Context of 'September-November 2005: Questions Raised over NIST’s Claims of Pre-Collapse Tilt of WTC Towers'

Flight 175 hits the WTC South Tower. The picture was taken from a traffic helicopter. [Source: WABC 7/ Salient Stills]Flight 175 hits the South Tower of the World Trade Center (Tower Two). Seismic records pinpoint the time at six seconds before 9:03 a.m. (rounded to 9:03 a.m.). Hijackers Marwan Alshehhi, Fayez Ahmed Banihammad, Mohand Alshehri, Hamza Alghamdi, and Ahmed Alghamdi presumably are killed instantly, and many more in the tower will die over the next few hours. [New York Times, 9/12/2001; CNN, 9/12/2001; CNN, 9/17/2001; North American Aerospace Defense Command, 9/18/2001; USA Today, 12/20/2001; Federal Emergency Management Agency, 5/1/2002, pp. 1-10; New York Times, 5/26/2002; Associated Press, 8/21/2002; USA Today, 9/2/2002] According to the NIST report, the crash time is 9:02:59. [National Institute of Standards and Technology, 9/2005, pp. 38] According to the 9/11 Commission Report, the crash time is 9:03:11. [9/11 Commission, 7/24/2004, pp. 8] Millions watch the crash live on television. The plane strikes the 77th through 85th floors in the 110-story building. Approximately 100 people are killed or injured in the initial impact; 600 people in the tower eventually die. The death toll is far lower than in the North Tower because about two-thirds of the South Tower’s occupants have evacuated the building in the 17 minutes since the first tower was struck. [USA Today, 12/20/2001; National Institute of Standards and Technology, 9/2005, pp. 5-9, 41] The combined death toll from the two towers is estimated at 2,819, not including the hijackers. [Associated Press, 8/21/2002] The impact severs some columns on the south side of the South Tower. Each of the Twin Towers is designed as a “tube-in-tube” structure and the steel columns which support its weight are arranged around the perimeter and in the core. The plane, which is traveling at an estimated speed of around 500 mph (see October 2002-October 2005), severs 33 of the building’s 236 perimeter columns and damages another one. [National Institute of Standards and Technology, 9/2005, pp. 39] The perimeter columns bear about half of the tower’s weight, so the damage to them reduces the tower’s ability to bear gravity loads by about 7.1 percent. [National Institute of Standards and Technology, 9/2005, pp. 6] The actual damage to the 47 core columns is not known, as there are no photographs or videos of it, but there will be much speculation about this after 9/11. It will be suggested that some parts of the aircraft may be able to damage the core even after crashing through the exterior wall (see 8:46 a.m. September 11, 2001). [National Institute of Standards and Technology, 9/2005, pp. 107] According to NIST’s base case model, five of the core columns are severed and another five suffer some damage. [National Institute of Standards & Technology, 9/2005, pp. 235 ] This may reduce the tower’s ability to bear loads by a further approximately 8 percent, meaning that the aircraft impact accounted for a loss of about 15 percent of the building’s strength. This damage will be cited as an event contributing to the building’s collapse after 9/11 (see October 23, 2002 and October 19, 2004). NIST’s base case estimate of damage to the North Tower’s core will be similar, even though the aircraft impact there was dissimilar (see 8:46 a.m. September 11, 2001). Flight 11 hit the North Tower’s core head on, whereas Flight 175 only hits the corner of the South Tower’s core. [National Institute of Standards and Technology, 9/2005, pp. 20-23, 38-41] In addition, some of the fireproofing on the steel columns and trusses may be dislodged (see 8:46 a.m. September 11, 2001). [National Institute of Standards & Technology, 9/2005, pp. xxxvi, 83 ] Photographs and videos of the towers will not show the state of fireproofing inside the buildings, but the National Institute of Standards and Technology (NIST) will try to estimate the damage to fireproofing using a series of computer models. Its severe case model (see (October 2002-October 2005)) will predict that 39 of the 47 core columns are stripped of their fireproofing on one or more floors and that fireproofing is stripped from trusses covering 80,000 ft2 of floor area, the equivalent of about two floors. NIST will say that the loss of fireproofing is a major cause of the collapse (see April 5, 2005), but only performs 15 tests on fireproofing samples (see October 26, 2005). [National Institute of Standards and Technology, 9/2005, pp. 41] According to NIST, less fireproofing is stripped from the North Tower (see 8:46 a.m. September 11, 2001).

A man who is on the 105th floor of the South Tower calls emergency 9-1-1 to report that floors below his location, “in the 90-something floor,” have collapsed. The 9-1-1 operator types a record of this call into the Special Police Radio Inquiry Network (SPRINT) data link, which will be passed on to the New York fire department’s Emergency Medical Service (EMS). It isn’t known when the call is made exactly, but the EMS Dispatch computer apparently receives the call record at this time. However, because it is classified as a “supplement message,” it is not yet read by anyone. The police dispatcher dealing with the area around the WTC also receives the call record, but misinterprets it as meaning that the floor the person is on has collapsed. EMS dispatchers are dealing with an enormous volume of calls as well as performing many other tasks under extreme pressure during the crisis, so a report later concludes that the EMS operators didn’t have the time to review the information before the collapse of the South Tower at 9:59 (see 9:59 a.m. September 11, 2001), and the fire chiefs never received the information. [New York City Fire Department, 8/19/2002]

The South Tower of the World Trade Center collapses. [Source: Associated Press]The South Tower of the World Trade Center tilts to the southeast and then collapses. It was hit by Flight 175 at 9:03 a.m., 56 minutes earlier (see 9:03 a.m. September 11, 2001). [Washington Post, 9/12/2001; New York Times, 9/12/2001; MSNBC, 9/22/2001; USA Today, 12/20/2001; Associated Press, 8/21/2002; ABC News, 9/11/2002; National Institute of Standards and Technology, 9/2005, pp. 44] The first sign of the collapse is visible on floor 82. [National Institute of Standards and Technology, 9/2005, pp. 87] The angle of the tilt will be disputed after 9/11 (see September-November 2005), as will the time it takes the towers to fall to the ground (see September 12, 2001-September 2005). [Scientific American, 10/9/2001; Eagar and Musso, 12/2001; PBS Nova, 5/2002; National Institute of Standards and Technology, 8/30/2006]

A helicopter flying above the burning World Trade Center. [Source: History Channel]Minutes after the collapse of the south WTC tower, police helicopters fly near the North Tower to check on its condition. The pilot of one helicopter radios, “About 15 floors down from the top, it looks like its glowing red,” and adds, “It’s inevitable.” Seconds later, another helicopter pilot reports, “I don’t think this has too much longer to go. I would evacuate all people within the area of that second building.” While these warnings are relayed to police officers, fire and rescue personnel do not hear them, as they operate on a different radio system. [New York Times, 7/7/2002; Inter Press Service, 8/25/2005; National Institute of Standards and Technology, 9/2005, pp. 224] The North Tower will collapse 21 minutes later (see 10:28 a.m. September 11, 2001).

Andi Ball. [Source: White House]Laura Bush, the president’s wife, and her entourage are driven from Capitol Hill to the Secret Service headquarters in Washington, DC, for their own security, but their journey is slowed by the heavy traffic. [Woodward, 2002, pp. 17; National Journal, 8/31/2002; Kessler, 2006, pp. 136; Bush, 2010, pp. 200-201] Bush has been at the Russell Senate Office Building in Washington, where she was originally scheduled to testify before a Senate committee. [CNN, 9/12/2001; Woodward, 2002, pp. 16-17] Her Secret Service agents have said they are going to take the first lady and her staff to a secure location. [Kessler, 2006, pp. 136] After the Secret Service emergency response team arrived for her, Bush was escorted out of the Russell Senate Office Building and to her limousine (see (Shortly After 10:00 a.m.) September 11, 2001). Bush and those accompanying her leave Capitol Hill at 10:10 a.m., according to Noelia Rodriguez, the first lady’s press secretary.
Agents with Guns Drawn Protect Motorcade - Secret Service agents protect Bush’s motorcade with their guns as it heads to the secure location. Ashleigh Adams, the first lady’s deputy press secretary, will later recall, “It felt like we were in a war, because the Secret Service was driving next to the motorcade and they were hanging out of the windows with their machine guns out.” She will add that she has “been around the agents” before, but has “never seen them with their guns.”
Motorcade Delayed by Traffic - However, the motorcade is slowed by the heavy traffic. Bush will describe, “Outside our convoy windows, the city streets were clogged with people evacuating their workplaces and trying to reach their own homes.” Rodriguez will say, “In the car, we seemed to be going in slow motion.” [National Journal, 8/31/2002; Bush, 2010, pp. 200] “The traffic was so bad that everything was stopped,” Andi Ball, Bush’s chief of staff, will recall. One of the Secret Service agents escorting Bush and her staff will later say a car sideswiped them during the journey.
Secure Location Is Secret Service Headquarters - The “secure location” that Bush and her staff are being taken to turns out to be the Secret Service headquarters. [Kessler, 2006, pp. 136] The Secret Service headquarters, according to journalist and author Ronald Kessler, is “an anonymous nine-story tan brick building on H Street at Ninth Street NW in Washington.” [Kessler, 2009, pp. 23] It is located a few blocks from the White House. After the Oklahoma City bombing in 1995 (see 8:35 a.m. - 9:02 a.m. April 19, 1995) it was reinforced to survive a large-scale blast. Bush and her entourage arrive there through an underground entrance. [Washington Post, 8/23/2009; Bush, 2010, pp. 200-201]
Journey Reportedly Takes 45 Minutes - The exact time they arrive at is unclear. According to Washington Post reporter Bob Woodward, “In the traffic jam from the Capitol, it took 45 minutes to get [Bush] to Secret Service headquarters.” This would mean the first lady arrives there at around 10:55 a.m. [Woodward, 2002, pp. 17] However, Bush will write that she watches the North Tower of the World Trade Center collapsing “live in front of my eyes,” on a screen at the headquarters. [Bush, 2010, pp. 201] If this is correct, she must arrive at the headquarters sometime before 10:28 a.m., when the North Tower comes down (see 10:28 a.m. September 11, 2001). [9/11 Commission, 7/24/2004, pp. 311] White House spokespeople will refuse to disclose where the first lady has been taken to, only saying she is at a “secure location.” [Associated Press, 9/11/2001; CNN, 9/12/2001; CNN, 9/11/2002]

The North Tower collapses in a matter of seconds. [Source: Ray Stubblebine/ Reuters/ Landov] (click image to enlarge)The North Tower of the World Trade Center tilts to the south and then collapses. Its north side was hit by Flight 11 at 8:46, 102 minutes earlier. [CNN, 9/12/2001; New York Times, 9/12/2001; MSNBC, 9/22/2001; Kim and Baum, 2002 ; Associated Press, 8/21/2002; National Institute of Standards and Technology, 9/2005, pp. 33] After the antenna starts to move (see (10:28 a.m. September 11, 2001)), the next sign that the top section of the building is moving downward is on floor 98, at the top of the impact zone. [National Institute of Standards and Technology, 9/2005, pp. 22, 87] The angle of the tilt will be disputed after 9/11 (see September-November 2005), as will the time it takes the towers to fall to the ground (see September 12, 2001-September 2005). [Scientific American, 10/9/2001; Eagar and Musso, 12/2001; PBS Nova, 5/2002; National Institute of Standards and Technology, 8/30/2006] The death toll could have been much worse—an estimated 15,000 people made it out of the WTC to safety after 8:46 a.m. [St. Petersburg Times, 9/8/2002]

At around 8:00 p.m., Afghanistan time (11:30 a.m., New York time), Taliban leader Mullah Omar allegedly says, “Things have gone much further than expected.” This is according to what the New Yorker will describe as “Afghan intelligence sources” who monitor the call. (It is unclear what “Afghan intelligence sources” means, since the Taliban control nearly all of Afghanistan at this time, but it could be a reference to Northern Alliance forces; the CIA gave them equipment to monitor the Taliban (see Winter 1999-March 2000).) Omar’s comment takes place over an hour after one of the World Trade Center towers collapsed, which means thousands have been killed in the attacks, not hundreds (see 9:59 a.m. September 11, 2001). An Afghan intelligence official will later say: “They were expecting a reaction. But they thought it would be a Clinton-type reaction. They didn’t anticipate the kind of revenge that occurred.” [New Yorker, 6/10/2002] The “Clinton-type reaction” presumably is a reference to the August 1998 missile strikes on Sudan and Afghanistan during the Clinton administration (see August 20, 1998).

Jeffrey Scott Shapiro. [Source: University of Florida]Larry Silverstein, who recently took over the lease of the World Trade Center complex (see July 24, 2001), discusses possibly bringing down WTC Building 7 in a controlled demolition in a telephone conversation with his insurance carrier, according to a reporter who is at the WTC site this afternoon. [Fox News, 4/22/2010] WTC 7 is a 47-story office building located just north of the Twin Towers. The National Institute of Standards and Technology (NIST) will say it suffered some structural damage (see 10:28 a.m. September 11, 2001) when the North Tower collapsed (see 10:28 a.m. September 11, 2001) and it has fires on several floors (see (10:28 a.m.-5:20 p.m.) September 11, 2001). It will collapse at 5:20 p.m., apparently becoming the first tall building ever to come down primarily as a result of fire (see (5:20 p.m.) September 11, 2001). [National Institute of Standards and Technology, 11/2008, pp. xxxv-xxxvi ]
Silverstein Allegedly Wants WTC 7 Demolished - Investigative reporter Jeffrey Scott Shapiro, who is at the scene of the attacks in New York, will later recall: “Shortly before [WTC 7] collapsed, several NYPD officers and Con Edison workers told me that Larry Silverstein… was on the phone with his insurance carrier to see if they would authorize the controlled demolition of the building, since its foundation was already unstable and expected to fall. A controlled demolition would have minimized the damage caused by the building’s imminent collapse and potentially save lives.” Shapiro will add: “Many law enforcement personnel, firefighters, and other journalists were aware of this possible option. There was no secret.” [Fox News, 4/22/2010] Preparing a large building for demolition usually takes weeks, or even months. This time is spent on operations such as wrapping concrete columns to ensure pieces do not fly off. [Baltimore Sun, 2/26/1995; PBS NOVA, 12/1996; Construction Equipment Guide, 2/24/2005]
Discussion of Demolition Later Denied - Silverstein will later recall discussing WTC 7 over the phone with the commander of the New York Fire Department, and telling him, “We’ve had such terrible loss of life, maybe the smartest thing to do is pull it” (see After 12:00 Noon September 11, 2001), but a spokesman will subsequently claim that Silverstein was referring to withdrawing firefighters from the building, not bringing WTC 7 down with explosives. [US Department of State, 9/16/2005; BBC, 7/4/2008] At the end of a three-year investigation into the building’s collapse, NIST will say WTC 7 “did not collapse from explosives,” but critics will dispute this conclusion (see August 21, 2008). [Associated Press, 8/21/2008; New York Times, 8/21/2008]

Following the attacks, there is no agreement on the speed with which the Twin Towers of the World Trade Center collapsed on 9/11 (see 9:59 a.m. September 11, 2001 and 10:28 a.m. September 11, 2001). The 9/11 Commission says that the South Tower collapsed in “ten seconds” and the National Institute of Standards and Technology says that tops of the buildings came down “essentially in free fall.” [9/11 Commission, 7/24/2004, pp. 305; National Institute of Standards and Technology, 9/2005, pp. 146] In the dispute over the reason for the WTC’s collapse after 9/11, it is claimed that, if the towers fell in ten seconds, then this is evidence they were destroyed by explosives. For example, David Ray Griffin, a theologian and outspoken critic of commission’s report, writes, “For a 1,300-foot building, however, ten seconds is almost free-fall speed. But if each floor produced just a little resistance, so that breaking through each one took half a second, the collapse of all those floors—80 or 95 of them—would have taken 40 to 47 seconds. Can we really believe that the upper part of the buildings encountered virtually no resistance from the lower part?” [Griffin, 2004, pp. 16] But according to Canadian scientist Frank Greening, who studies the Twin Towers’ collapse, the freefall time would be about 9.6 seconds, and he calculates that it takes longer for the buildings to fall—twelve to thirteen and a half seconds—and states this does not indicate that explosives were used. [Canadian Broadcasting Corporation, 8/25/2005]

FEMA’s World Trade Center Building Performance Study. [Source: FEMA]FEMA releases its report of the WTC collapses. It concludes, “[W]ith the information and time available, the sequence of events leading to the collapse of each tower could not be definitively determined.” On Building 7: “The specifics of the fires in WTC 7 and how they caused the building to collapse remain unknown at this time.” [Federal Emergency Management Agency, 5/1/2002]

The National Institute of Standards and Technology (NIST)—an agency of the US Commerce Department’s Technology Administration—announces details of its forthcoming investigation into the collapses of the World Trade Center Twin Towers and Building 7 on 9/11. The NIST investigation aims “to investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the World Trade Center (WTC) disaster.” It also aims to examine the activities of building occupants and emergency responders on 9/11, studying such issues as emergency communications within the WTC, the movement of people during the evacuations, and issues around persons with disabilities. Leading technical experts from industry, academia, and other laboratories, alongside NIST’s own expert staff, will participate in the investigation. Expert professionals from the private sector will also be involved. Glenn Corbett, a fire science professor at John Jay College, says, “This is going to be the most extensive building disaster investigation ever performed.” [Associated Press, 8/21/2002; National Institute of Standards and Technology, 8/21/2002; National Institute of Standards and Technology, 8/3/2005] The investigation is formally authorized in October 2002, when the National Construction Safety Team Act is signed into law. The act, which gives NIST authorization to investigate major building failures in the US, is written largely as a result of the World Trade Center collapses. [National Institute of Standards and Technology, 10/2/2002; New York Times, 5/8/2003] NIST’s investigation is originally proposed to last two years, with a budget of $16 million. [National Institute of Standards and Technology, 8/21/2002] However, it eventually will last three years, with its final report into the collapses of the Twin Towers being released in October 2005 (see October 26, 2005). A previous analysis of the WTC collapses conducted by FEMA and the American Society of Civil Engineers (ASCE) was completed earlier in 2002 (see May 1, 2002), but this had a budget of just $1.1 million. By the time NIST starts its investigation, much of the crucial steel debris from the WTC collapses has already been destroyed (see September 12-October 2001). They later refer to there being a “scarcity of physical evidence that is typically available in place for reconstruction of a disaster.” [National Institute of Standards and Technology, 9/2005, pp. xxxvi]

Matthys Levy. [Source: PBS]A report is made publicly available, which the Engineering News-Record calls the “most comprehensive study yet on the destruction of the World Trade Center.” The study was commissioned by WTC leaseholder Silverstein Properties Inc. to support a $7 billion insurance claim, and conducted by a team of engineers from several leading firms, including Weidlinger Associates, LZA Technology/Thornton-Tomasetti, and ARUPFire. It is intended to build on a previous study sponsored by FEMA (see May 1, 2002). The report’s findings are based on an analysis of original structural drawings, thousands of photos, and dozens of videos. Investigators used fire evaluation techniques and powerful computer software to simulate the condition of each tower at critical times between the planes’ impacts and the towers’ collapses. The earlier FEMA investigators had no access to such computer modeling. Matthys Levy, the chairman of Weidlinger Associates and one of the engineers on the study team, says, “The buildings had tremendous reserve capacity and that was reflected in all of the elements we analyzed. In fact, because there were so much excess capacity, the columns even in the impact floors did not buckle immediately, but failed as the result of the fire.” The report states that failure of the WTC’s steel floor supports (“trusses”) did not contribute to the collapses. Instead, the collapses were caused by the failure of steel structural columns that were either destroyed when the planes hit or lost fireproofing, leaving them vulnerable to the weakening effects of the ensuing fires. It says that debris and dust distributed by the plane crashes inhibited the fires, such that the average air temperatures on the impact floors were between 400 and 700°C (750-1,300°F): significantly lower than those associated with typical “fully developed” office fires. However, says Matthys Levy, “By the time the temperature inside the buildings reached 400 degrees, the steel would have lost approximately 50% of its strength. Eventually, gravity took over and the towers began to fall.” Then, according to the analysis led by researchers from LZA Technology/Thornton-Tomasetti, “Once collapse initiated in each tower, essentially all of the interior structure of the tower fell straight down with floors pancaking on top of one another. The network of perimeter steel columns and spandrels acted like a chute to funnel the interior contents into the tower footprint.” According to the computer simulations, the damage to the South Tower’s steel core columns was so severe that the tower should have collapsed immediately after the plane hit. Civil engineer John Osteraas says this incorrect result casts doubt upon some of the study’s predictions. The report concludes that the collapse of the South Tower did not cause or contribute to the subsequent collapse of the North Tower, thus supporting Silverstein Properties’ claim that the terrorist attack represented two occurrences, entitling it to two $3.5 billion insurance policy limits. A separate study commissioned by the insurers contradicts this (see October 23, 2002). The Silverstein report apparently does not examine the collapse of WTC Building 7, a 47-story skyscraper that also collapsed on 9/11 (see (5:20 p.m.) September 11, 2001). It has been passed on to the National Institute of Standards and Technology (NIST), which is undertaking its own investigation of the WTC collapses (see August 21, 2002). [New York Times, 9/30/2002; Business Insurance, 10/23/2002; Silverstein Properties, Inc., 10/23/2002 ; Engineering News-Record, 10/25/2002; New York Times, 10/29/2002; Engineering News-Record, 11/4/2002; Real Estate Weekly, 4/30/2003]

Insulated trusses in the World Trade Center. [Source: Gilsanz Murray Steficek]At a press briefing in New York City, the National Institute of Standards and Technology (NIST) releases a 122-page progress report on its investigation into the WTC collapses. NIST began its study in August 2002 (see August 21, 2002). Investigators say they believe that the Port Authority of New York and New Jersey, who built the Twin Towers, failed to carry out vital tests to establish how the buildings would cope with a major fire. They have been unable to find evidence that tests were conducted on the fireproofing material used in the buildings. Their report also states that in 1969, builders directed contractors to coat the WTC floor supports with half an inch of spray-on fireproofing. In 1999, the Port Authority issued guidelines to triple the thickness of the fireproofing, and by 9/11, about 30 floors in the upper areas of the two towers had been upgraded. Almost all the floors in the impact zone of the North Tower had their fireproofing upgraded, while in the South Tower just the 78th floor—the lowest in its impact zone—had been upgraded. As the New York Times states, though, “investigators took great care… to say they were nowhere close to definitively determining how and why the towers collapsed after they were struck by hijacked airliners.” [National Institute of Standards and Technology, 5/2003, pp. 81 ; National Institute of Standards and Technology, 5/7/2003; Guardian, 5/8/2003; New York Times, 5/8/2003]

The National Institute of Standards and Technology (NIST) issues a progress report on its investigation into the World Trade Center collapses. Since 2002, NIST has been investigating the collapses of the Twin Towers and WTC Building 7 (see August 21, 2002). The progress report includes its “working hypothesis” for the collapse of WTC 7. This was a 47-story building, located about 350 feet from the north side of WTC 1, which collapsed completely at around 5:20 in the afternoon of 9/11. The report claims that “fire appears to have played a key role” in the collapse, though it points out, “No fire was observed or reported in the afternoon on floors 1-5, 10, or above Floor 13.” It also says, “there may have been some physical damage on the south side of the building.” NIST summarizes its working hypothesis of the WTC 7 collapse as follows:
“An initial local failure at the lower floors (below Floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event), which supported a large span floor bay with an area of about 2,000 square feet.”
“Vertical progression of the initial local failure up to the east penthouse, as large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse.”
“Horizontal progression of the failure across the lower floors… triggered by damage due to the vertical failure, resulting in the disproportionate collapse of the entire structure.”
NIST claims this hypothesis “is consistent with all evidence currently held by NIST, including photographs and videos, eyewitness accounts and emergency communication records,” but says it “will be revised and updated as results of ongoing, more comprehensive analyses become available.” [National Institute of Standards and Technology, 6/2004, pp. L1, L3, L17, L34, L38, L51-L52 ; National Institute of Standards and Technology, 6/18/2004] NIST will release its final reports on the collapses of the Twin Towers in October 2005 (see October 26, 2005). As of mid-2007, it has yet to release a final report on the collapse of WTC 7. In early 2006, Dr. S. Shyam Sunder, who is the lead investigator for NIST’s WTC investigation, will admit, “truthfully, I don’t really know” what caused WTC 7 to collapse. He will add, “We’ve had trouble getting a handle on building no. 7” (see March 20, 2006). [New York Magazine, 3/20/2006] An earlier report on the WTC collapses, released by the Federal Emergency Management Agency (FEMA) in May 2002, had been inconclusive on what caused WTC 7 to collapse, and stated that “Further research, investigation, and analyses” were necessary (see May 1, 2002). [Federal Emergency Management Agency, 5/1/2002, pp. 5-31]

Both towers of the World Trade Center tilted to one side before beginning to fall on 9/11 (see 9:59 a.m. September 11, 2001 and 10:28 a.m. September 11, 2001). The National Institute of Standards and Technology, which investigated the collapses (see August 21, 2002), states that the South Tower, which was hit on its south side, tilted about 7-8 degrees to the east and 3-4 degrees to the south, and the North Tower, which was hit on its north side, tilted about 8 degrees to the south, before starting to fall. [National Institute of Standards and Technology, 9/2005, pp. 300, 308 ] However, these findings are criticized by Canadian scientist Frank Greening. After examining photo and video evidence, Greening says that the South Tower tilted by no more than two degrees and the North Tower by no more than three degrees before collapse initiation. Greening also says that the tilt angles NIST gives cannot correspond to the downward movement of the towers’ walls NIST claims before they started to fall, and points out that the tilt angles NIST uses are inconsistent throughout its reports. Though Greening agrees with NIST that the towers were destroyed by the plane impacts and fire damage, he concludes that its computer model is “highly inaccurate and therefore of no value in explaining the demise of the Twin Towers.” [Greening, 11/2005 ] Greening is a leading figure in the post-9/11 dispute over why the WTC collapsed and publishes a series of papers dealing with various aspects of the Twin Towers’ collapse. For example, a CBC documentary uses Greening for analysis of the WTC’s fall. [Canadian Broadcasting Corporation, 8/25/2005]

The 43 NIST reports ran to over 10,000 pages. [Source: NIST]The National Institute of Standards and Technology (NIST) issues the final report of its three-year, $16 million study into the WTC collapses on 9/11. NIST has produced over 10,000 pages of findings, and its report includes 30 recommendations for improving building safety, such as having wider stairwells and structurally hardened elevators for use in emergencies. The recommendations are mostly the same as those outlined in an earlier draft of the report (see June 23, 2005). [Engineering News-Record, 10/27/2005; New York Times, 10/27/2005] NIST has made some amendments and clarifications, though, based upon nearly 500 comments received during a six-week public review period. [National Institute of Standards and Technology, 10/26/2005] NIST’s theory about what caused the Twin Towers to collapse remains the same as that described in its previously released findings (see October 19, 2004). However, the NIST’s account only examines events up to the initiation of each collapse; the investigation “does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable.” [National Institute of Standards and Technology, 9/2005, pp. 82] NIST makes no mention of molten metal found at the collapse site in the weeks and months after 9/11, which has been described in numerous reports (see September 12, 2001-February 2002). The “NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001.” [National Institute of Standards and Technology, 9/2005, pp. 146] Members of Congress are critical of NIST’s recommendations, saying they are not detailed enough, or adequately documented, to be rapidly incorporated into standard building code publications. [New York Times, 10/27/2005] According to Glenn Corbett, a technical adviser to NIST and fire science professor at John Jay College, NIST is not aggressive enough to carry out major forensic investigations. He says, “Instead of a gumshoe inquiry that left no stone unturned, I believe the investigations were treated more like research projects in which they waited for information to flow to them.” [Associated Press, 10/26/2005; US Congress, 10/26/2005 ] NIST will release its final report on the collapse of Building 7 of the WTC separately, at a later date. [National Institute of Standards and Technology, 9/2005, pp. xiii]

NIST lead investigator Shyam Sunder answering questions about NIST’s three-year study of the collapse of WTC 7. [Source: Don Berkemeyer / National Institute of Standards and Technology]The National Institute of Standards and Technology (NIST) announces the findings of its study of the collapse of World Trade Center Building 7, and says the 47-story tower fell late in the afternoon of 9/11 primarily due to fires. [National Institute of Standards and Technology, 8/21/2008; National Institute of Standards and Technology, 8/21/2008] NIST releases its findings as part of a 915-page report, which is the result of three years’ work by over 50 federal investigators and a dozen contractors (see August 21, 2008). [New York Times, 8/21/2008]
Collapse Is 'No Longer a Mystery' - In a news conference at NIST’s headquarters in Gaithersburg, Maryland, lead investigator Shyam Sunder admits: “[W]e knew from the beginning of our study that understanding what happened to Building 7 on 9/11 would be difficult. It did not fit any textbook description that you could readily point to and say, yes, that’s why the building failed.” But, he says, “[T]he reason for the collapse of World Trade Center 7 is no longer a mystery.” [National Institute of Standards and Technology, 8/21/2008; New York Times, 8/21/2008]
'New Phenomenon' Caused Collapse - Sunder says the “critical factor” that initiated the collapse was “thermal expansion of long-span floor systems located in the east side of the building,” and adds that NIST’s study “has identified thermal expansion as a new phenomenon that can cause structural collapse. For the first time we have shown that fire can induce a progressive collapse.”
Collapse Sequence - Sunder describes the sequence of events NIST believes led to the collapse of WTC 7. He says debris from the collapse of the north WTC tower “started fires on at least 10 floors of the building. The fires burned out of control on six of these 10 floors for about seven hours. The city water main had been cut by the collapse of the two WTC towers, so the sprinklers in Building 7 did not function for much of the bottom half of the building.” He continues: “Fires on floors 7 through 9 and 11 through 13 were particularly severe.… Eventually, a girder on floor 13 lost its connection to a critical interior column.” Floor 13 collapsed, beginning a cascade of floor failures down to the fifth floor. “With the support of these floors gone, column 79 buckled, which initiated the fire-induced progressive collapse of the building.… This in turn caused the failure of nearby columns 80 and 81 and floor failures up to the roof line.… As the roof line begins to fall adjacent columns buckle as well. In quick succession, the remaining interior columns failed from east to west across WTC 7, until the entire core began moving downward. Finally, the remaining outer shell or façade of the building fell.”
NIST Created 'Virtual WTC 7' Model - Sunder says that NIST reached its conclusions about the collapse “by reconstructing the entire building, beam by beam, column by column, connection by connection into a computer model, a virtual WTC 7 building. We then filled that virtual building with as much detail as possible about exactly what types of furnishings were on each floor. Then we set fire to those virtual offices on the floors where video and other visual evidence told us the fires burned.” The investigators “used a well-validated computer program developed at NIST, for studying the growth and spread of fires, to calculate temperatures throughout the building.… And we used well-established data on the properties of structural steel, the sprayed fire resistive material or fireproofing, and other building materials to determine how those temperatures affected the structure.”
Explosives Not Used - Sunder says that the investigators “did not find any evidence that explosives were used to bring the building down” (see August 21, 2008), nor was the collapse “due to fires from the substantial amount of diesel fuel stored in the building” (see August 21, 2008). NIST commenced its investigation of the WTC collapses in 2002 (see August 21, 2002) and issued its findings on the collapses of the Twin Towers in October 2005 (see October 26, 2005). Since then it has been focused on WTC 7. [Government Computer News, 8/21/2008; National Institute of Standards and Technology, 8/21/2008]
Final Report to Be Released - After suggestions are made by members of the public in response to its current report, NIST will release a finished version of the same report in November 2008, thereby completing its WTC investigation (see November 20, 2008). [National Institute of Standards and Technology, 11/20/2008]

NIST’s ‘Final Report on the Collapse of World Trade Center Building 7.’ [Source: National Institute of Standards and Technology.]The National Institute of Standards and Technology (NIST) releases the final report of its three-year investigation of the collapse of World Trade Center Building 7, the 47-story skyscraper which collapsed late in the afternoon of 9/11 (see (5:20 p.m.) September 11, 2001). This is the completed version of the report, and comes three months after a draft version was released for public comment (see August 21, 2008). NIST states that the new report “is strengthened by clarifications and supplemental text suggested by organizations and individuals worldwide in response to the draft WTC 7 report.” NIST conducted an additional computer analysis in response to comments from the building community, and made several minor amendments to the report. But, it says, “the revisions did not alter the investigation team’s major findings and recommendations, which include identification of fire as the primary cause for the building’s failure.” With the release of this report, NIST has completed its six-year investigation of the World Trade Center collapses, which it commenced in August 2002 (see August 21, 2002). The final report of its investigation of the Twin Towers’ collapses was published in October 2005 (see October 26, 2005). [National Institute of Standards and Technology, 11/20/2008; Occupational Health and Safety, 11/25/2008]