# Collapse of 7 World Trade Center

In the afternoon hours of September 11, 2001, 7 World Trade Center caught on fire collapsed onto the streets below after being heavily damaged during the collapse of the World Trade Center that had occurred several hours prior. Although nobody died, the collapse spawned several conspiracy theories and sparked a long investigation into the structure of the building.

## Background

On September 11, 2001, four coordinated Islamist terrorist suicide attacks were carried out by al-Qaeda against the United States. 19 terrorists hijacked four aircraft and crashed two of the planes, American Airlines Flight 11 and United Airlines Flight 175, into the Twin Towers of the World Trade Center in New York City. The September 11 attacks ultimately killed 2,977 people, making it the deadliest terrorist attack in history.

## Collapse
### Collapse

As the North Tower collapsed on September 11, 2001, heavy debris hit 7 World Trade Center, damaging the south face of the building$^{:18(PDFp.22)}$ and starting fires that continued to burn throughout the afternoon.$^{:16,18}$ The collapse also caused damage to the southwest corner between floors 7 and 17 and on the south face between floor 44 and the roof; other possible structural damage included a large vertical gash near the center of the south face between floors 24 and 41.$^{:17}$ The building was equipped with a sprinkler system, but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps instead of being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser, and the sprinkler system required some power for the fire pump to deliver water.$^{:11}$ Additionally, water pressure was low, with little or no water to feed sprinklers.$^{:23–30}$
After the North Tower collapsed, some firefighters entered 7 World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts. Over the course of the day, fires burned out of control on several floors of 7 World Trade Center, the flames visible on the east side of the building. During the afternoon, the fire was also seen on floors 6–10, 13–14, 19–22, and 29–30.$^{:24(PDFp.28)}$ In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon. At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might collapse. During the afternoon, firefighters also heard creaking sounds coming from the building. Around 3:30 pm, FDNY Chief of Operations Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel.
The fire expanded the girders of the building, causing some to collapse. This led to the northeast corner core column (Column 79), which was especially large, to buckle below the 13th floor. This caused the floors above it to collapse to the transfer floor at the fifth level. The structure also developed cracks in the facade just before the entire building started to fall.$^{:21}$ According to FEMA, this collapse started at 5:20:33 pm EDT when the east mechanical penthouse started crumbling.$^{:23}$ Differing times are given as to what time the building completely collapsed: at 5:21:10 pm EDT according to FEMA,$^{:23}$ and at 5:20:52 pm EDT according to NIST.$^{:19,21,50–51}$
There were no casualties associated with the collapse. NIST found no evidence to support conspiracy theories such as the collapse being the result of explosives; it found that a combination of factors including physical damage, fire, and the building's unusual construction set off a chain-reaction collapse.

### Reports

In May 2002, the Federal Emergency Management Agency (FEMA) issued a report on the collapse based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers under the leadership of Dr. W. Gene Corley, P.E. FEMA made preliminary findings that the collapse was not primarily caused by actual impact damage from the collapse of 1 WTC and 2 WTC but by fires on multiple stories ignited by debris from the other two towers that continued burning unabated due to lack of water for sprinklers or manual firefighting. The report did not reach conclusions about the cause of the collapse and called for further investigation.$^{:3}$
Subsequently, the National Institute of Standards and Technology (NIST) was authorized to lead an investigation into the structural failure and collapse of the World Trade Center Twin Towers and 7 World Trade Center. The investigation, led by Dr S. Shyam Sunder, drew upon in-house technical expertise as well as the knowledge of several outside private institutions, including the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE); the Society of Fire Protection Engineers (SFPE); the National Fire Protection Association (NFPA); the American Institute of Steel Construction (AISC); the Council on Tall Buildings and Urban Habitat (CTBUH); and the Structural Engineers Association of New York (SEAoNY).
The bulk of the investigation of 7 World Trade Center was delayed until after reports were completed on the Twin Towers. In the meantime, NIST provided a preliminary report about 7 WTC in June 2004, and thereafter released occasional updates on the investigation. According to NIST, the investigation of 7 World Trade Center was delayed for a number of reasons, including that NIST staff who had been working on 7 World Trade Center were assigned full-time from June 2004 to September 2005 to work on the investigation of the collapse of the Twin Towers. In June 2007, Shyam Sunder explained,
   We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible. The 7 WTC investigation is in some respects just as challenging, if not more so than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers.
In November 2008, NIST released its final report on the causes of the collapse of 7 World Trade Center. This followed NIST's August 21, 2008, draft report, which included a period for public comments, and was followed in 2012 by a peer-reviewed summary in the Journal of Structural Engineering. In its investigation, NIST utilized ANSYS to model events leading up to collapse initiation and LS-DYNA models to simulate the global response to the initiating events.$^{:6–7}$ NIST determined that diesel fuel did not play an important role, nor did the structural damage from the collapse of the Twin Towers or the transfer elements (trusses, girders, and cantilever overhangs). The lack of water to fight the fire was an important factor. The fires burned out of control during the afternoon, causing floor beams near column 79 to expand and push a key girder off its seat, triggering the floors to fail around column 79 on floors 8 to 14. With a loss of lateral support across nine floors, column 79 buckled – pulling the east penthouse and nearby columns down with it. With the buckling of these critical columns, the collapse then progressed east-to-west across the core, ultimately overloading the perimeter support, which buckled between Floors 7 and 17, causing the remaining portion of the building above to fall down as a single unit. The fires, which were fueled by office contents and burned for 7 hours, along with the lack of water, were the key reasons for the collapse.$^{:21–22}$ At the time, this made the old 7 WTC the only steel skyscraper to have collapsed from fire.
Files relating to numerous federal investigations had been housed at 7 World Trade Center. The Equal Employment Opportunity Commission estimated over 10,000 of its cases were affected. Investigative files in the Secret Service's largest field office were lost, with one Secret Service agent saying, "All the evidence that we stored at 7 World Trade, in all our cases, went down with the building." Copies of emails in connection with the WorldCom scandal that were later requested by the SEC from Salomon Brothers, a subsidiary of Citigroup housed in the building, were also destroyed.
The NIST report found no evidence supporting the conspiracy theories that 7 World Trade Center was brought down by controlled demolition. Specifically, the window breakage pattern and blast sounds that would have resulted from the use of explosives were not observed.$^{:26–28}$ The suggestion that an incendiary material such as thermite was used instead of explosives was considered unlikely by NIST because of the building's structural response to the fire, the nature of the fire, and the unlikelihood that a sufficient amount of thermite could be planted without discovery. Based on its investigation, NIST reiterated several recommendations it had made in its earlier report on the collapse of the Twin Towers.$^{:63–73}$ It urged immediate action on a further recommendation: that fire resistance should be evaluated under the assumption that sprinklers are unavailable;$^{:65–66}$ and that the effects of thermal expansion on floor support systems be considered.$^{:65,69}$ Recognizing that current building codes are drawn to prevent loss of life rather than building collapse, the main point of NIST's recommendations was that buildings should not collapse from fire even if sprinklers are unavailable.$^{:63–73}$

### Aftermath

The collapse of 7 WTC sent debris flying into the nearby Borough of Manhattan Community College's Fiterman Hall building, located at 30 West Broadway, damaging and contaminating it beyond repair. A revised plan called for demolition in 2009 and completion of the new Fiterman Hall in 2012, at a cost of $325 million. The collapse also damaged the eastern facade of the Barclay–Vesey Building, an Art Deco building to the west; it was restored at a cost of US$1.4 billion. Construction on the new 7 World Trade Center began in 2002, and the new structure opened in 2006.