Building a Better Mirage

Introduction

NIST's Theory

Remaining strictly within the confines of the officially prescribed theory, NIST crafts an explanation for the "initiation of the collapse of each Tower" that avoids faulting the Towers' construction: The aircraft impacts dislodged insulation from the steel, and the exposed steel succumbed to the fires. Sagging trusses pulled in portions of the perimeter walls, causing a rapid spread of "column instability" in perimeter columns, which in turned strained the fire-weakened core columns. The "tremendous energy" of the floors above the collapse zone led to "global collapse."

Challenges

In this critique I challenge NIST's explanation on two levels:

• Its theory about the effects of crash and fire damage is deeply flawed.
• Its presumption that "collapse initiation" will automatically lead to "global collapse" is unfounded.

Passing Off "Global Collapse"

To explain the collapses of the Twin Towers, both NIST's theory of the "collapse initiation" and its supposition that "global collapse" automatically follows from such an event would have to be true. The Report simply asserts the supposition without any supporting argument, and subtly reinforces it without drawing attention to it.

Circumscribing the Investigation

This is the final report on the National Institute of Standards and Technology (NIST) reconstruction of the collapses of the World Trade Center (WTC) towers, the results of an investigation conducted under the National Construction Safety Team Act. This reports [sic] describes how the aircraft impacts and subsequent fires led to the collapses of the towers after terrorists flew jet fuel laden commercial airliners into the buildings; whether the fatalities were low or high, including an evaluation of the building evacuation and emergency response procedures; what procedures and practices were used in the design, construction, operation, and maintenance of the towers; and areas in current building and fire codes, standards, and practices that warrant revision. (p xiii/15)
[emphasis added]

The Executive Summary is less candid about the pre-ordained conclusion of the investigation.

1. Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the aircraft and why and how WTC 7 collapsed; (p xli/43)

Hiding Engineering History

NIST's Report gives the reader no clue of how unprecedented and unexpected the total collapses of these skyscrapers were. Rather, it suggests that total collapses of high-rise buildings are normal events, but usually happen less suddenly.

In our cities, there has been no experience with a disaster of such magnitude, nor has there been any in which the total collapse of a high-rise building occurred so rapidly and with little warning. (p xlvii/49)

Indeed, buildings are normally evacuated and cordoned off before being taken down by controlled demolition, so the statement is literally true. But the Report doesn't contain the word demolition, so the statement seems crafted to mislead. In fact, there appear to be no examples of total collapse of skyscrapers anywhere in the world except through controlled demolition. There are examples of steel-framed buildings about 20 stories in height being knocked over by severe earthquakes, but large portions of earthquake-destroyed buildings remain intact. In contrast, the steel skeletons of the Twin Towers were shredded into thousands of pieces, and their non-metallic constituents and contents were pulverized into fine dust.

Severe earthquakes have caused collapses of steel-frame buildings but the buildings were not shredded into small pieces. The earthquake in Kobe, Japan, knocked buildings off their foundations (inset). In contrast the Twin Towers were thoroughly shredded, leaving a crater that smoked for over three months.

Keeping the reader in the dark about the history of steel-framed high-rise buildings is essential to passing off the notion that partial collapse automatically leads to total collapse.

Selling Progressive Collapse

The Report mentions "progressive collapse" 16 times, mostly in sections describing recommendations. It defines progressive collapse as when "a building or portion of a building collapses due to disproportionate spread of an initial local failure" but does not mention how rare the phenomenon is or that there are no examples of total progressive collapse of steel-framed buildings outside of 9/11/01.

By repeatedly invoking the specter of "progressive collapse" while concealing the phenomenon's lack of repeatability outside of "terrorist incidents," the Report surreptitiously bolsters its supposition that "global collapse" automatically follows from "collapse initiation."

NIST's Amazing Column Failure Theory

The truss failure theory was in vogue in 2002, having won the big PBS and Discovery Channel endorsements, and it eclipsed early column failure advocates. But now in 2005 the column failure theory is back, with a new advocate (NIST) sporting a $20 million budget and computer models galore.

A Mountain of Distracting Details

Finding NIST's theory of the collapse takes some work because of the size of the Report. As I note above, NIST does not actually provide a complete theory of the collapse, only a theory of events that led up to "collapse initiation." However, the casual reader may conclude that NIST does provide a complete theory from phrases in section titles such as "Collapse Analysis," "Global Analysis," and "Probable Collapse Sequences."

Mixing Observation and Speculation

A key technique the Report uses to add realism to its theory is to mix observational data with speculation, while failing to note the difference. For example, in numerous places the Report juxtaposes its estimates of core column damage (which are highly speculative) next to estimates of perimeter column damage (which can be inferred directly from photographic evidence).

The debris cut a shallow path through the west and center array of trusses, damaging the insulation up to the north wall of the building core. This devastation took 0.7 s. The structural and insulation damage was considerable and was estimated to be:

• 35 exterior columns severed, 2 heavily damaged.
• 6 core columns severed, 3 heavily damaged.
• 43 of 47 core columns stripped of insulation on one or more floors.
• Insulation stripped from trusses covering 60,000 ft2 of floor area.

(p 22/76)

Cinematic Emphasis

While providing extreme detail in certain areas, the Report makes key assertions without any detail or supporting evidence, often couched in language seemingly calculated for emotive effect. For example, the Report suggests that there was something very unusual about the construction of the Towers' trussed floor diaphragms.

... each tenant floor consisted of 4 in. thick, lightweight cast-in-place concrete on a fluted steel deck, but that is where "ordinary" ended. Supporting the slab was a grid of lightweight steel bar trusses. (p 10/64)

Elsewhere the Report notes that the Towers used "an innovative framed-tube concept." While it is true that this design was relatively novel when the Towers were built, today, most of the tallest skyscrapers employ such a design -- a framed tube with long trussed floor spans connecting the core to the perimeter.

The Report repeatedly assures us that steel heats rapidly when exposed to fire.

Bare steel in fire places and wood stoves "can heat quickly when exposed to a fire of even moderate intensity," but we don't often see fireplace gratings or wood stoves collapsing.

Bare structural steel components, when exposed to a large and sustained fire, can heat rapidly to the point where their ability to support their load is compromised. (p 11/65)

Bare structural steel components can heat quickly when exposed to a fire of even moderate intensity. Therefore, some sort of thermal protection, or insulation, is necessary. (p 68/122)

Imagined Heat

The Report repeatedly makes claims that amazingly high fire temperatures were extant in the Towers, without any evidence. The Report itself contains evidence contradicting the claims.

Observations of paint cracking due to thermal expansion. Of the more than 170 areas examined on 16 perimeter column panels, only three columns had evidence that the steel reached temperatures above 250 ºC: east face, floor 98, inner web; east face, floor 92, inner web; and north face, floor 98, floor truss connector. Only two core column specimens had sufficient paint remaining to make such an analysis, and their temperatures did not reach 250 ºC. ... Using metallographic analysis, NIST determined that there was no evidence that any of the samples had reached temperatures above 600 ºC. (p 88/142)

The highest temperatures estimated for the samples was 250 ºC (482 ºF). That's consistent with the results of fire tests in uninsulated steel-framed parking garages, which showed maximum steel temperatures of 360 ºC (680 ºF). How interesting then, that NIST's sagging truss model has the truss heated to 700 ºC (1292 ºF).

A floor section was modeled to investigate failure modes and sequences of failures under combined gravity and thermal loads. The floor section was heated to 700 ºC (300 ºC at the top surface of the slab) over a period of 30 min. Initially the thermal expansion of the floor pushed the columns outward, but with increased temperatures, the floor sagged and the columns were pulled inward. (p 96/150)

Where does NIST get the idea that steel temperatures should be more than 450 degrees Celsius (or 842 degrees Fahrenheit) higher than their own evidence indicates? This passage provides some insight into their experimental method.

A spray burner generating 1.9 MW or 3.4 MW of power was ignited in a 23 ft by 11.8 ft by 12.5 ft high compartment. The temperatures near the ceiling approached 900 ºC. (p 121/175)

1.9 to 3.4 MW (megawatts) is the heat output of about 500 wood stoves -- that in a living-room-sized space!

The jet fuel greatly accelerated the fire growth. Only about 60 percent of the combustible mass of the rubblized workstations was consumed. The near-ceiling temperatures varied between 800 ºC and 1,100 ºC. (p 123/177)

Temperatures of 800 ºC to 1,100 ºC (1472 ºF to 2012 ºF) are normally observed only for brief times in building fires, in a phenomenon known as flashover. Flashover occurs when uncombusted gases accumulate near the ceilings and then suddenly ignite. Since flame consumes the pre-heated fuel-air mixture in an instant, very high temperatures are produced for a few seconds. Note that this temperature range includes the 900 ºC recorded using the megawatt super-burner, so they must have had to pour on quite a lot of jet fuel.

The first section of the Report describing the fires deceptively implies that 1,000 ºC (1832 ºF) temperatures (rarely seen in even momentary flashovers) were sustained, and that they were in the building's core.

Aside from isolated areas, perhaps protected by surviving gypsum walls, the cooler parts of this upper layer were at about 500 ºC, and in the vicinity of the active fires, the upper layer air temperatures reached 1,000 ºC. The aircraft fragments had broken through the core walls on the 94th through the 97th floors, and temperatures in the upper layers there were similar to those in the tenant spaces. (p 28/82)

Note the absurdity of asserting that the fires in the core were as intense as those in the tenant spaces when the core:

• Had very little fuel
• Was far from any source of fresh air
• Had huge steel columns to wick away the heat
• Does not show evidence of fires in any of the photographs or videos

Furthermore, NIST's suggestion of extremely high core temperatures is contradicted by its own fire temperature simulations, such as the one illustrated on the right, which show upper-level air temperatures in the core of mostly below 300 ºC.

Ignored Conduction?

NIST calibrated its computer model of heat transfer to the steel structure using thermally isolated pieces of steel. NIST does not appear to have taken into account the role of heat conduction within the steel structure in lowering the temperatures of the fire-exposed steel.

NIST apparently ignored thermal conduction within its model of the steel structure. Since steel is a good conductor of heat, and the steel in the Twin Towers' structures was well connected, their massive steel structures would have drawn heat away from the parts that were exposed to fire. The Report describes a model of "The Fire-Structure Interface", and describes the computation of heat transfer between the air and the steel structure, but it does not mention the conduction of heat along spans of the steel structure. (p 129/183) The suspicion that NIST simply ignored the conduction of heat within the steel is corroborated by the Report's disclosure that they used heat transfer tests on isolated steel elements to calibrate their model. (p 132/186)

Bowed Columns, or Refracted Light?

This illustration is in the slide presentations predating the Report, and is included in the final Report draft (p 33/87). NIST assigns inward displacements of every fifth column at each floor based on their appearance in the photograph.

A key part in NIST's theory of the collapse initiation is that the perimeter columns on one of the faces of each Tower bowed inward, pulled by sagging trusses. The Report contends that the columns on the south face of the North Tower bowed inward in the moments before its collapse and that the columns on the east face of the South Tower bowed inward some time before its collapse. As evidence for the supposed bowed columns NIST cites photographs. The Report includes one annotated photograph allegedly showing bowing in the North Tower, but no such photographs of allegedly bowing of columns in the South Tower. There are two photographs of alleged South Tower column bowing in an earlier slide presentation.

NIST fails to consider an alternative explanation for the bowed appearance of columns in its selected photographs of the Twin Towers: light refraction caused by the layer of hot air adjacent to the Towers. Such atmospheric conditions would refract light in a way that is consistent with apparent distortion of the columns seen in the photographs.

NIST's "Global Analysis"

The nearly-300-page Report has one and a half pages describing the "Results of Global Analysis" of the collapses. They describe the results of simulations of the "structural deterioration" of each Tower "from the time of the aircraft impact to the time at which the building became unstable." This is the only detail the Report gives on the supposed sequence of events leading up to collapse initiation. It seems that the closer the Report gets to the collapses, the more vague it becomes.

6.14.2 Results of Global Analysis of WTC 1

After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed to other columns. The north wall lost about 7 percent of its loads after impact. Most of the load was transferred by the hat truss, and the rest was redistributed to the adjacent exterior walls by spandrels. Due to the impact damage and the tilting of the building to the north after impact, the south wall also lost gravity load, and about 7 percent was transferred by the hat truss. As a result, the east and west walls and the core gained the redistributed loads through the hat truss.

Structural steel expands when heated. In the early stages of the fire, structural temperatures in the core rose, and the resulting thermal expansion of the core was greater than the thermal expansion of the (cooler) exterior walls. About 20 min. after the aircraft impact, the difference in the thermal expansion between the core and exterior walls, which was resisted by the hat truss, caused the core column loads to increase. As the fires continued to heat the core areas without insulation, the columns were thermally weakened and shortened and began to transfer their loads to the exterior walls through the hat truss until the south wall started to bow inward. At about 100 min, approximately 20 percent of the core loads were transferred by the hat truss to the exterior walls due to thermal weakening of the core; the north and south walls each gained about 10 percent more loads, and the east and west walls each gained about 25 percent higher loads. Since the hat truss outriggers to the east and west walls were stiffer than the outriggers to the north and south walls, they transferred more loads to the east and west exterior walls.

The inward bowing of the south wall caused failure of exterior column splices and spandrels, and these columns became unstable. The instability spread horizontally across the entire south face. The south wall, now unable to bear its gravity loads, redistributed these loads to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. The building section above the impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to carry the increased loads. This further increased the gravity loads on the core columns. Once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued. (p 143/197)

6.14.3 Results of Global Analysis of WTC 2

Before aircraft impact, the load distribution across the exterior walls and core was symmetric with respect to the centerline of each exterior wall. After aircraft impact, the exterior column loads on the south side of the east and west walls and on the east side of south wall increased. This was due to the leaning of the building towards the southeast. After aircraft impact, the core carried 6 percent less load. The north wall load reduced by 6 percent and the east face load increased by 24 percent. The south and west walls carried 2 percent to 3 percent more load.

In contrast to the fires in WTC 1, which generally progressed from the north side to the south side over approximately an hour, the fires in WTC 2 were located on the east side of the core and floors the entire time, with the fires spreading somewhat from south to north. With insulation dislodged over much of the same area, the structural temperatures became elevated in the core, floors, and exterior walls at similar times. During the early stages of the fires, columns with dislodged insulation elongated due to thermal expansion. As the structural temperatures continued to rise, the columns thermally weakened and consequently shortened.

The south exterior wall displaced downward following the aircraft impact, but did not displace further until the east wall became unstable 43 min later. The inward bowing of the east wall caused failure of exterior column splices and spandrels and resulted in the east wall columns becoming unstable. The instability progressed horizontally across the entire east face. The east wall, now unable to bear its gravity loads, redistributed them to the thermally weakened core through the hat truss and to the east and west walls through the spandrels.

The building section above the impact zone began tilting to the east and south as column instability progressed rapidly from the east wall along the adjacent north and south walls, and increased the gravity load on the weakened east core columns. As with WTC 1, once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued. (p 143/197)

So we get detailed computer simulations of how the planes were shredded by the impacts, but when it comes to the collapses, the most quantitative thing we get is "tremendous energy of the falling building section." Why are there no calculations of the approximate amount of energy? Also, did NIST's global model actually show that that the "building section above the impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to carry the increased loads..."? From the Report it is impossible to know. There are illustrations of its global floor model, such as to the right, but they show only the effects of crash damage. None show "column instability" or tilting.

NIST's Opaque Investigation

The Report makes impressive-sounding claims about the thoroughness of its investigation. For example, it describes its collection of visual evidence:

The assembled collection included:

• 6,977 segments of video footage, totaling in excess of 300 hours. The media videos included both broadcast material and outtakes. Additionally, NIST received videotapes recorded by more than 20 individuals.
• 6,899 photographs from at least 200 photographers. As with the videos, many of the photographs were unpublished.

This vast amount of visual material was organized into a searchable database in which each frame was characterized by a set of attributes: photographer (name and location), time of shot/video, copyright status, content (including building, face(s), key events (plane strike, fireballs, collapse), the presence of FDNY or NYPD people or apparatus, and other details, such as falling debris, people, and building damage). (p 81/135)

Unfortunately NIST does not share its visual database with the public. Are there any videos in NIST's archive that show the alleged column bowing? They would immediately show whether the effect was due to refraction or to column distortion.

The Report boasts about the computer models it used to simulate the crashes and fires, but it does not make its models available for download, nor does it publish any of the data sets generated by the models.

The Report does not contain footnotes. It is filled with claims, the basis for which the reader can only guess. It leaves the public with no way to compare its conclusions with the evidence on which it was purportedly based.

Correcting the ASCE's Candor

NIST's is the second of only two official government investigations of the collapses of the World Trade Center buildings. The Report makes the following reference to the earlier investigation, in which FEMA assembled a team of volunteers from the American Society of Civil Engineers (ASCE):

Immediately following the terrorist attack on the World Trade Center (WTC) on September 11, 2001, the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began planning a building performance study of the disaster. The week of October 7, as soon as the rescue and search efforts ceased, the Building Performance Study Team went to the site and began their assessment. This was to be a brief effort, as the study team consisted of experts who largely volunteered their time away from their other professional commitments. The Building Performance Study Team issued their report in May 2002, fulfilling their goal "to determine probable failure mechanisms and to identify areas of future investigation that could lead to practical measures for improving the damage resistance of buildings against such unforeseen events." (p xxxi/33)

This is misleading for several reasons:

The ASCE's Equivocation

About the Twin Towers, FEMA's report stated:

With the information and time available, the sequence of events leading to the collapse of each Tower could not be definitively determined.

On Building 7 they were even more reserved:

The specifics of the fires in WTC 7 and how they caused the building to collapse remain unknown at this time. ... the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue.

It is, therefore, misleading for NIST to say that the FEMA-ASCE report determined the "probable failure mechanisms."

Like NIST's Report, FEMA's report did not attempt to provide an explanation for the total collapses of the Twin Towers. However, it did venture slightly beyond the NIST's cutoff of "collapse initiation," stating:

As the floors collapsed, this left tall freestanding portions of the exterior wall and possibly central core columns. As the unsupported height of these freestanding exterior wall elements increased, they buckled at the bolted column splice connections, and also collapsed.
[emphasis added]

Note that no photographs show "tall freestanding portions" or any substantial portions of the exterior walls above the descending rubble clouds. Also, even without the floors surrounding them, the core columns were not freestanding, but were connected by horizontal I-beams into lattices of steel. The authors' use of the non-committal "possibly central core columns" suggests they don't believe their collapse theory.

NIST's Report shows no such equivocation or doubt-revealing ambiguity.

The ASCE's Disclosures of Steel Sulfidation

Omissions and Distortions

The Privatization of the World Trade Center

After providing a fairly detailed overview of the history of the World Trade Center, the Report mentions that WTC 7 "was completed in 1987 and was operated by Silverstein Properties, Inc." (p 2/56) However, the Report makes no mention of the fact that a private consortium headed by Silverstein Properties acquired a 99-year lease of the main World Center complex on July 24, 2001. Nor does it mention that the new landlord secured an array of insurance policies that included a special provision for loss due to terrorist attacks, and, subsequent to the attack, successfully sued the insurers to obtain twice the value of the policy based on its being "two occurrences" (two airplane crashes).

Chief Palmer's Radio Call

The Report conceals one of the most vivid accounts of heroism in responding to the attack. Battalion Chief Orio J. Palmer had reached the 78th floor of the South Tower by 9:48 -- 11 minutes before the explosive collapse began -- and reported via radio "two isolated pockets of fire." In contrast to Palmer's communication, NIST's Report implies that no firefighters were able to reach the crash zones.

However, there was insufficient time and no path to reach any survivors on the impact floors and above. Any attempts to mitigate the fires would have been fruitless due to the lack of water supply and the difficulty in reaching the fire floors within the time interval before the building collapse. (p 45/99)

It would take hours to accumulate sufficient people and equipment to access the impact zones. (p 163/217)

NIST gets the closest to admitting Palmer's account here:

From radio communications and first-person interviews, it appears that there were responders as high as floors in the 50s in WTC 1 and the 78th floor in WTC 2. (p 166/220)

Here's a transcript of a portion of the radio communication with Chief Palmer:

Battalion 7 Chief: Battalion Seven ... Ladder 15, we've got two isolated pockets of fire. We should be able to knock it down with two lines. Radio that, 78th floor numerous 10-40 Code Ones.
...
Ladder 15: Floor 78?

Battalion 7 Chief: Ten-four, numerous civilians, we gonna need two engines up here.
...
Battalion 7 Chief: I'm going to need two of your firefighters Adam, stairway to knock down two fires. We have house line stretched we could use some water on it, knock it down, kay.

Excuses, Excuses

Light wind from the north bathed the northern portion of the North Tower's roof with cool, fresh air.

Of the 1,344 people estimated to have been on or above the 91st floor of the North Tower when the plane hit, not a single person survived, the crash having blocked all three stairwells. But many might have been rescued from the roof, had not the doors been locked and helicopter rescue barred. Two choppers arrived within 5 minutes of the crash, one of which was a Bell 412 equipped with a 250-foot hoist and capable of carrying as many as 10 survivors at a time, and carrying a three-man crew specially trained for rooftop rescues. One of the choppers was piloted by Greg Semendinger, who had helped to rescue 28 people after the 1993 WTC parking garage bombing. Semendinger and other veteran pilots have stated that rescue from the North Tower roof would have been difficult but possible. But on 9/11/01, no rooftop rescues were allowed.

NIST avoids any mention of the 1993 rooftop rescues and the opinions of pilots that rescue was an option.

Some of the people went toward the roof. However, there was no hope because roof evacuation was neither planned nor practical, and the exit doors to the roof were locked. .. Even had the roof been accessible, the helicopters could not have landed due to the severe heat and smoke. (p 26/80)

NIST excuses the locked doors and lack of notification to the occupants:

The 2003 code does not intend roof access to be used for evacuation and has no prohibition on locking this access. (p 164/218)

NIST excuses the amazing prohibition of rooftop rescue by misrepresenting the condition of the roof, (whose accessibility is documented by photographs and the words of the helicopter pilots) and by falsely implying that a helicopter would have had to land on the roof to effect any rescue.

NYPD helicopters reached the scene by 8:52 to assess the possibility of roof rescue. They were unable to land on the roof due to heavy smoke conditions. During the first hour, FDNY did not consider the option of roof rescue. When the aircraft struck WTC 2, it was clear that this was criminal activity, and the decision regarding roof top operations became the responsibility of NYPD. The NYPD First Deputy Commissioner ordered that no roof rescues were to be attempted, and at 9:43 a.m., this directive was passed to all units. (p 164/218)

This implies that an hour instead of 18 minutes passed between the North Tower strike (8:46) and the South Tower strike (9:03). Also, it was clear almost immediately after the first strike that people could not evacuate downward from above the crash zone. Why then did the unnamed First Deputy Commissioner prohibit rooftop rescue? NIST shows no curiosity at this decision, but makes further excuses, suggesting that a few lives weren't worth the effort:

Even if it had been possible for a helicopter to gain access to the roof, only a very small fraction of the large number of people trapped above the impact zone could have been rescued before the Towers collapsed. (p 169/219)

Given the great lengths and expense to which public officials often go to save a single life, it is striking that the Report's authors suggest that there was nothing wrong with the NYPD decision to prohibit attempts to rescue people from the roof. This, like the Report as a whole, is evidence that the authors would defend the authorities no matter what their conduct.

Fudging the Models

The Report contains a lengthy accounting of how the models performed under various assumptions about the buildings and the planes. One assumption common to all their simulations is the following:

The two Tower models included the core columns, the floor beams, and the concrete slabs from the impact and fire zones to the highest floor below the hat truss structure: from the 89th floor to the 106th floor for WTC 1 and from the 73rd floor to the 106th floor for WTC 2. Within these floors, aircraft-damaged structural components were removed. (p 98/152)
[emphasis added]

Apparently, any structural component estimated to have been damaged to any degree was removed from the model -- as if it contributed nothing to the structure. In other words, if NIST's crash simulation predicted that a column had lost 10% of its load-bearing capacity, it was treated as if it had lost 100% of its capacity.

For each Tower, NIST created two cases. The first set of cases, North Tower case A and South Tower case C, were based on the averages of NIST's estimates of building and plane strength, impact trajectories and speeds, etc. The second set of cases, North Tower case B and South Tower case D, assumed conditions more favorable to the failure of the buildings. The enhancements adopted for Cases B and D over cases A and C are described in the following table:

The Report noted that cases A and C did not produce results matching observations, so cases B and D were selected for use in its four-step modeling.

Altering Flight 175's Path

This illustration from WAKING UP FROM OUR NIGHTMARE shows the probable path of Flight 175's fuselage through the South Tower based on the appearance of a fragment the diameter of the fuselage exiting its east corner.

This photograph shows a piece of fuselage from Flight 175, which exited the South Tower.

Whereas NIST admits some of the liberties it took in adjusting its models' parameters to fit the desired result, such as their substitution of cases B and D for the more accurate cases A and C described above, it hides others.

This illustration (p 112/167) shows NIST's estimate of damage to the South Tower's columns, red indicating severed columns.

For example, NIST estimates that the crash of Flight 175 severed 10 core columns and damaged 11 others. That damage estimate assumes that the plane impacted the right side of the core nearly head-on.

NIST's assumption contradicts FEMA's estimate of Flight 175's trajectory, as well as the simple analysis of the plane's path through the building based on the entry and exit points of the fuselage. The illustration to the right shows the South Tower's impact gash, a video frame showing a fragment of fuselage exiting the Tower's east corner, and the path connecting the centers of the entry and exit holes. That trajectory left only the left wing and engine to do nearly all of the alleged damage to the Tower's core structure.

In contrast, NIST estimates that Flight 11 severed only six of the North Tower's core columns. Thus, NIST estimates that the North Tower had less core damage than the South Tower, which is completely implausible because:

• Flight 11 impacted the North Tower's core in a direct, centered fashion, contrasting with Flight 175's off-centered impact, glancing the core.
• The core columns at the North Tower's 95th floor impact zone were only about half as thick as the core columns at the South Tower's 80th floor impact zone.

Shrinking the Core Columns

Conclusion

Assuming the premise of the official explanation, the total collapses of the Twin Towers and Building 7 were the largest, most unexpected, and least understood failures of engineered steel structures in the history of the world. NIST's Report, like FEMA's 2002 report, presents the appearance of explaining the collapses of the Twin Towers, but in reality it doesn't explain them at all. Flatly asserting that "global collapse" inevitably follows "collapse initiation," the Report implies that the only issue worthy of study is how the jet impacts and fires led to collapse initiation -- an issue to which it devotes well over one hundred pages. Thus, the Report makes two fundamental claims, the first explicit and the second implicit:

• The impact damage and fires caused the tops of the Towers to lean and then begin to fall (collapse initiation).
• Once initiated, the collapses proceded to total collapses.

NIST goes to great lengths to support the first claim, but commits numerous omissions and distortions in the process. It remains quiet about the second claim, which is indefensible. Accepting that claim requires us to believe:

• That the collapses of WTC 1, 2, and 7 are the only examples of total progressive collapse of steel-framed structures in history.
• That those collapses were gravity-driven despite showing all the common physical features of controlled demolitions. In the cases of the Twin Towers, those features included the following: