structure  explosion  wtc  north tower
[1,2,3,4]

Failure: World Trade Center
Location: New York, New York
Year: 2001
Type: Terrorist Attack
Deaths: 2,763.
Architect: Minoru Yamasaki
Structural Engineer: John Skilling and Leslie E. Robertson
Construction Firm: Tishman Realty & Construction

The World Trade Center Towers in New York City were victims of a terrorist attack on September 11, 2001, which caused their collapse. The two, 110-story buildings, were actually designed to withstand the forces cause by a horizontal impact of a large commercial aircraft [a]. The structure was composed of a core of columns in the middle interior section as shown in figure 1. The perimeter of the building was composed of metal tubing that aided in the support of the floors, as horizontal floor trusses spanned from the tubing to the central core. The structure was also designed with redundancy, so that if a column were to fail, the building would still remain intact. Ronald Hamburger, a structural engineering studying the failure of the World Trade Center, said “because of its great structural redundancy, the load was distributed to other parts of the building [b].” He also stated that he has reason to believe that without fire, the building could have remained standing and be repaired [b].

The World Trade Center failed for multiple reasons. The fire and explosion (as seen in image 2) caused by the jet fuel of a Boeing 767 (as seen in image 4) was unaccounted for when designing the structure to support a collision of an airplane, and it ultimately led to the collapse of the World Trade Center. The prolonged heating of the steel columns caused the columns to experience creep, and for the yield strength of the columns to lower significantly [a]. The fire also caused the trusses that supported the floors to fall apart and because the fire had affected the columns and trusses on the top floor, the upper section of the building failed first. A domino effect occurred as the top floors fell straight down and crushed the floors beneath. The south tower failed after 56 minutes of being hit, and the north tower failed after 102 minutes [c]. An image of the aftermath of the world trade center collapse can be seen in image 3.

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