Historical Context and Attack Sequence
The attacks on September 11, 2001, were a series of coordinated terrorist events that led to the collapse of the World Trade Center’s Twin Towers.
These moments unfolded with a chilling precision that shocked the world and permanently altered the skyline of New York City.
Chronology of the Attacks
On the morning of September 11, 2001, four commercial airliners were hijacked by terrorists affiliated with al-Qaeda.
The sequence began with American Airlines Flight 11 departing from Boston at 7:59 AM, en route to Los Angeles, which was crashed into the North Tower of the World Trade Center at 8:46 AM.
Shortly after, at 9:03 AM, United Airlines Flight 175, also flying from Boston to Los Angeles, hit the South Tower.
This precise timeline of events triggered a catastrophic chain reaction leading to the towers’ eventual collapse.
| Time (EDT) | Event |
|---|---|
| 7:59 AM | American Airlines Flight 11 departs Boston |
| 8:46 AM | Flight 11 crashes into the North Tower |
| 9:03 AM | United Airlines Flight 175 crashes into the South Tower |
Profile of the World Trade Center
The World Trade Center complex was an iconic feature of New York City’s skyline.
The Twin Towers, each standing at 110 stories tall, were marvels of modern architecture constructed of lightweight steel and designed with a central core.
The World Trade Center towers were approximately 95 percent air and, despite their size, their structural vulnerability became the focal point of the attack.
Details of the Hijacked Flights
Both American Airlines Flight 11 and United Airlines Flight 175 were transcontinental flights hijacked by terrorists shortly after takeoff from Boston.
The flights carried a total of 10 hijackers, five in each plane.
Flight 11 carried 81 passengers and 11 crew members, and Flight 175 carried 56 passengers and 9 crew members.
The impact zones were between floors 93 to 99 in the North Tower and floors 77 to 85 in the South Tower.
These impacts caused damage that compromised the buildings’ structural integrity and, combined with the ensuing fires, led to their total progressive collapse.
The catastrophic events claimed the lives of nearly 3,000 people, including all passengers on both flights.
The Collapse Mechanism and Structural Analysis
The intricate interplay between structural damage from aircraft impacts and subsequent fires led to the collapse of the World Trade Center towers.
Detailed investigation revealed how each factor contributed to weakening the building’s steel framework, culminating in the towers’ tragic fall.
Critical Damage to the Towers
On September 11, 2001, hijacked airplanes struck the World Trade Center towers in Lower Manhattan, causing severe damage.
The aircraft sliced through the perimeter columns and the building’s central core, compromising the structural integrity of both the North and South Towers.
Each plane carried thousands of gallons of fuel, which exacerbated the damage.
The force of impact dislocated crucial fireproofing materials and severed key structural elements including bolts and floor trusses.
The Fire and Its Contributions
The resulting infernos, fed by the jet fuel, generated extreme heat, further weakening the steel framework.
Flames spread through the elevator shafts, accelerating the breakdown of critical components.
The high heat compromised the viscoelastic dampers and the steel, reducing their ability to bear the gravity load.
Engineering Insights on the Collapse
Investigations by the National Institute of Standards and Technology (NIST) provided insights into the collapse process.
They noted that the combination of structural damage from the impact and weakening of steel from the fires triggered a progressive collapse.
The failure started with the floor trusses, which led to the floors pancaking down onto each other.
This put an untenable load on the central columns, causing them to buckle.
As these components failed, the remaining structure could not support the gravity load, leading to the complete collapse of the towers.