As horrifying as the collapse of the World Trade Center has been, with more than 6,000 dead or missing, some structural design experts say it could have been worse. Had the buildings fallen over, they say, the damage to nearby structures would have been far more extensive and the death toll much higher.

Steve Ruotolo was a construction superintendent at the World Trade Center for 25 years, until his retirement in 1999. When he saw the 110 story towers fall on September 11, the impact was deeply personal. "To me it was particularly tough because I probably built [330,000 square meters] within the confines of the building," Mr. Ruotolo said. "So it was tough seeing all that hard work and effort destroyed so quickly and so unfortunately."

Mr. Ruotolo said the Trade Center towers were a marvel of engineering: strong and sturdy enough to have withstood the 1993 terrorist bombing attempt, but also flexible enough to sway. He said the buildings collapsed, but did not fall over, because of their unique design. "The way the Trade Center was built, and it was built to collapse the way it did, it was built on a truss system," he explained. "All the steel from the exterior of the building was smaller members all tied together into the central spine, almost like an exo-skeleton. If you saw the plane as it penetrated the exterior of the facade, it literally cut it in half because there was no major steel components within the structure to stop the plane."

Mr. Ruotolo says the burning jet fuel then warped the supports so that the top floors fell straight down and the entire structure collapsed downward. Had it tipped over, he adds, the tall towers would have crushed nearby buildings and sent hazardous debris flying for many blocks around.

Mr. Ruotolo says most other tall buildings in New York have interior steel structures that would allow them to take the impact of an airplane and the fire from the jet fuel, without collapsing. He says the Trade Center was a marvel of engineering, but it also had its vulnerabilities, a point that was made tragically obvious after the planes struck.

Photo by VOA's Greg Flakus.