Protecting Bridges from Terrorist Attacks

Eric Williamson was at the dentist having his teeth cleaned when the first plane hit the World Trade Center.

Initial reports on the morning of Sept. 11 made it sound like a tragic, but unintentional event: a commuter plane had accidentally crashed into one of the twin towers. But by the time Williamson left the dentist for his office at The University of Texas at Austin's J.J. Pickle Research Campus, another plane had hit.

“It was clear something more than just an accident was taking place,” said Williamson, an associate professor in structural engineering at the Cockrell School of Engineering.

Williamson had chosen his field of study based on his own childhood fascination with how structures respond to impact and blasts. But now he — along with others around the world — watched in horror as the response played out in the worst possible way, and the towers collapsed. In an instant, the world was transformed, and so with it was the trajectory of Williamson's research.

The Sept. 11 attacks on the Pentagon and World Trade Center dramatically illustrated the catastrophic damage that terrorists can inflict on civil structures. Because of this, engineers like Williamson have worked in the decade since then to design structures and transportation systems that are more resistant to attacks.

Over the centuries, one of the most common targets for terrorists has been transportation infrastructure, like roads, bridges and trains – a reality that was most recently underscored by the 2004 and 2005 bombings in Madrid and London.

Through research funded by the National Cooperative Highway Research Program (NCHRP), Williamson and Cockrell School Professor Oguzhan Bayrak turned their attention to improving the design of bridges to make them harder to attack, and more resistant to collapsing during an explosion or bomb blast.

The two researchers developed the first-ever national guidelines for building and retrofitting bridges to better withstand an attack. The guidelines are now available to engineers around the country and serve as a crucial but easy resource for designing and building safer bridges.

Williamson is also designing computer software for the Department of Homeland Security, which will allow engineers to program the schematics of their bridge design and determine how factors, like structural safety and costs, are affected when certain variables are changed in the design.

“There is lots of general information available to the public on how blasts can affect structures, but as far as specific guidance that a practicing engineer can use to improve a bridge’s resistance to an explosion or blast,” Williamson said, “this is the only thing out there.”

Getting in the mind of a terrorist

Part of Williamson’s research required him to think like a terrorist and, at times, make decisions based more on psychology than on hard engineering principles. This meant asking questions like, “Why were previous targets chosen and how catastrophic was the outcome?”

“Any time you’re trying to put your head into the mind of the attackers, you’re speculating as to what they’d do,” Williamson said. “You want to think about how someone might approach the target. What’s the terrain, how difficult is it to access, and is the bridge the target itself or is there an iconic type building or gathering place nearby?”

Transportation infrastructure is typically an attractive and easy target for terrorists, Williamson said, because it's easily accessible to the public, has minimal security, and provides the opportunity for a high number of casualties and injuries. The attacks on transportation infrastructure, 60 percent of which involved explosives between 1920 and 2000, can also have a profound economic impact.

Mitigating damage through improved guidelines

Shortly after Sept. 11, Williamson led one of the first research projects on bridge security in the nation. The study, funded by the Texas Department of Transportation and seven other state transportation agencies, identified several major areas of concern in transportation infrastructure. Using analytical models, the study also investigated cost-effective and unobtrusive design and retrofit options for a variety of bridges.

Based on the success of the research findings, Williamson and Bayrak were tapped by the federal NCHRP to evaluate the effectiveness of blast-resistant guidelines, which were intended for buildings but used at the time on U.S. bridges. The goal of their research was to develop a set of guidelines specifically for bridges.

The researchers collaborated with the U.S. Army Corps of Engineers and private consultants specializing in bridge design and structural response to blast loads.

Using computer simulations and blast tests on 16 half-scale bridge columns, the researchers were able to see how these critical bridge components respond to various explosion scenarios.

Among the major findings is that the shape of a bridge column can play a large role in how well a bridge withstands blast pressure. In the experiments, circular columns decreased the blast pressure on a bridge by up to 34 percent compared to a square or rectangular column of the same size.

The dimensions of bridge columns also had a major impact on the column’s capacity to withstand a close blast. Consequently, the researchers recommended a minimum bridge column diameter of 30 inches in their design guidelines.

Williamson said the researchers made guideline recommendations with the medical ethics principle of “do no harm” in mind. The recommendations also had to be economical and easy to apply to a wide range of bridge designs.

In states prone to earthquakes, like California, the impact of the guidelines is not as significant because bridges there are already designed and built to withstand tremors from an earthquake, Williamson said. But in states like Texas, where there is little or no seismic activity, bridge construction now will require improved column detailing. In these states, the new guidelines can make a significant difference in improving bridge safety.

“The general public is probably not aware of the risks associated with bridges and other transportation infrastructure,” Williamson said. “But the good news is that many people are taking a close look at how to improve their safety.”


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