Seven days after the World Trade Center attack on September 11, 2001, Professor Abolhassen Astaneh-Asl was in New York, armed with a digital camera and a notebook and documenting what he could of the devastation. A Professor of structural engineering at the University of California Berkeley working under a grant from the National Science Foundation to collect perishable data and conduct a reconnaissance of the collapsed towers from a structural engineering point of view, Astaneh-Asl knew he had a very brief window of opportunity in which to gather evidence that might be useful in any future analysis of the buildings’ collapse.
What he did not realize, however, was that he was embarking on a half-a-decade-long odyssey that would have him scrambling for resources and trying to obtain construction drawings and design documents for the towers from the Port Authority of New York and New Jersey, the owner of the WTC.
The release of these documents—which ultimately required an order from Congress’ Committee on Science --was critical because they contained design specifics that Astaneh-Asl needed in order to develop a detailed structural model to simulate the impact of the airplanes on the twin towers. “Basically we wanted to find out what role the buildings themselves played in the tragic events of that day and to learn lessons that can be used in protection of other structures,” he explains.
Now, as the five-year anniversary of the World Trade Center attack draws near, Astaneh-Asl finally expects to have the results of his analysis published in an academic journal. The project, requiring thousands of hours to complete, was self-funded and conducted by Astaneh-Asl, his students, and analysts from the MSC.Software Corporation, which donated the structural analysis software (MD Nastran and Dytran).
Engineers use this kind of software to simulate the behavior of objects subjected to any number of punishing scenarios, including bending, shaking, and collisions.
A roomful of engineers and analysts—whose own simulation studies revolve around more practical, everyday concerns like how much torque a new wrench design can sustain—got a sneak preview of those upcoming results at MSC. Software’s2006 Virtual Product Development Conference in Huntington Beach, CA, in July. http://vpd.mscsoftware.com/americas/
“The simulation model shows the plane slicing right through the outer walls of the as-built building like it was a thin soda can,” Astaneh-Asl explained to the spellbound crowd.
He described the issue in a nutshell: “Because of their unique design and the use of the so called “steel bearing wall” tube structural system, which as far as we know has never been used before or after its application in the WTC towers, the buildings essentially showed no resistance to the impact of a medium-sized plane flying into them at about 450miles per hour.”
Elaborating on the novelty of the design, he said that the notion of a ‘structural framing system’ simply didn’t apply in the case of the twin towers. “Rather than traditional columns and beams, the designers employed a steel bearing wall tube system for the perimeter and steel truss joists in the floors that connected the gravity load-carrying inner core columns to the outside perimeter steel bearing walls. The relatively thin steel bearing wall pre-fabricated units of the perimeter bearing tube were bolted together in a Lego-like fashion to expedite construction” he explained.
He also noted that designers chose to fabricate many of the building columns out of very high strength steel [90 ksi steel as opposed to the more typical 36-65 ksi steel]. “This is not allowed by the structural design codes then and is still notallowed in current codes,” he stressed. “But the World Trade Center did not need to obtain a permit from City Hall. Because of special status as Port Authority of New York and New Jersey, they could make such choices outside the prevailing codes.”
This choice, he argued, allowed builders to use less steel in the columns [two to three times thinner than typical columns] presumably to save cost.
But by using high strength steel and thin cross sections, he pointed out, on impact the plane was able to cut through the outside steel bearing wall and enter the building--delivering thousands of gallons of jet fuel to the interior. During the ensuing fire, he said, the thin outside columns of the steel bearing walls were quite vulnerable to the rapid rise of temperature in them and reduction of their strength as a result of rising temperature of the steel.
“When the fires started, they heated up the steel. In my opinion, the truss joists collapsed first, leaving the exterior columns of probably two floors in the impact area with no bracing but still under gravity load from the floors above. As the columns heated up and reached temperatures of nearly 1,000F, their strength was reduced to less than half the design strength and they started to buckle. When the columns buckled, the top portion of the building, losing its supports, was pulled down by gravity and dropping on the floors below, pancaking the floors one after another and leading to progressive collapse in an almost perfect vertical direction of the pull of gravity force.”
In a 2001 article Why Did the World Trade Center Collapse? Science, Engineering, and Speculation, published in the Journal of Metal, MIT Material Engineering Professor Thomas Eagar and Graduate Student Christopher Musso concluded that the failure of the steel resulted from loss of strength due to the temperature of the fire and the loss of structural integrity due to distortion of the steel from non-uniform temperatures in the fire. They did not comment on the type of steel used in the design.
In that paper, they concluded that the World Trade Center was not defectively designed, and that “no designer of the WTC anticipated, nor should have anticipated, a 90,000 L Molotov cocktail on one of the building floors.”
Astaneh-Asl says that the reason for undertaking his studies is not to implicate the designers, but rather to look into the design and answer the basic question that has bothered him since September 11: “Why did these towers collapse so quickly and so completely while other steel structures, including skyscrapers, under intense fire for hours, have not failed?”
He says that he feels he is closing in on the answer. “These structures were so unique that their collapse does not represent the performance expected of any other existing steel high-rise structure subjected to the same scenario,” he says.
Berkeley Researcher Believes Fire Led to Collapse of Towers
Why Did the World Trade Center Collapse? Science, Engineering, and Speculation
Skyscraper Safety Campaign Website