THE NEW SKYSCRAPER GOING UP ON THE WORLD TRADE CENTER SITE IS EMPLOYING ALL SORTS OF ENGINEERING INNOVATIONS TO THWART TERRORISTS.

Standing on the 30th floor of Seven World Trade Center is both exhilarating and terrifying. If you can push aside feelings of vertigo and haunting memories of the tragedy that felled the original building on Sept. 11, 2001, the view of New York harbor is stunning, and the rebuilding is an impressive monument to the resilience of the human spirit. Seven was the last building of the old WTC complex to fall down and the first of the new complex to go up. A year from now, when tenants start moving in, they will enjoy a bird's eye view of the construction of the Freedom Tower. Silverstein Properties, the developer that owns the lease on both building sites, is employing state-of-the-art engineering techniques to protect the properties from potential terrorist attacks and natural disasters. The buildings are influencing the revamping of municipal building codes and the design and construction of tall buildings throughout the world.

"Why did the towers go down?" asks Carl Galioto, the partner in charge of the technical group at Skidmore, Owings & Merrill, the architecture firm that designed Seven, the Freedom Tower, and the Sears Tower in Chicago (the country's tallest building). "There are a number of theories. Some say it was the perimeter columns, some say the core columns, and some say the fireproofing. We've learned several lessons just by thinking about them."

Some of the lessons have already been applied to the construction of Seven. On its ground floor is the Consolidated Edison substation that powers lower Manhattan. The building is still only half finished, and parts of the 30th floor are open to the elements with only a pair of cables cordoning them off. After the view, the first thing visitors notice is the building's reassuringly solid core. Two feet thick, the core is made of reinforced concrete, which is "much more impact-resistant than steel with dry wall," Galioto says. Within this protected core are the exit stairs, the electrical closets, and the elevators.

Designed with evacuation in mind, the stairs are 20 percent wider than required by the building code, allowing two people to pass side by side or go down them together. The landings are large enough for someone in a wheelchair to wait for assistance without impeding other traffic. The stairs ultimately exit at four different locations directly onto the street, not into the building lobby. Other safety measures include stair pressurization, which prevents smoke from entering and remaining in stairwells (and is required by city codes throughout the world but not in New York), and enhanced lighting for the stairs and photo-luminescent paint on hand rails and markers in case power goes out.

The sprinkler and fire alarm systems also include redundancies, explains Scott Frank, associate partner of Jaros, Baum & Bolles, the mechanical and electrical consulting engineering firm. There are two standpipes to deliver water to the sprinklers; most high-rises have only one. Similarly, the fire alarm system uses two paths of communication. The air filtration systems have also been designed to thwart would-be terrorists who may use chemical, biological, or nuclear agents. All outside air comes from the top of the building, which makes the air supply less accessible to anyone wishing to do harm and gives airborne contaminants more opportunity to dissipate before reaching the air supply.

No building will ever be completely terrorist-proof, but when they're finished, Seven World Trade Center and the Freedom Tower may well be the most terrorist-resistant buildings on Earth.

Mary Kathleen Flynn has covered technology for more than 15 years for a variety of media outlets, including Newsweek, the New York Times, U.S. News & World Report, CNN and MSNBC.