Structural engineer Jeremy Isenberg has a vivid appreciation for how the 9/11 attacks have affected his profession: He was in Manhattan on the day two jetliners exploded into the Twin Towers of the World Trade Center.

“I was riding the subway into work when they stopped the train and unloaded us,” recalls Isenberg, who graduated from Stanford with a degree in civil engineering in 1962 and is now a senior principal with the AECOM engineering consultancy firm. “I came up above ground and saw smoke billowing up but I didn’t know what had happened. It was a 20-minute walk to my office, and during thattime the second plane struck.”

Ten years after the attacks that claimed more than 2,700 lives, Isenberg and other Stanford engineers are at the center of ongoing efforts to learn from the disaster and make it less likely that another large-scale terrorist attack could ever exact a similar toll in life and property.

“There has never been anything on the order of what happened to the Trade Center, and it focused attention on the potential vulnerabilities of tall buildings,” says Gregory Deierlein, the John A. Blume Professor of Engineering and Director of the Blume Earthquake Engineering Center at Stanford.

Possible changes to building design

Engineers approach terror studies with a sense of humility because the threat is so nebulous and the defensive challenge is as vast as the inventory of high-rises that dominate urban landscapes.

“It’s not like we can design buildings to resist airplane impacts,” Deierlein says.

Instead, he says, practitioners have sought to deter lower-intensity threats through simple design fixes, such as keeping trucks away from buildings, a lesson learned in the 1993 World Trade Center bombing and again in the 1995 Oklahoma City blast. Damage-resistant materials, such as glass panels laminated with layers of plastic to reduce shattering have come into greater use.

More complex initiatives under consideration envision creating shelters inside high-rises where occupants could wait out fires or providing multiple exits so people don’t get trapped as tragically occurred on 9/11.

Attacks spawn threat assessment specialty

Stephanie King, a former Blume Center Associate Director who earned her structural engineering Ph.D. from Stanford in 1994, says the Trade Center attacks redirected her career. She left campus in late 2000 to join Weidlinger Associates, expecting to work in the realm of earthquakes and other natural hazards.

“After 9/11 a few of our clients started asking us about terrorism risk,” King says. “Within the first few months I got involved in security assessment, and since then I’ve spent most of my time on that.”

Most of King’s clients manage large infrastructure facilities and tend to be based on the East Coast where public concerns about terrorism are most pronounced. What clients want to know, she says, is where they are most vulnerable and how to cost-effectively reduce the risk. Engineers have experience answering such questions with regard to earthquakes and hurricanes because they can cite U.S. Geological Survey and National Weather Service data when they assess the probability of occurrences and make damages estimates.

“But an intentional hazard is so different,” King says. “How do you come up with a way to estimate which types of buildings and structures are at greatest risk?”

Security controls tightened on building plans

King, who has sat on expert committees that considered building codes changes in the wake of 9/11, says that uncertainty has impeded efforts to change governing documents like the International Building Code. “We spent a long time drafting code language but we couldn’t come to a consensus on what types of buildings should be included,” she says.

One post-9/11 change affecting design professionals involves tighter controls on building plans and sites for large or noteworthy structures.

The concern is that terrorists will use reverse-engineering techniques to figure out how to bring buildings down. “Most (building) documents have to be stamped with a control number and secured,” King says “I can’t take a tour of the construction site without setting it up in advance and having a need to know.”

Computer models aim to contain damage, prevent ‘progressive collapse’

Meanwhile, on the research level, the attacks have spurred computer models to show how engineers might design buildings so that intentional damage to key structural elements does not cause a cascade effect known as progressive collapse.

Isenberg, who studied the progressive collapse of the Twin Towers as an expert in an insurance dispute, says computer models of structural responses to blast effects were first used to design missile silos that could survive nuclear attacks. These models were soon adapted to study earthquake effects.

Beginning in the late 1980s and accelerating after the Oklahoma City bombing, researchers and software developers used structural modeling to improve building safety in the face of deliberate attempts to cause damage or destruction. This technology was helpful in understanding the collapses on 9/11 and further advances could help prevent recurrences.

“The technology has matured over the last 15 years to the point where we can have some confidence of being able to simulate attacks and the damage that might ensue,” Isenberg says. A simple modeling problem might involve three columns supporting a floor beam. If the middle column were knocked out by a blast, could the beam be kept in place, even if it sagged, by stronger connections with the two surviving columns?

“The idea is to restrict the effects of damage to where the damage occurs,” Isenberg says.

Chris Poland, who earned his MS in civil engineering in 1974 and is now chairman of San Francisco-based Degenkolb Engineers, helped pioneer the application of computer-based techniques to the field of seismic upgrades. In the mid-1990s, he oversaw the earthquake retrofit of Stanford’s Ruth Wattis Mitchell Earth Sciences Building, using these new analytical tools to show how to retrofit the building using only exterior concrete walls at the corners. Traditional techniques would have called for more expensive and intrusive interior fixes.

“Instead of a $7 million seismic upgrade that would have involved a shutdown, we did a $700,000 retrofit and used the building the whole time,” he says.

Studies inspired by attacks have seismic implications

Cross-pollination between seismic and terror studies continues. Deierlein says advances in computer modeling inspired by 9/11 could suggest ways to make high rises more earthquake-resistant. High-rises are already designed to remain standing after earthquakes, but that does not necessarily mean they could be reoccupied right after a seismic event.

Deierlein says this becomes of greater concern as high-rise apartments spring up in San Francisco and other urban areas. After a quake, will tall dwellings suffer damage to exterior surfaces, mechanical systems or structural elements that might render them unfit to occupy? “Will an earthquake displace a lot of people?” he asks.

Engineers can offer only tentative answers to such questions but the computer modeling work spurred by 9/11 helps create the basis for reliable risk assessment and cost-effective mitigation to improve safety and survivability in the face of any disaster. Given the expenses involved in adding extra layers of safety, however, and the vast inventory of structures at potential risk, changes percolate into practice very slowly.

“What we can do for now is increase our knowledge, explore design solutions, and raise the visibility of these issues,” Deierlein says.

Gregory Deierlein, professor of civil and environmental engineering at Stanford University