NIST-reports - decrypted version from which you can copy&paste can be found here:
http://ift.tt/1OUorVQ
(Thanks to MrKoenig from The911Forum.)
The short story of the NIST's collapse initiation is that the fire-induced failure of G79-44 on floor 13 triggered progressive floor collapses, leaving column 79 critically unbraced.
NIST arrived at this by the following sequence of models and their results:
Now everybody always focuses on the G79-44 girder on floor 13, that "walked off" to the west, in NIST's story, being pushed off its seat by epanding secondary beams to its east.
Much debate ensues over whether such a "walk off" to west is plausible, with ARUP finding a "pull off" to either east or north plausible as well, and Truthers denying whatever they can.
BUT
What everybody seems to be ignoring is that this G79-44 girder on floor 13 wasn't the only girder or beam falling!
NIST also reports on a failure on the 14th floor of the connection of the G77-80 girder at column 79, which fell onto floor 13 and also apparently cascaded down.
To verify this is true, turn to Figure 11–36 of NIST NCSTAR 1-9 (see link at the beginning of the post) on page 512 (p578 of the PDF):
Column 80 is the center column of the three right-most core columns, and the G77-80 girder is the one to its left (west). Its connection on column 80 is drawn as a black dot, black meaning it has failed completely. The girder is drawn red, meaning it has lost vertical support. The secondary floor beams extending to both sides (north and south) are also drawn red, as their vertical support depends on G77-80 being vertically supported, which it isn't.
The LS-DYNA model starts running at a time t=0 without loads and damages. During the first 8.5 seconds, NIST applies gravity, external damage and temperatures and allows for the structure to settle. This is described in Section 12.3.2 on page 563 (p629 of the PDF) and following.
Then, at 8.5 s, NIST applies all the Case B fire damage that ANSYS determined after 4 h (Section 11.3.2, start at page 503), to the LS-DYNA model.
Failed connections are let fall.
Figure 12-42 shows the result at 9.5 seconds - 1 s later:
Note that this Figure shows floor structures from both floor 13 and floor 14 dropping on the respective floor below inside the area circled red on the right.
Figure 12-43 then shows the situation another 6 seconds later:
(Note that this image is only a PART of Figure 12-43 - the rest of the Figure has plots with the vertical stress and displacement of columns 79, 80 and 81).
In this image, the floor failures on floor 13 and floor 14 have already reached floor 5.
I am wondering:
http://ift.tt/1OUorVQ
(Thanks to MrKoenig from The911Forum.)
The short story of the NIST's collapse initiation is that the fire-induced failure of G79-44 on floor 13 triggered progressive floor collapses, leaving column 79 critically unbraced.
NIST arrived at this by the following sequence of models and their results:
- They modeled fire progression through floors 5-16 to get a couple of heating regimes
- They applied those heating regimes to a 16-story model in ANSYS
- ANSYS
- They found that fire Case B best fit observations
- After 4 hours of Case B fires, ANSYS had identified which girders, beams and connections were damaged and failed
- In a separate LS-DYNA model of 47 floors, they first applied gravity loads, then external damage, then temperatures prevalent after 4 hours of the ANSYS model, then, suddenly and at the same time, all damages that ANSYS found.
- Girder and beam connections determined to have "failed" started to fall in that model.
- The dynamic impact of falling masses on floors below was computed by LS-DYNA and found to punch out floor slabs below, such that after a few seconds, column 79 (and 80, 81) were unbraced over several stories and failed
- Total progressive collapse followed.
Now everybody always focuses on the G79-44 girder on floor 13, that "walked off" to the west, in NIST's story, being pushed off its seat by epanding secondary beams to its east.
Much debate ensues over whether such a "walk off" to west is plausible, with ARUP finding a "pull off" to either east or north plausible as well, and Truthers denying whatever they can.
BUT
What everybody seems to be ignoring is that this G79-44 girder on floor 13 wasn't the only girder or beam falling!
NIST also reports on a failure on the 14th floor of the connection of the G77-80 girder at column 79, which fell onto floor 13 and also apparently cascaded down.
To verify this is true, turn to Figure 11–36 of NIST NCSTAR 1-9 (see link at the beginning of the post) on page 512 (p578 of the PDF):
Column 80 is the center column of the three right-most core columns, and the G77-80 girder is the one to its left (west). Its connection on column 80 is drawn as a black dot, black meaning it has failed completely. The girder is drawn red, meaning it has lost vertical support. The secondary floor beams extending to both sides (north and south) are also drawn red, as their vertical support depends on G77-80 being vertically supported, which it isn't.
The LS-DYNA model starts running at a time t=0 without loads and damages. During the first 8.5 seconds, NIST applies gravity, external damage and temperatures and allows for the structure to settle. This is described in Section 12.3.2 on page 563 (p629 of the PDF) and following.
Then, at 8.5 s, NIST applies all the Case B fire damage that ANSYS determined after 4 h (Section 11.3.2, start at page 503), to the LS-DYNA model.
Failed connections are let fall.
Figure 12-42 shows the result at 9.5 seconds - 1 s later:
Note that this Figure shows floor structures from both floor 13 and floor 14 dropping on the respective floor below inside the area circled red on the right.
Figure 12-43 then shows the situation another 6 seconds later:
(Note that this image is only a PART of Figure 12-43 - the rest of the Figure has plots with the vertical stress and displacement of columns 79, 80 and 81).
In this image, the floor failures on floor 13 and floor 14 have already reached floor 5.
I am wondering:
- What if only floor 14 had collapsed - or had collapsed ahead of the floor 13 G79-44? Could this have triggered the fl13 G79-44 collapse even if that girder had not walked off?
- Has NIST, or anyone, considered this?
- Is that a scenario that we could ask Dr. Hulsey to check out?
- How appropriate was if of NIST to let all ANSYS failures occur at the same time in LS-DYNA, given that they most likely would have occurred at different points in time?
via International Skeptics Forum http://ift.tt/1QSo4l3
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