AGG

Volume 21, Issue 2 - April/May/June 2007

Testing Against Terrorism
More Than One Option Can Work
By Steven Samuels

Blast testing has become a common discussion topic in government agencies and engineering and architectural firms. Today, two tests, full-scale arena and shock tube testing, are used commonly, and the science of computer modeling for blast resistance is emerging. 

Blast prevention of any type is inherently difficult. Terrorist activity requires human creativity and is unpredictable. It’s impossible to predict when or where someone may set off a bomb, how large it might be, and what target it might be aimed at. Just over a decade ago, buildings were designed with natural disasters in mind. In the years since the Oklahoma City bombing in 1995, the landscape of threats widened considerably. Despite such unpredictability, building designers must endeavor to protect those who are at perceived risk from the effects of explosions, both planned and accidental. 

The Blast Effect 
Much has been written about the effects of the blast wave following an explosion. When a high explosive charge detonates, it releases a tremendous amount of energy. The energy is expressed in the form of sound, heat, light and a shock, or blast, wave. The rapid release of energy from the explosion compresses the air. The compressed air then moves away from the explosion as a blast wave. A blast wave is characterized at distant points by an instantaneous rise in pressure, followed by a decay over a period of time called the positive phase. 

Immediately following the positive phase is a void of air. Like water in a lake on a windy day, the air rushes to equalize itself in this void. This “bouncing back” effect is the negative phase, which provides a secondary blast effect after the initial positive phase. While the negative, secondary effect is far less powerful than the initial blast, its duration is much longer.

Shock-Tube Testing 
As you can imagine, these blasts can be potent. So designing a blast-resistant building requires significant expense. Glass is one of hundreds of factors that must be considered. In searching for an economical means to test blast performance, many designers opt for the shock-tube testing method. During this type of test, the specimen is mounted at the end of a large tube. A pressurized pulse is generated at the opposite end of the tube and travels down it.

“Shock-tube testing can be quite valuable when you need to test one or two lites quickly,” says Ed Conrath, structural engineer for the U.S. Corps of Army Engineers. “A shock tube test will give you a ‘quick look and see’ as to whether a product will really perform,” says Ron McCann, a security specialist at Viracon, a glass fabricator headquartered in Owatonna, Minn. “Some manufacturers also like shock-tube testing because it gives a good read on how the glass behaves by itself. When we are not looking at total system performance, we’ll often use a shock-tube test.” 

There are some drawbacks to shock-tube testing, according to Scott Norville, professor and chairman of civil and environmental engineering at Texas Tech University. “The process only tests the positive phase of the blast wave,” he says. “The negative effect is typically not present in shock-tube tests, leaving a void in the modeling of the full blast. Secondary effects, because of their longer duration, may inflict additional damage to the glazing system.” 

Conrath agrees, citing instances he has experienced when more damage has been caused by the negative effect than the positive during live testing. 

Shock-tube testing also can be subject to testing inaccuracies. A thin diaphragm barrier is inserted into the shock tube and pressure is increased behind. The diaphragm is ruptured at a certain pressure and the pressure behind is released instantaneously, essentially replicating the blast wave. To be effective, the test relies on the diaphragm’s bursting completely and at exactly the right time. If there are any flaws in the diaphragm, the test is flawed. Additionally, lighter weight materials such as glass are more affected during the negative phase, thus there is a greater chance of inaccuracies when the negative phase is not present.

Conrath also believes shock-tube tests can produce tests results that predict worse performance than a product would provide in a real blast. “During the positive phase of the explosion, the decay is slower in a shock tube than in a live test, which can offset results,” he says. 

Computer Modeling
Computer modeling is another option to evaluate blast resistance of glass. Norville, however, believes the currently available software is not always accurate. “Often an actual specimen will differ significantly from the modeling, if any, in the software,” he says. “When the computer tries to interpolate between different glass sizes and constructions, there is no guarantee that the results will be accurate or verifiable.” 

Conrath is hopeful that computer modeling systems will improve greatly in the next 5 to 10 years. “We simply do not have enough data yet for computer modeling to be a true indicator,” he says.

Conrath contrasts the data on glass with that of concrete. “The United States has been collecting data on the performance of concrete since the 1950s and ‘60s. The Department of Defense was designing buildings to withstand nuclear loads and warfare. Windows weren’t even in the equation during those days,” he says. Because of the vast amount of data on the subject, Conrath says the Army Corps of Engineers is confident using computer modeling to measure the performance of concrete. 

Glass is a different matter. The human suffering from glass damage following the Oklahoma City bombing in 1995 suddenly put studying the blast performance of glass on the radar screen of the government and private industry. “Glass is still a relatively new material to be studied for blast performance,” Conrath says. “We don’t have nearly the data we need yet. We need to keep learning.” 

Full-Scale Arena Testing
Full-scale arena testing involves an actual explosion in front of glazed glass panels that can be of any size or shape. Witnessing an actual explosion and examining the glazed test panels provides real time data that is evidenced by actual events. 

“One test is worth a million calculations,” says Norville. “There is no other way to be sure that a large curtainwall is going to perform unless you conduct a live test.” Because an actual explosion produces both positive and negative phases, the test is deemed more reliable. 

If more than one specimen must be tested, full scale arena testing may actually be more economical than shock-tube testing. “You can test multiple lites, configurations and framing systems with one explosion,” says Norville.

Conrath also emphasizes the importance of the ability to test a complete system with an actual explosion, not just the glass, in certain instances. 

The ability to capture high-speed, high-quality video during an explosion is extremely important both from the standpoint of obtaining quality scientific data and extending peace of mind to end users. 

While no method of testing is completely perfect, it is significant that the industry is now taking the role of glass in explosions very seriously. “It’s impossible to harden every building against every potential event, but we can use the science we have to do the best job possible,” Conrath says. 

Because every building being built today has some level of risk, the industry must continue to evaluate and discover the most appropriate solutions possible in testing for blast performance. Part of the solution is to fully understand how specific systems will perform under various blast conditions. 

Whether it is a simulated computer test, a shock-tube test or a test by live explosion, the industry has the responsibility to use the best methods available to fully demonstrate a product’s performance. 

Steven Samuels is operations manager of HTL Texas, a Lubbock-based company that specializes in full-scale arena blast testing of architectural building components and assemblies. 



Architects' Guide to Glass & Metal
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