US: In the most detailed examination of a building failure ever conducted, an official inquiry has found that the twin towers of the World Trade Centre in New York were able to withstand the impact of aircraft on 9/11, but that the support columns buckled under the heat of fires from aviation fuel and office furniture.
However, the buildings were solidly constructed and would not have collapsed under the combined effect of the impact and fire if fireproofing had not been considerably dislodged.
"While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts," said Shyam Sunder, lead investigator for the National Institute of Standards and Technology (NIST).
"This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled."
The probable collapse sequences in both towers were presented by Mr Sunder at a press briefing in New York city.
"Each aircraft severed perimeter columns, damaged interior core columns and knocked off fireproofing from steel as the planes penetrated the buildings," he concluded.
"The weight carried by the severed columns was distributed to other columns. Subsequently, fires began that were initiated by the aircraft's jet fuel but were fed for the most part by the building contents and the air supply resulting from breached walls and fire-induced window breakage.
"These fires, in combination with the dislodged fireproofing, were responsible for a chain of events in which the building core weakened and began losing its ability to carry loads. The floors weakened and sagged from the fires, pulling inward on the perimeter columns.
"Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle-a process that spread across the faces of the buildings. Collapse then ensued."
The inquiry found that several factors confined the loss of life to 2,767 in buildings where the full capacity was 25,000 occupants.
Principally the towers were only about one-third occupied, and the number and width of exits and stairways were adequate for escape. The agency was asked by Congress after 9/11 to establish why the buildings collapsed.
The inquiry determined that there were some 8,900 people in the first World Trade Centre tower and 8,500 in the second and 13 in every 100 people perished.
Of the victims, one in five is thought to have been alive just before the towers collapsed.