Sharjah24 – Reuters: The two skyscrapers were both designed around two vertical systems that were held together by a weaker horizontal link.
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.
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