New York, Sep 6 : Investigators have finally determined that fire caused Building 7 at the World Trade Center to collapse on September 11, 2001.
The fall of the 47-story World Trade Center's Building 7 at 5:20 p.m. on Sept. 11, 2001, was primarily due to fires from the building itself, NIST announced Friday, following an extensive, three-year investigation. This was the first known instance of fire causing the total collapse of a tall building, the agency said.
Dr. Shyam Sunder with the National Institutes of Standards and Technology said it was primarily due to fire.
After a three year investigation, he discovered debris from Tower 1 started fires on at least 10 floors of Building 7.
They burned out of control for hours and caused what he termed a "fire induced progressive collapse." The fires created a thermal expansion in building support beams which later buckled in the heat.
Water to the building was not available because of the situation involving the Twin Towers.
If water to the sprinklers hadn't been cut off, Sunder said there's a chance Building 7 would still be standing today.
The trapezoid-shaped Building 7 was connected to the World Trade Center plaza by an elevated walkway. The building was damaged and set afire by debris - including enormous broadcast antennae - from the collapse of the nearby North Tower.
"Video and photographic evidence combined with detailed computer simulations show that neither explosives nor fuel oil fires played a role in the collapse of WTC 7," the Washington Times quoted Sunder, as saying.
The NIST investigation team also determined that other elements of the building's construction - namely trusses, girders and cantilever overhangs - did not play a significant role in the collapse.