Yesterday, the National Institute of Standards and Technology (NIST) issued its report on the collapse of World Trade Center Building 7 (WTC 7) on September 11, 2001. Collapse was caused by the rupturing of the building’s metal framework due to the thermal expansion of its floor beams, which were heated by uncontrolled fires because the water main that supplied the building’s fire suppression system had been cut by the collapse of WTC 1. NIST has posted a press release web page that includes a video presentation about, and two animations of their collapse model.
Nine excerpts from the NIST press release of August 21, 2008 follow, then my comments.
GAITHERSBURG, Md.—The fall of the 47-story World Trade Center building 7 (WTC 7) in New York City late in the afternoon of Sept. 11, 2001, was primarily due to fires, the Commerce Department’s National Institute of Standards and Technology (NIST) announced today following an extensive, three-year scientific and technical building and fire safety investigation. This was the first known instance of fire causing the total collapse of a tall building, the agency stated as it released for public comment its WTC investigation report and 13 recommendations for improving building and fire safety.
According to the report, a key factor leading to the eventual collapse of WTC 7 was thermal expansion of long-span floor systems at temperatures “hundreds of degrees below those typically considered in current practice for fire resistance ratings.”
The investigators also reported that if the city water main had not been cut by the collapse of World Trade Center towers 1 and 2 (WTC 1 and WTC 2), operating sprinklers in WTC 7 would likely have prevented its collapse.
Determining the probable collapse sequence for WTC 7, NIST found that the impact of debris from the collapse of WTC 1 ignited fires on at least 10 floors of WTC 7, and the fires burned out of control on six lower floors. 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. Eventually, a girder on Floor 13 lost its connection to a critical interior column that provided support for the long floor spans on the east side of the building. The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the fifth floor. Many of these floors had already been at least partially weakened by the fires in the vicinity of the critical column. This collapse of floors left the critical column unsupported over nine stories.
“When this critical column buckled due to lack of floor supports, it was the first domino in the chain,” Sunder explained. “What followed in rapid succession was a progression of structural failures. Failure first occurred all the way to the roof line—involving all three interior columns on the most eastern side of the building. Then, progressing from east to west across WTC 7, all of the columns in the core of the building failed. Finally, the entire façade collapsed.”
The investigation team considered the possibility of other factors playing a role in the collapse of WTC 7, including the possible use of explosives, fires fed by the fuel supply tanks in and under the building, and damage from the falling debris of WTC 1.
The team said that the smallest blast event capable of crippling the critical column would have produced a “sound level of 130 to 140 decibels at a distance of half a mile,” yet no noise this loud was reported by witnesses or recorded on videos.
As for fuel fires, the team found that they could not have been sustained long enough, could not have generated sufficient heat to fail a critical column, and/or would have produced “large amounts of visible smoke” from Floors 5 and 6, which was not observed.
Finally, the report notes that “while debris impact from the collapse of WTC 1 initiated fires in WTC 7, the resulting structural damage had little effect in causing the collapse of WTC 7.”
I had thought the diesel fuel stored in the building would be needed to supply the necessary heat to weaken the metal framing near Column 80 through the first seven floors to the point of failure, and wrote this scenario in my article of November 2006.
Instead, NIST found that the beams of Floor 13 near Column 79 (one column north of Column 80, and closer to the building exterior) experienced the most heating, supplied entirely by the combustion of the furnishings, fixtures, paneling, and tenant materials on Floors 11 and 12, where the most intense burning had been observed. “Fires burned in sections of Floors 6 through 30 at different times, and they migrated along their floors independently, seeking new sources of fuel. From the street the fires on Floors 11 and 12 appeared most intense. Many fires in the area went unchecked because utility power for electrical pumps, and water pressure for fire engines had either diminished or been lost,” I noted in 2006.
The beams of Floor 13 soaked in heat and expanded against their end joints, sagging under the increased compression along their lengths until the beam between perimeter Column 44 (on the east side of the north face) and interior Column 79 ruptured its joint to Column 79, initiating the collapse of the eastern end of Floor 13. The beams for eight floors below this point had been similarly weakened by fire, so the initial collapse sheared away nine floors of lateral supports to Column 79. Weakened by heat and now unable to transfer the load of the upper 34 floors laterally over a nine floor height, Column 79 buckled and this initiated the collapse of the entire eastern end of the building. See the NIST animation “Collapse Initiation — Physics Based Model.”
In rapid succession, the failure of Column 79 led to the loss of lateral supports to Column 80, then its subsequent failure and so on to Column 81, which three columns supported the eastern end of the building. Once the eastern end had fallen, collapse proceeded along the core of the building from east to west, and finally the exterior columns of the façade failed. See the NIST animation “Visualization Model of WTC Collapse.”
NIST notes that any explosive capable of damaging Column 79 sufficiently to cause the observed collapse would be exceedingly loud for quite some distance (half mile), and no such blast was observed. Sound at 130 to 140 decibels is about as loud as humans can tolerate, beyond this power one is really encountering a blast wave, a jump in pressure that delivers sensible force. Examples of loud sounds and their effects include: a jet engine at 100 meters (110-140 dB), hearing damage due to short term exposure, for example front row at a rock concert (120 dB), threshold of pain (130 dB), a rifle being fired at 1 meter (140 dB).
NIST used to be called the NBS, the National Bureau of Standards. This was the agency within the Department of Commerce that was charged with keeping the standard clocks and rulers, against which all clocks and rulers were measured and calibrated. NBS never gave the impression of being a dynamic organization because it was expected to be unerringly reliable and trustworthy, and its published data on any topic or phenomenon was accepted as being the most accurate, complete, and exhaustively tested. The word “standards” is key to understanding the detail and pace of NIST work. A person without a grounding in science might not appreciate the value of this type of detailed, methodical work, and express impatience with the NIST “plodding” before arriving at conclusions on a topic of popular interest, like the phenomenology of the WTC building fires and collapses. The NIST scientists are clearly proud of their work on WTC 7, and it shows in the polish of the press release web site.
I am pleased that I got some of the story right (thermal expansion), and my error (oil fire) was a physically reasonable assumption for the time. I am consoled to the flaws of my analysis by the fact that the NIST effort to arrive at such accurate and detailed conclusions was a major effort of over three years. Learning is both humbling and interesting. The omniscient never learn because learning is the recognition of one’s errors. I could rebel against learning by deciding my earlier ideas are an unappreciated revelation, which needs preservation despite the findings of later science, and I could devise elaborate arguments to protect my unrecognized ignorance. But defending ignorance is the essence of stupidity. A better approach would be to follow Socrates’ dictum “The only thing I know is the fact that I am ignorant.” Keep thinking, test your ideas, and don’t become overly attached to them.
Linus Pauling (1901-1994) was a great scientist and peace activist who won two unshared Nobel prizes, for chemistry in 1954, and for peace in 1962. He made major contributions in quantum chemistry, and nearly unraveled the structure of the DNA molecule before Watson and Crick. In a recorded interview in his later life he was asked “how do you get so many good ideas?” His reply was “just keep having lots of ideas, and throw away the bad ones.”
MANUEL GARCIA, Jr. is a retired physicist. E-mail = firstname.lastname@example.org