World Trade Center history and significance

World Trade Center Collapse

The Architecture and Design of the Twin Towers

The September 11, 2001 attacks changed the face of the Manhattan skyline and will remain as one of the worst days of 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 that originated out of hatred towards the government and military presence of the United States elsewhere in the world. Thousands of people got up and went to work that morning like it was any other morning, only to witness one of the greatest tragedies of 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 propelled fire, leading to their collapse and the demise of thousands of workers, fireman, and police on that tragic September morning. Although the design of the two twin towers really could not have been improved that much based on the conditions of when they were built, modern architects can learn from the failures of the buildings as well as their strengths to help in the construction of modern skyscrapers to be the strongest and safest buildings possible with the materials available to us.

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 twin towers of the World Trade Center were some of the biggest buildings in New York. They dominated over the Manhattan skyline, and became an important image 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 had 110 floors and stood over 1,300 feet high in the Manhattan sky (Wilkinson 1). Each separate tower had around 104 passenger elevators and 21,800 windows, allowing for thousands of workers to commute and enjoy the modernity of the giant skyscrapers. They were built in the fashion of a tube building in order to promote the highest level of safety and strength. In design, "Tube buildings are strengthened by closely placed columns and beams in the outer walls," (PBS Online). These close proximity columns helped strengthen the support of the massive buildings and helped redistribute the weight of the floors and external pressure placed on the structure by such outside factors as wind. The columns served an important mechanical purpose in the safety of the building, but also set the tone for the original design and aesthetic look of the buildings themselves; "The columns, finished with a silver-colored aluminum alloy, were 18 ae" wide and set only 22" apart, making the towers appear from afar to have no windows at all," (Wilkinson 1).

They helped maintain the building core even through the extreme loads that buildings of this magnitude would naturally withstand. High winds were the biggest danger considered to place force and pressure on the buildings, but the idea of a commercial jet plane crash also came into play during the early stages of the design and construction, which later helped the buildings stand for as long as they did after the initial crashes on September 11, 2009. The columns were also built to withstand 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). Therefore, the two towers had a very sturdy base that went far own into the earth, past the subway lines underneath, and way into the original bedrock of lower Manhattan. This was an amazing feat of engineering, and all designed without the use of masonry (Wilkinson 1). Together, the two buildings were a symbol of American engineering and capitalism, which is ultimately the reason they were targeted for the attacks which occurred on September 11, 2001.

Although they were built to withstand the impact of a commercial plane crash, the main reason for the buildings completely destruction and collapse was the raging fires that weakened the steel frame and support columns; "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 after the impact of the plane could have blown away the fire insulation from the steel beams, which then exposed the steel to the intense heat of the jet fuel caused fire, (Buyukozturk & Ulm 1). The explosion of the planes itself also helped weaken 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). However, this was not the initial cause of the collapse. According to later research conducted after the attacks, after the beams and frame were exposed to the intense fire, they began to weaken. This fire was definitely not an ordinary office fire. It was caused by the explosion of the fuel tanks in the commercial jets which 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 a high intensity fire raging for hours and hours on end, it was only a matter of time before the integrity of the steel columns and structure were compromised. The fire itself did not melt the steel in a way that would cause it to collapse, but it did prove hot enough to change the chemical make up of the steel as to threaten its 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 chemical make up of the steel altered, it became brittle and weak; unable to hold the weight of the debris and numerous floors above the site of the initial plane crash. On top of the severity of the fires, the plane crash had damaged the internal fire safety precautions, making it next to impossible to fight the raging hot fire, "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 loosing strength, was blown off by the blast at impact," (Wilkinson 1). And so, under weakened steel and jet fuel propelled fires, both the towers collapsed within hours of the initial plane crash.

Almost immediately after the crash, investigators and curious minds began wondering if anything could have been avoided within the elements of the World Trade Center design. Could the exterior skin or structure of the building have prevented the plane from breaking into the interior? Could the steel have been placed in a way to have avoided the collapse? These were typical questions arising from the public regarding the fate of the beloved twin towers. However, the design of the building was relatively strong and held up well within the given context. The two towers were built for holding their own during the impact of a Boeing 707 aircraft. Research conducted years after the tragic event has shown that the design did hold up well, "The towers were ingeniously designed for the conditions prior to September 11. Even on this momentous day, the towers did not significantly tilt throughout their failure, which no doubt avoided an even greater catastrophe of massive destruction far beyond lower Manhattan," (Buyukozturk & Ulm 1). Rather than collapsing sideways, which would have caused even more damage, the two towers collapsed internally. Also, the collapse was not a direct result of the plane crashes, but of the immense heat caused by the fires started by the jet fueled explosion. In terms of designing for preparation for such a catastrophe, the 1960s had no concept of such an attack, "Prior to 9/11, the deliberate use of a commercial jet airplane as a weapon was, thankfully, beyond the collective imagination of the nation, much less design-builders," (Mellon 1). So to have the structural design hold up as well as it did for an attack on such a magnitude is a testament to the initial design and construction. However, even the greatest of designs will fail under such extreme circumstances, and unfortunately the twin towers were no exception. The extreme heat of the fires were the final blow for the massive buildings, eventually leading to their collapse hours after the planes struck; yet, those hours in between allowed for many survivors to escape.