...after the fall
The collapse of the World Trade Center towers left thousands dead and a mountain
of debris to clean up. For engineers, it also left behind a troubling mystery:
what caused two of the world's tallest steel-framed building to fall? Jonathan
Barnett '74 and a team of researchers from WPI played
a central role in helping to find the answers
Professor Jonathan Barnett is an expert in structural and fire protection engineering, whose research has focused on building performance in fires and failure analysis. But that expertise didn't prepare him for the images that flashed across his television screen on Sept. 11, 2001.
He knew that the world had never seen the collapse of a protected steel-framed building. And yet, there were two of the world's tallest steel-framed towers crumbling into piles of rubble. Barnett's extensive research left him uniquely qualified to understand what was happening inside the blazing structures from the moment they were struck by speeding jetliners to the horrifying seconds when they dropped onto the streets of lower Manhattan, but, in truth, he was as surprised as anyone.
In the days following the terrorist attacks, the American Society of Civil Engineers (ASCE), in cooperation with the Federal Emergency Management Agency (FEMA), began assembling a "dream team" of engineers to investigate the causes of the destruction, not only of the main towers of the Trade Center, but of Building 7, the 47-story structure that collapsed in flames seven hours after the loss of the towers, and of Buildings 3, 4 and 5, which suffered extensive damage and partial collapse as a result of fire and impacts from falling debris.
Barnett was approached early on, but was unprepared when his cell phone rang on Oct. 5, in the middle of a meeting, summoning him to join the World Trade Center Building Performance Assessment Team (BPAT) in New York City the next day for a week of fieldwork. "It was 5 o'clock on a Friday," Barnett recalls, "and I had no steel-toed boots."
Not one to let a pair of boots stand between him and the professional service opportunity of a lifetime, he scoured the attic of the Auburn Fire Department and found a usable pair. The next day he met up with the 24 other team members--the country's foremost structural, seismic and fire protection engineers. One was an alumnus, Christopher Marrion, who holds a master's degree in fire protection engineering from WPI. (See Marrion at the Controls)
Barnett's credentials to serve as one of the two BPAT core members in fire protection engineering include three WPI engineering degrees (his master's thesis in civil engineering focused on seismic design of buildings; his doctoral dissertation in mechanical engineering, completed before WPI began granting Ph.D.s in FPE, explored the effect of fire on steel structures). Barnett joined WPI in 1979 as the first assistant director of the Center for Firesafety Studies, and in 1989 became a tenure-track assistant professor in the discipline he helped create. Today he is a full professor of fire protection engineering and co-founder and co-director of the Melbourne (Australia) Project Center.
At Ground Zero, almost a month after the attacks, the stench of destruction and death was still strong. Across the bay at the Fresh Kills Landfill on Staten Island, recovery teams were at work screening debris down to a quarter of an inch--the size of the smallest human bone. "As an engineer," Barnett says, "you tell yourself, OK, I have to be professional, I have to take notes, I have to ignore the death around me. At the same time, as a human being, that's not easy to do."
Team members toured what was left of the 16-acre World Trade Center Plaza, interviewed officials and eyewitnesses, and examined remnants of fallen structures at the Staten Island landfill and at salvage yards. Steel samples were cut and cataloged for further study, and some were taken back to WPI for analysis (see The "Deep Mystery" of Melted Steel).
Besides asbestos dust and bio-contamination, the investigators faced physical dangers in the unstable buildings. On a walk-through of Building 5, Barnett's group noticed that the floor slab beneath them was severed. When they checked from below, they discovered that they had been standing on unsupported rubble. Later, while taking measurements in the building's subterranean parking garage, the roof started to collapse, and they fled to safety.
In addition to his work at Ground Zero, Barnett drove to the Fresh Kills Landfill with teammates Marrion, Venkatesh Kodur and Saw-Teen See (wife of the towers' designer, Leslie E. Roberston, and a partner in Robertson's firm) to see the steel recovered from the Trade Center. After showing his pass to the guard at the gatehouse, Barnett was directed to the appropriate area, where he parked his two-week-old Acura.
"I've been to landfills," he says, "and this one didn't smell right to me." Knowing that the rubble brought to the site contained human remains, he quickly urged See back into the car, and when Marrion and Kodur resisted, Barnett insisted that he was getting his car and his teammates out of there, right away. As they closed the doors, a dozen workers in full Tyvek biohazard gear walked by. "See that?" said Barnett, feeling vindicated. "I think maybe we're underdressed for the occasion."
He drove to New Jersey as quickly as he could. "We took out the floor mats and wiped them on the grass, and we all wiped our feet. Then we took the car to a carwash."
Despite the grim nature of its task, the BPAT members were warmly welcomed. Barnett was thanked by strangers in the street, and ushered to a seat on a packed subway car when his ID badge slipped out from under his shirt. At the upscale Tribeca Grille, the grimy engineers, still in their work clothes, were escorted to a center table, once the maitre d' learned who they were.
"New Yorkers were just so friendly and willing to support our efforts in any way they could, even if it was just with a smile," Barnett says.
Barnett, at left, with other members of the building performance team at the Fresh Kills Landfill, were piles of steel from the World Trade Center towers were stored.
The complex science of fire modeling can be reduced to two questions: "How hot?" and "Where?"
Those were the questions facing Barnett and his team back at WPI when he returned to campus to begin analysis of the data--which included two cartons of videotapes, thousands of photographs and detailed construction documents. While other members of the BPAT looked at seismic data, emergency response and evacuation, Barnett simulated the fires, focusing on the floors of impact.
"To understand the collapse, we needed to know how the structural elements of the towers stood up to the stresses inflicted on the morning of Sept. 11," explains doctoral candidate James A. (Jay) Ierardi '97 ('99 M.S.), who previously worked with Barnett on the analysis of the 1999 Worcester Cold Storage warehouse fire. As the FEMA report indicates, the twin towers withstood the mechanical insult of the planes' impact, but were then subjected to interior fires, with temperatures ranging from 200 to 2,000 degrees Fahrenheit.
The WTC fires were remarkable in two ways: first, for their sheer size, and second, for the fact that such a large area was ignited instantaneously. (Typical office fires start small and spread slowly, Ierardi says.) The towers were penetrated by planes canted at a 30-degree angle and a 45-degree angle, which immediately set four or five floors--each about an acre in area--ablaze. Barnett compares the jet fuel that doused those floors and flowed down elevator shafts to charcoal lighter fluid. With rapid flashover on so many floors, sprinkler pressure would have been inadequate, even if the water supply lines had not been severed by the aircraft. Ierardi speculates that the hijackers knowingly calculated the angle of their hits to overwhelm the buildings' fire-suppression mechanisms.
To compute the size of the fires, Barnett needed to know how much oxygen was available to burn the 10,000-gallon fuel load in each 767. His calculations included the enormous holes ripped open by the planes, and the dimensions and location of every window, stairwell, and elevator or utility shaft. He also plotted the layout of offices, the location of partitions and furnishings, and flammability specification of the building materials, furnishings and other contents.
To determine which windows were open during the fire, Barnett examined more than 120 hours of videotape to see where smoke was venting. WPI undergraduates pitched in, taking home tapes to screen over the Thanksgiving break. One of these students was Patrick T. Spencer '05, son of fallen firefighter Thomas E. Spencer; Patrick came to WPI on a scholarship set up for children of victims of the Worcester warehouse fire. Ironically, he was the one who first informed Barnett of the terrorist strikes on the morning of Sept. 11. Graduate students in Barnett's failure analysis class helped calculate how much jet fuel the initial fireballs consumed.
To quantify and compute all of these variables in such a large, complex space-- a space that no longer exists-- is a mammoth task that requires painstaking research and a certain amount of informed speculation. The size and complexity of the problem challenged even WPI's fastest computers. Barnett says it took one week to simulate 10 minutes' worth of the fire. During the three weeks of report writing, only 40 minutes of the fire event could be modeled. The complete simulations won't be available until fall.
Barnett estimates that on top of his academic and civic activities, he's put more than 600 hours into the BPAT investigation. In the months between the October fieldwork and the May 1 release of the FEMA report, he made one or two trips per week, sometimes flying back and forth between WPI and Washington in a single day to teach classes and attend meetings.
He is the lead author on the section of the report that describes the metallurgy work done by WPI professors Ronald Biederman and Richard Sisson, as well as the chapters about Buildings 4, 5 and 6. He is a co-author on the chapters about the collapse of Buildings 1 and 2 (the twin towers). "I think the most important outcome of the FEMA report is that we've identified areas that need to be studied," Barnett says. "Before you spend millions of dollars [on further investigations], you need to know what to spend it on."
A bigger budget, more time and earlier access to the scrap yards, where steel was being cut up and sold, would have enhanced the investigation, he says. "You do the best you can, with the available resources. I think we did a very credible job." Efforts are under way to address factors that hindered FEMA's BPAT investigation. The proposed "National Construction Safety Team Act of 2002" outlines procedures to ensure that evidence is preserved in the event of another attack of this magnitude.
In interviews, Barnett has repeatedly stressed that the public does not need to worry about living and working in high-rise buildings. "Our buildings are generally safe," he reiterates. "If we were doing things that were unsafe, then periodically we would have had failures. In fact, I would suggest, because we've never had failures, we probably over-design."
On May 1, Barnett accompanied BPAT leader Gene Corely to Washington to respond to questions as Corley presented the team's findings to Congress. FEMA has proposed a $16-million, multiyear follow-up investigation, to be headed by the National Institute of Standards and Technology (NIST). Two areas earmarked in the FEMA report for further study are the metallurgical examinations performed by Biederman and Sisson, and the fire modeling computations done by Barnett. WPI hopes to obtain NIST funding to pursue these investigations. The report also calls for further examination of the building and fire codes, but recommends against considering aircraft impact as a design parameter for every structure. "I think the lessons for ordinary buildings are few and far between," says Barnett.
The terrorist attacks and their aftermath highlight the importance of fire protection engineering as a discipline, and the need for closer ties with the field of structural engineering, Barnett says. The FEMA report specifically recommends cross training between the disciplines, to ensure that the impact of fire is adequately addressed in the design process.
Barnett says he is grateful to have had the chance to participate in an important national study, working with a team of professionals to tackle questions that are important to his profession and the country (and that provide a real-world case study to bring into the classroom). "In my career," he says, "I've never had the privilege of working with so many awesome practitioners."
- World Trade Center Building Performance Study
- WPI Center for Firesafety Studies
- WPI Materials Science and Engineering Program
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Last modified: Sep 15, 2004, 12:04 EDT