A Berkeley Engineer Searches for the Truth About the Twin Towers' Collapse
Abolhassan Astaneh-Asl's computer simulations suggest more-conventional skyscrapers might have withstood the attacks
By JEFFREY R. YOUNG
When the World Trade Center's burning south tower crumbled to the ground five years ago, just 56 minutes after terrorists crashed a Boeing 767 passenger jet into its upper floors, Abolhassan Astaneh-Asl's horror was mixed with professional surprise.
As a professor of structural engineering at the University of California at Berkeley and an expert on steel structures, he thought that the buildings should have stood longer, even after such a catastrophic impact, and that the collapse should not have been so nearly vertical.
"From the day that I stood there and watched it collapse" on television, he says, "I was thinking that this is impossible. That there's something strange here."
Mr. Astaneh-Asl says he knew immediately that he wanted to be a part of the scientific response to the tragedy. He felt that his unique expertise could help in understanding how the two towers collapsed. He was well versed in the effects of terrorist bombings on buildings, having conducted research on blast effects after a car bomber brought down the Alfred P. Murrah Federal Building in Oklahoma City. And he had long studied how buildings responded to earthquakes and other natural disasters, so he knew that researchers must act fast to get the clues needed to understand what had happened.
The day after the attacks of September 11, 2001, Mr. Astaneh-Asl submitted an emergency grant proposal to the National Science Foundation asking for money to examine the steel at ground zero firsthand. Only days later, the request granted, Mr. Astaneh-Asl flew to New York and spent weeks at a recycling center where the towers' remains were being scrapped. There he inspected and collected samples of joints and other scraps of steel from what were once two of the tallest buildings in the world.
Though his NSF grant soon ended, questions about the collapse remained. Mr. Astaneh-Asl decided to create a computer simulation of the plane attacks, with as much detail as possible, in the hope that the unprecedented tragedy might yield lessons that could be used in the design of future skyscrapers.
He did not expect the discoveries he would make, the political obstacles he would face — or that, five years later, he would be involved in a struggle for skyscraper safety.
This week Mr. Astaneh-Asl is scheduled to present findings from his latest simulations in a lecture at Berkeley, making an argument that is sure to raise eyebrows in the engineering community and beyond.
If the World Trade Center towers had been built in a more conventional way and in strict accordance with New York City building codes — from which they were exempt because they were built under the auspices of the Port Authority of New York and New Jersey — the buildings probably would not have collapsed, he argues, and thousands of lives might have been saved.
Two extensive government investigations found no fault with the towers' structural design, and many engineers say that no engineer could have anticipated or shielded against kamikaze attacks by fuel-laden jetliners. And some say that what-if scenarios are fruitless lines of research.
Leslie E. Robertson, who helped design the twin towers, could not be reached for comment. But William Faschan, a partner at Leslie E. Robertson Associates, defended the building's performance. "It's extraordinary that any building could withstand that event and remain standing," he says. Mr. Faschan says he is not familiar with Mr. Astaneh-Asl's research and would not comment on its findings.
Even Mr. Astaneh-Asl is careful when discussing his findings, stressing that the people who perished in the buildings' collapse "were murdered by terrorists." But he insists that it is his obligation as an engineer to seek "the truth" about the buildings' history and structure. He speaks excitedly about the importance of his findings, and talks eagerly for hours about the topic.
"We can avoid this happening to someone's loved ones in the future," he says.
His work examining steel near ground zero gave Mr. Astaneh-Asl a few short minutes of fame. He was interviewed on The NewsHour With Jim Lehrer, CNN, and National Public Radio, as well as by several newspapers, including The Chronicle. At the time, he stressed the positive aspects of the twin towers' performance on the day of the attacks, noting that the length of time the buildings stood allowed most occupants to escape.
Mr. Astaneh-Asl says that from his inspection of the steel, he decided the collapse was not due to faulty welding or poor workmanship. That meant he was still not sure exactly how the collapse happened.
Then he got a call from an analyst from MSC Software Corporation, which makes high-end computer-modeling software used by carmakers and other businesses. Company officials had seen the Berkeley professor quoted in the news media, and offered to donate the company's software for his efforts. He quickly took them up on their offer and began the next phase of his research.
Mr. Astaneh-Asl wanted the computer simulation to be as true to life as possible. That would require the blueprints and construction specifications for the twin towers.
"Basically you build the towers inside your computer with all the dimensions — you represent each element of your structure," he says. "It's really a virtual replica of your physical structure."
But the building plans for the towers turned out to be hard to come by, as the developers kept them sealed from public view. Experts say that most developers keep such documents private, viewing them as proprietary information, but Mr. Astaneh-Asl says he had hoped that, considering the circumstances, the plans would be made available to researchers.
He nearly got access by joining an investigation team led by the Federal Emergency Management Agency and the American Society of Civil Engineers, which brought together some two dozen researchers and engineers in late 2001.
Mr. Astaneh-Asl was initially asked to participate, but he says he was troubled that team members were all required to sign a nondisclosure form promising to keep certain details of the investigation, including the buildings' architectural plans, to themselves. Mr. Astaneh-Asl's says he felt the agreement violated his academic freedom, and so he resigned from the team before its investigation got under way.
(The leader of that investigation, W. Gene Corley, says he believes the wording of the nondisclosure agreement would not have stopped any participant in the investigation from publishing academic papers about the structures. "It essentially said that we would not use information we obtained there to be used in a lawsuit against the owners and designers of the building," says Mr. Corley, who is senior vice president of the CTL Group, in Skokie, Ill.)
After the Berkeley professor had nearly lost hope of obtaining the blueprints, he was invited to testify before the U.S. House of Representatives' Committee on Science, in March of 2002, at a hearing titled "Learning From 9/11: Understanding the Collapse of the World Trade Center."
There, he was asked what "impediments" he had encountered in his research, and he replied that the largest one was his inability to get the design and engineering documents. Soon afterward, he was sent a copy of the plans by an official from FEMA. (He jokes that his wife wishes he had also asked for money to support his research.)
As Mr. Astaneh-Asl examined the construction documents, however, he was horrified by aspects of the design. He says the structure essentially threw out the rule book on skyscraper construction. "This building was so strange, and so many violations of practice and code were introduced," he says.
The design contains at least 10 unusual elements, he says. For example, rather than using a traditional skeletal framework of vertical and horizontal columns, the twin towers relied partly on a "bearing wall" system in which the floors and walls worked together to support each other, says Mr. Astaneh-Asl. That system allowed designers to use thinner steel in the buildings' columns and exterior than would be used in a traditional design, he says, adding that in some places the steel in columns was only one-quarter of an inch thick. And he says the designers used stronger steel (measured in what is known as "yield strength") in some columns than is allowed by any U.S. building codes, and that such steel is less flexible — and therefore more brittle — than the type traditionally used in such buildings.
As a result of such design elements, he argues, when the two airliners smashed into the upper floors of the towers, both planes plunged all the way in, wings and all. Airliners carry much of their fuel in their wings. His model clearly shows that in the initial fight between the plane and the building's exterior, the plane won, easily breaching the structure.
"It's like a soda can hit with a pencil," says Mr. Astaneh-Asl. "It was so easy that the plane went in without any damage and took the thousands of gallons of jet fuel in."
The structural innovations meant the developers saved money because they could use less steel, says Mr. Astaneh-Asl.
Efforts were made at the time of construction to verify the buildings could withstand anticipated forces, including high winds. The towers were among the first buildings ever to be modeled and tested in a wind tunnel before they were built. The buildings were widely praised for the efficiency of their construction.
But Mr. Astaneh-Asl argues that in engineering, innovations should — and usually do — emerge slowly, through evolutionary processes that follow time-tested practices. "Structural engineering is something that evolved," he says. "It was not invented."
"Unfortunately and tragically, when [this design] was subjected to this terrorist attack, there's no way this building could stand it."
Ross B. Corotis, a professor of civil engineering at the University of Colorado at Boulder, disagrees, arguing that the innovations did not mean that the towers were poorly designed. "I think our understanding of materials and our ability to analyze structural behavior gives us the ability to innovate more without introducing additional risk," he says.
Would a traditional structure have done better?
To try to answer that question, Mr. Astaneh-Asl and his team made another computer model in which they altered the design of the north tower's structure to make it more consistent with what the researcher calls standard engineering-design practice. Then he ran the same simulated plane into the structure in the same place it hit on September 11, 2001.
In that scenario, the airplane's wings are torn off, and therefore kept out of the building, when they hit the outer wall, while the fuselage still pierces the wall. "When it gets inside, there's not very much fuel," he says. Government reports found that it was not the damage from the planes, but the subsequent fires that weakened the steel and caused the buildings' collapse.
Mr. Astaneh-Asl says he cannot be certain whether a more-traditional building would have survived the smaller fire that would have followed because he is not an expert on fires. Even so, he argues, if the World Trade Center towers had been designed "using the codes and traditional systems, the building most likely would have survived — it most likely would not have collapsed."
The Prevailing View
Many engineers disagree with Mr. Astaneh-Asl's conclusions.
Mr. Corotis argues that "that particular design probably did better than most traditional designs would have done." One feature that helped, he says, was a cap across the tops of the towers that helped the buildings redistribute weight after the planes knocked gaps in them. Though he has not seen the blueprints for the structures, he says he is familiar with the building's innovative design, which has been widely publicized.
"Given the size of the planes and given the fire," he says, "the fact that they did come down is not surprising — but it's still shocking."
The most extensive investigation of the towers' collapse, completed by the National Institute of Standards and Technology, also found no fault with the structure, but it did acknowledge the buildings' unique history and design.
"The buildings were unlike any others previously built, both in their height and in their innovative structural features," a report on the investigation says. "Nevertheless, the actual design and approval process produced two buildings that generally were consistent with nearly all of the provisions of the New York City Building Code and other building codes of that time that were reviewed by NIST. ... The departures from the building codes and standards identified by NIST did not have a significant effect on the outcome of September 11."
And the other government investigation into the collapse, led by FEMA, reached the same conclusion.
"We didn't really cite anything we thought the designers should have known about at that time," says Mr. Corley, who led that investigation. "It would have made no difference to what happened regardless of what building code it was built under."
Mr. Corley says Mr. Astaneh-Asl's simulations do not prove that the design was flawed. "If I know what's going to happen, I can design something that can do better" under those circumstances, he says.
Mr. Astaneh-Asl responds that his modified version of the towers was not designed specifically for an airplane strike. "We designed this building assuming that they were building this building in 1970 following the [New York City] code without any consideration of an airplane," he says.
He sounds exasperated by what has come to be the accepted wisdom among engineers: that there was nothing wrong with the buildings. "I cannot see why the entire profession has agreed to sit in this convenient seat of saying that there is nothing wrong with our work," he says.
For the most part, Mr. Astaneh-Asl has done little to publicize his findings so far, especially since he still hopes to publish a scientific paper about his latest simulation. He agreed to talk to The Chronicle only after a reporter called him to follow up on its previous coverage of his research.
He did present the findings in July at MSC Software's Virtual Product Development Conference, in Huntington Beach, Calif. An article that ran in Design News says the presentation had audience members "spellbound."
But Mr. Astaneh-Asl has been drawn into the political fight over the new Freedom Tower that is slated to be built at ground zero. Last year he joined an advisory panel of a group started by families of 9/11 victims. The group, the Skyscraper Safety Campaign, is lobbying to, among other things, require the new office tower to adhere to local building codes, rather than to the Port Authority's guidelines.
"They're going to design this building without going to City Hall and getting permits," says Mr. Astaneh-Asl, his voice rising. "Even if you want to change your kitchen, you have to get a permit."
The Berkeley researcher says he initially declined the group's invitation to join because he wanted to remain completely independent. Aside from the free software, Mr. Astaneh-Asl says that his simulations research is not financially supported by anyone, and that he and the graduate students who helped with the project have volunteered their time, even using their personal computers. He says he later agreed to join the Skyscraper Safety Campaign's advisory panel because he supports their argument about building codes, but that the group has not given him any money.
Sally Regenhard is the leader of the Skyscraper Safety Campaign. Her son was a firefighter who perished responding to the attacks. Ms. Regenhard says that the government was slow to investigate the performance of the World Trade Center on September 11, 2001, and of the emergency response that followed. "There was a huge, huge force of don't ask, don't tell — don't ask questions," she says.
Many people outside engineering and government have developed their own theories about how and why the World Trade Center buildings fell. Some, wondering how buildings that easily withstood fierce wind gusts for decades were so quickly brought down by airplanes, argue that explosives planted before the attacks must also have been involved. Even some college professors have advanced such theories, though they have largely been dismissed (The Chronicle, June 23).
Mr. Astaneh-Asl also rejects such alternative theories. "I certainly don't buy into any of the conspiracy stuff," he says.
"Those are lightweight buildings," he adds. "There was no need for explosives to bring them down."