Citat:
Jodå - se NIST FAQ #10:
Jodå - se NIST FAQ #11:
10. Some people have said that a failure at one column should not have produced a symmetrical fall like this one. What is NIST's answer to those assertions?https://www.nist.gov/topics/disaster-failure-studies/faqs-nist-wtc-7-investigation
WTC 7's collapse, viewed from the exterior (most videos were taken from the north), did appear to fall almost uniformly as a single unit. This occurred because the interior failures that took place did not cause the exterior framing to fail until the final stages of the building collapse. The interior floor framing and columns collapsed downward and pulled away from the exterior frame. There were clues that internal damage was taking place prior to the downward movement of the exterior frame, such as when the east penthouse fell downward into the building and windows broke out on the north face at the ends of the building core. The symmetric appearance of the downward fall of WTC 7 was primarily due to the greater stiffness and strength of its exterior frame relative to the interior framing.
Jodå - se NIST FAQ #11:
11. In a video, it appears that WTC 7 is descending in free fall, something that would not occur in the structural collapse that you describe. How can NIST ignore basic laws of physics?https://www.nist.gov/topics/disaster-failure-studies/faqs-nist-wtc-7-investigation
In the draft WTC 7 report (released Aug. 21, 2008; available at http://www.nist.gov/el/disasterstudies/wtc/wtc_draftreports.cfm), 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 façade to the lighter color of the sky.
The approach taken by NIST is summarized in NIST NCSTAR Report 1A, Section 3.6, and detailed in NIST NCSTAR Report 1-9, Section 12.5.3.
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.
Hm alltså jag läser inte riktigt det där som ”förklaringar”. Även om vi skulle anta att NIST har rätt i att hela inre strukturen redan har mer eller mindre försvunnit när fasaden börjar kollapsa så är ju inte det en ”förklaring” till symmetrin? Den inre strukturen kollapsade ju inte alls symmetriskt i NISTs modell?
EDIT:
Ok. De ”förklarar” fritt fall med att fasaden vek sig (jag antar helt symmetriskt?) över typ 17 våningar?
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Senast redigerad av fffffuuuuuu 2019-10-05 kl. 23:27.
Senast redigerad av fffffuuuuuu 2019-10-05 kl. 23:27.
