Glass and the Hidden Terrorist Threat

By DENIS WRIGHT

The threat of terror—real or imagined—has focused the minds of facilities management professionals, building owners, architects, engineers, technologists, and planners to better design buildings that can withstand a new array of risks. It’s led to design teams taking a multidisciplinary approach to assessing hazards such as power failures, cyber attacks, civil disorder, and fire and explosive detonations, arriving at risk assessments that, hopefully, illuminate how those buildings should be designed and built.

Wrightstyle Limited supplies glass and glazing systems internationally to contain fire and provide safe evacuation routes. Requirements to guard against fire are integral elements in building regulations everywhere. However, neither guidelines on building safety nor a comprehensive understanding among architects on the options to make buildings safer during a terrorist attack are included in these regulations.

Designing for building safety starts with determining the possible risks against occupants, structure, resources, and continuity of operations. There are several assessment methods for understanding the potential threats, identifying the assets to be protected, and how best to mitigate those risks. These assessments guide the design team in determining acceptable risks and the cost-effectiveness of the measures proposed. However, there still is no accepted methodology for assessing or guarding against terrorist threats.

High explosives alone are not the main cause of death and injury; in urban areas, between 80 and 85 percent of all secondary blast injuries are caused by flying glass. When a bomb detonates, it produces gases at very high temperatures. This leads to a rapid expansion of air and the creation of a shock wave that travels at supersonic speeds. The shock wave lasts only a few milliseconds and is then followed by an equally sudden but longer lasting drop in pressure. It’s the enormous impact of the shock wave and the subsequent suction that shatter the glass and distort the framing.

The day after the Oklahoma City bombing, President Bill Clinton instructed the Department of Justice (DOJ) to see what conclusions could be drawn in terms of protecting federal buildings. One of the DOJ’s key missions was “to provide for [the] application of shatter-resistant material to protect personnel and citizens from the hazards of flying glass.” At Oklahoma, glass fragments were found six miles from the detonation. In New York City, 15,500 windows were damaged within a mile of Ground Zero—nearly 9,000 within half that distance.

However, in the wake of Oklahoma City, researchers from the Glass Research and Testing Laboratory at Texas Tech University found that damage to property and person could have been reduced if laminated glass (photo 1), at the very least, had been used in the buildings that surrounded the Alfred P. Murrah Federal Building. It’s a lesson being learned across the globe as architects struggle to balance form and function with the new requirement of additional security. After Oklahoma City, the U.S. State Department started to make windows smaller and less numerous in several embassy projects. However, no one wants to live or work in windowless environments, and architects don’t want to design buildings where form and function are severely imbalanced.

(1) Photo courtesy of Wrightstyle Limited.

Specialist glazing companies are now carrying out much research and development coupled with high-pressure, blast-resistant testing. The new systems that the industry is bringing to market offer enormous advantages over older systems as well as some peace of mind to the occupants of buildings where such systems are fitted. However, anyone specifying a glazing system to protect against explosions must ensure that both the glass and framing systems have been tested together. The two components form part of one assembly; it doesn’t matter how strong the glass is if it’s held in an inappropriate frame—or vice versa.

Ingrained attitudes toward blast resistance are changing. More building designers are specifying rigorous test accreditation. However, too many architects are using aluminum framing systems in large spans of curtain walling and are covering the interior surface with a plastic coating to hold the broken glass in place. This may be cheap, but it could also be deadly. Aluminum curtain walling is fine, but not for blast resistance. Simply, it’s not strong enough, and its structural integrity will be compromised.

Wrightstyle Limited’s steel system has been tested independently by detonating 500 kg of TNT-equivalent explosive adjacent to the glazing system; that’s the size of a truck bomb. We immediately followed that with a simulated car bomb attack on the same assembly (100 kg of TNT). The truck-style bomb was detonated 75 meters from the test rig. The car bomb was detonated from 20 meters, producing a higher loading on the façade. Both tests were equally successful.

DENIS WRIGHT is the founder and chairman of Wrightstyle Limited. He has been involved in the glazing systems industry for the past 35 years and has worked many years with aluminum systems, developing applications in steel through the technical advantages of steel for specialist applications for fire, blast, and ballistic protection. He continues to develop and design the company’s steel glazing systems.

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