Terrorist Targets Need Structural Security

Eve Hinman, PE, is president of San Francisco-based Hinman Consulting Engineers, which she founded in 1997. In 1983, after earning BS/MS degrees in civil engineering and a PhD in engineering mechanics from Columbia, she became one of the first structural engineers to focus on terrorist bombings and helped establish Weidlinger Associates' anti-terrorist design practice. She is the author of Lessons from the Oklahoma City Bombing: Defensive Design Techniques and principal author of a FEMA primer on designing commercial buildings to resist attack.

BD+C: How do you view a building for its security risk?

Eve Hinman : We evaluate the risk of attack on the basis of target attractiveness, asset value, and vulnerability.

Target attractiveness is the big-picture issue?the symbolic value of the building, its visibility, and its historic significance. Security professionals will tell you that if a building has been attacked once, there's a high probability that it will be attacked again.

For asset value, we look at how many people are in the building. If that building were to cease to function, how would that affect the situation locally and nationally?

Vulnerability deals with the building system itself: the perimeter, the landscape architecture, the exterior envelope, the interior layout. How close can a bomb get to that building? How deep could a hand-carried weapon get into the building? What about high-speed vehicles?

The distance between the building and public roadways is a key factor, because a blast decays very quickly with distance. Façade materials can be localized, anti-shatter film and wall systems can be used.

BD+C: What should building owners be doing about terrorism?

EH : Most buildings are not terrorist targets, but they may be near a building that is a potential target. Also, terrorist attacks are very rare, and buildings have to function on a daily basis, so they must be comfortable, pleasant places for people to do their business. Owners need to balance those issues.

Reasonably cost-effective measures?such as visual deterrents, a newspaper stand, attractive planters, a combination of things?can be taken to prevent truck bombings. Where we have enough setback, we'll use staircases, statues, fountains, etc., to make it appear that you can't get at the building.

BD+C: What else should we be looking for?

EH : The most devastating exposure to a building is progressive collapse. So if there is something we can do to prevent the building from totally collapsing, that's important.

When you go to a lower level of casualties, glass lacerations are very common. They don't cause as many fatalities, but they are a drain on the medical system.

BD+C: Can you retrofit high-rises to prevent progressive collapse?

EH : It's difficult and very expensive, and causes a lot of of interruption. Our approach is to do a balanced design. We look at the sequence of failure that occurs. The skeletal structure of the building is crucial, and it may be very difficult to retrofit. We want to make sure that the exterior wall is weaker than the skeletal structure, so that it doesn't put pressure on the skeleton. We've been able to sell that concept to a lot of clients who don't have a lot of money and don't want to disturb their tenants.

BD+C: What other measures should Building Teams be thinking about?

EH : Evacuation plans. They need to think about how they're going to get people out of the building?multiple ways, not just the main lobby or loading docks, so that if one part of the building becomes damaged, people can still get out. And you want those routes to be continuous. The World Trade Center routes were not continuous. Current codes have been changed to make evacuation routes continuous.

BD+C: What's the biggest security-design mistake you see?

EH : Often architects will put special spaces above loading docks because of the longer spans that are needed to accommodate trucks. Sometimes it's a VIP's office, sometimes it's an auditorium, and sometimes?and this really gets me?it's a child care center. This is possibly the worst place to put such spaces because they have a higher risk of being attacked and are structurally more apt to collapse. I see this quite a lot in new design.

BD+C: Any last bit of advice for our readers?

EH : There are a lot of easy, cost-effective things the building community can do to improve security that don't have to be ugly. Designers need to look at the layers of security, starting with the perimeter and working their way in. Keep vehicles and weapons from getting to the building and into it?that's your most cost-effective way to protect the building. The primary thing is to prevent progressive collapse.

The last thing I want to do as a structural engineer is to make the building clunky.

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