It is natural to examine the other 2 collapsed buildings more carefully to see whether similar gross errors and omissions exist.
The NIST representation of WTC7 is shown on the right. The actual object being studied is shown on the left:
In this case no dust obscures the view of the actual motion, so how can NIST researchers justify using the model on the right to represent the physical object being studied on the left?
INDEPENDENT MAPPING AND MEASUREMENTS OF WTC7 BUILDING MOVEMENT THROUGH THE COLLAPSE INITIATION SEQUENCE BACK TO THE EARLIEST DETECTABLE MOVEMENT
The collective visual record of the WTC7 collapse is examined directly and independently of all other sources, groups or individuals. The movement of the structure during the initial column failure sequence is mapped and traced back to the earliest point of detectable movement from multiple angles. Features of the initial failure sequence, including
1) Deformations
2) Ejections and overpressurization leading into the collapse initiation sequence
3) Collapse initiation motion and deformity traced back to the earliest detectable motion
4) Failure sequence penthouse and perimeter walls
are mapped, documented and compared to NIST observations and measurements. All claims are verifiable and all methods reproducible.
SUMMARY OF EARLY WTC7 MOVEMENT
As was shown in section 2.5, features of the initial failure sequence can be understood as a rapid succession of 7 identifiable events occurring in the following order:
1) Movement Detected from 2 Minutes before Collapse
2) Increase of rocking 6 seconds before visible collapse
3) Ejections and overpressurizations
4) Collapse of the East Penthouse
5) Collective core failure
6) Perimeter response
7) Acceleration downward
1) THE EARLIEST MOVEMENT BECAME DETECTABLE ABOUT 90 SECONDS BEFORE VISIBLE COLLAPSE
The NIST did not notice the earliest movement leading into the collapse initiation sequence seen in the gif images or mention it in their report.
A quick normalised view of NW corner horizontal motion...
A comparison with the West edge of the East Penthouse.
1) Doesn't show the same to-and-fro as the NW corner.
2) Uses the same static region subtraction data, so the to-and-fro is not a side-effect of the static point data.
3) Does show similar magnitude of movement shortly before release.
(And useful data resulting from sub+/-0.2 pixel variance data.
Cam3 - Bottom Field - NW Corner Horizontal - Lowpass high res
Comparison between horizontal and vertical motion...
1) Both start to move around the 60s mark.
2) Significant motion rate increase around the 160s mark.
3) Significant motion is at an inflexion in direction.
4) real world scales are not equal.
The establishment of a static reference point is addressed in footnote1.
2) MORE INTENSE ROCKING BEGINNING ABOUT 6 SECONDS BEFORE VISIBLE COLLAPSE
West edge trace by NIST, with raw data overlaid...
NCSTAR 1-9 Vol 2 Appendix C - VIDEO ANALYSIS OF WTC 7 BUILDING VIBRATIONS BEFORE COLLAPSE
A few direct quotes from the NIST report:
The west edge of WTC 7 (to the right in the frame) was of the most interest in this analysis. This was the northwest corner of the building, which was clear of smoke throughout the recorded period.
From the camera’s perspective, the angle of this edge appears close to vertical.
The tripod mount kept fluctuations in viewpoint to a minimum, although they were not be negligible.
In the previous WTC 2 moiré analysis, frame-to-frame and slow motions of a tripod-mounted camera were found to be a source of error.
Due to the camera location, points on the north face near the northwest edge were closer to the camera than points near the northeast edge. However, this distortion was small, since the width of the north face was much smaller than the distance of WTC 7 from the camera.
The perspective view of the camera looking up at WTC 7 also introduced some error into the measurement of the number of pixels for the width, as did the uncertainty of a couple of pixels in the exact location of the edges defining the north face.
Given these sources of error, an estimate for this video of the width of the north face of WTC was 301 ± 4 horizontal pixels.
Since the true dimension of the north face was 329 ft, the conversion factor was 1.09 ft ± 0.02 ft per horizontal pixel. Combining this with the equivalence of 100 ± 10 vertical pixels for each horizontal pixel gave the final conversion factor of 1.1 ft ± 0.1 ft (13 in. ± 1 in.) for each 100 pixels of vertical marker motion.
To prepare for analysis, the video clip was exported into a sequence of images, with each image carrying the data for a single frame of the video. Each frame was then converted from the original RGB color into grayscale values.
One source of uncertainty in this analysis was the curve-fitting process; another arose from defects in the video images.
Many of the frames from this video contained defects, such as the color and black-and-white patterns in the lower center and lower left of Figure C-1. Similar defects occurring along the northwest edge being used for the analysis were responsible for some outlier points in the results.
To find the intersection point of the pixel intensity plots for these two pixel columns, the data for each column were first fitted to a smooth curve. A third-order polynomial least-squares fit gave a good compromise between the variance (a measure of the distance of each data point from the curve) and an estimate of the error in determining the intersection point.
Starting at about 6 s before the penthouse began its downward movement into the building, there was an abrupt change in the slope of the data that marked the beginning of oscillations that continue until collapse. There was also a second abrupt change in the data at about 1.5 s before the penthouse started moving downward.
there are major changes in the location of the marker point that occurred over long time intervals of 20 s or greater. These changes were likely due to movement of the camera.
Of primary interest in this analysis was any information that could shed light on the collapse sequence of WTC 7.
There are multiple problems with the NIST analysis:
They didn't deinterlace video before analysis
This is a sample frame from the NIST camera 3 view interlaced:
It is impossible to locate the NW corner with precision before deinterlacing the video and separating the fields as anyone can see from the interlaced frame above.
Not clear of smoke
Not clear of smoke. No attempt to quantify the effect of smoke distortion upon their method. Their method is very sensitive to such minor pixel intensity fluctuations.
Edge not vertical
The edge was not vertical, and no attempt to correct their readings for it was performed.
Tripod movement not negligible
Yet no attempt was made to extract camera movement from their data.
Upward camera 3 view introduces perspective errors
Yet no perspective correction was applied to the data.
Perimeter deformation as a response to the failure of the core is clearly observable:
There is nothing within the NIST computer simulations that can account for this perimeter behavior.
Mapping the deformations from various angles allows for rough global model of perimeter flexure. The model describes the witnessed flexure of the perimeter as being the result of the core dropping in three sections as shown:
In contrast to the actual perimeter flexure as a response to collective core failure, consider the perimeter response in the NIST models:
From these models NIST nevertheless concluded: “the global collapse analyses matched the observed behavior reasonably well".
Drop curves for the center of building, northwest corner and northeast corner compared
Coupled reaction between core and perimeter
Drop rates and velocities of core and perimeter compared
Here are the two elevations NIST used in their camera 3 drop data:
The very same elevations are measured below but using the Dan Rather video viewpoint instead.
It appears that the core was slowed down by the perimeter. The core appears to have strained the floors, took down the perimeter and catapulted the outer shell downwards until the floors were once again less strained. After this it appears the whole interconnected unit fell together
The NW corner of the west penthouse just fell until it was slowed down.
So how is it possible for the perimeter to drop faster than free fall?
Core and perimeter were still connected through the floor system. Once the core dropped at gravitational acceleration the core-floor-perimeter could have acted like a spring system. First, the core pulled the perimeter inwards. Secondly, the perimeter failed at a very low elevation and was then pulled downwards by the "floor-springs". The falling core was slowed down at the same time until the entire system fell as one unit.
Of course the center of mass of the entire system cannot exceed gravitational acceleration, but the perimeter can.
The NIST claimed:
The time the roofline took to fall 18 stories was 5.4 seconds...
...which equals 135 frames at 25fps!
The NW corner of the roof reached the 29th floor in frame 397 of this measurement. The time segment measured by NIST is marked here:
NIST began measuring at exactly the moment when the NE corner of remaining west penthouse started to move and stopped measuring when the NW corner reached the 29th floor.
7) Measurements of Downward Acceleration
The NIST measurement in green compared to actual acceleration trend in red:
Multiple methods of extracting downward acceleration from displacement data yield the same trends:
How did the NIST measure free-fall acceleration?
The NIST Method in their own words:
Mistakes with the NIST downward displacement data
Mistakes within the NIST measurement of drop motion, downward acceleration are numerous:
The following image shows the roofline above Region B for the first 5 NIST datapoints (Including T0)...
As one can easily see, the roofline structures are still visible for the first 4 datapoints, and the roofline cannot be determined by automated tracing techniques.
The T[sub]0[/sub] pixel is obscured behind foreground buildings around 4.7s after that time...which coincides with the change in datapoint separation seen for the last 5 samples in 12-76, shown below:
It seems NIST started the trace from the T0 pixel location, then swapped tracking point for those last 5 samples to a location above Region B.
The NIST data suffers from the following (non-exhaustive) series of technical issues, each of which reduce the quality, validity and relevance of the data in various measures...
NIST did not deinterlace their source video. This has two main detrimental effects: 1) Each image they look at is actually a composite of two separate points in time, and 2) Instant halving of the number of frames available...half the available video data information. Tracing features using interlaced video is not a good idea. I have gone into detail on issues related to tracing of features using interlaced video data previously.
NIST did not sample every frame, reducing the sampling rate considerably and reducing available data redundancy for the purposes of noise reduction and derivation of velocity and acceleration profile data.
NIST used an inconsistent inter-sample time-step, skipping roughly every 56 out of 60 available unique images. They ignored over 90% of the available positional data.
NIST likely used a manual (by hand-eye) tracking process using a single pixel column, rather than a tried and tested feature tracking method such as those provided in systems such as SynthEyes. Manual tracking introduces a raft of accuracy issues. Feature tracking systems such as SynthEyes employ an automated region-based system which entails upscaling of the target region, application of LancZos3 filtering and pattern matching (with FOM) to provide a sub-pixel accurate relative location of initial feature pattern in subsequent frames in video.
NIST tracked the *roofline* using a single pixel column, rather than an actual feature of the building. This means that the trace is not actually of a point of the building, as the building does not descend completely vertically. This means the tracked pixel column is actually a rather meaningless point on the roofline which wanders left and right as the building moves East and West.
NIST used the Cam#3 viewpoint which includes significant perspective effects (such as early motion being north-south rather than up-down and yet appearing to be vertical motion). It also means that each horizontal position across the facade requires calculation of a unique scaling metric, which NIST do not appear to have bothered to do.
NIST did not perform perspective correction upon the resultant trace data.
NIST did not appear to recognise that the initial movement at their chosen pixel column was primarily north-south movement resulting from twisting of the building before the release point of the north facade.
NIST did not perform static point extraction(H, V). Even when the camera appears static, there is still (at least) fine movement. Subtraction of static point movement from trace data significantly reduces camera shake noise, and so reduces track data noise.
NIST did not choose a track point which could actually be identified from the beginning to the end of the trace, and so they needed to splice together information from separate points. Without perspective correction the scaling metrics for these two points resulted in data skewing, especially of the early motion.
Measured drop rate and downward velocity compared to the NIST measurements
high res
Note: t=0 was placed as early as could possibly be justified by the derived velocity plot, and yet it is still ~1s after the NIST T0.
The NIST blew their stage 1 analysis
The NIST description of a "stage 1" of collapse initiation is dependent upon their interpretation of downward acceleration of the roofline.
NIST stated that the north face of the building descended 18 stories (the portion of the collapse visible in the video) in 5.4 seconds, based on video analysis of the building collapse. This time period is 40 percent longer than the 3.9 seconds this process would have taken if the north face of the building had descended solely under free fall conditions. During the public comment period on the draft report, NIST was asked to confirm this time difference and define the reasons for it in greater detail.
To further clarify the descent of the north face, NIST recorded the downward displacement of a point near the center of the roofline from first movement until the north face was no longer visible in the video. Numerical analyses were conducted to calculate the velocity and acceleration of the roofline point from the time-dependent displacement data. The instant at which vertical motion of the roofline first occurred was determined by tracking the numerical value of the brightness of a pixel (a single element in the video image) at the roofline. This pixel became brighter as the roofline began to descend because the color of the pixel started to change from that of the building facade to the lighter color of the sky.
The approach taken by NIST is summarized in Section 3.6 of the final summary report, NCSTAR 1A (released Nov. 20, 2008; available at http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201A.pdf) and detailed in Section 12.5.3 of NIST NCSTAR 1-9 (reproduced earlier).
The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:
* Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
* Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)
* Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity
This analysis showed that the 40 percent longer descent time - compared to the 3.9 second free fall time - was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below.
As was demonstrated, the initial downward acceleration was measured incorrectly by the NIST. The 3 stages of collapse as described by the NIST have little meaning when more accurate acceleration measurements are taken into account.
According to NIST_NCSTAR_1-9_vol2 12.5.3. NIST used the "camera 3" viewpoimt.
In the Draft Report NIST states:
"The elevation of the top of the parapet wall was +925 ft. 4 in. The lowest point on the north face of WTC7 visible on the camera 3 video (section 5.7.1) prior to any downward movement was the top of windows on floor 29, which had an approximate elevation of +683 ft 6 in."
However, just a few of the floor 29 windows are visible and the ones that are visible are located directly below a place where the penthouse is seen above the perimeter wall. To measure the fall of the building NIST had to measure a vertical path above the visible top of the windows.
First problem: Confusing paraphet wall, screenwall and roofline
NIST defined "parapet wall" for the same elevation they defined as "roofline" in the final report.
Hence, we have to measure this path (or swap data points as mentioned earlier).
The "screenwall" is visible above the roofline and had an elevation of about two additional floors and the lack of contrast allows no direct tracking of the roofline.
So how was NIST able to measure the fall of the perimeter wall?
They answer the question in the FAQ:
The instant at which vertical motion of the roofline first occurred was determined by tracking the numerical value of the brightness of a pixel (a single element in the video image) at the roofline. This pixel became brighter as the roofline began to descend because the color of the pixel started to change from that of the building facade to the lighter color of the sky.
NIST did not measure the parapet wall. They measured the fall of the screenwall about 2 floor heights above the parapet wall.
Second problem: Inward flexure of roofline during earliest movement.
The perimeter wall didn't bow downwards as visible from a different vantage.
Instead, the perimeter wall stayed vertically straight even during the fall for several floors from this viewpoint
.
The motion of the perimeter wall as visible from "camera 3" is nothing but the bowing of the perimeter toward the core. The bowing was shown earlier and is reproduced here. Early deformation of the perimeter wall from a different vantage point can also be seen in this image.
What the NIST actually measured was the drop of the "screenwall" + the transition of the falling screenwall into the horizontal bowing of the perimeter wall + the transition into the vertical motion of the perimeter wall.
The red curve shows the motion NIST measured:
The thicker bright blue curve is a calculated free fall. The lower curves are the trackings of several floors along the path the NIST measured. The slow onset of motion of these lower curves is the result of the perimeter bowing away from the camera.
One can easily see how the roofline point tracked by the NIST gives a deceptive result during the earliest detectable motion:
In the Final Report as well as in the FAQ NIST described their result the following way:
* Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
* Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)
* Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity
The NIST "stage 1" includes about the frames 150 ... 202 of this motion tracking measurement.
The NIST "slower than free fall" stage 1 includes the following real events:
the screenwall on top of the core started to move (frame 150...155)
the screenwall reached about free fall (frame 156...170)
the screenwall disappeared behind the parapet wall (frame 170)
the perimeter wall bows towards the core (frame 170...180)
the perimeter wall dropped above gravity (frame 180...200)
That is where stage 2 at "gravitational acceleration (free fall)" begins according to the NIST.
On August 26, 2008, the NIST's lead investigator, Dr. Shyam Sunder, hosted a NIST Technical Briefing to answer questions regarding the final draft of the Final Report on the Collapse of World Trade Center Building 7.
Note that Dr. Sunder shared authorship of the most important chapters of the WTC7 Final Report; Chapter 13, Probable Collapse Sequence and Chapter 14, Principal Findings.
During this briefing of a few hours, he was confronted with this question; "Any number of measurements using a variety of methods indicate the northwest corner of WTC 7 fell with an acceleration within a few percent of the acceleration of gravity.
Yet your report contradicts this, claiming 40% slower than free fall based on a single data point?"
His answer...
Dr. Sunder: "Well...um...the...first of all gravity...um...gravity is the loading function that applies to the structure...um...at...um...applies....to every body...every...uh...on...all bodies on...ah...on...um... this particular...on this planet not just...um...uh...in ground zero...um...the...uh�the analysis shows a difference in time between a free fall time, a free fall time would be an object that has no...uh... structural components below it. And if you look at the analysis of the video it shows that the time it takes for the...17...uh...for the roof line of the video to collapse down the 17 floors that you can actually see in the video below which you can't see anything in the video is about...uh... 3.9 seconds. What the analysis shows...and...uh...the structural analysis shows, the collapse analysis shows that same time that it took for the structural model to come down from the roof line all the way for those 17 floors to disappear is...um... 5.4 seconds. It's...uh..., about one point...uh...five seconds or roughly 40% more time for that free fall to happen. And that is not at all unusual because there was structural resistance that was provided in this particular case. And you had...you had a sequence of structural failures that had to take place and everything was not instantaneous."
He was simply quoting from the section of the report that he helped write:
The stages listed by Dr Sunder to defend his position do not exist in reality. The are merely artifacts of poor measuring techniques and a poorly chosen viewing angle.
Horizontal vs vertical motion of the northwest corner
This is a trace of the NW corner using a larger copy of this stabilised Animated GIF]
Even though the viewpoint is obviously affected by the NW camera position, the southward motion is rather well behaved and gradual.
NIST PROPOSED COLLAPSE MECHANISM AND COMPARISON WITH OBSERVABLES
As mentioned earlier, the stated purpose of the NIST reports is to determine how and why the buildings collapsed, or the collapse initiation mechanism that led to building failure. A summary of some of the key problems with the NIST explanation of the collapse of WTC7 is contained within a written comment submitted to the NIST over the draft report by Frank Greening. The letter is reproduced below and a summary of the main technical problems follows.
Comments on the Draft Report NIST NCSTAR 1-9: Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7”, issued by NIST August 21st, 2008
By F. R. Greening
1.0 Introduction
A preliminary (draft) version of NIST's final report on the collapse of WTC 7 was issued on August 21st 2008 together with a call by NIST's Investigation Team for the submission of comments on the Draft Report from interested parties within the general public. First I wish to thank NIST for producing such a detailed technical report on the collapse of WTC 7 and secondly, I applaud NIST for allowing researchers from around the world to offer technical feedback that hopefully will be duly considered by NIST before a final version of the report is issued.
In reading the Draft WTC 7 Report a number of issues emerge that are crucial to the credibility of NIST's proposal as to how and why building 7 collapsed on September 11th, 2001. These key issues center on the narrative surrounding the ignition of the fires in WTC 7 and the spreading of these fires within the building prior to its collapse. The accuracy of NIST's account of what transpired within the confines of building 7 during 9/11, is vital to NIST's entire WTC 7 Report because it provides the basis for the computer modeling/simulation of the heating of structural elements on the fire-affected floors, which in turn, leads to NIST's proposed collapse initiation and propagation mechanism.
In the following comments I will attempt to address each of the key topics - fire ignition and spreading, fire intensities and durations, structural heating, collapse initiation and propagation - and in so doing, highlight my concerns or objections to NIST's position on these topics as presented in its Draft WTC 7 Report.
2.0 The Ignition and Spreading of the Fires in WTC 7
In Chapter 9 of NIST NCSTAR 1-9 we encounter one of the most significant problems with attempts to unravel the mystery of why and how WTC 7 collapsed late in the afternoon of September 11th, 2001 " the question of where and how fires started in building 7. On page 376 of NCSTAR 1-9 we read:
"the ignition and early course of the fires (in WTC 7) were unknown because they were presumed to have occurred in the damaged and heavily smoke-shrouded southern portion of the building."
NIST's knowledge of the fires in WTC 7 is therefore based on images of the exterior faces of the buildings. Unfortunately however, as acknowledged by NIST, most of the burning of combustible materials at the WTC on 9/11 took place beyond the views available through exterior windows well inside the buildings.
NIST propose, and it appears to be a reasonable assumption, that the fires in WTC 7 started near the south face as a result of the collapse of WTC 1 at about 10:29 on the morning of 9/11. However, even this assumption is problematical because fires on the crucial 12th and 13th floors of WTC 7 were not in fact observed until after 2:00 p.m., and then only on the east face of the building.
Faced with the problem of modeling the spreading of the fires in WTC 7, NIST begins its computer simulation with a set of 2 MW fires, presumably one per floor, for floors 7, 8, 9, 11, 12 and 13. These hypothetical fires are stated to be 'roughly equivalent to small, single workstation fires'�, but NIST is quite vague about where these fires were located other than “near the southern face of the building”. What is more, for the fire to spread to NIST's satisfaction on floor 8, two fires were hypothesized to start at this level within the building.
Other aspects of NIST's simulation also appear to be quite arbitrary and unphysical. Thus the fire on floor 12 was prescribed to start "near the center of the south face at an assigned time of 12:00 noon." This is a strange choice of ignition time given that the WTC 7 fires were supposedly started by flaming debris from the collapse of WTC 1 at 10:29 a.m. It implies that some of the flaming material in the WTC 1 debris that settled near WTC 7 remained dormant for about an hour and a half before spontaneously igniting fires that were subsequently observed on floor 12.
1.0 Fire Intensities and Durations
The way the fires spread in WTC 7 during 9/11 was largely determined by the distribution of combustible materials throughout the building. In NIST's fire simulations this distribution was approximated by an average fuel load for each fire-affected floor of 20 - 32 kg/m2 or 4.0 and 6.4 lb/ft2, (See NIST NCSTAR 1-9 pages 59, 60). As shown in Figures 10-15 and 10-16 of NCSTAR 1-9, this fuel loading is calculated by NIST to have been sufficient to sustain temperatures above 400 °C for the floor beams and concrete slab on the east side of floors 12 and 13 for about 2 hours. According to NIST's fire simulations, floors 12 and 13 were the most severely heated floors in WTC 7; however, there are reasons to question the level of heating claimed by NIST.
NIST's fire simulation would have us believe that a very substantial heat release rate was sustained for over 2 hours over a floor area of about 500 m2 in building 7. Thus Figure 9-13 of NCSTAR 1-9 shows that a heat release rate of 200 MW was attained on floor 12 at about 3:00 p.m. on September 11th and remained above 200 MW until well after 5:00 p.m. But we need to ask: Is a 200 MW fire consistent with a fuel loading of 32 kg/m2 - the value used by NIST for its floor 12 fire simulations? The answer appears to be no. Thus a 200 MW heat release rate for 2 hours implies a total energy release of 1,440 GJ. If the combustible material on the 12th floor of WTC 7 is assumed to release 20 MJ/kg, we have to conclude that 72,000 kg of office material was combusted over an area of 500 m2, or there was a fuel loading in WTC 7 of 144 kg/m2 - a value over four times NIST's assumed fuel loading.
That there is a problem with NIST's predicted fire intensities in WTC 7 compared to the assumed fuel loading is supported by comparisons to other studies of fires in steel framed buildings. For example, the well-known Cardington tests conducted in the U.K. in 1999 measured a maximum heat flux of about 200 kW/m2 over a period of about 1 hour from the combustion of 6000 kg of cellulose-based fuel inside a 144 m2 steel framed structure, giving a fuel loading of 42 kg/m2. Thus we see that in the Cardington tests the total energy release is predicted to be 144 x 200 kW for one hour which equals 28.8 MW for 3600 seconds or 104 GJ. The heat of combustion of the fuel was 17 MJ/kg, so for 6000 kg we would expect a heat release of 102 GJ in good agreement with the predicted energy release.
The main problem with the NIST fire simulation appears to be the calculated duration of the fire on the 12th and 13th floors of WTC 7. For example, if we assume a more reasonable fire duration of 30 minutes, rather than NIST's excessive 2 hours, we may revise the energy release down from 1,440 GJ to a mere 360 GJ in which case the combustion of 20 MJ/kg fuel would have consumed 18,000 kg of material and the fuel loading would have been 36 kg/m2 in much better agreement with NIST's assumed fuel load. That these are more realistic figures is also supported by some of NIST's own studies of the relationship between combustible loads in buildings and classifications of fire severity. Thus M G. Goode in NIST Report No. GCR-04-872, published in July 2004, provided a table showing that fire durations of 0.5 and 0.75 hours are to be expected for fuel loads of 20 kg/m2 and 30 kg/m2, respectively.
4.0 Structural Heating
In view of the fact that NIST appears to have overestimated the intensity and duration of the fires in WTC 7, particularly on floors 12 and 13, it follows that the heating of the structural steel is also overestimated in the WTC 7 Draft Report. This is fatal to the overall validity of NIST's collapse initiation hypothesis because NIST's hypothesis is predicated on the thermal expansion of long span beams and girders on floors 12/13 and their eventual loss of connectivity with column 79, (See Chapter 8 of NCSTAR 1-9).
NIST's computer simulation of the thermal response of floors 12/13 is described in Chapter 10 of NCSTAR 1-9 and estimates temperatures as high as 675 °C “on much of the east side and the east portion of the south side of (floor 12)”. NIST also concludes that the temperatures of floor beams and girders on floors 12/13 were 600 °C or higher for 1 - 2 hours.
The temperature vs. time profile of a structural steel member exposed to a fire and protected by a layer of insulation of thickness di is given by the formula:
DTs / Dt = [ ki / (di cs rs) ] (Ap / V) ( Tg - Ts )
where,
DTs / Dt is the rate of change of the temperature of the steel
ki is the thermal conductivity of the insulation material
cs is the heat capacity of the steel
rs is the density of the steel
Ap / V is the section factor of the steel member
Tg - Ts is the temperature difference between the steel and the combustion gases
Values for the quantities ki , di , cs , rs and Ap / V appropriate for calculations of the heating of structural members in WTC 7 are as follows:
ki = 0.12 W/m.°C (Monokote MK-5)
di = 0.015 m
cs = 660 J/kg.°C
rs = 7800 kg/m3
Ap / V = 100 m-1 (W33x130 girder)
As discussed in Section 3.0 above, the duration and intensity of the fires on floors 12 and 13 of WTC 7 discussed by NIST in Chapter 10 of NCSTAR 1-9, appear to be inconsistent with the fuel loads used in NIST's simulations. However, based on data from A. Jowsey's thesis: Fire Imposed Heat Fluxes for Structural Analysis, (Edinburgh 2006), an upper layer gas temperature of 800 °C sustained for 40 minutes would appear to provide a more realistic description of the fires at the east side of floors 12 and 13 prior to the collapse of WTC 7. This leads to a predicted heating rate of 7.46 °C/min and a maximum temperature for the floor framing beams and girders near the critical column 79 of about 300 °C, or barely half the temperatures estimated for these structural members in the NIST WTC 7 Draft Report.
5.0 Collapse Initiation and Propagation
NIST's computer simulation of the collapse of WTC 7, as presented in Chapters 8 and 12 of NCSTAR 1-9, is remarkable for the low temperatures - as low as 100 °C - at which failures of connecting elements such as bolts and studs are predicted to have first occurred in WTC 7 after about 3:00 p.m. on 9/11. These failures were caused, so NIST asserts, by the thermal expansion of asymmetrical framing beams and girders on the east side of floors 12/13. Nevertheless, in NIST's model, complete separation of column 79 from lateral restraints to buckling is predicted to occur only at temperatures well above 300 °C. Thus NIST's collapse initiation hypothesis requires that structural steel temperatures on floors 12/13 significantly exceeded 300 °C - a condition I believe that could never have been realized with NIST's postulated 32 kg/m2 or lower fuel loading.
However, assume for a moment that collapse initiation in WTC 7 did in fact occur as NIST states: by a thermally induced buckling failure of column 79 on floors 12/13. It would then be appropriate to ask: Is the collapse propagation mechanism proposed by NIST consistent with the observed collapse of WTC 7? If the answer to this question is indeed "Yes", it would add credibility to NIST's account of what happened to building 7 on 9/11 even if an inappropriate fuel loading was used. However, I would suggest that NIST's account of the last ½ minute of the life of WTC 7 not only lacks crucial physical detail, but is also at odds with what was observed in the well-known collapse videos of WTC 7.
In NIST's WTC 7 collapse simulation, the fires in the lower part of the building severely heat floors 12 and 13 near column 79 causing it to lose lateral support and buckle. Then, according to NIST, the entire section of column 79 above floor 14 began to descend and trigger a global 'disproportionate' collapse of WTC 7. In NCSTAR 1-9, Chapter 12, page 57, it is claimed that the top of column 79 was moving downward within 0.2 seconds of its buckling between floor 5 and 14.
Let's consider this alleged motion of column 79 in more detail. Figure 12-43 in Chapter 12 of NCSTAR 1-9 NIST shows column 79 buckling between floors 5 and 14 starting about 14.9 seconds into NIST's collapse initiation simulation. The lateral displacement of column 79 is shown to be about 5.5 meters to the east of its normal, fully vertical position at floors 9/10 at 15.5 seconds into the simulation. A consideration of the geometry of a column buckling over a length of about 36 meters shows that a lateral displacement of 5.5 meters should lower the top of the column by about 0.8 meters. In the same collapse simulation timeframe, (14.9 �" 15.5 seconds), NIST show in Figure 12-45 that the vertical displacement of column 79 at the roof level was in fact 0.83 meters in 0.6 seconds. This implies that column 79 was moving downwards with an acceleration of 4.6 m/s2 or about ½ g which is a very dramatic motion for a column that was restrained by several framing beams and girders on all the undamaged and unheated floors above floor 14 just moments before collapse initiation. I would therefore ask NIST to explain how and why all lateral supports acting on column 79, from more than 30 upper floors, were simply ripped out or otherwise detached from their very secure connections in only 0.2 seconds?
To conclude this section I would like to briefly mention NIST's simulation of the final global collapse of WTC 7. Of course we are all very familiar with what actually transpired during the final moments in the life of WTC 7 because of the numerous well-known videos of this dramatic event, as discussed in Chapter 5 of NCSTAR 1-9. These videos typically present an unobstructed view of at least the upper third of WTC 7 and permit the collapse to be followed for 4 - 5 seconds. The videos show the upper section of WTC descending very smoothly as an intact structure with the roofline remaining essentially horizontal until it passes behind buildings in the foreground. The only significant distortion of the boxed-shaped Building 7 that is noticeable after the façade begins its downward motion, is the formation of a slight kink on the eastern side of the north face.
Now consider NIST's version of the final moments of WTC 7 as exemplified by the computer-generated simulacra of Figure 12-69 of NCSTAR 1-9. These images of the final collapse of WTC 7 from the north, west and south show very extensive buckling of the exterior columns especially near the mid-height of the building. It is simply astounding that, even though these computer generated images of a crumpled and severely distorted Building 7 look nothing like the video images of the real thing, NIST nevertheless concludes: "the global collapse analyses matched the observed behavior reasonably well."
5.0 Conclusions
I believe there are many problems with the material presented in NIST's Draft WTC 7 Report; most of these problems stem from the fuel loading assumed by NIST but I would add that NIST's collapse hypothesis is not physically realistic and is not well supported by observations of the behavior of Building 7 during its collapse. I certainly believe that an alternative collapse initiation and propagation hypothesis is called for; an hypothesis that more accurately reflects the reality of what happened to WTC 7 on September 11th 2001.
Dr. F. R. Greening
Hamilton, Ontario, Canada
The comments by Frank Greening do not mention any of the numerous problems with the NIST measurements and descriptions of early motion as listed earlier.
The items mentioned within the Greening criticism can be grouped into 3 main categories:
Fires: The Ignition and Spreading, Intensities and Durations
Structural Heating and Response
Collapse Initiation and Propagation
Consider the NIST fire simulation of floor 12 around 4:00pm with the visual record of the fires:
Consider characteristics from the NIST collapse simulations:
Compared to the actual buiding behavior:
The key global geometric characteristics of collective core failure and flexing of the perimeter are not expressed or explained at all by the simulation. The only similarities between the simulation and the observed collapse is that both collapsed.
To requote Greening:
"It is simply astounding that, even though these computer generated images of a crumpled and severely distorted Building 7 look nothing like the video images of the real thing, NIST nevertheless concludes: “the global collapse analyses matched the observed behavior reasonably well.”
It seems even more astounding how many people seem to uncritically accept the NIST explanation as adequate.
The NIST simulation was gamed to collapse
The NIST descent simulation was based upon a model which was never going to do anything except collapse.
The simulations:
Consider:
The base global model had gravity applied over 4.5s
had all impact damage applied instantly
was left 2s to settle
had 4 hours worth of heat applied in 2s
then all fire induced damage estimates applied instantly
and had collapsed under 17s later.
Explained by the NIST:
The fire-induced damage estimated from the ANSYS model was, therefore, input to the LS-DYNA model as the final step before analyzing the structural response. The damage was applied immediately after the temperatures were applied, by a sudden removal of damaged or failed elements calculated in the ANSYS analysis.
The global model basically implies global collapse 3hrs prior to it actually descending.
CONCLUSIONS ON WTC7
The simulation was set up to fail. Even though the simulation bears almost no resemblance to the collapsed as documented, it is passively accepted as convincing by many.
It was gamed to collapse, and the collapse as simulated shows no key geometric features such as the collective core failure or flexure of the perimeter.
The issue of collective core failure leading to perimeter flexure and an extremely well-ordered collapse is not addressed at all. Instead, the public is asked to accept the simulated model even though it lacks an detail of the key geometric global features clearly visible in the collapse including:
1) Collective core dropping
2) Perimeter flexure as a response to the core falling
3) Building movement detectable from about 90 seconds before visible movement
COMPARISON OF NIST DESCRIPTION OF EARLY MOVEMENT OF WTC7 WITH THE ACTUAL VISUAL RECORD
1) Movement Detected from 2 Minutes before Collapse
Was never noticed by the NIST
2) Increase of rocking 6 seconds before visible collapse
This movement was measured by the NIST yet there is no explanation for it within their computer simulations of the collapse.
3) Collective core failure
Not noted or modeled in the NIST report
4) Perimeter response
Not noted in the NIST report. The Core-perimeteer action that is such an important feature in the early collapse process is not noted in the NIST report
5) Acceleration downward
Was measured incorrectly within the reports. The core-perimeter interaction was not understood and there were multiple problems with the NIST camera #3 tracking as listed earlier.
SIMILARITIES BETWEEN THE WTC1 AND WTC7 COLLAPSE INITIATIONS
Just as in the case of WTC1, WTC7 experienced collective core failure which pulled the perimeter walls down behind it. In the case of WTC1, there no evidence that collective core failure was not recognized by the NIST. Instead, the early motion of the building was grossly misrepresented within the recorded history of the collapse. Since no official sources acknowledge collective core failure within WTC1, it never had to be explained and remains ignored to this day.
In the case of WTC7, due to the very visible movement of the penthouse, collective core failure is impossible to ignore completely. Yet it is ignored within the NIST explanation of the collapse as analysis demonstrates.
Just as with WTC1, signs of rapid, collective core failure are ignored. The perimeter response to early core movement is also ignored. And just as with WTC1, the earliest detectable movement leading into the collapse initiation sequence went unnoticed.
1) The spike around the 95s mark appears to be caused by someone walking past the camera. His head is seen momentarily.
2) There doesn't seem to be any obvious shake caused by descent of the building.
Cam#3 - Bottom Field - Static Region - Vertical - Raw - 29.97 samples/s
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![~1s period of over-[i]g[/i] acceleration](https://web.archive.org/web/20210924092313/http://femr2.ucoz.com/_ph/7/238197312.png){kind=link}