Shatter-Proof: Blast-resistant glazing
When a truck bomb exploded outside the Alfred P. Murrah Federal Building the morning of April 19, 1995, hundreds of people were either killed or injured by flying or falling glass shards.
The majority of the 168 deaths were linked to the partial collapse of the nine-story building, but collateral damage from shattered glass — some of which was found more than a mile from the site — accounted for more than 60% of all injuries.
Nearly nine years after this horrific event, and with the terror alert at an all-time high, the Building Team for the replacement Federal facility in Oklahoma City is determined to avoid a similar tragedy in the future. Led by Chicago-based design architect Ross Barney + Jankowski Architects, the Building Team is putting the final touches on the $33 million horseshoe-shaped office facility. The three-story structure received its first tenants — two dozen employees with the Small Business Administration — last month, and it will be officially dedicated in May.
The key challenge for design architect Carol Ross Barney was integrating the building's stringent security requirements into the aesthetic and functional design of the facility.
"It's true that buildings need to be secure," says Barney. "But when you start taking other important things out of buildings or make them feel unfriendly, then you're paying an awful high price for security."
She points to the extensive use of blast-resistant glass throughout the facility to permit plenty of daylight to enter the workspaces. "It's an environment that's meant to be highly productive, with a big daylighting initiative and an underfloor air system that gives people more control of temperature in their workspaces," she says.
The blast-resistant glazing is designed to meet the second-highest design criteria for blast resistance (performance condition 2) established by the General Services Administration (see table). Designed and installed by window contractor Masonry Arts, Bessemer, Ala., the system consists of laminated glass bonded with structural silicone adhesive to structural steel framing. During a blast, the glass will crack, but will be retained by the laminate interlayer and hardened frame.
The grand lobby of the replacement Federal facility in Oklahoma City permits people to enter from either the north or south before reaching the security check station. Multiple entrances “make the facility more friendly to the surrounding urban environment,” says architect Carol Ross Barney. The lobby itself was made blast resistant to protect the connecting office spaces from an explosion.
More demand, new standards
Demand for blast-resistant glazing has grown significantly during the past decade, mostly from the GSA and the Departments of Defense and State, which have established mandates for installation of protective glazing in new and renovated buildings that meet certain threat criteria. Typical projects: military barracks, Federal office complexes, courthouses, post office buildings, and our embassies overseas. With the exception of airports, few commercial or institutional projects have employed the technology.
"Not every owner needs install blast-resistant glazing," says Scott Norville, PhD, professor of civil engineering and director of the Glass Research and Testing Laboratory at Texas Tech University, Lubbock. "If a building is a target or near a target, then owners should investigate designing blast-resistant glazing."
Norville recently headed the development of the first non-government-related standard for the design of blast-resistant glazing. Issued last October, ASTM F2248 - 03 ("Standard Practice for Specifying an Equivalent 3-Second Duration Design Loading for Blast Resistant Glazing Fabrication with Laminated Glass"), provides a methodology for designing blast-resistant glazing for car bombs and larger bombs at greater distances.
"The standard says that if you have a particular bomb size at a certain distance, then here's the loads you need to design a laminated glass system for," says Norville. Based on the blast load criteria, the standard provides guidance on the proper thickness and type of laminated glazing, as well as the ideal methods for attaching the glass to the frame and anchoring the frame to the wall.
The development of such standards is a direct result of the bombing in Oklahoma City. Before that event, "we relied on testing that was done for high wind and hurricanes," says Kirk A. Marchand, P.E., principal and director of Secure Design services with structural engineer Walter P. Moore Engineers + Consultants, Houston. "During the past five to eight years, there's been thousands of experiments conducted for blast resistance, and both computational and mathematical models have been created that allow us to do a much better job of predicting a response."
The building is set back 50 feet from the perimeter, which is lined with concrete bollards to restrict vehicles from getting too close.
Even so, Norville says blast-resistant design has inherent limitations. "The crux of the problem is that once you design a system to a particular bomb size and standoff distance, someone can come along with a bigger bomb or get it a lot closer to the building, and the design can fail," he says.
Although the fundamental technology behind blast-resistant glazing has not changed much during the past decade — laminated glass is bonded to the window frame, which is anchored to the wall system — vendors like OldcastleArpal Protective Glazing, Arlington, Va., have tweeked it to improve performance. The high-end version of its Blast-Tec product, for instance, incorporates a cable-catch system that provides additional support to help retain the glass pane during a blast.
Joseph L. Smith, VP and director of security consulting services with Applied Research Associates Inc., Vicksburg, Miss., says more vendors offer completely engineered and tested windows systems that also incorporate light control and energy saving features.
"Early on after the Oklahoma City bombing, companies were coming out with individual components, like window film and laminated glass," says Smith, who's firm is involved in the testing of protective glazing systems. "Today, we're testing entire window system assemblies."
GSA protection levels for glazing response to glass
|Performance condition||Protection level||Hazard level||Description of window glazing response|
|1||Safe||None||Glass does not break. No visible damage to glazing or frame.|
|2||Very high||None||Glass cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.|
|3a||High||Very low||Glass cracks. Fragments enter space and land on floor no further than 3.3 feet from the window.|
|3b||High||Low||Glass cracks. Fragments enter space and land on floor no further than 10 feet from the window.|
|4||Medium||Medium||Glass cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than 10 feet from the window at a height no greater than 2 feet above the floor.|
|5||Low||High||Glass cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 10 feet from the window at a height greater than 2 feet above the floor.|