24 April 2009
The Collapse of World Trade Center 7
The 47-story World Trade Center (WTC) 7 building, which was heavily damaged by the collapse of the nearby WTC north tower, collapsed at 5:20 pm on September 11. A three-year-long investigation by the National Institute of Standards and Technology (NIST) concluded that the collapse of the WTC north tower ignited fires in WTC 7, which burned out of control on several floors because the water supply from the city water main had been damaged by the collapse of the twin towers. These fires caused the steel structure of the WTC 7 to expand and eventually buckle, causing the collapse of the building. NIST found no evidence of controlled demolition.
See the NIST videos on the collapse of WTC 7.
The Silverstein Comment
An often-cited piece of “evidence” for the conspiracy theory claim that WTC 7 was intentionally demolished is a comment that Mr. Larry Silverstein, who owned the World Trade Center complex, made on the September 2002 television documentary American Rebuilds. Mr. Silverstein said:
I remember getting a call from the Fire Department commander, telling me they were not sure they were going to be able to contain the fire. I said, you know, “We've had such terrible loss of life that the smartest thing to do is just pull it.” And they made that decision to pull it and we watched the [World Trade Center 7] building collapse.
Conspiracy theorists put forward the notion that Mr. Silverstein’s suggestion to “pull it” is slang for intentionally demolishing the WTC 7 building.
On September 9, 2005, Mr. Dara McQuillan, a spokesman for Silverstein Properties, issued the following statement on this issue:
Seven World Trade Center collapsed at 5:20 p.m. on September 11, 2001, after burning for seven hours. There were no casualties, thanks to the heroism of the Fire Department and the work of Silverstein Properties employees who evacuated tenants from the building.
The Federal Emergency Management Agency (FEMA) conducted a thorough investigation of the collapse of all the World Trade Center buildings. The FEMA report concluded that the collapse of Seven World Trade Center was a direct result of fires triggered by debris from the collapse of WTC Tower 1.
In the afternoon of September 11, Mr. Silverstein spoke to the Fire Department Commander on site at Seven World Trade Center. The Commander told Mr. Silverstein that there were several firefighters in the building working to contain the fires. Mr. Silverstein expressed his view that the most important thing was to protect the safety of those firefighters, including, if necessary, to have them withdraw from the building.
Later in the day, the Fire Commander ordered his firefighters out of the building and at 5:20 p.m. the building collapsed. No lives were lost at Seven World Trade Center on September 11, 2001.
As noted above, when Mr. Silverstein was recounting these events for a television documentary he stated, “I said, you know, we've had such terrible loss of life. Maybe the smartest thing to do is to pull it.” Mr. McQuillan has stated that by “it,” Mr. Silverstein meant the contingent of firefighters remaining in the building.
The NIST Report
In addition to the FEMA investigation, the National Institute of Standards and Technology conducted a three-year-long investigation of the collapse of WTC 7, from 2005 to 2008. It concluded that fires burning out of control for seven hours caused the thermal expansion of steel beams, which eventually dislodged a critical floor beam on the 13th floor and led to that floor’s failure. This, in turn, triggered the collapse of the seven floors below it. This chain of events left a critical vertical column unsupported over a nine-story length, causing it to buckle and triggering the progressive collapse of the building.
In conducting its study, NIST stated it
… complemented in-house expertise with private sector technical experts; accumulated copious documents, photographs, and videos of the disaster; conducted first-person interviews of building occupants and emergency responders; … performed computer simulations of the behavior of WTC 7 on September 11, 2001; and combined the knowledge gained into a probable collapse sequence. (p. xxxv)
The NIST Final Report on the Collapse of World Trade Center Building 7 states:
The fires in WTC 7 were ignited as a result of the impact of debris from the collapse of WTC 1, which was approximately 110 meters (350 feet) to the south. … The fires were ignited on at least 10 floors; however, only the fires on Floors 7 through 9 and 11 through 13 grew and lasted until the time of the building collapse. … Had a water supply for the automatic sprinkler system been available and had the sprinkler system operated as designed, it is likely that fires in WTC 7 would have been controlled and the collapse prevented. (p. xxxvi)
… both the primary and back-up source of water for the sprinkler system in the lower 20 floors of WTC 7 was the city water main. Since the collapses of the WTC towers had damaged the water main, there was no secondary supply of water available … to control those fires that eventually led to the building collapse. (p. xxxvii)
… As the fires progressed, some of the structural steel began to heat. … The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7 … damaging the floor framing on multiple floors.
The initiating local failure that began the probable WTC 7 collapse sequence was the buckling of Column 79. … When steel (or any other metal) is heated, it expands. If thermal expansion in steel beams is resisted by columns or other steel members, forces develop in the structural members that can result in buckling of beams or failures of connections. (p. 21)
Fire-induced thermal expansion of the floor system surrounding Column 79 led to the collapse of Floor 13, which triggered a cascade of floor failures … down to the 5th floor (which … was much thicker and stronger). … This left Column 79 with insufficient lateral support, and as a consequence, the column buckled eastward, becoming the initial local failure for collapse initiation. (p. 22)
The NIST videos on the collapse of WTC 7 show this sequence as it was reconstructed.
No Evidence of Controlled Demolition
The NIST investigators examined the evidence for a controlled demolition. They concluded:
Blast events did not play a role in the collapse of WTC 7. Based on visual and audio evidence and the use of specialized computer modeling to simulate hypothetical blast events, NIST concluded that blast events did not occur, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing a critical column (i.e., Column 79) would have resulted in a sound level of 130 dB [decibels] to 140 dB at a distance of at least half a mile [805 meters] if unobstructed by surrounding building (such as along Greenwich Street or West Broadway). This sound level is consistent with standing next to a jet plane engine and more than ten times louder than being in front of the speakers at a rock concert. There were no witness reports of such a loud noise, nor was such a noise heard on the audio tracks of video recordings of the WTC collapse. (p. 49)
