On 11 September 2001, two Boeing aircraft 767 collided with the Twin Towers, which with their 102 floors were the tallest buildings in New York.
The first plane reached the North Tower at 8am 56 in the morning. The building caught fire for 102 minutes and then only 11h11 in the morning, it collapsed, collapsing in just 28 seconds.
Eighteen minutes after the first accident, at 9 am 000, the second plane hit the South Tower. The skyscraper resisted the flames by 56 minutes, and then at 9 am56, it collapsed in 9 seconds.
“After the incredible noise of the building collapsing, in a few seconds everything was more than the dark night, without sound, and I couldn’t breathe”, remembers Bruno Dellinger, a survivor who worked at 45 North Tower floor.
“I was convinced I was dead, because the brain can’t process something like that,” Dellinger said in his testimony shared by the Memorial and Museum of the 11 September in New York.
The balance was 2.606 dead.
Why the Are the towers dead?
Immediately after the attacks, civil engineer Eduardo Kausel, professor emeritus in the Department of Civil and Environmental Engineering at the Massachusetts Institute of Technology (MIT), led a series of studies and publications in which MIT experts have analyzed the causes of collapses from a structural, engineering and architectural point of view.
Kausel’s answer contains a series of physical and chemical phenomena that triggered a catastrophe like no one else. , at that time, I was able to imagine.
The MIT studies, published in 2002 , largely coincide with the conclusions of the report that the US government commissioned the National Institute of Fathers and Technology (Nist) to find out why the towers collapsed, and whose final version was published in 2002.
Both MIT and Nist conclude that the towers collapsed primarily due to a combination of two factors: the severe structural damage caused by aircraft collisions in each building and the chain of fires that spread over several floors
“If there had been no fire, the buildings would not have collapsed,” says Kausel. “And if there had been just one fire, without the structural damage, they wouldn’t have collapsed either.”
“The towers showed a lot of strength,” says the engineer.
The Nist report, in turn, states that there are official documents indicating that the towers were designed to withstand the impact of a Boeing 707, which was the largest aircraft existing commercial at the time of their project.
The researchers, however, warn that they have not found any information on the criteria and methods used to reach this conclusion.
O It is clear that, together, the impact and the fire produced a devastating result: the collapse of the two towers.
How the towers were built
The Twin Towers had a design that was standard in the decade of 1960, when they started to be built.
Each building had a vertical core of steel and concrete in the center that housed the elevators and stairs.
Each floor was formed by a series of windows. steel gas (horizontal) that departed from this core and connected with steel columns (vertical) to form the external walls of the building.
Interspersed with beams distribute the weight of each floor towards the pillars, while each floor, in turn, served as a lateral support that prevented the pillars from twisting, which in civil engineering is known as buckling.
The entire metallic structure was covered with concrete, which worked as a protector of beams and pillars in case of fire.
The beams and columns were also covered by a thin insulating layer, fireproof.
Impact, fire and air
Both towers were hit by different models of Boeing aircraft 767, larger than a Boeing 707.
The impact, according to the Nist report, “severely damaged” the columns and dislodged the fire insulation covering the structure’s beams and columns. ao.
“The vibration of the shock caused the fracture of the anterior coating. fire from the steel, leaving the beams more exposed to fire,” explains Kausel.
Thus, the structural damage opened the way for the flames, which in turn caused more structural damage.
Meanwhile, the temperatures, which reached 1. C, caused the window panes dilated and broke, which increased the flow of air, fueling the fire.
“The fire fed on air and so it spread,” says Kausel.
Official data estimate that each plane carried about 37.850 liters of fuel.
“They were flying bombs,” says Kausel.
Much of this fuel was burned during the fireball that formed on impact, but some of it was spilled on the lower floors of the towers.
This caused the fire to expand, encountering various objects flammables in his path that allowed him to continue advancing.
This fire had two main effects, explains the MIT engineer.
First, the intense heat caused the beams and slabs on each floor to expand. This caused the beams to separate from their slabs.
In addition, the expansion of the beams also pushed the columns outward.
But then there was a second effect . The flames began to soften the steel in the beams, making them malleable.
This made what were once rigid structures now look like ropes that, when bent, began to push inward the columns to which they were attached.
“This was fatal to the towers,” says Kausel.
At that moment, all the ingredients came together to trigger the collapse.
The columns were no longer fully vertical as the beams first pushed them out and then pulled them in, so they started to give way.
Thus, according to the Nist report, the columns began to collapse by arching, while the beams to which they were connected pulled them inwards.
*) Kausel’s analysis, on the other hand, adds that, in some cases, the beams pulled the columns so hard that they destroyed the screws that held them to the columns, which caused these floors to collapse. The debris caused overweight on the lower floors.
This put additional pressure on the capacity of the already weakened columns.
The result was a cascading drop.
After the building went into free fall, explains Kausel, the collapse progressively pushed the air between the floors, causing a strong wind.
This caused the collapse to be enveloped by a cloud of dust and the external walls collapsed outwards, “like someone peeling a banana”, says the expert.
Both buildings disappeared in seconds, but the fire in the rubble continued to burn for 47 days.
Twenty years later, the horror and pain caused by the attacks are still frightening.