How did the twin towers fall?
Questions remain

NIST's 9/11 reports show:

Fireproofing scenario is puzzling
Theory supported by scant evidence
'Mystery blasts' glossed over
Inquiry considered a 'small bomb'
WTC7 collapse baffles probers
NIST adds brief note on 'controlled demolition'
Also: Physics prof challenges NIST theory

NIST web site
Defense contractor aids stalled WTC7 probe
NOVA special on collapses, with views of MIT Professor Thomas Eagar who offers yet another theory
Weapons that affect cognitive functions
Weldon: 9/11 hijackers 'known' in 2000 (NY Times report)
GSN's report on 'Able Danger'
Weldon's 'Meet the Press' remarks in June 2005
Jim Hoffman's criticism of the NIST report
WMD's: debunking the myths (presidential commission conclusions)
Ex-FBI chief blasts 9/11 probe
Professors doubt 'official' exit poll story
Further criticism of 'exit poll' line
Collapse time data: Omissions and disparities
About Znewz1
Weapons lab scientist tries to debunk 9/11 skeptics
Chomsky tries to derail new 9/11 investigations
Scientists clash over 9/11 collapses
The symmetry problem in the WTC collapses

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Citations refer to NIST reports. In many cases, page numbers are given. However, some NIST reports use two systems of page numbers. So, it may be necessary to check the alternate page number in order to find a reference.
To search NIST reports, reach NIST's search engine through the media link and view the text version with the keyword 'NCSTAR' (and other keywords) highlighted.

This page went on-line in July 2005 and has been updated since.

By PAUL CONANT

Jetliners crashing into World Trade Center towers blew off much of the fireproofing on floor supports and girders but left intact most of the fireproofing on floor undersides, according to the official theory.

Investigators for the National Institute of Standards and Technology were unable to come up with a 'credible' collapse scenario without resorting to the seemingly contradictory assumptions. This difficulty is not apparent in the main report [NCSTAR 1] but only emerged after extensive examination of the multitude of supporting reports.

A review of the NIST investigation of the World Trade Center disaster also shows that probers were puzzled by numerous 'pressure pulses' of smoke that preceded each collapse but were reluctant to speculate on the origin of the blasts.

And, of the small amount of steel preserved as evidence, very little shows signs of the temperatures needed to critically weaken it, and photographic evidence also fails to corroborate the government's conjecture that, on the fire floors, a superhot upper layer of fiery gases caused girders to weaken critically and a large number of floor connectors to weaken and trigger a collapse sequence. Even so, in order for such a thing to happen, the fireproofing had to have been stripped from core columns and floor assembly components but not from the underside of the metal floor deck directly above, the reports -- which seem contradictory on a number of points -- show.

The NIST's main report fails to account for the fact that ceiling tile, though falling, would still have intercepted much of the high-velocity rubble before it reached the floor assembly fireproofing [Tile and debris velocities are discussed in a footnote below.]

The NIST shelved a report on the collapse of the 47-story WTC7, which has baffled investigators, until after the twin towers report's release. The twin towers report was made final in September 2005 [see footnote below] and some editorial changes were made.

However, it is unreasonable to assess probabilities of failure without including the WTC7 collapse. The NIST fails to offer a probability analysis for the collapse of three skyscrapers. [This reporter's comments on probabilities are found in a footnote below.]

John Young, a veteran architect and founder of the activist site Cryptome.org, believes that the towers were structurally unsound and shook to pieces following the impact of the jets. He says that coverup of design flaws is a routine result of building disaster investigations. [Young's remarks and can be found in a footnote below.]

Glenn P. Corbett, a fire safety expert and NIST adviser, said in October 2005 that the overall NIST effort was unsound and that in future the agency should be disqualified from forensic investigations, according to an Associated Press report.

"Instead of a gumshoe job that left no stone unturned, I believe the investigations were treated more like research projects in which they would wait for information to flow to them," Corbett, a fire science professor at John Jay College in New York, is reported to have said. An email request for further comment went unanswered.

In November 2005, Steven E. Jones, a Brigham Young University physics professor, challenged the government theory and argued that the physics pointed to the likelihood of a bomb plot [In the fall of 2006, Jones was pressured to retire as a result of his paper, which is no longer easily accesible on the internet.] He timed the mysterious pressure pulses in WTC7 and found that they had occurred too closely together to have been the result of interior collapses (more detail below). No response was received to an email query for further comment.

The NIST inserted a note in NCSTAR 1 saying that probers had found no evidence of controlled demolition of the skyscrapers. The draft NCSTAR 1 report ignored the alternative hypothesis altogether and there is no evidence in the various studies that the NIST ever examined that possibility. In fact, there is no indication in NIST supporting data that experts on controlled demolition were consulted to see whether such a scenario had any credibility.

'Able Danger' ignored
The point that government 9/11 reports are unreliable was reinforced with a furor that erupted over a claim by Rep. Curt Weldon, R-Pa., that a supersecret Pentagon 'data mining' operation called 'Able Danger' had in 1999 or 2000 identified Mohamed Atta and three other future 9/11 hijackers as suspected members of an al Qaeda cell operating in the United States. Lee Hamilton, a co-chairman of the 9/11 commission, said that though the panel had heard something about the operation, Atta's name hadn't come up. However, a 9/11 panel spokesman then said that Atta's name had been mentioned during a briefing to a staffer but that the panel decided against mentioning the information because of credibility problems. Still, the panel's reports take pains to debunk numerous theories and innuendos that came to the attention of staffers.
At any rate, the commission never mentioned the operation or the hijack suspects. Defense Secretary Donald Rumsfeld denied in August 2004 ever having heard of 'Able Danger,' raising the question of why the Pentagon did not vigorously check into Weldon's comments about Able Danger that were broadcast in June 2004. In addition, the New York Times reports that an observer from the Pentagon sat in on a commission briefing about 'Able Danger.'
As of Aug. 11, the White House was staying silent on the subject, even though Weldon said he had in October 2001 told White House national security aide Stephen J. Hadley of the operation.
However, in October 2004, Rep. Dan Burton, R-Ind., told the Times that he and Weldon had met with Hadley on Sept. 25, 2001, when Hadley was shown a chart containing pre-attack information collected by Able Danger on suspected al Qaeda operatives. Weldon said he gave Hadley the chart. At this point, a Hadley spokesman confirmed that Hadley had seen such a chart but that it could not be found after a search of White House files.
In November 2004, former FBI Director Louis J. Freeh assailed the 9/11 commission in its handling of the Able Danger matter. He noted that the FBI would normally pounce on tips of that type in order to thwart terrorist attacks. In January 2007, the Senate Intelligence Committee said that its investigators had been unable to substantiate that the Pentagon had pinpointed Atta long before Sept. 11, 2001.

Following are discussions of the government theory, the scantiness of supporting evidence, the large and small pre-collapse blasts, the NIST's lack of clarity and the WTC7 collapse problems.

THE GOVERNMENT THEORY

The government theory is that the jet impacts and ensuing fires combined to trigger the collapse of the towers, with the fires causing most weakening in World Trade Center 1 and the impact doing most of the damage in WTC2, which was struck second but collapsed first.

And the fact that inward bowing of the east face of WTC2 occurred 18.5 minutes after impact while such bowing did not occur for WTC1 until just before collapse tends to support that possibility.

Essentially, the NIST decided that core column shortening strained superheated floor links, which failed, causing the floor assemblies to sag and drag the exterior walls inward, though it also seems to favor the idea that wrecked core columns toppled, pulling floors down with them. However, this idea is problematic since an excessive number of core columns would have to have been superheated, contrary to fire analyses.

Though a number of wide-flange columns were found to have been milled from substandard steel [NCSTAR 1-3, p107], the NIST does not believe that this issue was a highly significant factor in the collapses.

The NIST's assumptions and computer models showed that key structural responses leading to collapse were:

*Floor sagging caused by failure of thermally weakened truss members, resulting in pull-in forces.

*Downward displacement of the core [the reinforced interior structure of the tower] due to jet impact and shortening of core columns due to increased load and heat effects.

The high-temperature gases primarily heated floor trusses and the bottom face of the concrete and metal floor slabs through convection and the top face through radiation. As the floor system heat increased, web diagonals buckled and truss seats failed, allowing floor assemblies to sag, the NIST conjectured [NCSTAR 1-6D, p31, NCSTAR 1-6, p319; for a similar description, see NCSTAR 1-6, page 286.

The computer models covered floor trusses, core beams, perimeter and core columns and concrete floor slabs [NCSTAR 1-5G].

However, tests showed that steel coated with spray-on concrete-and-fiber insulation held up with no significant weakening, even under intense, prolonged heating [NIST 1-5B]. Thus, the NIST decided that fireproofing must have been dislodged by the jetliner crashes. Various possibilities were examined, with probers eventually settling on 'Case B' -- which required fireproofing shorn off core columns and a wide swath of floor trusses, but not from the underside of the adjacent floor deck.

In the main report, a Case B condition is that the 'soffit remains' in order to provide a hot enough upper layer of gases to critically damage the floor assembly parts [NCSTAR 1, p124 (or alternate p178)]. ('Soffit' is a builder's word that refers to a covering of a floor underside or to an enclosure between a floor underside and a ceiling.)

However, one must go to NCSTAR 1-5B (p110 ff) to learn that the soffit refers to 48-inch (1.2-meter) Marinite fireproofing boards used in experiments to determine heat flows that would critically damage floor assembly parts. (When this reporter checked, the 'text' version of NCSTAR 1-5B was unavailable, making the word 'soffit' difficult to locate in that report.)

NCSTAR 1-5's executive summary asserts that in Cases B and D, which are WTC1 and WTC2 scenarios leading to collapse, 'a more severe representation was to leave a 1.2m soffit that would maintain a hot upper layer on each fire floor.'

The summary adds, 'This produced a fire of longer duration near the core columns and the attached core membranes.'

It then becomes apparent that the Case B condition means that the soffit represents the fireproofing on the deck underside. Without insulation, the floor deck steel would transmit the high temperatures upward, possibly crumbling the lightweight concrete slab, with the heat then radiating into the air of the floor above, NIST data indicate.

A photo of a floor assembly test unit shows fireproofing coating the deck underside [NCSTAR 1-6, p49], implying that such protection was standard for both towers. Also, a none-too-clear 1993 photo of a WTC floor assembly shows at least some spray-on insulation (or SFRM) on the deck underside [NCSTAR 1-6, p26]. For a description of floor assemblies, see NCSTAR 1-6, p65ff.

Another NIST report found that, for Case B, it was necessary to have partially opened walls in order to have enough oxygen for the fires while still trapping gases in order to raise temperatures [NCSTAR 1-5G, p179], but fails to make clear that the closed walls in the scenario are ceiling firewalls. [Also see NCSTAR 1-5F, p105.]

In order for fireproofing to be stripped off, debris must have struck the steel components with accelerations of 40 g's or more (40 x 32ft/s² or 40 x 9.8 m/s²), the NIST found [NCSTAR 1-6A, p110, p162], with debris velocities on the order of 350 mph [NCSTAR 1, p117]. The NIST pointed out that the non-fire-rated tile hanging from ceilings would likely have fallen when the buildings were jarred by the impacts, a suggestion that accords with survivor observations. The ceiling tiles would have fallen at a rate of less than 1 g (less than 32 ft/s² or 9.8 m/s², meaning many wouldn't have fallen far before being struck by the spray of debris.

In addition, experiments showed that it took substantially more force to blast SFRM off a narrow-diameter bar than off flat-surface steel [NCSTAR 1-6A, p102]. Yet the government claims the SFRM was dislodged from the narrow-diameter web truss diagonal bars but not the planar decks.

Hence the government theory requires that at least some of the ceiling tiles be pulverized by the debris and become part of the debris spray, which would then strip the floor supports of insulation but not the floor deck [see below for a brief discussion of the velocities]. This reporter could find no detailed discussion of such a scenario.

Probers also suggest SFRM could have been shaken off steel elements as the building vibrated from the jet collision but decided against using the idea in their models. But, even if that suggestion were valid, the question would remain as to why the SFRM didn't also rattle loose from the floor decks.

The contention that the floor decks retained their insulation is supported by the 'cold spot' across floors 80, 81 and 82 of WTC2. Photos show what appear to be diagonally draped floor slabs showing through windows. Infrared images showed that this area didn't heat up, though it was surrounded by high temperatures [NCSTAR 1-5, p30].

Saying there was insufficient data to explain the cold spot [NIST 1-5, p36], the government probers managed not to draw attention to the point that undamaged floor deck fireproofing tended to imply undamaged floor support fireproofing.

HEAT ISSUES

Probers found that WTC steels tend to lose structural integrity if exposed to temperatures of 500^oC or more for a period of at least 18 minutes. Generally, a temperature of 650^oC is held to be critical.

The fuel available for the fires was estimated from the jet debris, jet fuel presumed not to have burned in the initial fireballs and from typical workstation contents. In the main report and elsewhere, NIST probers seem quite confident in their estimate of 4 pounds per square foot of combustibles on the fire floors, though NCSTAR 1-5's executive summary says that, for the relevant case, probers required 5 pounds per square foot in order to generate enough heat for each floor [NCSTAR 1-5, page xliv].

NIST is careful to debunk the myth that jet fuel raised temperatures to steel-bending levels. Its tests found that jet fuel accelerated burning of workstation contents but had little effect on temperatures [NCSTAR 1, p180].

There is very little physical evidence to support the government's theory. Of the 200,000 tons of steel from the twin towers, the NIST ended up with a paltry 236 pieces of steel for use as evidence.

However, a large number of still and video photos greatly assisted the investigation. Yet, the photographic evidence is weak when it comes to the NIST hypothesis, with investigators admitting that the trade center fires, despite variability, are mostly low-heat fires [NCSTAR 1-5, p16], but they argue that smoke may have obscured evidence of the extent and heat of the fires in the two towers.

Except for one case, fires behind windows on WTC1's east face last 6 minutes to 16 minutes. One window on floor 92 shows a fire lasting 28 minutes, which is long enough in duration to have the potential to warp steel [NCSTAR 1-3, p58].

The NIST scenarios would have oxygen-fed, but air-cooled fires near blown windows, with the hottest gases gathering near the core floor supports, with the insulated floor deck acting as a horizontal fire wall.

Also, fires at most sites on WTC1's west and north faces appeared to be of low intensity, though flames were seen on occasion belching from north and south face windows [NCSTAR 1-5, p19]. Though fires blazed across 90 percent of WTC1 floors 96, 97 and 98, they tended to die out as they went, sweeping a floor in a somewhat circular fashion dictated by still-standing interior walls [NCSTAR 1-5, p19] and by the availability of oxygen as fire pressures blew out windows.

Fires generally did not spread through WTC2 [NCSTAR 1-5, p29].

Probers theorized that about 20 percent of floor units on WTC1's 97th and 98th floors failed because of thermal weakening of vertical supports [NCSTAR 1-6, p289]. Yet, 90 percent of the 31 core floor truss connectors (core seats) recovered were intact, though probers say much damage could still have occurred [NCSTAR 1-5, p130,131].

Examination of steel columns known to come from the fire floors proved virtually fruitless.

Twenty-six columns were identified by code numbers as coming from WTC1 fire floors. Yet only one showed clearcut evidence of temperatures in excess of 250^o C [NCSTAR 1-3C, Appendix E, p448ff].

Both analysis of paint cracking and microscopic differences in steel surfaces showed very little indication that WTC1 fire-floor steels were exposed to sustained temperatures of 650^o C or more [NCSTAR 1, p86, NCSTAR 1-3, p101].

For example, the two WTC1 core columns subjected to paint analysis showed no sign of temperatures above 250^o C [NCSTAR 1, p86].

Of perimeter panels that failed, fire played a negligible role [NCSTAR 1-3, p70].

The NIST says that its sample of columns, which includes only a few core columns, doesn't provide statistically meaningful data [NCSTAR 1-6, p86].

Additionally, the NIST's Case B fire simulation shows more than 50 percent of columns on fire floors sustaining temperatures above 250^oC for various time periods [NCSTAR 1-5, p112 ff], but the sample of perimeter and core columns recovered -- if generalized statistically -- would indicate well below 50 percent of fire-floor columns reaching such temperatures. [See NCSTAR 1-3's appendix E, p448ff.]

After an outcry over the FBI's failure to obtain an order to preserve the steel evidence, volunteer experts and a professor with a National Science Foundation grant combed salvage yards and tagged steel pieces deemed to be 'structurally significant' -- suggesting that the experts did not run across core columns that showed fire damage or that piqued their interest.

Probers, citing scantiness of data, found that they faced 'challenges' in simulating WTC2 fires, forcing them to concentrate the fuel load and volatility estimates onto two floors,[citation to come] even though the aircraft impact analysis distributed these variables more widely [NCSTAR 1-2, p62].

MYSTERY BLASTS

'Pressure pulses' expelled anomalous puffs of smoke -- or possibly dust -- from both towers at various times prior to collapse, with many more blasts observed at WTC2 than WTC1 [NCSTAR 1-6, p154] -- though this difference may be because photographic evidence for WTC1 diminished sharply after WTC2 collapsed.

NIST probers still have no explanation for what they admit were highly unusual 'correlated puffs of smoke' that came from the towers, though they conjecture that another cluster of major blasts was due to ignition of pools of jet fuel, but they do not vouch for that notion.

In WTC2, at least '65 occurrences' of smaller smoke puffs were recorded along with seven large blasts of smoke and flame that lasted about one minute each. However, the lesser puffs were considered to represent forces that were 'much too small to affect the tower's structural components' [NCSTAR 1-5, p37,38] -- though other NIST probers conclude that 'numerous puffs of smoke may indicate internal changes in architectural or structural features' [NCSTAR 1-6, p166]. Basically, the blasts are ignored in assessment of the probable collapse sequences.

No response was received to an email query sent to Richard G. Gann, who led the NCSTAR 1-5 research team and who edited the main report, asking for the reasoning used to conclude that the forces behind the smaller puffs were insufficient to damage steel components [letter appears below].

At 10:18:43 a.m., smoke suddenly billowed from floors 92 through 98 on the north and west faces of WTC1. At 10:22:59, inward bowing of the south wall occurred and at 10:28:20 the building collapsed [NCSTAR 1-6, p154]. The NCSTAR 1-5 team guessed that the event was triggered by settling of the core or collapse of floors [NCSTAR 1-5, p17]. Elsewhere, the NIST's chief investigator, Shyam Sunder, had said that it would take failure of a large number of floors to initiate collapse, though later his probers put the lower limit at 3 consecutive floors (the number of apparent tilted slabs seen through WTC2 windows).

The main report says the large bursts of smoke emitted from WTC2's 79th and 80th floors between 9:30 and 9:34 a.m. may have been caused by ignited jet fuel or by shifting floor slabs [NCSTAR 1, p43]. WTC2 fell at 9:59.

Also, at 10:21:15 a.m., 7 minutes before collapse, an intense burst of fiery light, lasting 3 seconds, appeared through WTC1's 98th floor windows [NCSTAR 1-5, p17].

NIST scientists estimated that well over half of each jet's fuel pooled inside the buildings rather than being immediately burned up in the fireballs that erupted upon impact [NCSTAR 1, p24, p42]. Two conditions favored rapid fuel burning, though another condition works against it. The jet fuel was all in the wings [NCSTAR 1, p104], which could be expected to disintegrate completely on impact, and the affected floors were mostly large open areas with few firewalls [NCSTAR 1, p57]. On the other hand, there would be some delay for used-up oxygen to be replenished as air rushed in through the hole created by the jet.

New York City firefighters reported that jet fuel had flowed into elevator shafts and elsewhere [NCSTAR 1, p163]. Even so, there seems to be a difficulty with the jet fuel distributions and the WTC2 blasts, which may explain why probers expressed restrained confidence in the jet fuel conjecture.

When reviewing the reports, one must of course be careful to distinguish blasts of smoke or dust that occur very near or at the times of collapse, which would be expected, from the other blasts.

However, physicist Jones noticed that in WTC7 a sequence of downard pulses occurred 0.2 seconds apart. Using the free-fall equation (y = 1/2gt²) he found that it requires at least 0.6 seconds for a floor to drop and strike another. The NIST gave scanty data on the pressure pulses.

The NIST's computer simulations included a scenario in which a tower is sabotaged with a 'small bomb,' numerous arson fires and a wrecked sprinkler system. The simulation left the building standing [NCSTAR 1, p144].

However, the NIST did not publicly consider the possibility that the towers in fact may have been sabotaged with high-energy explosives. No scenario in which larger bombs are attached to core columns and smaller ones attached to floor connectors is considered -- though such a simulation would have been closer to observations than the 'small bomb' simulation. Of course, such a possibility requires that the Port Authority police security system have been compromised so that radio-linked explosives could be planted on many floors, the plotters having no way of knowing for sure where the planes would hit.

Because 'available information, as extensive as it was, was neither complete nor of assured precision,' the NIST 'took steps to ensure that the conclusions of the effort were credible explanations for how the buildings collapsed' [NCSTAR 1, p141].

Does this mean a sabotage scenario was ruled out in advance?

Architect Young says he photographed the Ground Zero rubble pile two weeks after collapse and saw no steel with signs of melting, an indicator of use of explosives [See his full remarks below]. However, the NIST does not report that anti-terror probers had seen no signs of molten steel but simply keeps silent. On the other hand, Jones cites reports from expert eyewitnesses in credible publications to the contrary. They saw pools of molten metal at Ground Zero.

Jones says that the molten metal is "consistent with the use of a high-temperature thermite reaction, used to cut or demolish steel," adding, "The end products of the thermite reaction are aluminum oxide and molten iron."

Jones cites a number of other factors that convince him that the towers were demolished with explosives, including the point that the WTC7 pressure pulses occurred sequentially downward, which led him to the timing problem, as well as sequentially upward, meaning interior collapse could not be a cause.

"Delayed firing is used to help direct and control the direction of fall" of big buildings, according to Gary H. Hemphill's book Blasting Operations (McGraw-Hill, 1981). Hemphill, an expert in industrial use of explosives, noted that a sequential blasting machine in use in the 1980s could be set to fire at intervals of 10 milliseconds to 200 milliseconds.

The NIST points out that it was not permitted to impute blame to individuals or institutions [NCSTAR 1, page xxxi]. Would conclusions pointing to a conspiracy within the federal government be construed as imputing blame to an institution?

The NIST's initial unwillingness to discuss a 'large bomb' sabotage scenario can only add to concerns over the federal government's failure to control the evidence. Less than 5 percent of major steel elements from the main fire floors of WTC1 were held for analysis, with virtually no steel held from WTC2 or WTC7 -- which also collapsed under mysterious circumstances.

Why the FBI -- whose forensics expertise helped crack the 1993 World Trade Center bombing and, reportedly, the 1995 Oklahoma City bombing -- would not think to sift the rubble and seek an order to preserve steel component evidence is puzzling. In addition, it is notable that then-Mayor Rudolph Giulani, a former federal prosecutor, was evidently initially unaware of a failure to preserve evidence.

And the NIST cites no reports from either the FBI, CIA or Pentagon about on-site analysis by their investigators who presumably would have checked rubble for telltale signs of bomb explosives. The CIA and Pentagon had offices in WTC7. The 9/11 commission, which sought documents from federal investigative agencies, relied on the NIST research for the technical issues concerning the collapse of the towers.

Failure to control the evidence is the more disturbing in light of the fact that -- to cite a Federal Emergency Management Agency study of the disaster -- 'many knowledgeable structural engineers' were astonished by the collapses.

In fact, NIST adviser Corbett had previously called for a major probe to replace FEMA's inquiry, which he decried as poorly funded and lacking subpoena power. The journal Fire Engineering, with which Corbett is affiliated, first raised the alarm in January 2002 about the disposal of the steel evidence.

Corbett was listed by Popular Mechanics as one of the experts it consulted for its March 2005 article: "9/11: debunking the myths" which backed the official position. In a CNN interview about the PM piece, he said that there was insufficient evidence to prove the towers had been blown by explosives, but he also said it is understandable that the issue lacks closure "because there's still a lot of unanswered questions."

A LACK OF CLARITY

The quality of much of the NIST's experimental work appears to be highly credible, but the need to create a dubious narrative seems to have clouded professional standards.

Readers of the NIST's main report are not helped to understand that the '1.2m soffit' mentioned in a table and accompanying paragraph [NCSTAR 1, p124] implies that the NIST's 'credible' collapse model requires that the floor deck underside fireproofing remain intact, in contrast to nearby floor joist and girder fireproofing. In fact, the NIST is unclear about whether fireproofing remained on joists in its computer simulation.

In addition, the various scenarios or 'cases' cited show variations among the reports and it is not easy to discern whether apparent inconsistencies are important.

Here are some instances:

* NCSTAR 1-5, p105, gives the most influential variable in the scenarios as density of combustibles, with 5 1b/ft² cited as necessary, though elsewhere 4 lb/ft² is indicated for the same scenario [NCSTAR 1, p76].

* According to NCSTAR 1, p142, six scenarios, or cases were considered for WTC1 and WTC2. But the two base cases were tossed out as not conforming to observations. Remaining were Cases A and B for WTC1 and C and D for WTC2 which gave two levels of assumed pre-collapse damage for each building. Modelers found that only cases B and D led to collapse.

But NCSTAR 1-5G, p179, tells the reader that fire simulation results for Case B were 'not qualitatively different' from those for Case A, which posits the less-severe pre-collapse conditions. This is surprising in that the main report says Case A was disregarded once it was clear it wouldn't trigger collapse [NCSTAR 1].

* NCSTAR 1-6D, p31, distinguishes two sets of cases for aircraft damage estimates and fire damage estimates and aircraft damage cases are label A_i, C_i and so on.

* NCSTAR 1-6, p224, relates that simulations for WTC1 'used Case B impact damage and temperature histories' and for WTC2 'used Case D impact damage and temperature histories, as described in previous chapters,' where earlier scenarios dubbed A_i and so forth are discussed [NCSTAR 1-6, p121].

Still, NCSTAR 1-5G, p179, discloses that the researchers did not use Case B aircraft damage to get Case B fire damage. That is, less severe aircraft damage was needed to preserve key areas of fireproofing.

Also, the base case simulation 'provided a much better match to the observed damage' than did the worst case, NCSTAR 1-2, p48, relates.

At any rate, readers of the main report are not well informed as to the essential meanings of the various cases.

UNWARRANTED CONFIDENCE?

The NIST's narrative of twin tower events is simply a conjecture, but a casual reader who missed the disclaimers might think its 'facts' have mostly been proved.

NIST, though warning that the destruction of records in the collapse and the failure to preserve the steel left holes in the input data used for their computer simulations, asserts that it was 'able to gather sufficient evidence and documentation to reach firm findings and recommendations' [NCSTAR 1, p19]. However, the air of confidence rests on selective presentation of evidence and issues in part 2 of the main report [NCSTAR 1].

For example, the reader sees the table that refers to 'soffit' in NCSTAR 1, p142, but is left with little further information in the main report.

A reader desiring further guidance is directed to NCSTAR 1's appendix B, which lists some supporting reports. However, the main report is not properly footnoted or indexed, making it difficult to closely examine assertions.

In addition, NCSTAR 1-6 notes that the NIST's analytical methods strained the limits of structural engineering experience and training [NCSTAR 1-6, p9] and NCSTAR 1 says each step of the simulations 'stretched the state of technology and tested the limits of software tools' [NCSTAR 1, page xlii].

THE THIRD COLLAPSE

At 5:20 p.m., some seven hours after WTC1 fell, the 47-story WTC7 collapsed almost straight down, meaning collapse must have begun on a lower floor.

The NIST's principal analysis of that collapse has been 'decoupled' from twin towers analyses and postponed; the agency says staff workload necessitated the separation.

In a March 2005 Popular Mechanics article titled, '9/11: debunking the myths,' lead investigator Sunder is quoted as saying that new evidence indicates that WTC7 showed severe structural damage following the tower collapses and that this weakening, abetted by a longterm fire, was the agency's working hypothesis.

Some 10 lower stories, or about 25 percent of the building vertically, was 'scooped out,' he is reported to have said. (A Federal Emergency Management Agency, or FEMA, report also cited such damage, but did not view it as compelling.)

Yet NCSTAR 1-3, p114, says that NIST made no effort to check high-strain or impact properties of the type of steel used in WTC7 because 'WTC7 did not suffer any high strain rate events.'

Sunder said that an oddball design implied that failure of even one column on a lower floor might trigger collapse and suggested that a fuel-oil-fed fire contributed critical weakening.

However, NCSTAR 1-1J, found that the standard safeguards for the building's several fuel oil systems would likely have blocked a longterm fuel-oil-fed fire, an idea first mentioned by skeptical FEMA probers.

The most likely source of the leaking fuel oil would have been the Salomon Brothers system, NCSTAR 1-1J says, with probers citing two possibilities: a fuel spill from a 250-gallon 'day tank' on the fifth floor or fuel continually pumped up from an underground tank. But they suggest failsafes should have worked.

FEMA probers have said 250 gallons couldn't yield enough heat to inflict critical damage.

The NIST's contracted probers, Raymond A. Grill and Duane A. Johnson, say it is barely conceivable that an electrical malfunction caused pumps to keep bringing up fuel from a 6,000-gallon tank buried underground. But they are puzzled as to the source of the electricity. Power to the building would have been shut off the morning of Sept. 11, though the building's emergency generators were powered by fuel oil.

The electrical schematics for the fuel system are missing, along with building maintenance records that might have yielded clues to the electrical system. Grill and Johnson succeeded in finding much other WTC7 documentation, however.

In the May 2002 FEMA report, investigators wrote: 'Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis' for fire-fed building collapse 'has only a low probability of occurrence.' They demanded further inquiry as to how key supports could have given way.

In addition, the fuel oil had to pool in a mechanical room where possibly a truss was not firesheathed, they said.

In general, however, the FEMA report is not nearly so pointed. That report was edited by Theresa P. McAllister, who handled much of NIST's collapse analyses. She coauthored a lengthy report, NCSTAR 1-6, on the collapse scenarios that makes no mention of soffit.

It has been reported that Larry A. Silverstein, the real estate man who ran the trade center, was quoted in a PBS report as saying that he gave the go-ahead to the 'er-Fire Department' to 'pull' the building. A search of the PBS site for the interview proved fruitless, but Silverstein has put out a statement saying the FEMA report determined that fire was responsible for WTC7's collapse.

There is no record of steel-frame buildings over 10 stories high collapsing as a result of fire, probers say. The FEMA inquiry points out that in the 1990s the British Steel and Building Research Establishment fire-tested an eight-story steel structure, leaving secondary beams unsheathed by fireproofing. The building remained upright at the end of all six experiments.

The public comment period for the twin towers draft report ended Aug. 4, 2005, with the final version issued in September 2005.

During the public comment phase of the twin towers report, the NIST web site did not make clear that the principal WTC7 report had been omitted. Since then, the NIST has posted a sketchy document dated April 2005 that has a series of photos and a limited discussion with little supporting data. It contains a large disclaimer saying the agency had found no evidence of destruction by controlled demolition, missiles or bombs, but does not substantiate that assertion. [See 'NIST reports vanish' below]

Even so, the NIST has issed a set of findings and recommendations for building safety improvements without bringing in WTC7 data.

Please report errors to the email address below. Paul Conant's telephone number: 732-947-0749

Tile and debris velocities

We assume typical debris velocity is 350 mph and hold that number constant (though in reality the velocities vary with time).

We neglect the velocity of the compression wave triggered by impact, which is about 177,000 ft/s through steel.

A WTC floor was 209 feet on a side. So debris reaching the far wall from the point of initial impact does so in about 0.41s. We are safe to neglect diagonal distance, which is only slightly greater than the horizontal here, even though debris that could strike the upper floor assembly would mostly be at angles to the horizontal.

We now find that in 0.41s, a tile can have dropped no more than (1/2)(32)(0.41)² = 2.7 feet. Even then, the probability is strong that only one edge will have dropped that far, so that the tile would still intercept much debris before it reached floor joists.

However, the government theory requires most of the damage to steel connectors that link floors and core columns nearer the initial impact wall. So we use 100 feet for the horizontal (angled debris path lengths can also be neglected here). We have 0.2s for the debris to travel 100 feet, with ceiling tile dropping no more than 7 inches. Again, it is likely that only one edge would have dropped that far, meaning the tile would intercept debris.

On the other hand, it is quite possible that more debris might follow the intercepted debris and still reach the floor supports. So it is not implausible that some floor system areas were stripped of fireproofing -- but further consideration seems necessary for the assumptions that stripped from each fire floor were a 40-foot swath of floor supports [NCSTAR 1, p21] or perhaps a volume above 2/3 of a floor area [NCSTAR 1-6, p121].

NIST reports vanish

On its website, the NIST says that the main WTC7 report has been deferred until October 2005.

It is unclear whether the postponed report is the one titled 'Structural analysis of the response of World Trade Center 7 to debris damage and fire' which is cited in prefatory material to various NCSTAR reports but is nowhere to be found on the NIST website. The NIST awarded Ramon Gilsanz and his New York engineering firm a contract to do computer simulations of the WTC7 collapse. Omitted from the NIST website is NCSTAR 1-6F, the report by Gilsanz and nine others.

Also omitted was NCSTAR 1-6G: "Analysis of Sept. 11, 2001 seismogram data" by W. Kim. Won-Young Kim of Lamont-Doherty Earth Observatory has done previous analyses of 9/11 seismographic data.

No explanation is given for the seismic report's deletion, though it likely contains information concerning WTC7's collapse.

Bloomberg ignores the probabilities

It is not straightforward to estimate the single-event probability of the collapse of WTC1 or WTC2. We know that beforehand the probability of collapse for one tower would have been considered quite low, below 5%. Still, it is quite credible for a one-time fluke to occur.

We are aware now that the NIST gives two divergent scenarios for the collapse of each of the twin towers: for WTC1, the main cause is given as fire with a lesser contribution from load-structure problems; for WTC2 the main cause is given as load-structure problems with a lesser contribution from fire.

Hence one might construe that the collapses of WTC1 and WTC2 are effectively independent events.

As for WTC7, the event is only tenuously causally connected to the other collapses, and is easily regarded as independent of the other two.

Probabilities for independent events may be multiplied.

So, let's be very generous and estimate an 80% probability of collapse for WTC1 and likewise for WTC2. Knowing that government experts saw the probabilitiy of collapse for WTC7 as very low, we'll be generous and assign a 50% probability to that event.

In that case, the probability that all three buildings would collapse is 0.8²x 0.5 = 32%, implying a 68% probability of conspiracy.

However, a slightly more realistic collapse probability for WTC7 is 25%. Keeping the tower probabilities at 80%, the new calculation gives an 84% probability of conspiracy.

Now suppose we be extremely generous and consider the collapse of the twin towers as a single event with a 95% probability. We multiply 0.95 x 0.5 and get a 52.5% probability of conspiracy.

If we still consider the collapse of the twin towers as a single event but plug in a more realistic probability of 70% and plug in a more realistic 10% (still too high) for WTC7, we have a probability of conspiracy of 93%.

Michael Bloomberg, the mayor of New York and owner of a major financial news service, has raised no hue and cry over the problem of probability. Seemingly the many analysts at his beck and call failed to make him aware of such improbabilities.

Conant's NIST query

Date: Mon, 18 Jul 2005 09:44:25-0700 (PDT)

From: 'P.R. Conant' (prconant@yahoo.com)

Subject: Regarding Case B

To: rgannns@nist.gov, inquiries@nist.gov, director@nist.gov

Dear Dr. Ganns [sic],

I have reviewed the NCSTAR reports and can locate nothing of substance regarding the soffit accounted for in the Case B model [because the NIST search engine was defective].

Specifically, can you tell me what the soffit was composed of and precisely what its location was in relation to the floor system. Also, what does '1.2m' mean in the phrase '1.2m soffit'? Is this a side length or vertical length from the metal deck?

NIST's main report also uses the table with the 'soffit remains' note. Yet after reading NCSTAR 1-6, I was unable to be sure whether the 'soffit remains' scenario was used in the final computer models that describe the sequences for WTCI and II.

At another point in your report [NCSTAR 1-5], your team says that the pressure pulses from the anomalous puffs of smoke did not imply a force sufficient to cause structural damage. What reasoning was used to substantiate this assertion?

I am taking the liberty of sending copies of this email to two other NIST addresses.

I may quote from this email and your response for future publication.

Thank you.

Best regards,

Paul Conant

732 947 0749

Architect counters Conant but doubts government

Date: Wed, 27 Jul 2005 19:57:11-0700

To: Paul Conant [and others]

From: 'John Young' (jya@pipeline.org)

Re: A troubling WTC fireproofing scenario

Paul,

I appreciate your forwarding these comments on the WTC collapse, but I'll offer counter points to these latest.

Fireproofing on the floor supports was probably dislodged by vibration passing through the slender rods and top and bottom chords of the bar joists because the floor plate would have been more resistant due to its comparatively greater mass and solidity. I have seen fireproofing come off such bar joists in non-impacted structures due to the lesser quality application often given to such joists due to their hard to reach upper surfaces, compared, say, to the easily sprayed deck underside. Fireproofing inspectors regularly ask for corrective work on such joists for this reason.

The 'smoke pulses' were probably dust pulses indicating the early stages of collapse before the whole floor succumbed. Various parts of the structure are weaker than others, and these give way according to their weakness, say, for example, gypsum board partitions being crushed explosively by the descending much heavier floor parts. This kind of dust can be seen on ordinary demolition jobs when the big hammers and balls cave-in walls onto light-weight interior elements.

I looked at the collapse heap about two weeks after the attack (on October 3, 2001), and took several dozen photos. I was looking for evidence of what caused the hard to believe collapse. By then a lot of steel had been carted away, and apparently sold to recyclers overseas, a hard to understand dispersal of evidence. Still, from what I could see from shelf angles which supported the bar joists, the breakage was clean and not due to melting. No doubt I could see only what was on top of the heap, but many of the giant columns were still in the pile jutting out of it, and the exterior lattice frame was visible in large sections.

No melting could be seen in any of the structural parts. This is not to say that there was no evidence buried under the pile.

My supposition is that the collapse was due in large part to separation of the floor support joists from their supporting columns, from impact and from subsequent swaying beyond design limits, along with oscillation uniquely fostered by the tubular design of the towers -- this oscillation would not be so prominent in ordinary framed-steel structures.

Basically, the towers shook themselves apart once the initial impact set off vibration, oscillation and separation of the various components, with the floor joist-to-column connection being the weakest part of the system.

Two 3/4-inch bolts at each end of the bar joist attached the joist to the shelf angle, with the angle welded or bolted to the column (as noted above, many of these shelf angles remained attached to columns on October 3). These are very weak joints with almost no capacity to resist lateral loading, and certainly not violent sway and oscillation.

Note that the exterior tube remained intact the longest, with the floor structures collapsing inside. This tube was whiplashing the floor structures due to it keeping its integrity as a lattice, and heaving the floors against the stiffer interior core.

The floors collapsed onto one another until they created a force strong enough to blow out the enclosing lattice, and during this sequence a lot of explosive dust and debris would have been created.

WTC7 is indeed a special case, and its delayed investigation is due to the building not being owned by the Port Authority. Silverstein and his insurers are under no obligation to make the investigation public, and no doubt have many liability reasons to keep it confidential.

Court cases may eventually reveal part of the story, but it is very common for building collapse suits to be sealed by agreement of all parties. I have tried to get several such cases unsealed so the public can learn more about unsafe conditions, but so far none have been unsealed, and it is likely that a lot about WTC will never be revealed as well -- so remain skeptical of whatever is officially released due to the long-standing practice of concealing building hazards but releasing only the information that fits conventional wisdom and sustains the inflated value of urban high-rise property.

We design professionals are quite complicit in this unsavory practice.

Conant responds to Young

Your idea that the fireproofing vibrated off the joists but not the decks is not addressed in the NIST analyses. NIST mentioned the vibration possibility but decided against using it because probers felt it was too iffy.

As for bar fireproofing being less reliable because of application problems, the NIST does mention that point, but goes on to say that in recent years all fireproofing in the floor assemblies had been upgraded to a greater thickness and reapplied.

Regarding the 'correlated' smaller blasts of smoke or dust, it was NIST investigators who described them as very unusual.

Your notion that the towers essentially shook themselves apart is of interest in light of the fact that the fires were largely of low heat. However, NIST analyzed the sway of WTC2 after impact and found that its period of oscillation was within what would be expected for a very windy day (there was no wind on Sept. 11). Of course, you still could be right.

As for WTC7, the NIST says its researchers were simply too busy with the twin towers to complete their WTC7 work.

Both FEMA and NIST investigators are clearly highly disconcerted by that collapse.

With respect to the possibility that government probers were shielding builders, the report goes to great lengths to accentuate the positive, particularly underscoring that most steel components met or exceeded specifications and that the use of closely-packed columns tended to provide redundant load-carriers, so that failure of a few would not compromise the building.

The probers note that two documents from the 1960s reported that each tower was designed to withstand the impact of a Boeing 707 flying at 600 mph (thus carrying significantly more potential energy than the planes that actually hit), but could find no detailed calculations. The NIST probers also cited an MIT study that found that the Sept. 11 impacts were enough to topple the towers without fire but said that their analysis showed the towers should stand; the probers suggested that MIT had not taken into account the fact that the floor slabs would absorb much of the initial impact.

The possibility that the government probers were responding to a perceived need to protect Rockefeller family prestige cannot be altogether dismissed, considering that the World Trade Center was the pet project of David Rockefeller, the powerful New York banker.

Still, it seems unlikely that scientists would risk being part of a coverup related to such a cataclysmic event without being told that 'national security' required the evasions.