Structural engineers generally agree that the collapse of the World Trade Center (WTC) Twin Towers on September 11, 2001, was due to a combination of factors involving the structural damage caused by the airplane impacts and the subsequent fires.

Here are the key reasons commonly cited by structural engineers:

1. Initial Structural Damage from Airplane Impact

The planes that hit the North Tower (WTC 1) and the South Tower (WTC 2) caused significant damage to the structural framework. Each airplane severed key load-bearing columns and compromised the towers' ability to distribute the weight of the floors above the impact zone. According to engineers, this immediate damage weakened the overall stability of the buildings.

• Core and Perimeter Column Damage: The Twin Towers had a unique tube-like structure with a strong steel core and perimeter columns. The impact destroyed portions of the external and internal columns, which supported much of the towers' weight.
• Damage to the Fireproofing: Engineers believe that the impact also dislodged or destroyed the fireproofing materials protecting the steel columns. This would later contribute to the failure of the steel structure under prolonged heat.

2. Intense and Prolonged Fires

While the initial damage was critical, it was the fires ignited by the airplanes' jet fuel that ultimately led to the towers' collapse. Engineers explain that:

• High-Temperature Fires: The burning jet fuel created extremely high temperatures (over 1,000°C or 1,832°F). Although the steel columns themselves wouldn’t melt at these temperatures, they significantly weakened.
• Weakened Structural Steel: Structural engineers emphasize that steel loses much of its strength when exposed to extreme heat, even without melting. The intense fires caused the steel to soften, lose strength, and eventually buckle under the weight of the upper floors.

3. Progressive Collapse

Once the structural integrity was sufficiently weakened, a "progressive collapse" ensued. This means that once one floor collapsed, the combined weight of the upper floors caused a chain reaction of floor failures:

• Pancake Effect: The floors of the towers collapsed one after the other in a sequence, often referred to as the "pancake" effect. This rapid vertical collapse overwhelmed the remaining structural components, which could not withstand the enormous downward force.

4. Design Considerations and Limitations

While the towers were designed to withstand large-scale forces, such as high winds and the impact of smaller aircraft (like a Boeing 707), the size, speed, and fuel load of the Boeing 767s that hit the towers were outside the parameters originally considered.

• Innovative but Vulnerable Design: The towers’ tube-in-tube design provided them with remarkable resistance to lateral forces (like wind), but made them more vulnerable to localized failures, especially when the fireproofing was damaged.
• Fireproofing Insufficiencies: Engineers believe the fireproofing material on the steel columns was insufficient to handle such intense, prolonged heat. The absence of proper fire protection played a crucial role in the failure of the structural components under high temperatures.

5. Structural Asymmetry Post-Impact

Engineers point out that the structural damage was not symmetrical in either tower, leading to an uneven distribution of loads. In the South Tower (WTC 2), the impact was lower and more off-center than in the North Tower (WTC 1), which is why it collapsed faster despite being hit later.

• Impact Zones: The varying heights and locations of the impacts caused different degrees of structural damage. In the South Tower, the core of the building was damaged more severely, contributing to its earlier collapse.

Final Reports

Extensive investigations, most notably by the National Institute of Standards and Technology (NIST), confirmed the views held by most structural engineers. Their findings align with the aforementioned reasons—specifically, the combination of initial structural damage, the intense heat from fires, and the progressive collapse mechanism.

These factors led to a catastrophic failure that could not be stopped once the process of collapse began. The findings were published in a detailed report by NIST in 2005 and have been widely accepted within the engineering community as a comprehensive explanation of the collapse.