Blast-Resistant Facilities: Even Terrorists Can’t Break The Laws of Physics
Sept. 11 changed everything, even the architecture that surrounds American’s daily lives. While the concept of creating blast-resistant structures is nothing new, the emphasis on its use has surged dramatically. Barricades, guards and new security protocols have suddenly surrounded sensitive structures that had originally only been scheduled for blast-resistant upgrades at some vague, unspecified time in the future. Many government agencies — from the Department of Defense to the Federal Aviation Administration — rapidly developed new standards and criteria to reduce the threat to facilities and occupants. By many measures, sensitive government facilities are safer and more prepared than they were on Sept. 11, but how safe?
“Everybody’s got to realize that you can never completely protect any type of facility against a determined, dedicated, committed terrorist. For every measure there’s a countermeasure, and it’s been that way throughout history,” says blast-resistance and force-protection expert James Weeks, PE, a senior associate and structural engineer with architecture and engineering firm DMJM. “However, the physics of explosions has not changed. And we can design against explosive effects. Committed terrorists are serious foes, but we can make their targets much less attractive to them. To do that, you’ve got to look at the physics of blast loads. A blast shock load — a terrorist’s bomb, for example — is essentially a high-pressure front that moves out radially and decays very quickly. It’s a load of very short duration. So everything centers on charge-weight and standoff distance. This isn’t designing for conventional loads. We typically design these structures to withstand one-time-event loads by allowing large permanent deformations and structural yielding short of collapse. The design intent is to prevent catastrophic failure or collapse and allow people time to get out. It saves lives.”
Saving lives is what blast resistance is all about. Convention dictates designing a wall or a panel or a column to withstand a wind load, for example, within what is called the elastic region. The elastic region is the range within which the wall, panel or column will withstand a load and then return to its original shape. There is no permanent structural damage. But as Weeks points out, blast-resistant design must actually prepare a structure to exceed the elastic limit — essentially it must be allowed to lose its structural integrity and fail before a structure or a structural element collapses. However, blast-resistant design actually begins at a considerable and measured distance from a structure.
At A Standoff Distance
While terrorists may employ a myriad of tactics, they often choose explosive devices to target large numbers of people. Whether the device of choice is a shoe bomb or a pipe bomb or a large vehicular bomb, terrorists rely on proximity. To be effective, they must be close. So, charge weight (the size of the explosive) and standoff distance (how far the explosive is from its target) can often determine the success or failure of a terrorist attempt. In the terrorist bombing of the Murrah Federal Building in Oklahoma City, for example, the equivalent of approximately 4,000 pounds of TNT was delivered to within about 10 feet of the front of the building. The results, as we know, were devastating.
Today, concrete barricades surround significant government facilities and structures around the world. According to Weeks, enforcing standoff distance is often the first choice for blast protection. “You have to start any blast-resistance process by determining the threat a particular facility faces, and whether or not it needs these protective measures,” he says. Creating standoff distance is usually the best first option. It is usually economically efficient to increase the standoff distance and keep the threat away from a facility in the first place. That’s certainly much less expensive than trying to harden a facility to withstand an attack. And cost is undeniably a factor in creating protection.
“Remember, after Sept. 11, every airport terminal in the country cordoned off its parking areas so that a vehicle could not be parked within 300 feet of the terminal (as per TSA requirements). That was a quick, simple way to make sure that a terminal could withstand a potential blast load, by just moving that potential blast load further away. Unfortunately, it’s not always that simple.”
Example: Southwest Florida Airport
In some instances, there simply isn’t enough room to create necessary standoff distance. That said, other blast-resistant technologies and methods must provide the necessary safety margin. DMJM’s blast analysis and design efforts for the Southwest Florida International Airport in Fort Myers, Fla., serves as a good example. With construction slated to start on a $437 million terminal around Sept. 11, 2001, the airport had originally planned to build a parking garage 225 feet from the new terminal. While that proximity was troublesome in the long term, it wasn’t the airport’s immediate worry. Their existing terminal was less than 300 feet from a short-term parking facility.
“We have a very large ‘meeter and greeter ratio’ in our customer base, one of the largest in the country,” explains Bob Ball, Southwest Florida International Airport director. “So short-term parking is very important to us. We were faced with two major questions: In the short term, how do we get our facility up to speed in terms of safety? And over the long term, how do we adjust our new facility’s design so that we don’t end up building a new terminal that has to be renovated the day it opens?
“Our first step was to initiate a blast analysis and mitigation study for both situations. And that proved critical. We discovered that given the blast-resistant measures we already had in place — supplemented with certain alternative security implementation procedures — we could meet TSA requirements and re-open our short-term parking facility. For our new facility, computer models determined that we needed to replace the entire steel structure of the roof. We embarked on a redesign of the roof structure that added about $1.5 million to the project. With our designer’s help, we presented the plan to the TSA and received approval. That was crucial. Without that approval, we would have had to move everything 75 feet — a frighteningly expensive proposition. The redesign saved us millions of dollars.”
The Pentagon Model
Another case in point is the Pentagon. When it was attacked on Sept. 11, the Pentagon was undergoing its first major, large-scale renovation ever. And part of that renovation process was the implementation of blast-resistant design and materials. By sheer chance, the airliner that crashed into the Pentagon hit the one wedge of the Pentagon that had been renovated, before continuing to slice into a second wedge.
A recently released study, “The Pentagon Building Performance Report” published by the American Society of Civil Engineers, credits the building’s original design and construction as the major factors accounting for the minimal loss of life: “Despite the extensive column damage on the first floor, the collapse of the floors above was extremely limited.” However, the report also shows that blast-resistant improvements played a considerable role in limiting damaging and aiding survivability.
“The superior performance of the improved window system incorporated during the renovation is evident,” the report says. “The structural upgrades of the exterior wall performed reasonably well, considering that they were not specifically designed for aircraft impact. The only window frames removed by the impact were those struck directly by the wings or the fuselage. On the second floor, immediately adjacent to where the fuselage entered the building, upgraded windows remained in their frames even though the surrounding masonry façade was completely removed. Upgraded glass was generally not broken immediately after the impact or after the ensuing fire had been extinguished. By contrast, most of the original windows in a vast area of Wedge Two [the non-renovated wedge] were broken.”
Essentially, while the original design and construction features of the Pentagon served as the major factors preventing the loss of life, blast-resistant modifications helped save lives by reducing shards and debris and preventing the spread of fire. Pentagon renovation manager Lee Evey believes that the blast-resistant modifications made a critical difference.
“That fire went nowhere in Wedge One [the renovated wedge]. I did get a heck of a lot of water damage in Wedge One as a result of that, but the fire went nowhere,” he says. “In Wedge Two, the fire just took off. The heat of the fire was so intense that it damaged the concrete, and it damaged it further than we had initially thought it had. In some areas, the fire was intense enough that the windows had actually melted.
“The [blast resistant windows] cost us about $10,000 apiece when you put all the stuff together: the steel, the Kevlar, the windows themselves, and so forth,” Evey continues. “When the time came for these windows to account for themselves — in less than a second — they worked extraordinarily well. We strongly believe they helped reduce the injuries and loss of life in the building. I know this: 2,600 people were in the immediate area when the plane hit, and we had 125 casualties. It is certainly unfortunate that we had those casualties, but the building did a remarkable job of protecting people.”
Evey is not alone in his assessment. “The renovation involves several elements that undoubtedly saved many lives,” adds Lester M. Hunkele III, PE, M.ASCE. Hunkele is a senior vice president with DMJM, and the program manager of the DMJM — 3D/I joint venture supporting the Pentagon Renovation Program Office. “First, the walls have been structurally reinforced. This reinforcement stopped the plane from penetrating further. And the strengthening of the walls enabled the internal structure of the penetrated rings to remain standing for a while. In an explosion, shards of flying glass can be deadly. The blast-resistant windows on the exterior ring minimized glass-related injuries.
“Three other elements of the renovation helped as well. The fire that followed the crash was held in check both by the new sprinkler system and by accordion fire doors that slammed shut immediately after the crash. These doors, along with fire dampers in the ducts, stopped smoke from circulating throughout the building. All told, there were five different features of the renovation that helped to protect those who were in the immediate vicinity of the crash. The renovation bought them additional time to escape the building.”
Blast-resistance technology saves lives. But it is not a panacea. And it is impractical and impossible to design and construct every facility to withstand every possible terrorist attack. But these methods and materials do make a significant difference.
“These types of designs and materials do not necessarily save money. But they do save lives,” Weeks explains. “And there is an expected standard of care in our society. People expect a certain level of protection. What it comes down to is this: We have to do the right thing to protect our people.”
This article was contributed by DMJM H+N (www.dmjm.com), a 75-year-old global architecture, engineering, and operations firm.
