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What caused the World Trade Center towers to collapse on 9/11?

World Trade Center Construction

To understand why the World Trade Center towers collapsed on Sept. 11, 2001, you need to first understand how they were constructed. The design was conceived in the early 1960s by architects from Minoru, Yamasaki & Associates and Emery Roth & Sons, along with structural engineers from Worthington, Skilling, Helle & Jackson. At that time most skyscrapers (like the Empire State Building) were supported by a dense grid of steel beams that was sturdy, but limited the open floor space on each level. The World Trade Center's architects and engineers employed revolutionary construction methods to maximize their buildings' floor space and build higher than anyone had before.

The most significant advancement in the World Trade Center's design had to do with the towers' steel framework. Instead of spacing the vertical support beams evenly across the floors, the designers moved all of them to the exterior walls and the central core of the structure. These columns supported all of the buildings' weight, but without lateral, or side-to-side, support from the floors, these columns would have buckled. The floors were built upon trusses, which bridged the distance between the exterior and core columns. Connected with two bolts on each end, these spans of rigid steel framework prevented the columns from bowing inward or outward. They also supported a 4-inch-thick (10 centimeters) floor made of reinforced concrete (reinforced concrete is embedded with steel for increased strength). Spray-on fireproofing, made from material similar to the rolled insulation in your home, further protected the integrity of the steel floor trusses, while the central columns were shielded by fire-resistant drywall. This economical design required less concrete and created nearly an acre of rentable office space on each of the buildings' 110 floors.

On Aug. 5, 1966, crews broke ground for the construction of the new World Trade Center towers. The north tower welcomed its first tenant in December 1970, while the south tower was first occupied in January 1972. The ribbon cutting for the entire complex occurred on April 4, 1973. Despite their innovative design, the buildings would be reduced to rubble just 28 years later. Read on to discover how the impact of the airplanes and the resulting fires ultimately brought down these massive skyscrapers.