- 17:28 06 April 2005
- NewScientist.com news service
- Will Knight
The NIST report concludes that a combination of factors caused both buildings to collapse shortly after terrorists flew hijacked commercial airliners into them on 11 September 2001. Computer simulations have been used to help piece together the chain of events that unfolded between impact and the collapse of each structure. Several videos of the simulations can be seen here and here (both require Realplayer).
The report says the initial collisions severely damaged several of the columns at the core of each building. Critically, they are also thought to have dislodged fireproofing on both the columns and the floors - the floors linked the inner columns to the supports on the outer structure.
"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," says Shyam Sunder, who led the NIST investigation. "This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and [became] buckled."
Photographs show that the walls of the north tower to have deformed by as much as 140 centimetres just a few minutes before collapse and the walls of the south tower to have arched by 50 cm.
Sunder told a press conference that newly developed fireproofing could perhaps have sustained the structure for longer. "Even with the aeroplane impact and jet-fuel-ignited multi-floor fires - which are not normal building fires - the buildings would likely not have collapsed had it not been for the fireproofing that had been dislodged," he says.
The report further concludes that more lives might have been saved if both structures had been built with more than just three staircases. And the stairs were also surrounded by lightweight drywall that was immediately destroyed upon impact. Reinforced surrounding walls "might have provided greater opportunities for escape", Sunder says.
But some experts remain unconvinced by the study's conclusions. James Quintiere, of the University of Maryland, US, says he does not understand how fireproof insulation could have been dislodged from the buildings' floors and columns.
"Everything I see points to the fact that there may not have been enough insulation," he told New Scientist, adding that the fuel loads used in the report's calculations may have been too low.
And Barbara Lane, leader of the Structural Fire Group at UK engineering company Arup, adds: "[We] don't believe that [the dislodging of fireproof material] has been substantiated in any of the published data to date.” She adds that it is difficult to extrapolate heat assessments of a material to what might happen when it is actually in place in a building.
Lane also questions recommendations concerning the use of thermally-resistant window assemblies to slow the spread of fire. "This is of considerable concern as even this form of glass can fail under direct flame impingement," she says.
The complete report is comprised of more than 10,000 pages, the preliminary 3400 pages of which were released on Tuesday. The remainder of the study is planned for release in July 2005, when the institute will also make recommendations concerning building design and construction.