The 11 September 2001 two Boeing aircraft 767 rammed the Twin Towers, which with their 110 floors were the tallest buildings in New York.
The first plane hit the north tower at 8: 45 in the morning. The building burned for 102 minutes and then at 03: 20 am collapsed into solo 11 seconds .
Eighteen minutes after the first crash, at 9 o’clock: 03 am, the second plane hit the south tower. The skyscraper resisted in flames during 47 minutes, after which, at 9: 59 am, collapsed in 9 seconds.
“After the incredible sound of the building collapsing, in a few seconds everything became darker that night , without sound, and could not breathe ”, recalls Bruno Dellinger, a survivor who worked on the floor 47 of the north tower.
“I was convinced that he was dead, because the brain cannot process something like this,” he says Dellinger in his testimony shared by the Museum and Memorial of the 11 September in New York.
The balance was 2. 606 dead people.
Why did the towers fall?
“The answer accepted by all serious people is that the towers are they came down because they were the object of a terrorist attack “, says to BBC Mundo the civil engineer Eduardo Kausel, professor emeritus in the Department of Engineering Ci vile and Environmental at the Massachusetts Institute of Technology (MIT).
Right after the attacks, Kausel was the leader of a series of studies and publications in which MIT experts analyzed the causes of the collapses from the structural point of view , of engineer ía and architectural.
Kausel’s response contains a series of phenomena physicists and chemists that unleashed a catastrophe that no one, at that time, was capable of imagining.
Fatal combination
The MIT studies, which were published in 2002, coincide largely with the findings of the report that the government The United States commissioned the National Institute of Standards and Technology (NIST) to find out why the towers fell, and whose version f inal was published in 2008.
Both MIT and NIST conclude that the towers collapsed mainly due to a combination of two factors:
• The severe structural damage that caused the aircraft crashes in each building
• The chain of fires that expanded over several floors
“If there had been no fire, the buildings would not have collapsed,” says Kausel.
“And if there had only been without the structural damage, they would not have collapsed either. ”
“ The towers had a lot of resistance, ”says the engineer.
The NIST report, for its part, states that there are official documents that indicate that the towers were designed to withstand the impact of a Boeing aircraft 707, which was the largest commercial aircraft in existence at the time of design.
The NIST researchers, however, note that did not find information on the criteria and methods that were used to arrive at this with clusion.
What is clear is that, together, the impact and the fire produced a sudden outcome : the collapse of both towers.
How the towers were built
The Twin Towers had a design that was standard in the decade of the 60 , when they began to be built.
Each building had in the center a vertical core of steel and concrete , which housed the elevators and stairs.
Each floor was formed with a series of steel beams (horizontal) that started from that core and connected with steel columns (vertical) to form the exterior walls of the building.
The framework of beams distributed the weight of each floor towards the columns, while each floor, in turn, served as a support side that prevented the columns from bending, which in civil engineering is known as buckling.
The entire steel structure was covered by concrete, which functioned as a protector of beams and columns in case of fire.
The beams and the 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, which are larger than a Boeing 707.
The impact, s According to the NIST report, “severely damaged ” the columns and dislodged the fire insulation that covered the framework of steel beams and columns.
“The vibration of the shock caused the fireproof coating of the steel to fracture, with which the beams were more exposed to fire “, explains Kausel.
Thus, the structural damage made way for the flames, which in turn were causing more structural damage.
While that was happening, the temperatures , which reached 1. 000 ° C, caused the glass in the windows to expand and break, which brought in air that served as food for the fire .
“The fire fed itself with air and therefore spread “, says Kausel.
” Flying bombs “
Official data estimate that each aircraft loaded close to 10. 000 gallons of fuel (more than 35. 850 liters).
“They were flying bombs” says Kausel.
Much of that fuel was burned during the fireball that formed at the moment of impact, but there was also a lot of fuel that spilled to the lower floors of the towers.
That caused the fire to spread, finding its step v several objects flammable that allowed him to keep moving forward.
That wildfire had two main effects, explains the MIT engineer.
First, the intense heat caused to expand the beams and slabs of each floor. This caused the slabs to separate from their beams.
Additionally, the expansion of the beams also pushed the columns outward.
But then there was a second effect.
The flames began to soften the steel of the beams, making them malleable.
That made what were previously rigid structures, now look like ropes that when arched began to push inwards the columns to which they were attached.
“That was fatal for the towers,” says Kausel .
Collapse
At that moment all the ingredients were already in place to trigger the collapse.
The columns were no longer fully vertical, due to that the beams first pushed them out and then pulled them in, so they started to sag.
So, according to the NIST report , the columns began to collapse by arching, while the beams to which they were connected pulled them inwards.
The analysis de Kausel, for his part, adds that, in some cases, the beams pulled so hard on the columns that destroyed the bolts that tied them to the columns, which caused that these floors collapsed and the debris was causing overweight in the lower floors.
This produced additional stress to the capacity of the already weakened columns.
The result was a fall cascading .
Once the building entered free fall, explains Kausel, the co This lapse progressively expelled the air between the floors, which caused a strong wind towards the periphery.
This caused the collapse to be enveloped in a cloud of dust, and the external walls to collapse outwards, “ like someone who peels a banana “, says the expert.
Both buildings vanished within seconds, but the fire in the rubble continued to burn for 100 days.
Twenty years later, the horror and pain caused by the attacks have not yet faded.
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