Question 3) Symmetrical Collapse from Asymmetrical Damage?
(I apologize, much as I tried to distill my reasoning, this is still a long post)
As I said in previous posts, I do not believe that the damage to the south face was the cause of collapse. The collapse initiation cause and location may well never be answered to a certainty, but the information and hypotheses available regarding the collapse mechanic and why the building stayed close to it's footprint is compelling.
From the current NIST status summary posted by host in post #31 (
http://wtc.nist.gov/media/WTC7_Appro...ec07-Final.pdf)
Quote:
page 6
•The current NIST working collapse hypothesis for WTC 7 is restated here:
- An initial local failure occurred at the lower floors (below floor 13) of the building due to fire and/or debris-induced structural damage of a critical column (the initiating event) which supported a large-span floor bay with an area of about 2,000 square feet;
- Vertical progressionof the initial local failure occurred up to the east penthouse,and as the large floor bays became unable to redistribute the loads, it brought down the interior structure below the east penthouse; and
- Triggered by damage due to the vertical failure, horizontal progression of the failure across the lower floors (in the region of floors 5 and 7 that were much thicker and more heavily reinforced than the rest of the floors) resulted in a disproportionate collapse of the entire structure.
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(My interpretation):
- An interior column (or columns) failed at a lower floor on the East side of the building.
- The floors above the column pancaked down until the East penthouse sinks into the roof.
- Meanwhile, the interior columns to the West of the failed section are subjected to more and more loading until they finally exceed their design limit, and they fail.
- The floors collapse, but since they are attached to the facade, they initiate a force vector that pulls the facade down over them.
The video below shows a real-time collapse of WTC7 from a Northeastern view:
Using my highly newbie screen capturing/annotation skills, I grabbed some shots that were of interest (yellow text is from the video, green annotations are mine):
Between the 4 and 5 second mark, the East penthouse begins to collapse. While it is possible that the failure began from the top and progressed downward , it seems more sensible that the failure began at a lower floor some time before this (since there is more weight to be supported the lower the floor, and it was the lower floor structural members that may have been subject to heating). This seems to support the "vertical progression of failure" in the hypothesis.
Between the 6 and 11 second mark, nothing appears to be happening externally. However, it is sensible to presume that some areas are still settling, and it is certain that the load of all the collapsed floors is now being distributed to the remaining columns on the West side.
Sometime between the 11 and 12 second mark, the West penthouse begins to collapse from the middle of the building towards the west, as evidenced by the tilt of the penthouse as it collapses through the roof. This tilt would seem to bolster a "horizontal progression of failure" in the hypothesis.
Another annotated picture that might be a little clearer:
My unofficial count from the time the penthouse is first seen to collapse (4-5 second mark) to the time it is about to leave the frame (17 second mark) is 12-13 seconds, which matches the value annotated in the video. Since the building has not hit the ground yet, and since we do not know at what point the collapse started before we see the east penthouse begin to sink, it is reasonable to assume that the entirety of the collapse mechanic was longer than 13 seconds.
Structure Magazine released an article in their November issue in which they commented on the WTC7 collapse, and modelled the building in order to try to determine the most likely member that failed. While it is safe to assume that their model was not nearly as detailed as the NIST one will be, their conclusions offer support to the hypothesis as well:
Article:
http://www.structuremag.org/Archives...sanz-Nov07.pdf
Quote:
(from page 2 right column)
...This sequence of events, with roof elements sinking into a building with an intact facade, suggest an interior failure. An internal failure would explain the appearance of a "controlled" collapse with a relatively small debris field...
(from page 3 right column)
...A kinematic model was created to test the final collapse hypothesis and isolate the structural elements that may have contributed to each phase of the failure...
...A collapse mechanism analysis performed for the removal of column 79 produced a deformed shape with a kink in the roof of the east penthouse, as captured in actual videos and photos taken that day...
...As shown in the computerized non-linear structural model, its [column 79] failure initiated the vertical collapse progression. WTC7s properties of load transfer at floors 5 and 7, when combined with the failure of column 79, led to horizontal collapse progression...
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Column 79 (right upper internal column):
Simulated model still (from page 4 of the article):
Based on the video evidence and the findings in the Structure Mag article, I don't think it unreasonable that the NIST hypothesis as to the method of collapse is incorrect, specifically that WTC7 collapsed due to an internal column failure which led to global collapse, and that a small footprint is indicative of such an internal failure (because the floors would be bringing the walls down on top of them).
Quote:
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Originally Posted by host
...The way WTC 7 was constructed, I don't see how it collapsed so quickly,or how even a neat, 20 stories tall, one or two windows wide, shallow verticle gouge out of the south face, could have caused or contributed to the failure pictured here:...
...WTC 7 seems to have been built of strong stuff that failed simultaneously, for as yet to be determined reasons.....
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Again, I don't believe that the gash or damage in the south side was the cause of collapse, other than providing a mechanism to starting the fires that were in the building. It seems there is sufficient evidence to suggest that it did *not* collapse very quickly, that the building was internally collapsing before the global (visible) collapse began. The question to me is whether:
1) the fires themselves were sufficient to initiate a collapse;
2) they ignited something else that finally initated a collapse (fuel tanks or transformers?);
3) A combination of 1) and 2) (However, as you posted NIST has discarded any fuel other than typical office items, so 2) and 3) would be an unlikely area to research);
4) A combination of the south face damage plus the fires contributed to the collapse.
5) None of the above (other)
I don't believe that there will ever be a 100% certainty as to the initiation cause or the exact location of initiation, only a high-percentage possibility. But I've invested this much time into it so I'm going to dig into the fire aspect as time permits just to satisfy my own curiosity
