- Carlos Serrano – @carliserrano
- BBC News World
On September 11, 2001, two Boeing 767 planes collided with the Twin Towers, which at 110 stories were the tallest buildings in New York.
The first plane hit the North Tower at 8:45 am. The building caught fire for 102 minutes and then, at 10:28 am, it collapsed, collapsing in just 11 seconds.
Eighteen minutes after the first accident, at 9:03 am, the second plane hit the South Tower. The skyscraper resisted the flames for 56 minutes, and then, at 9:59 am, collapsed in 9 seconds.
“After the incredible noise of the building collapsing, in a few seconds everything was darker than night, with no sound, and I couldn’t breathe,” recalls Bruno Dellinger, a survivor who worked on the 47th floor of the North Tower.
“I was convinced I was dead, because the brain can’t process something like that,” Dellinger said in his testimony shared by the 9/11 Memorial and Museum in New York.
The balance was 2,606 dead.
Why did the towers fall?
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 analyzed the causes of a point of collapse. structural, engineering and architectural view.
Kausel’s answer contains a series of physical and chemical phenomena that triggered a catastrophe that no one at that time was able to imagine.
The MIT studies, published in 2002, largely coincide with the findings of a report that the US government commissioned the National Institute of Standards and Technology (Nist) to find out why the towers fell, and whose final version was published in 2008.
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 hadn’t been a fire, the buildings wouldn’t 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 resistance”, 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 commercial aircraft existing at the time of its project.
The researchers, however, caution that they have not found any information about the criteria and methods used to reach this conclusion.
What is clear is 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 1960s, when they began 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 steel beams (horizontal) that started from this core and connected with steel columns (vertical) to form the outer walls of the building.
The interweaving of beams distributed 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 acted as a protection for beams and pillars in case of fire.
The beams and columns were also covered with a thin, fireproof insulating layer.
Impact, fire and air
Both towers were hit by different models of Boeing 767 aircraft, larger than a Boeing 707.
The impact, according to the Nist report, “severely damaged” the columns and dislodged the fire insulation covering the steel beam and column structure.
“The vibration from the shock caused the steel’s fireproof coating to fracture, leaving the beams more exposed to fire,” explains Kausel.
Thus, structural damage paved the way for flames, which in turn caused further structural damage.
Meanwhile, temperatures, which reached 1,000°C, caused the window panes to dilate and break, which increased the airflow, fueling the fire.
“The fire fed on air and so it spread,” says Kausel.
Official data estimate that each plane carried around 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 spilled into the lower floors of the towers.
This caused the fire to expand, finding several flammable objects in its path that allowed it 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 slabs to separate from their beams.
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 rafters, 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 for 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 that they began to give way.
So, according to the Nist report, the columns began to collapse by arching, as the beams to which they were connected pulled them inward.
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 those floors to collapse. Debris caused overweight on the bottom floors.
This put additional pressure on the capacity of the already weakened columns.
The result was a ripple effect.
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 to collapse outwards, “like someone who is peeling a banana”, says the expert.
Both buildings disappeared in seconds, but the fire in the rubble continued to burn for 100 days.
Twenty years later, the horror and pain caused by the attacks are still frightening.
Have watched our new videos on YouTube? Subscribe to our channel!