1. Context

1.1. Collapse of the World Trade Center

The collapse of the World Trade Center buildings on September 11, 2001 was as shocking as it was tragic. The complete destruction of such massive buildings was unthinkable; never before had a steel-framed multi-story building suffered a complete collapse as a result of fire.^[1] In the immediate aftermath, knowledgeable structural engineers began providing a range of explanations in an attempt to help the public understand these tragic events.^[2] However, a coordinated effort would need to be organized to investigate and analyze the complex series of events that led to each collapse.

1.2. FEMA World Trade Center Building Performance Study

In the aftermath of the World Trade Center complex, 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.^[3]

They were able to make many observations and finding, 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.^[4] FEMA's final report, issued in May 2002, also provides 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 recycle 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:^[3]

• No clear authority and the absence of an effective protocol for how the building performance investigators should conduct and coordinate their investigation with the concurrent search and rescue efforts, as well as any criminal investigation
• Difficulty obtaining documents essential to the investigation, including blueprints, design drawings, and maintenance records
• Uncertainty as a result of the confidential nature of the BPAT study
• Uncertainty as to the strategy for completing the investigation and applying the lessons learned

Because of these inadequacies, the Building Performance Study team could not "definitively determine the sequence of events leading to the collapse of each tower."^[5] The report was remarkably blunt in pointing out shortcomings and missteps in the investigation and its recommendation for another, more thorough and authoritative investigation.^[6]

1.3. 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.^[7] 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.^[8] 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.^[9]

The NCST act gives NIST a clear mandate to:^[10]

1. establish the likely technical cause of building failures;
2. evaluate the technical aspects of procedures used for evacuation and emergency response;
3. recommend specific changes to building codes, standards, and practices;
4. recommend any research or other appropriate actions needed to improve the structural safety of buildings; and/or changes in emergency response and evacuation procedures; and,
5. make final recommendations within 90 days of completing an investigation.

2. 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.^[11] 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.^[12] 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.

3. Findings

3.1. The 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:^[11]

1. A conventional fire should not have caused the collapse of the 110-story skyscrapers in the absence of structural and fire-proofing insulation damage.
2. The towers would likely not have collapsed if not for the impact and damage that the aircraft caused to the fire-proofing insulation.

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.^[13]

1. First, the floors that lost fire-proofing insulation due to debris impact began to sag as a result of the high temperature of the fire.
2. The sagging floors pulled inward on the girders and caused the exterior walls to deform.
3. The exterior walls began to buckle under the combined forces of the sagging floors, the fire, and the severed core columns from aircraft impact damage.
4. Finally, the exterior walls caved in and the buildings collapsed. The stories below provided little resistance to relatively tremendous energy of the falling building, allowing them to fall very quickly.

The NIST investigation's conclusions do not support the "pancake theory" of collapse, in which there is a progressive failure of the floor system.^[14]

3.2. 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.^[12]

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.^[15]

4. Reforms and Lessons Learned

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.^[16] 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.^[17]

4.1. 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.^[18]

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.^[19] As a result, new high-rise buildings, including One World Trade Center, are being constructed with reinforced, high-strength concrete.^[20]

4.2. 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.^[21]