The collapse of the World Trade Center (WTC) towers on September 11, 2001, remains one of the most shocking events in modern history. Numerous explanations have emerged since then, many of which are qualitative and, upon closer examination, scientifically inaccurate. This article delves into the quantitative analysis of the WTC collapse to separate fact from fiction, addressing common misconceptions such as steel melting and structural deficiencies. We aim to clarify the science and engineering principles that explain why the World Trade Center fell, offering insights into the factors that contributed to this catastrophic event and suggesting considerations for future structural designs.
Understanding the Original Design of the Towers
Constructed between the mid-1960s and early 1970s, the World Trade Center towers represented an innovative approach to skyscraper design, emphasizing lightweight construction and modular methods to expedite building and reduce costs. From a structural engineering perspective, each tower was essentially designed as a massive vertical cantilever column. Standing at 411 meters above street level and extending 21 meters below grade, each tower had a square base of 64 meters, resulting in a slender height-to-width ratio of 6.8. Despite a total structural weight of approximately 500,000 tons, the primary design consideration was wind load rather than gravity. The towers were engineered to withstand hurricane-force winds up to 225 km/h, equating to a lateral wind load of 2 kPa, or a total lateral force of 5,000 tons.
To achieve this wind resistance while maintaining a lightweight structure, the architects employed a “perimeter tube” design. This innovative system consisted of 244 exterior columns made from 36 cm square steel box sections, spaced on 100 cm centers. This design allowed for windows wider than half a meter, maximizing usable space and natural light. Within this outer tube was a central core, measuring 27 meters by 40 meters, designed to bear the gravity load of the tower. This core also housed essential building services, including elevators, stairwells, and mechanical utilities. Connecting the core to the perimeter at each story were 80 cm tall web joists, over which concrete slabs were poured to form the floors. This “egg-crate” construction method resulted in a structure that was approximately 95 percent air by volume, which explains the relatively low height of the rubble pile after the collapse.
This redundant “egg-crate” design was intended to ensure structural resilience. Theoretically, the building could withstand the loss of several columns because the load would redistribute to adjacent members. Before the advent of the lightweight perimeter tube design pioneered by the World Trade Center, most skyscrapers relied on massive columns spaced further apart (around 5 meters) and incorporated substantial masonry to bear structural loads. In contrast, the WTC was primarily a lightweight steel structure, but its numerous perimeter columns were designed to make it exceptionally robust and resilient.
The Impact of the Aircraft
Initial reports highlighted the towers’ ability to withstand the direct impact of the airplanes. Considering that the mass of each tower was over 1,000 times greater than that of the aircraft, and the towers were designed to resist sustained wind loads 30 times the weight of an airplane, their initial structural survival is not surprising. Furthermore, on September 11th, wind conditions were minimal, meaning the perimeter columns were stressed to only about one-third of their 200 MPa design allowable before the impact.
While the aircraft impacts undoubtedly damaged several perimeter columns, the number was not catastrophically large, and the structure’s redundancy allowed for load redistribution. The most critical consequence of the impact was the ignition of approximately 90,000 liters of jet fuel, nearly one-third of the aircraft’s weight, leading to massive explosions. The ensuing fires, not the initial impact, were the primary catalyst for the towers’ eventual collapse.
The Role of the Fire in the World Trade Center Collapse
The fires that erupted following the plane impacts are often misunderstood. A common misconception, even among some scientists and in media reports, is that the steel in the WTC melted due to the intense heat of the jet fuel. This belief, while widespread, is scientifically inaccurate. It’s crucial to differentiate between temperature and heat. Temperature, an intensive property, does not depend on the amount of material, whereas heat, an extensive property, does. Adding more fuel to a fire increases its duration and overall heat output, but not necessarily the peak temperature. Therefore, the large quantity of jet fuel at the WTC didn’t automatically translate to an exceptionally hotter fire capable of melting steel.
In combustion science, flames are categorized into jet burners, pre-mixed flames, and diffuse flames. Jet burners, like those in jet engines, achieve the highest heat intensity by mixing fuel and oxidant in near-perfect proportions within a confined space. Pre-mixed flames, such as in oxyacetylene torches, also involve pre-mixing fuel and oxidant but under constant pressure. Diffuse flames, like those in a fireplace or the WTC fires, occur when fuel and oxidant mix and combust in an uncontrolled manner. Diffuse flames produce the lowest heat intensities among these types.
The maximum theoretical flame temperature for burning hydrocarbons (like jet fuel) in pure oxygen is around 3,000°C. However, when burning in air, which is only about 21% oxygen, this temperature is significantly reduced to approximately 1,000°C. This is because a substantial amount of energy is used to heat the nitrogen in the air, which does not contribute to combustion. Furthermore, diffuse flames are often fuel-rich, meaning not all fuel is completely combusted, further reducing the temperature. The copious black smoke emanating from the WTC fires was a clear indicator of a fuel-rich, diffuse flame. Soot, a product of incomplete combustion, is responsible for the black smoke, confirming the fuel-rich nature of the fires. While factors like flame volume and soot can slightly increase temperatures by reducing radiative heat loss, it’s highly improbable that steel temperatures in the WTC reached beyond 750-800°C. The widely reported claim that steel melted at 1,500°C is, at best, a mischaracterization.
Some have speculated that the aluminum from the aircraft could have ignited, generating extremely high temperatures. While aluminum can burn under specific conditions, these conditions are not typical in a hydrocarbon-based diffuse flame. Burning aluminum produces a brilliant white flame, similar to a sparkler. There was no visual evidence of such a white-hot flame during the WTC fires, even through the dense smoke, making aluminum ignition an unlikely factor.
Structural steel begins to lose strength and stiffness at elevated temperatures, starting to soften around 425°C and losing approximately half of its strength by 650°C. This is why steel is stress-relieved within this temperature range in manufacturing processes. However, even a 50% reduction in steel strength alone is insufficient to explain the WTC collapse. As mentioned earlier, wind load was the primary design factor for the WTC, and on September 11th, the towers were likely stressed to only about one-third of their wind-load design capacity, or roughly one-fifth of the steel’s yield strength. Even with a 50% strength reduction due to fire, the steel could still support two to three times the imposed stresses at 650°C.
The critical factor, beyond strength reduction, was the distortion of the steel due to the non-uniform fire temperatures. Temperatures varied across the steel structures, with the exterior of box columns likely cooler than the fire-facing sides. Similarly, temperatures along the 18-meter floor joists were inconsistent. The thermal expansion of steel, coupled with temperature differentials as low as 150°C across structural members, can induce yield-level residual stresses. This thermal stress led to distortions and buckling failures in the slender steel components. Therefore, the structural failure was a combination of reduced steel strength due to elevated temperatures and, more critically, loss of structural integrity from steel distortion caused by uneven fire temperatures.
The Mechanics of the Collapse
Large buildings are generally designed with structural redundancy, allowing them to withstand the failure of a primary structural element, like a column. However, when multiple members fail, the redistributed loads can overwhelm adjacent members, leading to a cascading collapse, often described as a domino effect.
The perimeter tube design of the WTC, while highly redundant, was ultimately overcome by the fire-induced failures. While the towers survived the initial loss of exterior columns from the aircraft impact, the subsequent fires caused further steel failures. Many structural engineers believe that the crucial weak points were the angle clips connecting the floor joists to the perimeter wall columns and the core structure. Designed to support a floor load of 700 Pa, each floor should have been able to bear approximately 1,300 tons beyond its own weight. The total weight of each tower was about 500,000 tons.
As the floor joists on the most intensely burned floors gave way due to heat-induced weakening and distortion, the exterior columns began to buckle outward. This initiated the collapse of the floors above, which then exerted immense downward pressure on the floors below. A floor designed to support 1,300 tons could not withstand the sudden impact of approximately 45,000 tons or more from ten or more upper floors crashing down. This overload triggered a progressive, domino-like collapse, causing the buildings to fall in approximately ten seconds, reaching an estimated impact speed of 200 km/h. A true free-fall collapse, without any structural resistance, would have taken only about eight seconds with an impact speed of around 300 km/h.
It has been noted that the WTC towers collapsed almost vertically, rather than toppling sideways onto surrounding buildings. Several factors contributed to this vertical collapse. First, the buildings were mostly air (95%), allowing them to largely implode into their own footprint. Second, the inertia of a 500,000-ton structure is immense, making it extremely difficult to shift the center of gravity laterally to induce tipping, even with the force of an aircraft impact. Third, the rapid, near free-fall collapse speed left insufficient time for significant lateral movement to develop. In essence, the sheer mass and inertia of the towers dictated a predominantly vertical collapse.
Was the World Trade Center Design Flawed?
The design of the World Trade Center was not inherently flawed. The original designers could not have reasonably anticipated a scenario involving a massive, rapidly ignited jet fuel fire spanning multiple floors. Skyscrapers are typically designed to withstand standard office fires for about three hours, even if sprinkler systems fail, providing ample time for evacuation. The WTC towers stood for one to two hours after the impacts—less than the design fire resistance time, but only because the fuel load from the jet fuel was extraordinarily large and unprecedented. Normal office fires would take significantly longer to spread across such vast floor areas. The WTC fires were characterized by rapid ignition and intense heat release, although not exceptionally high temperatures in terms of steel melting.
Lessons Learned and Future Directions
The aftermath of the World Trade Center collapse involved a massive clean-up effort, dealing with approximately 1,000,000 tons of rubble, including asbestos fire insulation, and recyclable steel. The tragedy has prompted numerous changes in building codes and safety regulations. Improvements in emergency communication systems, enhanced evacuation procedures, and redundant emergency lighting systems are being implemented. Enhanced fire protection for structural members, smoke inhalation mitigation, energy-absorbing materials, and multiple egress routes are also under consideration for future building designs.
A fundamental engineering assessment of the WTC collapse dispels many myths. The perimeter tube design effectively protected the towers from immediate failure upon impact. The outer columns provided wind-load resistance and protected the gravity-load-bearing inner core. The removal of some exterior columns alone could not have caused the collapse. The stiffness of the perimeter design also prevented the aircraft impact from toppling the buildings.
However, the WTC towers were not designed to withstand the extreme heat generated by the jet fuel fires. While the fuel-rich, diffuse flames could not melt steel, the rapid ignition and intense heat weakened the steel, causing it to lose strength and distort. This weakening and distortion, particularly in the floor joist connections, initiated the collapse of several floors, leading to a progressive, domino-effect failure.
Designing buildings to withstand the fuel load of a burning commercial airliner is impractical. Instead of focusing on preventing structural collapse under such extreme circumstances, the emphasis should be on enhancing life safety through improved evacuation systems and building safety measures.
In the wake of such tragedies, a quantitative, science-based approach is crucial to understanding the facts and learning valuable lessons. As Lord Kelvin stated, “when you can measure what you are speaking about, and express it in numbers, you know something about it.” Moving forward, engineers and scientists must apply rigorous analysis and quantitative reasoning to design safer and more resilient buildings, drawing upon the lessons learned from the World Trade Center collapse. The emotional impact of the WTC tragedy stems from its intentional nature—an attack on innocent lives—underscoring the importance of applying our knowledge to build a safer future.
References
- Presentation on WTC Collapse, Civil Engineering Department, MIT, Cambridge, MA (October 3, 2001).
- D. Drysdale, An Introduction to Fire Dynamics (New York: Wiley Interscience, 1985), pp. 134–140.
- A.E. Cote, ed., Fire Protection Handbook 17th Edition (Quincy, MA: National Fire Protection Association, 1992), pp. 10–67.
- A.E. Cote, ed., Fire Protection Handbook 17th Edition (Quincy, MA: National Fire Protection Association, 1992), pp. 6-62 to 6-70.
- Steven Ashley, “When the Twin Towers Fell,” Scientific American Online (October 9, 2001); www.sciam.com/explorations/2001/100901wtc/
- Zdenek P. Bazant and Yong Zhou, “Why Did the World Trade Center Collapse?—Simple Analysis,” J. Engineering Mechanics ASCE, (September 28, 2001), also www.tam.uiuc.edu/news/200109wtc/
- Timothy Wilkinson, “World Trade Centre—New York—Some Engineering Aspects” (October 25, 2001), Univ. Sydney, Department of Civil Engineering; www.civil.usyd.edu.au/wtc.htm.
- G. Charles Clifton, “Collapse of the World Trade Centers,” CAD Headlines, tenlinks.com (October 8, 2001); www.tenlinks.com/NEWS/special/wtc/clifton/p1.htm.
