Group 17
Valerie Scott
Dan Kennedy

• World Trade Centre first proposed
-1943
• Minoru Yamasaki nominated as lead architect
-September 1962
• Design implemented by Worthington, Skilling, Helle & Jackson
Structural Engineers
• Original design of towers unveiled to the public
-January 1964.
• First ground breaking on site
-August 1966

• North tower completed
-December 1972
• South Tower completed
-July 1973
• Fire in the North Tower
-February 1975
• Bombing in underground garage of North Tower
-February 1993
• Terrorist Attacks
-September 11 2001

• Cost of Construction
– $400 million at the time (roughly equivalent to $2.2 billion today).
• 50,000 people worked in the towers daily.
• 200,000 passed through the buildings daily.
• Original design of 80 storeys was increased 110 storeys at the request of the Port Authority.

• Tube Frame Design
– steel core, columns around perimeter.
• Core contained elevators and utility shafts.
• Large column space between perimeter and core bridged by prefabricated floor trusses.
• Weight saved by using lightweight fire resistant materials.
• Windows were narrow as architect was afraid of heights
–caused objections

• On September 11 2001, a number of large commercial passenger aircraft were hijacked by terrorists. Two of these were later crashed into the twin towers.
• Both planes had departed Boston for California
– Full fuel tanks.
• Planes were ER variants
- Extended Range – more fuel

• North Tower struck between 93 rd and 99 th floors at 8:46am
• South Tower struck between 77 th and 85 th floors at 9:02am
• North Tower collapsed after 102 minutes at 10:28am
• South Tower collapsed after 56 minutes at 9:59am

• The towers, despite having been built to the same design spec failed in different ways.
• Both failures were from a combination of impact damage and structural components that were compromised and weakened by the heat from the fires.

• Fire weakened the core columns and caused the floors on the south side of the building to sag.
• The floors pulled the heated south perimeter columns inward, reducing their capacity to support the building above.
• Neighbouring columns quickly became overloaded as columns on the south wall buckled. The top section of the building tilted to the south and began its descent.

• The core was damaged severely at the southeast corner and was restrained by the east and south walls via the hat truss and the floors.
• The steady burning fires on the east side of the building caused the floors there to sag.
• The floors pulled the heated east perimeter columns inward, reducing their capacity to support the building above.
• The neighbouring columns quickly became overloaded as columns on the east wall buckled.

• The top section of the building tilted to the east and to the south and began its descent.
• WTC2 collapsed more quickly than WTC 1 because there was more aircraft damage to the building core, including one of the heavily loaded corner columns
• There were also early and persistent fires on the east side of the building, where the aircraft had extensively dislodged insulation from the structural steel.

• In the report carried out by NIST, it was found that neither the impact nor the fire alone would have been likely to have caused the towers to fail.
• The impact sustained by the towers from the aircraft was less than a quarter of the wind resisting capabilities of the towers.
• Although both towers collapsed vertically, damage was still caused to surrounding buildings, many of which either failed or had to be condemned.

• In the case of the twin towers, the failure was not as a result of inadequate design, but resulted from an attack which was designed to cause it to fail.
• The towers were in fact designed to withstand an aircraft strike, as the Empire state building had previously been struck accidentally by a B-25 bomber in 1945.
• It was for this reason that Extended Range planes were chosen to cause the fire damage necessary for failure.

• Fatalities :
• Clean-up:
• Rubble Removed:
2753
8 Months
1610000 t
• One World Trade Centre Building Due to open April 2013 at a cost of $3 Billion