9/11: Mind Numbing Attack
Tuesday Morning, 11 September 2001
Several coordinated terrorist attacks fumbled the highest skyscraper of its time “The twin towers: World Trade Center”. The building remained the icon for New York city in its short life-time of 28 years(1973–2001). Despite a lot of controversial issues, building sheltered 430 companies and engaged various commercial activities. After twin towers crashed, 5 other buildings collapsed at the same time. Leftover short rise buildings were also destroyed during the 8 months long cleanup and recovery process.
Structural aspects of World Trade Center :
One World Trade Center and Two World Trade Center are referred to as the Twin Towers, were designed by architect Minoru Yamasaki. Major factors that made Twin Towers of the World Trade Center structurally unique are its ‘Walls’ and its ‘Elevators’. Prior to its existence, skyscrapers were majorly supported by closely spaced large internal columns for load bearing. This caused the reduction in floor space and hence the financial value also. Twin towers’ walls were made of steel framed tube structures. The perimeter columns were placed closely together to form strong, rigid wall structure and supporting all virtual loads. Boosted by a single column at the center to provide more strength.
Now, as the height of buildings rises, so does the number of floors, residents and requirement of elevators, reducing the available floor space. This issue was solved in the construction of the twin towers through the use of express and local elevators. Express elevators would take passengers to “sky lobbies” placed on various floors throughout the building where they would then disembark and switch to local elevators to get to their required floor. The use of this system cut the number of required elevator shafts in half, thus preserving valuable floor space.
Engineering behind attack:
As shown in photograph, an American Airlines Boeing 767 loaded with 20,000 gallons of jet fuel crashed into the north of the World Trade Centre. It banked at 45-degree angle, damaging floors 92 to 95. About 40 minutes later, the second jet crashed into the south tower, hitting floors 78 to 84. The angle was kept intentional to destroy as many floors as possible. The impact of the jetliners shattered and fractured two-thirds of the support columns on one face of each tower, causing the partial collapse of several floors. But still, because of its great structural redundancy, the load was distributed to other parts of the building. If left so, the buildings could have stood indefinitely and been repaired.
But, the impact of the jet caused failure of the fireproofing system. Now, as per its property, as steel is reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength. Extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle, causing the failure of the structure.
Claimed structural defaults and adopted modifications:
In retrospect, the disaster at the World Trade Center became a lightning rod for a variety of building-safety issues.Narrow and less number of staircases increased the fatality. It was difficult for people to reach the closest staircase and save themselves. Since 9/11, national building codes have changed to include requirements such as providing an additional staircase, or a specially protected elevator and elevator shaft that can be used by building occupants for buildings over 420 feet high.
Fire protection for steel, firefighter-communications systems, sprinkler-system water supplies, elevator use and dozens of other concerns became the focus of a federal inquiry under the then-newly created National Construction Safety Team Act.
It is always hard to analyze the exact situation post such man-cause havoc. As an active human race and civil engineering enthusiasts, we tried to compile our findings from various available and trusted sources, from survivors’ experiences and Stanford’s official report. We would be glad to hear your thoughts and in case, you find something is wrong, do reach out to us!
