In this blog post, we will briefly examine the causes and implications of the collapse of the Twin Towers following the September 11 terrorist attacks.
Overview of the Incident
On September 11, 2001, two hijacked Boeing 767s crashed into the upper sections of the World Trade Center Twin Towers (WTC 1 and WTC 2) in New York at 8:46 a.m. and 9:03 a.m., respectively. Although the buildings did not collapse immediately after the collisions, a combination of fire and structural damage caused WTC 2 to collapse sequentially from the upper floors at 9:59 a.m. and WTC 1 to collapse at 10:29 a.m., reducing the 110-story buildings to piles of rubble.
Approximately 50,000 people worked in these buildings, and many visitors passed through them daily. As a result of the collapse, 2,752 of the approximately 58,000 people inside the buildings died, and about 500 rescue workers also lost their lives. The event unfolded as a catastrophic “progressive collapse,” shocking the entire world.
The Basics of Building Loads and the Nature of Collapse
Buildings are designed to withstand various loads, including dead load (the weight of the structure itself), live load (items and people inside), wind loads, and seismic loads. Among these, gravitational loads (dead load and live load) are the fundamental loads that are always present. While lateral wind and seismic loads are considered separately, regardless of the initial cause of collapse, a building ultimately collapses when the balance of gravitational forces is disrupted.
Structural defects or errors in design and construction are human factors that can be predicted and controlled, but external forces such as explosions or aircraft collisions are difficult to predict. While there have been cases of aircraft collisions in the past (e.g., the 1945 Empire State Building collision), none of them led to catastrophic cascading collapse. This demonstrates that total collapse can only occur when multiple factors combine.
The Concept of Cascading Collapse — The Domino Analogy
Cascading collapse is likened to the phenomenon of dominoes falling in sequence. It is a comprehensive failure mechanism in which damage originating from a localized failure transmits shock to the surrounding structure, and this accumulated shock ultimately leads to the collapse of the entire structure.
As in the domino example, placing a stable obstacle—such as a “thick encyclopedia”—in the middle to absorb the impact of the first domino breaks the chain. In architecture, such a “second line of defense” must be incorporated into the design to prevent localized damage from spreading into a total collapse.
WTC Collapse Mechanism: Professor Bazant’s Team’s Sequential Scenario
One of the most compelling scenarios proposed after extensive expert review is the sequential collapse model developed by Professor Bazant’s team at Northwestern University. The key points are as follows: While the plane crash caused localized damage to columns and floors, it did not immediately lead to total collapse. The problem was the massive fire caused by the fuel.
As the fire caused the temperature of the steel to rise, the stiffness and strength of the columns and beams were significantly reduced. In particular, areas where the fireproof coating had peeled off were exposed to high temperatures, causing the columns to buckle (a phenomenon where the cross-section becomes too small relative to the length, causing sudden bending under compressive load) or the joints to fail. As the upper floors partially collapsed, the lower columns could not withstand the kinetic energy of the falling mass, leading to a chain reaction of collapse.
Of the approximately 60 columns on the impact side, a significant number were severed or severely damaged; although the proportion of exterior wall columns (approximately 287 in total) directly damaged by the impact was small (about 16%), the load was concentrated on the undamaged columns. Furthermore, the combustion of jet fuel generated temperatures far higher than those of a typical office fire, accelerating damage to the fireproof cladding and reducing the strength of the steel.
A Brief Mechanical Analysis
One of the initial points of contention was whether “the impact force of the plane crash itself immediately triggered the chain reaction.” Based on Newton’s laws of motion (change in momentum = impact), the impact force can be roughly estimated as follows. Assuming a total aircraft weight of approximately 200 tons, a pre-impact speed of 250 m/s, and an impact duration of about 0.4 seconds (estimated to have traveled approximately 50 meters after impact before coming to a stop), the impact force is calculated to be roughly 12,500 tons.
This figure is relatively small compared to the wind loads considered in the building’s design (estimated to be approximately 30–40% of the design wind load), so the damage caused by the impact is likely to have been localized. In fact, the building did not collapse immediately after the impact; if the collapse had been caused solely by the impact force, a large moment would have acted on the lower floors, causing an immediate lateral collapse.
The lateral displacement caused by the impact is estimated to be only about 20–40 cm, which is very small—just 1/1,000 to 1/2,000 of the building’s total height. In other words, the dynamic deformation caused by the impact alone is insufficient to explain the total collapse.
On the other hand, local collapse caused by the buckling of columns whose strength has been reduced by high temperatures could result in the free fall of the upper-story mass. For the falling upper-story mass to come to rest, the decrease in potential energy must be absorbed as deformation energy by the columns. In the case of WTC 2 (estimated values: story height h ≈ 3.7 m, upper mass m ≈ 5.8×10^8 kg, effective stiffness of columns and structure k ≈ 7.1×10^10 N/m, g=9.81 m/s^2), a simple energy balance calculation indicates that a load tens of times greater than the design load was applied.
These approximate calculation results support the interpretation that, even when safety factors are considered, the compressive failure of columns would have occurred continuously, making a chain collapse inevitable. In other words, it is reasonable to conclude that the chain collapse began due to a combination of local damage caused by the impact and the weakening of the steel caused by the fire.
Prevention of Chain Collapse and Design Lessons
This incident had a significant impact not only on architectural engineering but also on design practices and disaster management.
In particular, design concepts emphasizing “alternative load paths” and structural integrity to prevent cascading collapse in super-tall buildings have been highlighted.
Generally, to prevent cascading collapse through design, one must assume “column loss scenarios” and ensure that even if one or several columns are lost, the surrounding beams and columns redistribute the load so that it does not lead to total collapse. Taking the example of a column on the first floor in the central section being lost, the columns and beams on either side must bear and transfer the column load that was previously supported by that column, and they require sufficient strength and ductility (the ability to deform) to resist deflection.
Since the WTC incident, there has been a reevaluation of how a lack of design margin—resulting from cost-cutting-focused optimization, the overuse of connection methods, and excessive reliance on precise analysis—can pose risks to structural safety. Securing design margins for strength and ductility, which act as a “second line of defense,” and ensuring alternative load paths have become critical during the design phase.
Conclusion
Although the WTC collapse was a complex event, the key point is that localized damage caused by the impact, combined with structural members weakened by massive fires, led to the free fall of the upper-story mass. Since the lower structure could not absorb this kinetic energy, it resulted in a cascade collapse. Even simple concepts of Newtonian mechanics—such as conservation of momentum and energy balance—can explain the core of this complex phenomenon.
Ultimately, the key lesson is that gravity is the “terminator,” and a building collapses when the balance of forces against gravity is disrupted. Therefore, designers must always keep in mind structural integrity, alternative load paths, and sufficient safety margins to prevent localized damage from propagating into a total collapse.
