STANFORD, Calif. – Vulnerabilities in the design of New York’s World Trade Center are likely to have contributed to the collapse of its two main towers and adjacent buildings, according to Ronald O. Hamburger, a structural engineer currently investigating the Sept. 11 disaster.
“These buildings were incredibly strong, especially with respect to resisting dead loads and wind loads, but they also had a number of vulnerabilities,” Hamburger said.
“What New York City experienced on Sept. 11 was very much like an earthquake,” he said. “Life loss exceeded anything we in the United States have experienced in an earthquake, and the financial loss exceeded anything we’ve experienced – and it all occurred within 1 square mile.”
Hamburger is a member of an engineering team commissioned by the Structural Engineers Institute of the American Society of Civil Engineers to assess the performance of the World Trade Center and surrounding buildings in the aftermath of the terrorist attacks.
Extended damage. Four buildings were immediately destroyed in the assault, and three others suffered irreparable damage and are in the process of being razed. Another half-dozen buildings were harmed structurally but can be repaired, and more than 50 others were damaged by the enormous debris cloud and the burning material that followed the collapse of the twin towers.
“Thirty million square feet of commercial space were affected, including 10 million square feet that were taken out permanently,” he said.
Hijackers’ plan. Using photographs of the World Trade Center taken before, during and after the Sept. 11 assaults, Hamburger noted the first jetliner banked into the north tower at a 45-degree angle, damaging floors 92 to 95. About 40 minutes later, the second jet crashed into the south tower, hitting floors 78 to 84.
“I believe that the hijackers flew the aircraft into the lowest part of the buildings they had access to,” Hamburger said. “If there had been no nearby structures, they would have hit the towers lower.
He believes the pilots intentionally banked the planes at an angle to take out as many floors as possible.
Fire weakened steel. According to Hamburger’s preliminary analysis, the impact of the jetliners shattered and fractured two-thirds of the support columns on one face of each tower, causing the partial collapse of several floors. Debris penetrated each building’s core and may have damaged the core columns located in the center of the 110-story structures.
“The damaged columns held up the weight of the building, so logic would dictate that the building would fall,” said Hamburger, “but that didn’t happen. Because of its great structural redundancy, the load was distributed to other parts of the building.
“We have reason to believe that, without the fire, the buildings could have stood indefinitely and been repaired. But we did have a fire.”
Born of fire. Hamburger said the fuel in both jetliners burned off rapidly, despite media reports that the aircraft continued burning long after the crash.
“The impact probably caused a failure of the fireproofing in the affected areas,” he said. “We think that the fuel ignited several floors in the building,” he added, which had a devastating effect on the steel support beams.
“Steel is born of fire,” Hamburger explained. “As it’s reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength.”
He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle.
Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.
Hamburger and his colleagues have not yet determined which of these scenarios occurred Sept. 11, but there is little doubt that the collapse of the upper floors of the towers brought down both structures.
“Think of the impact of dropping a 25-story building straight down,” Hamburger said. “It was like a pile driver, which is why it collapsed as it did.”
Vulnerabilities. While acknowledging the many innovations that went into the design of the towers in the 1960s – including one of the earliest applications of computer stress analysis – Hamburger also cited several features that made the buildings vulnerable to the intense fires that ultimately caused their collapse.
“The floor trusses [joists] were relatively flimsy. As the tower collapsed, the trusses just fell apart,” he observed, noting that trusses are difficult to fireproof.
Hamburger noted that each tower was constructed using a novel tube frame system designed to resist winds of up to 80 miles per hour. But the connections of the tube frame were weak, causing them to break apart and become three-pronged missiles that crashed into the street and into nearby buildings.
Future designs. He pointed out that fires frequently occur in high-rise buildings and noted that between 1994 and 1998, 30 fires occurred in the United States in buildings that were 50 stories or taller.
“The question is, should fire protection standards be changed in some significant way in the aftermath of Sept. 11,” Hamburger asked, “and should structural engineering designs include consideration of fire load and the response of structures? Right now, structural engineers know very little about fire.”
