At 8:46 AM, American Airlines Flight 11 slams into the North Tower of the World Trade Center on the morning of September 11, 2001. At 9:03 AM, United Airlines Flight 175 hits the South Tower. The South Tower fell at 9:59 AM. Twenty-nine minutes later, the North Tower collapsed. At 5:20 PM, Building Seven of the World Trade Center collapses (Meyerowitz, 247). “Prefabricated sections of the external steel columns tumbled down onto lesser buildings, piling onto terraces and rooftops, punching through parking structures, offices, and stores, inducing secondary collapses and igniting fires” (Langewiesche, 3).
On that same morning, American Airlines Flight 77, a Boeing 757, crashed into the Pentagon. The plane was flying from Dulles International Airport. After a close inspection minutes after the crash, Pentagon officials found only small pieces of aircraft. “No large debris – anything resembling the Boeing 757 wings, fuselage, or tail – is visible on the Pentagon lawn, and the lawn itself shows no sign that a Boeing 757 skidded across it or struck it” (Masud, 3).
CBS news reported that Flight 77 did a “downward spiral”, turning almost a complete circle and dropping 7,000 feet in two-and-a-half minutes. It plowed into the Pentagon at 460 mph (Masud, 7). Another report stated that the plane struck the first floor of the building.
In August 2002, the National Institute of Standards and Technology (NIST) began a technical investigation into the collapse of World Trade Center Buildings 1, 2, and 7 following the terrorist attacks of September 11, 2001 (Bukowski, 34). The investigation lasted for three years. It found that the fires on multiple floors in World Trade Center 7 caused the floor beams and girders to heat up. That “caused a critical support column to fall, initiating a fire-induced progressive collapse that brought the building down” (Science Daily, 1).
The World Trade Center 7’s Column 79 was identified as the one whose failure on September 11 started the progressive collapse. The goal was to see if the loss of this column “would still have led to a complete loss of the building if fire or damage from the falling debris of the nearby World Trade Center 1 tower were not factors” (Science Daily, 1). The investigation team found that that column’s failure under any circumstance would have led to the series of destructive events.
Another goal of the investigation was to determine the reason that fatalities and injuries were either very low or very high depending on the location. It was estimated that 17, 400 occupants were present in the World Trade Center towers on the morning of September 11 (Sunder, 1142). About 6 percent of those that survived reported pre-existing conditions that limited their mobility such as obesity, pregnancy, asthma, or a heart condition. About 7 percent of survivors reported having special knowledge about the building.
Approximately 87 percent of the towers’ occupants were about to evacuate successfully; 99 percent of them were below the floors of impact. “Two-thousand one-hundred fifty-nine building occupants (1,560 in WTC 1 and 599 in WTC 2) and an additional 433 first responders, including security guards but do not include aircraft passengers and crew or bystanders, were reported to have lost their lives on September 11, 2001” (Sunder, 1142). These victims were at or above the floors of impact.
As part of its investigation, NIST planned to interview 600 occupants to study the World Trade Center’s evacuation and emergency response procedures. These occupants would include those that were near the floors of impact and in elevators and lobbies of the Twin Towers. In WTC 7, they planned to interview the disabled, floor wardens, fire safety directors, and any family members of the victims that may have been called there (NIST, 8).
NIST also notes that once the first airplane struck WTC 1, those in WTC 2 had a chance to evacuate before the second plane struck that tower. WTC 2 had working elevators where as in WTC 1, survivors could only use the stairs. The rate of evacuation in WTC 2 was twice the rate of WTC 1.
Buildings aren’t designed for the fire protection and evacuation for the type of devastation caused by these terrorist attacks. The load conditions induced the by aircraft impact were not the norm of design loads considered in building codes (Sunder, 1138).
“Fires played a major role in further reducing the structural capacity of the buildings, initiating collapse” (Sunder, 1140). Aircraft impact damage alone did not initiate the building collapse. However, it did relinquish the sprinkler system and water supply. The scattering of jet fuel ignited fires. Some of these fires burned the entire time that the buildings remained standing. This was because both aircraft and building contents were burning.
On September 9, 2002, the Federal Emergency Management Agency (FEMA) transferred $16 million in funding to NIST to support the WTC investigation (NIST, 2). On October 1, 2002,
President Bush signed The National Construction Safety Team Act to establish NIST as the lead agency to investigate building failures. The act “provides for the establishment of investigative teams to assess building performance and emergency response and evacuation procedures in the wake of any building failures that has resulted in substantial loss of life or that posed significant potential of substantial loss of life” (NIST, 3).
In addition, NIST also used analytical, experimental, and numerical tools as part of the investigation. Factors that were taken into consideration were the mass of the steel, concrete, heavy machinery, and any building materials and contents that absorbed energy from the aircraft impact. Also, the growth and spread of fire through the towers and the resulting temperature of the structural steel. They also measured the performance of the steel components and connections under elevated temperatures along with the variability in the speed, direction, orientation, and point of impact of each aircraft (NIST, 5).
NIST conducted additional experimental studies of the mechanical properties of steel at high strain rates, the thermal-insulation properties of the fireproofing materials, and fire tests. NIST also reviewed previously completed tests of past performances based on available fire incident and insurance investigation reports (NIST, 5). They also worked with other organizations, such as the New York Police Department, to gain access to important information related to emergency response and evacuation on September 11 such as communication and operational records.
NIST conducted approximately 250 chemical analyses of World Trade Center steel. The results indicated that the perimeter columns were made of WEL-TEN 60, 70, or 80 steels. They were obtained from Nippon Steel. Primer paint used on the columns could withstand temperatures of 250° C with no visible damage or discoloration. However, the paint would become crumbly and powdery at 700° C. “Thus steel that shows little visible evidence of discoloration or damage to the primer paint still could have experienced high temperature levels due to the fires” (NIST, 9).
Of course, photographs and video images were very important to the investigation. Close-up photos of the aircraft can be used to determine exactly where each aircraft struck and its position. Video footage can capture the sight as well as the sounds of the events.
NIST had to recommend specific improvements to building standards, codes, and practices. The investigation was conducted under the National Construction Safety Team Act. NIST made recommendations such as enhanced fire protection, improved building evacuation, increased structural integrity, and improved emergency response.
September 11, 2001 changed the face of fire investigations across the country. Fires burned at Ground Zero until mid-December (Meyerowitz, 148). More bodies were still being recovered under the rubble. On March 30, 2002, the removal of the last column from the South Tower marked the end of the cleanup and recovery of Ground Zero.
1. Bukowski, Richard W. Applying the lessons of September 11, 2001 to the Built Environment”. Australian Building Regulation Bulletin, p. 34-37.
2. Langewiesche, William. (2002) American Ground Unbuilding: The World Trade Center. New York: North Point Press.
3. Masud, Enver. What Really Happened on September 11 at the Pentagon. Retrieved March 19, 2009 from http://www.twf.org/News/Y2005/0307-Pentagon.html.
4. Meyerowitz, Joel. (2006) Nine Months at Ground Zero. New York: Scribner.
5. National Institute of Standards and Technology. Progress Report on NIST December 9, 2002: Building and Fire Investigation into the World Trade Center Disaster. Retrieved March 16, 2009 from http://wtc.nist.gov/media/progress_report.htm.
6. Science Daily. Final World Trade Center 7 Investigation Report on September 11, 2001 Collapse Released. Retrieved March 19, 2009 from http://www.sciencedaily.com/releases/2008/11/081120144246.htm.
Sunder, S. Shyam. Building and Fire Safety: Responding to the World Trade