Twenty Years Later: Why the 9/11 Twin Towers Collapsed
The two skyscrapers were both designed around two vertical systems that were held together by a weaker horizontal link.
NEW YORK — In a few days it would have been 20 years since the 9/11 attacks killed thousands of people and brought down two of the world’s tallest office towers. Here is the official theory on how and why the towers collapsed:
The structural integrity of the World Trade Center depended on two connected systems of vertical pillars.
A central core of reinforced concrete conveyed vertical loads, while steel pillars were arranged around the perimeter. In combination with horizontal trusses, the outer pillars formed a system for absorption of horizontal loads, caused mainly by wind.
The horizontal trusses also supported 10 centimeter-thick cement slabs in each floor while connecting the two vertical systems, preventing the pillars around the perimeter from falling outwards.
In September of 2005, America’s National Institute of Standards and Technology released its report on why the buildings collapsed. It stated that when the planes struck the towers, the rupturing of their fuel tanks started a fire that buckled and weakened the buildings’ structural steel.
With time, the buckling caused the outer walls to deform, which caused the sudden collapse of the floors above the impact zones.
The report states that the floors below provided little resistance to the tremendous energy of the falling building, allowing the structures to disintegrate very quickly.
A senior materials scientist at independent Norwegian research organization SINTEF later said that it’s much more likely that the final blow was struck by molten aluminum from the fragments of the two planes.
Christian Simensen said such molten aluminum would have seeped into the building’s structure, where it would have made contact with pools of water from the building’s sprinkler system.
Such contact would have caused the aluminum to explode with enough force to greatly weaken the building’s structure.
He said this theory would also explain the many explosions that witnesses heard during the horrifying event.
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