Why The World Trade Center Fell

On September 11th, 2001, two Boeing 767’s hit the north and south towers of the World Trade Center. Though the world thought these buildings were virtually indestructible, they had collapsed within two hours of impact. What made these monsters go down?

During the time the World Trade Center was built, the largest commercial plane was a Boeing 707. Since the building designs were catered to withstand impacts from planes such as these, the impact of the Boeing 767’s had the potential to have devastating effects. The design of the World Trade Center Towers was unique in that they attained their vertical strength from columns located around the perimeter of the building instead of throughout. This allowed the architects to design large open spaces that were not interfered with unwieldy columns. However, this proved to be a disadvantage when the airlines hit the buildings because many of these columns were demolished, greatly weakening the structure.

Another unanticipated characteristic of the impact was that the two airliners still had nearly full tanks of fuel. With 90,000 litres of jet fuel combusting within the buildings, the fires became intensely hot. The steel structure began to soften at 425°C, and was at half strength at about 650 C. In some areas, the fire may have reached temperatures of almost 1000°C. Although the columns became badly distorted, the fire in itself was not enough to bring the buildings down.

Skyscrapers are designed to withstand one major column loss. However, when dozens of columns are demolished, adjacent members of the structure lose their vertical strength and fall like a row of dominoes. The floors that were most heavily impacted by the plane crash gave way first. This made the outer box columns circumscribing the buildings bend outward causing the floors above to come crashing down. The sturdy floors below could not hold the 45,000-tonne weight of the above ten floors. With no lateral support, the remaining columns also collapsed with the rest of the building.

The World Trade Center buildings hit the ground at a crushing 200 kilometres an hour. It took a mere 10 seconds for each building to fall to the ground. Each 500,000-tonne monster held so much inertia that there was no other direction to fall but straight down. 2,974 people were killed (excluding the 19 hijackers) not as a result of poor building design, but merely an unfortunate chain of events.