World Trade Center Collapse: Architecture and Engineering

Introduction: The Twin Towers and September 11

The September 11, 2001 attacks changed the face of the Manhattan skyline and will remain one of the worst days in American history in the minds of the American public. That day represented pure hatred and disregard for human life, with thousands of innocent working men and women falling victim to an attack rooted in hatred toward the government and military presence of the United States abroad. Thousands of people went to work that morning as they would on any other day, only to witness one of the greatest tragedies in American history.

The two World Trade Center towers, which were built to withstand the impact of a jet plane, fell victim to the intense heat of a jet-fuel fire, leading to their collapse and the deaths of thousands of workers, firefighters, and police officers on that tragic September morning. Although the design of the Twin Towers could not have been significantly improved given the conditions and knowledge of the era in which they were built, modern architects can learn from both the failures and the strengths of the buildings to help construct safer and stronger skyscrapers using the materials available today.

Architecture and Structural Design of the Twin Towers

The Twin Towers were an incredible feat of engineering, both at the time of their completion and even up until their collapse in 2001. Completed by 1973, the two towers of the World Trade Center were among the largest buildings in New York. They dominated the Manhattan skyline and became an important symbol of American engineering and capitalism. In total, the two buildings cost over $400 million to construct. Owned by the Port Authority of New York and New Jersey, they each had 110 floors and stood over 1,300 feet above the Manhattan street level (Wilkinson 1). Each tower contained approximately 104 passenger elevators and 21,800 windows, allowing thousands of workers to commute through and appreciate the modernity of these giant skyscrapers.

The towers were built in the form of a tube building in order to maximize safety and structural strength. In design, "tube buildings are strengthened by closely placed columns and beams in the outer walls" (PBS Online). These closely spaced columns helped reinforce the massive buildings and redistribute the weight of the floors, as well as external pressure from forces such as wind. The columns served an important structural purpose while also defining the distinctive aesthetic appearance of the buildings: "The columns, finished with a silver-colored aluminum alloy, were 18 inches wide and set only 22 inches apart, making the towers appear from afar to have no windows at all" (Wilkinson 1).

The columns helped maintain the building's core even under the extreme loads that structures of this magnitude naturally bear. High winds were the primary lateral force considered during design, but the possibility of a commercial jet plane crash also came into play during the early stages of planning — a consideration that later helped the buildings remain standing as long as they did after the initial impacts on September 11, 2001. The columns were also engineered to handle settlement loads: "Because the towers were also built on six acres of landfill, the foundation of each tower had to extend more than 70 feet below ground level to rest on solid bedrock" (PBS Online 1). The two towers therefore had a very sturdy base reaching far below the surface, past the subway lines underneath, and deep into the original bedrock of lower Manhattan. This was a remarkable feat of engineering, accomplished entirely without the use of masonry (Wilkinson 1). Together, the two buildings stood as a symbol of American engineering and capitalism — which is ultimately the reason they were targeted on September 11, 2001.

Although the towers were built to withstand the impact of a commercial plane crash, the primary cause of their complete destruction and collapse was the raging fires that weakened the steel frame and support columns. As one analysis explains, "although they were in fact designed to withstand being struck by an airplane, the resultant fires weakened the infrastructure of the building, collapsing the upper floors and creating too much load for the lower floors to bear" (PBS Online 1). The initial explosion following the plane impacts may have blown away the fire insulation from the steel beams, exposing the steel directly to the intense heat of the jet-fuel fire (Buyukozturk & Ulm 1). The explosion also directly compromised the enormous steel structures: "It appears likely that the impact of the plane crash destroyed a significant number of perimeter columns on several floors of the building, severely weakening the entire system" (Wilkinson 1).

The Role of Fire in the Collapse

However, the plane crashes alone were not the immediate cause of the collapse. According to research conducted after the attacks, the fires that ignited following the impacts were far from ordinary office fires. They were fueled by the explosion of the fuel tanks in the commercial jets that struck each tower: "The fire would have been initially fueled by large volumes of jet fuel, which then ignited any consumable material in the building" (Wilkinson 1). With such an intense fire burning for hours, it was only a matter of time before the integrity of the steel columns and overall structure was compromised.

The fire did not melt the steel outright, but it proved hot enough to alter the steel's material properties and threaten its load-bearing strength: "While the fire would not have been hot enough to melt any of the steel, the strength of the steel drops markedly with prolonged exposure to fire, while the elastic modulus of the steel reduces — stiffness drops — increasing deflections" (Wilkinson 1). With the steel's properties compromised, it became brittle and weak, unable to support the weight of the debris and the numerous floors above the crash site. Compounding the severity of the fires, the plane impacts had also disabled the internal fire-safety systems: "It is likely that the water pipes that supplied the fire sprinklers were severed by the plane impact, and much of the fire-protective material, designed to stop the steel from being heated and losing strength, was blown off by the blast at impact" (Wilkinson 1). Under the combined effects of weakened steel and jet-fuel-driven fires, both towers collapsed within hours of the initial impacts.

For a more detailed technical account of how the September 11 attacks unfolded and the engineering response they prompted, Encyclopædia Britannica provides a thorough overview of the events and their aftermath.

Almost immediately after the crashes, investigators and curious minds began asking whether anything in the World Trade Center's design could have prevented the outcome. Could the exterior skin or structure have stopped the planes from penetrating the interior? Could the steel have been arranged to prevent collapse? These were natural questions arising from the public's grief over the beloved Twin Towers. On the whole, however, the design of the buildings performed relatively well given the circumstances.