The mindset started to change with the 1993 explosion underneath the World Trade Center. The attack on the Murrah Federal Building in Oklahoma City was a pivotal turning point. Then came 9/11. Construction has not been the same since.

Today, the glass industry is working hand-in-hand with the architectural community to construct aesthetically pleasing buildings that also meet security needs.

“We try to balance off all the design and security factors to meet client needs and also make attractive buildings,” says David Varner, vice president at SmithGroup Inc., a Washington, D.C., architectural firm.

Safe & Secure
Tony Smith is the director of marketing and pre-construction for Tidewater Glazing, a contract glazing company in Maryland that focuses on government work. He charts the change in government construction. 
“After the Oklahoma City bombing we began to see domestic federal projects incorporate the use of security film to mitigate flying glass during a blast event. However, the September 11, attacks created a need for a greater protection of human life,” Smith says. “Designs for both federally-owned buildings and privately-owned buildings leased by the government began to incorporate blast resistance into the entire envelope—pre-cast, metal panel systems, glazing systems, entrances and roofing systems. The greatest effect this change has had on construction is cost, led primarily by the need for creative designs that serve both architectural and protection functions.” Blast-resistant systems are considerably more expensive than other types of glazing—an estimated 40 percent more. Price, though, is not the only consideration. Security glazing products are often larger, heavier and more difficult to handle.

Delicate Balance
One of the biggest trends within the architectural community right now is transparency. 

“We’re trying to work with the blast calculations to keep the transparency of the structures we design yet meet the blast criteria,” Varner says. “We have to find out how large a piece of glass we can use. The glass must allow us to keep within budget yet have daylighting properties and views. If we build fortresses, the government will have more trouble getting and retaining employees.” 

Luther Blair, vice president and lead designer, and Stephen Ours, vice president and senior project manager at Wisnewski Blair & Associates Ltd. in Arlington, Va., have similar views. Their company has shied away from curtainwall systems because there is no easy way to [enforce] the structure in a blast situation. It instead uses punched windows and structural silicone glazing. 

Christine Shaffer, marketing manager for Viracon Inc., agrees, but says the impact on designs for federal buildings has been subtle from a glazing perspective. 

“We continue to see architects use a combination of curtainwall and punched openings, just as we saw prior to Oklahoma City and 9/11,” she says. “Architects remain interested in daylight and open structures versus creating a bunker-like design based on perceived threats to a building. Better design and material choices to strengthen the structure can offer occupants significantly improved protection.”

Back Off
The standoff requirement is another factor that sometimes affects a structure’s design. 

“If the building site allows space for the 100-foot standoff area, designers are more inclined to use larger expanses of glass and curtainwall applications,” Shaffer says. “However, there are often applications in which a 100-foot standoff distance is not feasible, and these require a different evaluation to offer the appropriate level of security that goes beyond the glass composition.” 

In some cases, though, using less glass on building elevations next to the street is appropriate. “With the Murrah building in Oklahoma City, for example (which replaced the one destroyed by the bombing), the architect used punched openings on the side of the building facing the street and curtainwall for the areas of the building facing the interior courtyard,” Shaffer says. “This allowed the building to be light and bright, but also gave the occupants a sense of protection.”

Julie Schimmelpenningh, technical products manager for Solutia Inc., agrees that the standoff area is a significant design consideration. “One of the most important steps for secure design is mandating the use of laminated glass in new buildings,” she says. “Both the Department of Defense and General Services Administration (GSA) began to take steps toward this in the early 2000s. The fear of using glass in high-risk buildings has subsided and this is even truer when there is adequate space around the building for standoff.”

Critical Components
Most blast hazard mitigating designs incorporate laminated glass as one component of the insulating glass assembly for maximum impact energy absorption before breakage, says Steve Fronek, vice president of Wausau Window & Wall Systems in Wausau, Wis. This also helps to transfer the blast energy to framing materials.

“This means the interface between glass and frames must also transfer blast energy, using structural silicone glazing seals or deep glazing rebates to ensure that premature glass disengagement does not occur,” Fronek says. “Framing and its anchorage must, in turn, transfer loads and energy to the building structure at appropriate locations where they can be resisted.” 

“As we have evolved and become more educated, we have found that larger glass tends to absorb load while smaller glass distributes it to the framing system,” Smith says. “We now see not only the same levels of glass area that were prevalent ten years ago, but also larger units capable of absorbing more blast.”

Looking ahead, designers and installers agree: just because a project requires a certain level of security doesn’t mean it has to be without glass.

Protective Glazing Definitions
Do you know the difference between safety and security glazing? Is impact-resistance terminology new ground for you? If so, here’s a glossary of some common words and definitions.

Blast-Resistant Laminates: Blast-resistant laminates can substantially reduce injury from flying glass resulting from direct blast shock waves (over-pressures). When properly designed, framed and anchored, blast-resistant laminates are capable of maintaining the integrity of the building envelope following an explosion and reducing interior damage. Large scale arena testing indicates that ¼-inch laminated glass installed in a standard frame withstands over-pressures up to 16 psi with phase durations of 10-20 msec and positive phase impulses of around 100 psi-msec. Quarter-inch laminated glass in a wet-glazed frame (structural silicone) withstands over-pressures up to 15 psi with positive phase durations of 15-20 msec and phase impulses of 130-150 psi-msec. (source: GANA Laminated Glazing Reference Manual, 2003 edition.)

Bullet-Resistant Laminates (multiple ply): Bullet-resistant laminates are designed to resist penetration from medium- to super-power small arms and high-power rifles. There are numerous types of bullet-resistant laminates, including all-glass laminates, glass-clad polycarbonate laminates and laminated polycarbonate (or other plastics). It is important to understand these laminates must be tested to prove their performance to resist a certain ballistic threat; that a higher threat level than qualified may not offer similar performance attributes. It is therefore highly recommended that the designer select the highest threat level deemed to be required. (source: GANA Laminated Glazing Reference Manual, 2003 edition.)

Burglar-Resistant Laminates: Burglar-resistant laminates are deterrents to smash-and-grab crime, and resist penetration from hand-held or hand-thrown objects such as hammers, crowbars, bats, knives, bricks and rocks. Typical applications include storefronts, displays, museums, hotels and motels, homes, offices and government buildings. (source: GANA Laminated Glazing Reference Manual, 2003 edition.)

Charge Weight: Gross tare of a given explosive device, typically measured in pounds or kilograms. Usually, the device is expressed in TNT equivalent weight, using tables and calculations. Glass-Clad Polycarbonate and/or Laminated Polycarbonate: High and maximum security/institutional glass-clad polycarbonate laminates (multiple ply) are used in the same manner as institutional glazings and provide ballistic protection as well. The combination of materials will reduce the weight, thickness and frame requirements for a given level of security performance. (source: GANA Laminated Glazing Reference Manual, 2003 edition)

Impulse: Area on the blast wave graph under the pressure-time waveform, i.e. compounded decaying pressure over time (milliseconds). Typically expressed in pounds per square inch (psi) msec. (Source: Applied Research Associates.)

Medium Security/Institutional Lam-inates: Offer improved detention security and provide unobstructed vision while eliminating the confined look of bars and metal screens. Institutional glazings offer prolonged physical impact resistance and extend the time required for penetration. Typical applications include penal institutions, detention centers, psychiatric hospitals and police stations. In addition, institutional laminated architectural glass provides increased protection in other high security applications such as embassies, computer centers and sensitive research centers. (source: GANA Laminated Glazing Reference Manual, 2003 edition) 

Pressure: [Dynamic peak overpressure] the highest pressure generated instantaneously by an explosion and recorded at a specific point (usually the target), typically measured in pounds per square inch (psi). (source: Applied Research Associates.)

Safety Glass: Flat (including bent) glass so constructed, treated or combined with other materials that, if broken by human contact, the likelihood and/or severity of cutting and piercing injuries that might result from such contact is reduced. (Source: ASTM C 162-99 - Standard Terminology of Glass and Glass Products). 

Security Glass: [While very different from safety glass, security glass is more difficult to define empirically due to the breadth of areas covered. Security glass strives to protect against manmade or natural threats.] It is best understood through its specific categories, such as burglar resistant, bullet resistant, blast resistant, forced-entry resistant, impact resistant. Additional references: ASTM F 1233, Standard Test Method for Security Glazing Materials and Systems; ASTM F 1641, Standard Test Method for Measuring Penetration Resistance of Security Glazing Using a Pendulum Impactor; ASTM F 1915, Standard Test Methods for Glazing Detention Facilities; UL 972, Standard for Burglary Resistant Glazing Materials.

Stand Off Distance: Straight line distance from an explosive device to a target, typically measured in feet or meters. 

the author: Charles Cumpston is a contributing editor for USGlass magazine.

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