By Henry Burke
Buildings have long been designed to stand against and protect occupants from many natural elements: wind, rain, and the ravages of time. But in the years following the attacks of September 11, 2001, a new element has been added to the equation: blast protection.
Today, many public and government buildings need to be designed with the idea of withstanding the impact of an explosion. It is a sad reality, but one that engineers and architects must account for when designing structures that could become targets for attack.
Designing for blast resistance is far more complex than simply building thicker walls. A great deal of research has gone into finding the most effective techniques, building products, and applications to mitigate the impact of blasts and protect from progressive collapse.
“Especially since 9/11, there has been a great increase in the demand for blast resistance,” says Jack Fritz, P.E., M.ASCE, who is a structural engineer with CENTRIA. “While we had done a few blast-resistant designs early on, they were extremely conservative. We wanted to undertake additional research so we could be more precise and cost-effective with our designs.”
Without specific research into how building components respond to blast pressure, designers and engineers were playing it safe, which actually could result in overdesigning for the requirements of blast mitigation. And while single-skin profile panels had been tested in blast design, no real study had been done on how insulated metal panels, or IMPs, might respond in a blast situation. To learn more, CENTRIA teamed up with the protective structures team at Baker Engineering and Risk Consultants, Inc., in San Antonio, Texas. Together, they put IMPs to the ultimate test.
“We did this testing so we could learn what the response characteristics were for our Formawall® Dimension Series® panels, and along the way we actually developed a new methodology for determining the response characteristics of foam-filled composite panels,” Fritz says. “How the foam interacts with the skin is different in a blast situation than with wind loads. Since the duration of overpressure is relatively short, the panels may initially start deforming but they will survive appearing intact. We found that IMPs respond very well in a blast.”
When studying the expected performance of a panel in a blast, engineers approach testing from a dynamic loading standpoint. The key components of an explosion’s impact are blast load (or pressure) and duration. How much pressure does the blast apply to the building and for how much time? Understanding those pieces helps determine how much force is ultimately going to be applied to the panel.
The impact of a blast is extremely short but very powerful. “The pressure of a blast is significantly higher than a natural force like wind,” Fritz says. “When you’re designing a building for wind, you’re designing it to withstand maybe 40 pounds per square foot. Under a blast load, you may design for 10 pounds per square inch (psi). That translates statically to 1,444 pounds per square foot. It is a significant increase in pressures on the panel. What saves the panel and keeps it from collapsing is that the blast load is only applied for a set period of time.”
Duration is the area where blast testing differs from testing for something like wind. “When we test for wind loads, we look at what are called 3-second gusts,” Fritz says. “So if we say the wind is blowing at 110 miles per hour, we’re figuring it will get to that level for maybe 3 seconds. When we’re looking at blast loads, we’re talking about maybe 45 milliseconds.”
That may not sound like a long time, but because of the force at play in a blast, a lot can happen in those 45 milliseconds. The power of the blast impacts all the sides of the building directly in its path.
“Blast pressures actually work like fluid dynamics,” Fritz says. Imagine a blast like fluids flowing: When the blast hits the building, it goes up and around it, as well as through it. By the time that wave of force gets to the overpressure and gets around the building, it’s already gone from the face of the building, and now there is pressure on the back of the building and a surge on the back face. It is not like wind, which blows in one direction. The overpressure happens in every direction. If suddenly the air has 10 psi of pressure, it has that force in every direction, even if the blast wave is quickly moving in one direction. “That’s why as it envelops the building, it actually puts inward pressure on the back side as well as the side walls,” Fritz says.
In a wind storm, if you got behind a building, you wouldn’t feel much wind. That is not the case with a blast load. It might be 10 psi of pressure in the front and might dissipate to 8 psi on the back face, but you can still feel it everywhere.
To learn more about how IMPs would perform under these circumstances, testing had to be developed to simulate this kind of intense, encompassing pressure load. And in the case of IMPs, which are composite panels with layers of different materials, it was important to understand how those layers might function in a blast.
“It had never really been determined how well these parts would all work together in a blast, so to find out, we used the shock-tube method,” Fritz recalls. “Basically, we simulate the overpressure with a large amount of air pressure, and then we violently break a diaphragm so the air pressure comes racing out at the same rate that a blast would. We can record the duration of the overpressure and the amount of pressure and have a pretty good understanding of what kinds of pressures and durations the panels had experienced. And by recording the deflection of the panel itself, we also know what kind of response we got from the panel.”
How the panels respond is, of course, where the rubber meets the road. After this intense and exhaustive testing, IMPs proved to function even better than anticipated against this kind of powerful pressure. “We are much more comfortable installing IMPs at higher blast loads and at larger spans than we did previously,” Fritz says.
There are different levels of blast-resistant designs, which are set by a number of groups. “Blast loads are governed by whatever the specifying body is, and typically that is a government agency,” Fritz says. “The Veterans Administration has two levels, the Department of Defense has at least two levels, and the General Services Administration and Department of State all have their own levels, as well.”
Some of these levels have to do with the expected state of the building after a blast. The primary responsibility of any building component designed for antiterrorism protection is occupant safety. So, the first responsibility of blast-resistant design is to keep the people inside safe. Simply doing that is one level of blast protection.
“Under the Veterans Administration guidelines, the basic level of protection might be called a ‘life-safety protected facility,’” Fritz says. “In the event of an explosion, the intent would be to protect personnel and then get them out of the building so it can be rebuilt or repaired. A higher level of blast design is called ‘mission-critical.’ What that means is that the building not only has to take the blast, it also has to continue to function. This might be something like a hospital, because in the event of an attack or explosion, a hospital will need to operate to care for the injured. It may not have to look pretty, but it does have to keep the weather out and continue functioning.”
After examining the results of the blast tests, it was apparent that CENTRIA’s insulated metal panels could be utilized at many different levels of blast resistance. They held up well against the ravages of blast conditions and exceeded expectations. “IMPs can be applied to life-safety and mission-critical installations,” Fritz says. “On a mission-critical installation, following a blast, we would still suggest a schedule to have the panels examined and replaced, but this is something that could be redone over time.”
While materials like concrete may often come to mind for designers looking to design for blast resistance, this data shows that IMPs can be an excellent option on these kinds of projects.
For more information, review “Dynamic Analysis of Insulated Metal Panels for Blast Effects,” originally published in Structure magazine, at www.structuremag.org.
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