1. World Trade Centre Collapse Group 17 Valerie Scott Dan Kennedy

2. Timeline 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

3. Timeline 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

4. Information 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.

5. Structure 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

6. New York Skyline (prior to attacks)

7. Failure 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

8. Attack on Twin Towers

9. Failure 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

10. Failure

11. 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.

12. North Tower 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.

13. South Tower 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.

14. South Tower 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.

15. Failure

16. Findings 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.

17. Conclusions 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.

18. New York Skyline (present day)

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

20. Finally