2:55 p.m., May 6, 2009----Jeremy Isenberg, senior principal with AECOM and a member of the National Academy of Engineering, delivered the fourth annual Arnold D. Kerr Lecture in Engineering Mechanics to a crowd of about 250 in the University of Delaware's Mitchell Hall on Tuesday.

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The lecture, “Protective Design: 30 Years of Evolution,” addressed how changes in the threats to world and national security have influenced the analysis and design of protective structures. These changes, including the end of the Cold War and the rise of terrorist threats, have required new thinking from engineers responsible for designing safe structures, according to Isenberg.

“Structural engineering is at a crossroads,” he said. “The major issue we're facing now is whether to replace prescriptive design with performance-based design.” Currently, there are no codes or standards to guide performance-based design, and its implementation is limited to a small number of practitioners.

Isenberg pointed out that during the Cold War era, the defense strategy employed by the U.S. was to trick its enemies by playing a kind of “shell game,” in which missiles were housed in protective structures called silos. “We had more silos than we did missiles,” Isenberg explained, “in the hope that the Soviet Union would choose the wrong ones as targets.”

“Because they were defense structures,” he continued, “we wanted to protect them from any kind of damage. We didn't worry about the mode of failure -- failure was failure.”

With the end of the Cold War and the beginning of what Isenberg called the “Homeland Security Era,” concerns have shifted to managing risk. “We started to care about how buildings fail,” Isenberg said, “and we began to focus on controlling damage when failure occurred.”

As an example, the Alfred P. Murrah Federal Building in Oklahoma City -- target of a bombing in 1995 -- experienced what is known as progressive collapse, where the damage is disproportionate to the “insult.” Developments in modeling, testing, and construction over the past 15 years have created the technological foundation for controlling this type of damage.

“We can't prevent damage altogether,” Isenberg said, “but success lies in restricting it to the area of the insult.” However, he pointed out, while the technology is now available to do this, a major challenge lies in bringing that technology into the arena of professional practice.

Also, the issues associated with protecting our infrastructure go beyond technology development and implementation to encompass questions of politics -- who decides what we will protect? -- and economics -- how will we pay for it? Managing security risk in the 21st century involves organizing resources, conducting risk assessments, and identifying and implementing mitigation measures.

With regard to risk assessment, some degree of quantification has been introduced through the development of an index that allows infrastructure elements to be scored on various measures. But Isenberg emphasized that such an index is useful only for comparative purposes.

“Risk is the mathematical product of occurrence, vulnerability, and importance,” he explained. “It's not possible to develop absolute risk scores for terrorist attacks the way we can for natural hazards like hurricanes and earthquakes because we can't predict their occurrence in the same way.”

However, the index does help in making decisions abut where to allocate resources. Isenberg used the Golden Gate Bridge as an example of an iconic structure that should be protected to the extent possible because it cannot be replaced; in other words, its importance would give it a high risk index.

Other bridges might have high scores because their location makes them especially vulnerable to attack or because they are traveled by large numbers of people, creating the potential for a large loss of human life. Out of the approximately 600,000 bridges in the U.S., use of the index would probably yield about 1,000 that should be considered for protection, Isenberg said.

“The biggest challenge,” he concluded, “is keeping the effort going. As time elapses, our memory of 9/11 and other episodes fades, and we lose the motivation to retrofit buildings for protection from future attacks.”

Isenberg, who is a licensed engineer in five states, received his undergraduate education in civil engineering at Stanford University and his Ph.D. in structural engineering at Cambridge University, where he was a Fulbright Scholar.

Sponsored by the UD Department of Civil and Environmental Engineering, the engineering mechanics lecture series was initiated in honor of Arnold Kerr, Professor Emeritus of Civil Engineering, upon his retirement in 2004.

Past lectures in the Kerr series have been delivered by Henry Petroski, author of more than a dozen popular books on engineering-related topics; Donald Billington, professor at Princeton University; and Charles Thornton, a world-renowned structural engineer who has made significant contributions to the investigation of major failures including the collapse of the World Trade Center twin towers.

Article by Diane Kukich
Photo by Doug Baker