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...then why did they collapse?

  By Vikas Agrawal

The attack and following collapse of the World Trade Center Twin Towers on September 11, 2001 shocked the world. The enormous travesty occurred so fast and so surprisingly. But what may also shock you is that architect Minoru Yamasaki designed the World Trade Center towers to withstand a collision with a Boeing 707 airplane (Federal Emergency Management Agency 2002). The Boeing 707 is similar to the Boeing 767s that actually crashed into the towers, the main differences being that the 767 is slightly heavier and slower. The impact from the plane that hit Tower One was well within the force limits of the design and the impact from the second plane was only ten percent above the force that Tower Two was designed to absorb (“Nerdcities: The Guardian” 2002). So, from an engineering perspective, the World Trade Center towers, at least Tower One, should have been able to withstand the collisions on September 11th. Why, then, did the towers collapse? A government report entitled “World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations” by the Federal Emergency Management Agency (2002), may have the answers.


The Twin Towers had many novel safeguards and design elements to strengthen the buildings against a possible collision by an aircraft and prevent their collapse. Examining the structures of the buildings, one sees that the World Trade Center Towers were among the first to use a steel structure instead of masonry or concrete used amongst earlier high rises (Federal Emergency Management Agency 2002). The Twin Towers also used a new system called the tube structure for the majority of support of the building (Ashley 2001). They are made of a rigid hollow tube of closely packed steel columns with floor trusses (braces along the floor) that extend around the building to its center, further enhancing structural strength and the prevention of collapse (Federal Emergency Management Agency 2002). This tube structure enabled the Trade Center towers to withstand higher winds and higher lateral (horizontal) force loads, like those of high velocity impacts, and it also eliminated the need for interior columns (“Nerdcities: The Guardian” 2002). Additional lateral support came from exterior columns along the perimeter of the two towers, set only 22” apart (“Nerdcities: The Guardian” 2002). The five-inch-thick concrete floor and floor trusses supported most of the vertical weight of each story.


The most consequential designs that were not included in the Twin Towers were sufficient fire-suppression systems and fireproofing. Even though the towers were built to withstand the impact of a jetliner, they were not designed to withstand and remain standing during a fire of such great magnitude. The jet-fuel fire caused by the impact was impossible to contain in the Twin Towers. The World Trade Center had not been designed to fight hydrocarbon fires of such magnitude and high temperature – up to 1500 degrees Celsius. The fire-suppression system consisted of water sprinklers that were useless because water, at this temperature, would vaporize almost instantly. Instead, these fires had to be fought with chemical foam, which the Towers lacked (Ashley 2001).


The fireproofing system in the Towers was also insufficient. First, the Towers were lightweight because of their extensive use of steel and were devoid of masonry or concrete which made them difficult to insulate from the fire. Second, a more sophisticated fireproofing system could have been incorporated during the building process. Most of the supports and trusses could have been coated with extra fire proofing material (Ashley 2001). Third, the World Trade Center incorporated a novel, yet very flammable, elevator system (Wilkinson 2002). The engineers worried that, without masonry, the conventional elevator shafts would buckle and collapse with the intense air pressure exerted by the high speed elevators. To solve this problem the engineers used a drywall/plaster system fixed to a reinforced steel core; this made the shafts more flexible though much more flammable (Wilkinson 2002).


Another design shortcoming that made the ensuing fire even more destructive was the use of weak floor trusses which spanned abnormally long distances (Ashley 2001). In the Twin Towers the steel trusses spanned nearly sixty feet without any support and were only four inches thick (Federal Emergency Management Agency 2002). The extremely high-temperature fire heated the relatively thin floor rapidly, making the floor almost flexible because it lost most of its rigidity and consequently buckled. Since the floor buckled, the extra support needed to come from the remaining exterior perimeter columns, but many had been destroyed by the planes’ initial impact. But those columns also depended on the core steel columns for support, but these columns were being subjected to extremely harsh conditions of the fire and were failing themselves. The exterior columns began to buckle onto the floor which buckled on the floor beneath and started a gigantic domino effect of the plunging stories. So, in effect, the fire caused all structural supports to weaken and fail within the Twin Towers.


Fire was the Achilles heel of the World Trade Center Twin Towers, for they did not have sufficient fireproofing nor fire-suppression systems. Designers of future skyscrapers may install retrofitted aqueous film-forming foam extinguishers, similar to those used for aviation fires, to enhance fire safety in future projects. In addition, new high rises may have plans that have more evacuation sites as well as possible external ways like giant escape tubes or parachutes (Ashley 2001). In the future, architects, engineers, designers, and builders will look to further the safety and security to all those in skyscrapers and learn from the events of September 11.

References

Federal Emergency Management Agency. (2002). World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations. Retrieved January 30, 2002 from http://www.house.gov/science/hot/wtc/wtcreport.htm

Ashley, Steven. (2001). When the Twin Towers Fell. Scientific American. Retrieved January 23, 2003, from http://www.public-action.com/911/jmcm/sciam/


Wilkinson, Tom. (2001). Collapse of the World Trade Center. Retrieved January 23, 2003 from http://www.civil.usyd.edu.au/wtc.htm

(2002). World Trade Center Demolition. Retrieved January 23, 2003 from http://nerdcities.com/guardian/WTC/wtc-demolition.htm