On September 11, 2011, two Boeing 767 aircraft collided with the Twin Towers, the tallest 110-story building in New York City. The first plane hit the North Tower at 8.45 am. The fire continued for 102 minutes and then at 10.28 minutes the tower collapsed in just 11 seconds. The second plane collided with the second twin tower at 09.03 AM, 18 minutes after the plane hit the first tower. For 56 minutes, this tower also kept burning with fire, then collapsed in the next 9 seconds.
Bruno Dellinger, who works on the 47th floor of the North Tower, recalls the incident, "After the sound of the building collapsing, there was silence for a few seconds. Darker than the night, for a moment all the voices also disappeared. I couldn't even breathe." "I thought I was dead because the brain couldn't process anything like that." He said this in his examination from the Memorial and Museum on September 11.
Eduardo Cossel, a retired professor in the Massachusetts Institute of Technology's (MIT) Department of Civil and Environmental Engineering, explained that "all experts have accepted the answer that both towers fell because it was the object of a terrorist attack." After the tower collapsed in the attack, Kausel was the head of a team of experts at MIT that analyzed the collapse of the twin towers from the structural, engineering, and architectural perspectives of the building.
MIT's research published in 2002 closely matches the findings of the US government's National Institute of Standards and Technology (NIST), which was tasked with investigating the causes of buildings collapse. The results were published in 2008. Both MIT and NIST concluded that the two biggest reasons behind the tower's collapse had to do with one. First, the two buildings suffered severe structural damage when the planes collided. Secondly, the fire in the buildings after the collision had spread to several floors. "If there was no fire, these buildings would not have collapsed.
As NIST reports, official documents indicate that these towers were designed to withstand a collision with a Boeing 707 aircraft, the largest commercial aircraft existing at the time of the building's design. However, the NIST researchers did not provide any information about the criteria and methods used to reach this conclusion.
Steel beams were mounted horizontally on each of its floors, starting at the core and connected to the upright steel columns mounted in the original building to form the building's exterior walls. Groups of steel beams distribute the weight of each floor towards the pillar (center). Also, each floor used to support it (pillar) separately so that it would not bow down. In civil engineering, it is called buckling.
Both towers collided with a large Boeing. They were designed to withstand the collision of a Boeing 707. But a bigger Boeing 767 collided with it. According to the NIST report, the pillars of the building were severely damaged as a result of the collision, which removed the steel beams and fireproof insulator covering the pillar structure.
"The vibrations resulting from the collision broke the fireproof coating on the steel, and the beams were easily exposed to fire," explains Cosell. Thus the entire building was opened to flames and the structure was damaged. While the fire was spreading, the temperature in the building reached 1000 degrees centigrade and caused the windows to crack and break. As the windows were broken, air came in from outside, which proved to be helpful in spreading the fire. "The fire got air and it spread," says Kausel.
The pillars no longer stood completely upright as the beam pushed them outwards and then inwards, causing them to bend. Thus, as reported by NIST, the pillars tilted and started falling. The beams to which they were attached were pulling them inward. Kausal's analysis, on the other hand, states that in some cases the beams were pulling the poles so hard that it broke their nut bolts to which they were attached to the poles. Due to this these floors collapsed and the debris started creating a lot of weight on the underside of them.
Cosell explains that once the building collapsed, air came out between its floors and quickly spread around. Because of this very strong wind was blowing all around there. That is why there was a cloud of dust there. Within seconds both buildings disappeared, but the rubble fire continued to burn for the next several days. Even after 20 years, the horror and pain of that attack have not diminished.
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