Engineers Reconstruct WTC's Final Moments

Engineers Reconstruct WTC's Final Moments
By Jay Wrolstad

Some analysts maintain that constructing buildings to survive the type of assaults that occurred on September 11th is not a realistic goal.
Almost four months after terrorist attacks leveled the World Trade Center towers in Manhattan, work crews continue to sift through the tons of rubble left behind. And engineers continue to conduct forensic studies in an effort to determine how and why the twin towers collapsed after hijacked commercial airliners slammed into the 110-story buildings.

While the experts caution that it is premature to draw definitive conclusions about the causes of the buildings' destruction, most agree on some basic contributing factors.

"The bottom line, in my opinion, is that intense heat from the jet fuel fires melted the steel infrastructure, which went past its yield strength and led to the collapse of the buildings," Henry M. Koffman, director of the Construction Engineering and Management Program at the University of Southern California, told NewsFactor.

"These were evil geniuses. They had planned this attack carefully and knew how to inflict tremendous damage," Koffman said.

He suggested that stronger fire-protection standards for commercial buildings could result from an engineering analysis of the towers' collapse, pointing out that earthquake-prone California has adopted more stringent regulations that could serve as a model for national construction standards.

Towering Infernos

Although the impact of the plane crashes destroyed many perimeter support columns on several floors of the steel-frame buildings, severely weakening the entire structural system, many experts agree that was not enough to cause a collapse. The primary factor, by nearly all accounts, was extreme heat -- 1,000 to 3,000 degrees Fahrenheit -- from jet fuel fires ignited when the planes exploded on impact.

According to estimates, the planes -- which struck the buildings shortly after takeoff -- may have carried 18,000 gallons, or 500 tons, of jet fuel.

The heat generated by the fuel fires resulted in a weakening of the steel infrastructure. Combined with the impact damage, that weakening may have caused a failure of the truss system supporting a floor, or remaining perimeter columns, or even the internal core -- or possibly some combination of those factors.

With the flooring system gone, the perimeter columns would buckle outward, causing the collapse of at least one complete story of the building at the point of impact.

According to this scenario, when one story collapsed, all of the floors above would fall. The tremendous weight of falling structure would pick up speed, crushing the floors below and resulting in the complete collapse of the building.

Structural Scrutiny

"We are still in the process of collecting data, and I cannot offer any definite answers at this point, but heat from the jet fuel fires is one conclusion being offered," structural engineer Ronald Hamburger told NewsFactor.

Hamburger is a member of a building performance study team assembled by the American Society of Civil Engineers (ASCE) to assess the collapses at the World Trade Center, as well as chief structural engineer and senior vice president of ABS Consulting in Oakland, California.

Hamburger also said that vulnerabilities in the composition of the towers -- notably, weak floor trusses and weak connections in the tube-frame construction designed to resist high winds -- are being scrutinized.

"It is unlikely any commercial office building of this type could withstand such an attack, and I believe that the World Trade Center towers did better than other buildings would have under similar circumstances," he said.

Hamburger and other ASCE team members have been at ground zero in Manhattan since the attacks, collecting eyewitness accounts of the disaster, poring over video from news networks and examining the mountain of debris.

Blunt Trauma

Dr. Abi Aghayere, assistant professor of civil engineering technology at the Rochester Institute of Technology, also agreed with the initial assessment. "The intense fire from the impact of the airplanes carrying heavy loads of aviation fuel, together with the blunt trauma suffered by the buildings from the initial impact, most likely led to the collapse of the World Trade Center towers," he told NewsFactor.

The building materials used for fire protection of the structural elements in the towers withstood the fires for 45 to 60 minutes, allowing many occupants to escape. But once the fire protection material was destroyed by the intense heat, the steel structural elements supporting the floors became exposed to the fire, melted and fell on the floors below, Aghayere said.

"The floors below then collapsed under the weight of the falling structural elements from above, and the resulting chain reaction, called 'progressive collapse,' most likely led to the imploding-type collapse of the towers," he added.

Unrealistic Goal

Some analysts maintain that constructing buildings to survive the type of assaults that occurred on September 11th is not a realistic goal.

"Personally, I don't think this calls for more stringent building codes, but there could be recommendations for code changes," Hamburger said. "It's not clear that standards such as those used in California would have resulted in better performance of the buildings."

ASCE forensic investigators, working hand in hand with the Federal Emergency Management Agency (FEMA), hope to publish their initial findings on the disaster this spring, but engineers most likely will be studying the incident for years, Hamburger said.