other fire-rated glazing products, specially tempered glass does not perform well when used in conjunction with sprinklers. In an actual fire, if sprinklers activate nearby and water hits it, the hot glass may detach and fall out of its frame. It has a 20-minute fire rating.
Transparent wall units are ideal for areas such as lobbies, data centers and other spaces where large expanses of glass are used.
"These are typically not used in curtain wall, but rather in interior spaces where designers are trying to separate occupancies or get a fire rating in a corridor," says Rosenbaum. "They are tested to the same criteria as walls."
Different kinds of transparent wall units are available. Some are insulated units filled with a clear gel that turns into an opaque foam during a fire. Others are made of multiple layers of glass — similar to bullet-resistant glass — with intumescent interlayers that also turn opaque during a fire. Both styles have ratings of up to two hours, can work with sprinkler systems and offer high-impact safety ratings.
Moreover, thanks to new framing technology, designers can incorporate transparent wall and door units from floor to ceiling in their designs, and still offer two-hour fire protection.
An issue that certainly has been heightened since Sept. 11 is the need for protection against bomb blasts. Atrium and lobby spaces are considered prime targets because of their transparency and proximity to public streets.
In addition to stepped-up security in and around many buildings, some building owners are going as far as to retrofit windows and glass curtain wall with film to protect against bomb blasts and other impacts, such as those caused by hurricanes. Furthermore, laminated glass is being specified increasingly for new buildings. These glazing technologies not only provide strength, but also insulation, shading, protection against ultraviolet rays and, in some cases, a specific architectural appearance.
Laminated glass consists of a sheet of polyvinyl butyral sandwiched between two sheets of glass to create a strong unit that may shatter upon impact, but is designed to stay in place, much like an automobile windshield. Film is designed to act similarly, but is applied "on site" to existing windows.
New York City-based structural engineering firm Weidlinger Associates has researched various laminated glass and film for its responsiveness to blast loads. Peter DiMaggio, an associate with the firm, says the technology is "definitely changing and improving."
Breakthroughs for laminates center on the manufacturing processes to help bring down costs, says DiMaggio. "The other area is in the chemical properties of the laminate to make it much stronger and more adherent in holding more glass particles together," he adds.
Advances in film include more durability and an improved installation process.
A big step forward for both film and laminate, concludes DiMaggio, is in their use with silicone sealants, which better affix the glass to the frame to prevent the glass from flying into the lobby space in the event of a bomb.