Post-disaster report
The NIST World Trade Center Disaster Investigation was a report that the National Institute of Standards and Technology (NIST) conducted to establish the likely technical causes of the three building failures that occurred at the World Trade Center following the September 11, 2001 terrorist attacks. The report was mandated as part of the National Construction Safety Team Act (NCST Act), which was signed into law on October 1, 2002 by President George W. Bush. NIST issued its final report on the collapse of the World Trade Center's twin towers in September 2005, and the agency issued its final report on 7 World Trade Center in November 2008.
NIST concluded that the collapse of each tower resulted from the combined effects of airplane impact damage, widespread fireproofing dislodgment, and the fires that ensued. The sequence of failures that NIST concluded initiated the collapse of both towers involved the heat-induced sagging of floor trusses pulling some of the exterior columns on one side of each tower inward until they buckled, after which instability rapidly spread and the upper sections then fell onto the floors below. 7 World Trade Center, which was never directly hit by an airplane, collapsed as a result of thermal expansion of steel beams and girders that were heated by uncontrolled fires caused by the collapse of the North Tower and failure of the fire-resistive material.
Context.
Collapse of the World Trade Center.
During the September 11 attacks, two jet airliners struck the Twin Towers of the World Trade Center (WTC), one to each tower. As a result, the two 110 stories-tall skyscrapers collapsed, causing complete destruction to the entire WTC complex and killing 2,763 people. The South Tower collapsed at 9:59 a.m. (EDT) after burning for approximately 56 minutes. 29 minutes later, the North Tower collapsed having burned for 102 minutes. When the North Tower collapsed, debris fell on the nearby 7 World Trade Center, damaging it and starting fires that burned for almost seven hours, compromising the building's structural integrity. Seven World Trade Center collapsed at 5:21 p.m. (EDT).
The collapse of the World Trade Center buildings on September 11, 2001 was unprecedented; never before had a steel-framed multi-story building suffered a complete collapse as a result of fire. In the immediate aftermath, knowledgeable structural engineers began providing a range of explanations in an attempt to help the public understand these tragic events. However, a coordinated effort would need to be organized to investigate and analyze the complex series of events that led to each collapse.
FEMA World Trade Center Building Performance Study.
In the aftermath of the attacks, researchers responded immediately by traveling to ground zero where they began collecting data. Among the first was the Federal Emergency Management Agency (FEMA) and American Society of Civil Engineers (ASCE), who together formed a Building Performance Study Team to understand how the building structures failed and why. The team produced the first official government report attempting to explain the destruction of the World Trade Center complex.
They were able to make many observations and findings, including preliminary analysis of the damaged structures, analysis of the buildings' fire suppression systems, and recommendations to building codes and fire standards for including airplane impact into building design. FEMA's final report, FEMA 403 issued in May 2002, titled "World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations", also provided a substantial amount of data about the event not documented elsewhere. Their findings suggested that fires, in conjunction with damage to the structural members and fire suppression systems inflicted by the jet airliners, played a key role in the collapse of the buildings.
However, the team's investigation was hampered by a number of issues. The lack of authority of investigators to impound pieces of steel for examination before they were recycled led to the loss of important pieces of evidence that were destroyed early during the search and rescue effort, and much of the steel had already been recycled in the one month that had lapsed between the attack and the deployment of the team. The team was further impeded by ongoing criminal investigations by the FBI and NTSB. In a hearing to the U.S. House of Representatives' Committee on Science on March 6, 2002, a panel of witnesses and experts described the obstacles as:
Because of these inadequacies, the Building Performance Study team could not "definitively determine the sequence of events leading to the collapse of each tower." The report was remarkably blunt in pointing out shortcomings and missteps in the investigation and its recommendation for another, more thorough and authoritative investigation.
National Construction Safety Team act.
As a result of the inconclusive FEMA Building Performance Study team's findings and concerns over missteps in its investigation, the U.S. House of Representatives drafted legislation that would give wide powers, including the right to issue subpoenas, to teams investigating building failures. The bill also mandates that the teams would be centered at the National Institute of Standards and Technology (NIST) whose building and fire research laboratory in Maryland has conducted extensive investigations of building failures in the past. The bill, cited as the National Construction Safety Team (NCST) act, passed the House and the Senate and was signed into law on Oct. 1, 2002 by President George W. Bush.
The NCST act gives NIST a clear mandate to:
Investigation.
NIST began its investigation on 21 August 2002. Prior to this date, volunteers from NIST, FEMA, ASCE and others collected steel members important to the investigation from the four steel recycling facilities during the recovery effort. They collected and cataloged 236 steel artifacts, including exterior columns, core columns, floor trusses and other similar structural members. They were able to observe the metallurgical chemistry and structure and perform experiments on the recovered elements to measure their attributes such as mechanical properties under high temperatures.
NIST's Building and Fire Research Laboratory created a complex computer model to understand the collapse of the towers. Specifically, they wanted to know if the collapse of a core column could cause the progressive collapse of the whole building. They also modeled the dispersion of the jet fuel and damage to the interior of the building that was not visible from photographic evidence and eyewitnesses. The model was used to understand the hypothesis of the collapse.
Findings.
Twin Towers.
The investigation team integrated their metallurgy analysis, experimental results and computer simulation with video and photographs of the destruction and eyewitness accounts to form their understanding for how the buildings collapsed. They came to two conclusions:
The most probable collapse sequence was similar between the South Tower and North Tower, but they were not identical. However, they both involved all major structural systems of the building design: the core columns, the exterior columns and the building floors.
The NIST investigation's conclusions do not support the "pancake theory" of collapse initiation, in which the collapse is begun by a progressive failure of the floor system. However, "pancaking" was accepted as the mode of collapse progression.
7 World Trade Center.
NIST released the final report of their investigation into the collapse of the 47-story World Trade Center 7 building on 20 August 2008.
Their conclusion is that WTC7 collapsed primarily as a result of the fire that was started when the WTC1 collapsed. The collapse of the North Tower also damaged the south exterior wall of WTC7 but it was not a contributing factor. Because FDNY could not attempt to extinguish the fires that were burning on six floors of the building, the fire-proofing insulation began to fail. After several hours of uncontrolled fire, the steel columns, girders and trusses absorbed heat and rapidly lost their strength. As they began to sag, deform and buckle, the interior structure below the east penthouse was brought down. The failed core columns' load was distributed to the remaining columns which all failed. This led to the progressive collapse of the building.
Reforms.
The National Construction Safety Team Act of 2002, which mandated NIST to perform its investigation, specifically states that NIST, which is not a regulatory agency, is not authorized to require the adoption of building codes, standards or practices. However, NIST's final reports on the collapse of the WTC buildings provides the technical basis for new and improved standards, codes and practices on designing buildings to resist progressive collapse. Many NIST researchers are also key members of professional, standards-developing organizations, and NIST actively works with these organizations to ensure that lessons learned from investigations are put to use.
Fire Protection of Structural Members.
The National Fire Protection Association has adopted many of NIST's recommended improvements to the building code. NIST's recommendation for improving a building's structural frame and support system under severe loading conditions, such as during a fire event, resulted in the adoption of a new approach to high-rise building design. Much more scrutiny is to be given to the primary and secondary structural members as well as the connections that tie them together.
The steel columns of the WTC buildings significantly lost strength when they were subjected to the heat of the fire. Concrete heated to the same temperature, however, loses no strength at all. As a result, new high-rise buildings, including One World Trade Center, are being constructed with reinforced, high-strength concrete.
Emergency Communication Systems.
Due to the fact that New York City's Office of Emergency Management was located in World Trade Center 7 on the day of the attacks and vital communications equipment was located in the North Tower, their response was significantly impeded. They have since moved their offices to Brooklyn, and disaster and emergency management teams around the country are also moving away from possible targets of terrorist attacks, natural disasters and other emergency scenarios.
Conspiracy theories.
Critics of these conspiracy theories say they are a form of conspiracism common throughout history after a traumatic event in which conspiracy theories emerge as a mythic form of explanation.
A related criticism addresses the form of research on which the theories are based. Thomas W. Eagar, an engineering professor at MIT, suggested they "use the 'reverse scientific method'. They determine what happened, throw out all the data that doesn't fit their conclusion, and then hail their findings as the only possible conclusion."
References.
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