August 22, 2008
Fire, Not Explosives, Felled 3rd Tower on 9/11, Report Says
By ERIC LIPTON
GAITHERSBURG, Md. — Fires in the 47-story office tower at the edge of the World Trade Center site undermined floor beams and a critical structural column, federal investigators concluded on Thursday, as they attempted to curb still-rampant speculation that explosives caused the building’s collapse on Sept. 11, 2001.
No one died when the tower, 7 World Trade Center, tumbled, as the estimated 4,000 office workers there at the time had evacuated before it gave way, nearly seven hours after the second of the twin towers came down.
But the collapse of 7 World Trade Center — home at the time to branch offices of the Central Intelligence Agency, the Secret Service and the Giuliani administration’s emergency operations center — is cited in hundreds of Web sites and books as perhaps the most compelling evidence that an insider secretly planted explosives, intentionally destroying the tower.
A separate, preliminary report issued in 2002 by the Federal Emergency Management Agency questioned whether diesel fuel tanks installed in the tower to supply backup generators — including one that powered the Giuliani administration’s emergency “bunker” — might have been to blame.
But S. Shyam Sunder, the lead investigator from the National Institute of Standards and Technology, based here in the suburbs of Washington, also rejected that theory on Thursday, even as he acknowledged that the collapse had been something of a puzzle.
“Our take-home message today is the reason for the collapse of World Trade Center 7 is no longer a mystery,” Dr. Sunder said at a news conference at the institute’s headquarters. “It did not collapse from explosives or fuel oil fires.”
The institute’s findings were released on Thursday as part of a 915-page report resulting from the work of more than 50 federal investigators and a dozen contractors over three years.
Conspiracy theorists have pointed to the fact that the building fell straight down, instead of tumbling, as proof that explosives were used to topple it, as well as to bring down the twin towers. Sixteen percent of the respondents in a Scripps Howard/Ohio University poll said it was very likely or somewhat likely that explosives were planted.
During the last four decades, other towers in New York, Philadelphia and Los Angeles have remained standing through catastrophic blazes that burned out of control for hours because of malfunctioning or nonexistent sprinkler systems. But 7 World Trade Center, which was not struck by a plane, is the first skyscraper in modern times to collapse primarily as a result of a fire. Adding to the suspicion is the fact that in the rush to clean up the site, almost all of the steel remains of the tower were disposed of, leaving investigators in later years with little forensic evidence.
Using videos, photographs and building design documents, the investigators at the National Institute spent the last three years building an elaborate computer model of 7 World Trade Center that they used to test various chains of events to figure out what caused the collapse, Dr. Sunder said.
The investigators determined that debris from the falling twin towers damaged structural columns and ignited fires on at least 10 floors at 7 World Trade Center, which stood about 400 feet north of the twin towers. But the structural damage from the falling debris was not significant enough to threaten the tower’s stability, Dr. Sunder said.
The fires on six of the lower floors burned with particular intensity because the water supply for the sprinkler system had been cut off — the upper floors had a backup water supply — and the Fire Department, devastated by the collapse of the twin towers, stopped trying to fight the blaze.
Normally, fireproofing on a skyscraper should have been sufficient to allow such a blaze to burn itself out and leave the building damaged but still standing. But investigators determined that the heat from the fire caused girders in the steel floor of 7 World Trade Center to expand. As a result, steel beams underneath the floors that provided lateral support for the tower’s structural columns began to buckle or put pressure against the vertical structural columns.
These fires might have been fed partly by the diesel from tanks and a pressurized fuel line, which were on the fifth to the ninth floors, Dr. Sunder said. But the analysis showed that even in the worst case, the diesel fuel-fed fire would not have burned hot enough or long enough to have played a major role in weakening the structure. The investigators determined that the fire that day was fed mainly by office paper and furnishings.
The collapse started when a girder on the 13th floor disconnected from a critical column — listed as Column 79 — that supported a long open floor span, the report said. Once that floor gave way, the floors below it down to the fifth floor also collapsed, although this was not visible from the building’s exterior.
Without lateral support for nine stories, Column 79 buckled, and the floors above gave way all the way up to the roof. Only then did the collapse become visible from the exterior with a penthouse area on the roof first falling in, followed by what looked like the sudden implosion of the tower, Dr. Sunder said. “The physics is consistent, it is sound, it has been analyzed,” he said.
Skeptics have questioned whether explosives were planted at the three towers at ground zero, and at the Pentagon as well, often contending that the Bush administration had planned the catastrophes to provide a justification to invade Iraq and Afghanistan. What started as a small number of such conspiracy theorists ballooned into a movement of sorts, largely fed by Internet sites and homemade videos.
Dr. Sunder said the investigators considered the possibility that explosives were used, but ruled it out because the noise associated with such an explosion would have been 10 times louder than being in front of the speakers at a rock concert, he said, and detectable from as far as a half a mile away. He said that interviews with eyewitnesses and a review of video taken that day provided no evidence of a sound that loud just before the collapse.
The skeptics — including several who attended Thursday’s news conference — were unimpressed. They have long argued that an incendiary material called thermite, made of aluminum powder and a metal oxide, was used to take down the trade center towers, an approach that would not necessarily result in an explosive boom. They also have argued that a sulfur residue found at the World Trade Center site is evidence of an inside job.
Dr. Sunder said the investigators chose not to use the computer model to evaluate whether a thermite-fueled fire might have brought down the tower, since 100 pounds of it would have had to have been stacked directly against the critical column that gave way, which he said they did not believe had occurred.
To the skeptics, it was a glaring omission.
“It is very difficult to find what you are not looking for,” said Shane Geiger, who contributes to a Web site that follows the topic and who had come to Maryland from Texas to quiz Dr. Sunder about his findings, with a bumper sticker on his laptop computer that says, “9-11 was an inside job.”
Dr. Sunder attempted to patiently answer the questions that Mr. Geiger and another obvious critic presented to him during the news conference. Five armed police officers and a bomb-sniffing dog stood guard near the rear of the room.
Dr. Sunder said there were no apparent flaws in 7 World Trade Center’s design that contributed to its collapse and that it met New York City codes. But there are some important lessons for other skyscrapers, he said, as engineers and architects should consider how the heat from fires can weaken structural elements, potentially causing a so-called progressive collapse.
Owners of tall buildings with a similar floor design — he could not estimate how many such towers exist in the United States — should immediately consider whether to install reinforcements, he said, and perhaps codes should be changed to address the weakness.
A new, substantially different 7 World Trade Center — now 52 stories — reopened at roughly the same site in 2006. The new building has extra safety features, including wider emergency stairwells and a fire-resistant refuge area on each floor.
Within moments after the news conference ended, leaders of a group called Architects and Engineers for 9/11 Truth held their own telephone conference briefing, dismissing the investigation as flawed.
“How much longer do we have to endure the coverup of how Building 7 was destroyed?” said Richard Gage, a California architect and leader of the group.
Told of the doubts, Dr. Sunder said he could not explain why the skepticism would not die.
“I am really not a psychologist,” he said. “Our job was to come up with the best science.”
