A 'new phenomenon' known as thermal expansion caused World Trade Centre 7 to collapse September 11, 2001. This is the conclusion of a report released by an investigative team headed by Dr Sivaraj Shyam Sunder, director of Buildings and Fire Research Laboratory at the Nationals Institute of Standards and Technology in Gaithersburg, Maryland.
This was the first known instance of fire causing the total collapse of a tall building, the report said.
'Heating of floor beams and girders caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down.'
Dr Sunder says his team has set to rest many conspiracy theories about the fall of the building. After six year of investigation and spending millions of dollars, the report also suggested ways to prevent future tragedies, many of which are already part of building codes in many countries.
The team had completed the investigation on the collapse of the WTC Tower-1 (North Tower) and WTC Tower 2 (South Tower) two years ago. What remained was the 47 story WTC 7, which was not hit by a plane. It collapsed seven hours after the north and south towers collapsed.
The team found that a failure to fireproof the building was the main cause for the fall of the north and south towers. For WTC-7, the investigators found the new phenomenon.
'We conducted the study without bias, without interference from anyone. We have only one single-minded goal in this effort,' Dr Sunder told a press conference dismissing theories that the tower was brought down by a controlled explosion using explosives.
He said they also looked into this aspect, but found no evidence for that.
Debris falling from the North and South towers caused the fire in WTC-7, about 400 feet away. The fire on six lower floors was very intense as the water supply for the sprinkler system had been cut off. After the water main to the tower was cut off, the building's sprinkler system was unable to function, Dr Sunder said.
This allowed fires across 10 floors to burn uncontrolled for nearly seven hours. Due to the lack of water supply, an 'extraordinary event' occurred, and steel expanding due to heat from the flames caused columns to separate from structural concrete. Column 79 was the first to fail, which brought about a quick succession of failures in adjoining columns.
'If water had been available, it is likely that sprinklers would have operated and the building may still be here today,' he said.
While fireproofing may have prevented the tower from falling, in this case, the heat from the fire caused girders in the steel floor of 7 World Trade Center to expand. It had a chain effect. The WTC-7 housed the offices of the CIA, FBI, the Secret Service and the New York mayor's emergency operations centre. No one died there.
Many people theorized that the tower was destroyed using a controlled demolition. The video of the fall of the tower too created much suspicion.
"But these conspiracy theories have no validity," Michael Newman, an NIST spokesperson, told rediff.com.
"If there was a blast before the fall of the tower, there would have been materials strewn far and wide. Nobody heard any big sound of a major explosion too," he said.
There were also doubts whether the diesel fuel tanks for the emergency 'bunker' caused the explosion. 'It did not collapse from explosives or fuel oil fires," Dr Sunder said.
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. The WTC 7 became the first skyscraper in modern times to collapse primarily as a result of a fire.
More doubts were raised because the remains of the tower were removed quickly. The 50-member investigation team set an elaborate computer model of the tower and tested possibilities to figure out what caused the collapse, Dr Sunder said.
The team found that 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 fire was mainly due to the office paper and furnishings.
A girder on the 13th floor got disconnected from a critical column and several floors gave way, which was not seen from outside. From the outside the collapse may have looked like a sudden implosion, Dr Sunder said. 'The physics is consistent, it is sound, it has been analyzed,' he said.
Investigators also considered the possibility of the use of explosives, Dr Sunder said. If there was an explosion, the sound of it could have been ten times louder than what was actually heard.
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. But Sunder said the team did not make a computer model to evaluate whether a thermite-fuelled fire might have brought down the tower -- which critics claim is a crucial flaw.
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