Structural engineer describes collapse of the World Trade Center towers

Stanford Report, December 3, 2001

BY MARK SHWARTZ

Vulnerabilities in the design of New York's World Trade Center (WTC) 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 told a packed auditorium on Nov. 29 when he delivered the second John A. Blume Distinguished Lecture -- an annual event sponsored by Stanford's Blume Earthquake Engineering Center.

This graphic shows buildings in the New York Financial District affected by the Sept. 11 terrorist attack. Gray buildings were destroyed, red are in danger of collapse, blue suffered major structural damage and others shown were damaged, but can be repaired. Graphic courtesy of: Ronald Hamburger/ABS

"What New York City experienced on Sept. 11 was very much like an earthquake," he told the Stanford audience. "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 one square mile."

As chief structural engineer and senior vice president of ABS Consulting Inc. in Oakland, Calif., Hamburger is a member of an engineering team commissioned by the Structural Engineers Institute of the American Society of Civil Engineers (ASCE) to assess the performance of the WTC and surrounding buildings in the aftermath of the terrorist attacks.

He pointed out that four buildings were immediately destroyed in the WTC 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 -- the equivalent of all the financial office space in the city of San Francisco," he noted.

Hijackers' plan

Using photographs of the WTC taken before, during and after the Sept. 11 assaults, Hamburger presented a brief chronology of events. He noted that 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 commented. "If there had been no nearby structures, they would have hit the towers lower."

One member of the audience asked Hamburger if he believed that the pilots intentionally banked the planes at an angle to take out as many floors as possible.

"Yes," he replied.

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 noted that 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 on Sept. 11, but there is little doubt that the collapse of the upper floors of the WTC towers brought down both structures.

"Think of the impact of dropping a 25-story building straight down," Hamburger told the audience. "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 WTC 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.

Hamburger also discussed the collapse of WTC Building 7, which housed the offices of Con Edison, the FBI and the CIA.

"WTC-7 was a 47-story building and became a two-story pile of rubble," Hamburger said, "making it the first major structure in the United States to collapse because of fire."

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."

He and his colleagues will attempt to answer those questions when the ASCE report is made public next spring.

Does it make sense to design buildings to withstand such events as Sept. 11 in the future?

"In my opinion, no," Hamburger concluded, although he does support the decision implemented after the 1995 Oklahoma City bombing to redesign federal buildings to withstand the accidental loss of major structural elements.

The Blume Lecture series honors John A. Blume, often called "the father of earthquake engineering," who earned his undergraduate and doctoral degrees at Stanford. In 1974 he helped underwrite the John A. Blume Earthquake Engineering Center, which promotes earthquake engineering research and education. Today at age 92, Blume remains a consulting professor in Stanford's Department of Civil and Environmental Engineering.

Ronald Hamburger
Photo courtesy of: ABS