Originally Posted by chrismohr
It's not only "not fair" to interpret the NIST report ... etc.
It is absolutely, 100% wrong to say that.
NIST doesn't say much directly, but it is easy to interpret from their data. If you know a bit about structures.
The proof is buried in the data regarding the loads held up by the peripheral columns during the progression of collapse.
There is no doubt that NIST describes the multiple phases of the collapse, and gives time references to each. It describes the horizontal progression of the collapse, and refers to it occurring on the "middle floors". (See note shown below
in the middle of Fig 12-68, NCSTAR1-9 vol 2, pg 593, pdf pg 255)
The fundamental question is: when a group of columns fails at a lower floor, can the floors & columns above be held up & supported by their lateral connections? Or will the collapse of lower columns progress to the roof?
If, and only if, the columns & floors above a lower collapse can be supported by their lateral connections, can the remaining portion of the upper block "fall as a unit".
If the columns & floors above a lower collapse can not be supported by their lateral connections, but instead collapse as the lower columns collapse, then the upper block will NOT collapse as a single unit. But will instead collapse sequentially as the lower sections' collapse progressed. This will leave a hollowed out shell throughout the upper floors, with the upper floors.
There are two unequivocal proofs that NIST says that the upper core collapsed BEFORE the outer shell (i.e., NOT as a unit), and not simultaneously with the outer shell.
The first proof (and the place where I first realized that this had to be the case) is shown in Fig 12-61 of NCSTAR 1-9 vol 2, pg. 587, pdf pg. 249. This drawing shows the loads on the outer columns (in groups).
IF the lower internal columns collapsed, but the lateral supports sustained the upper inner core, then the loads on the north & south outer columns would HAVE TO increase dramatically (about 50%), because these columns would be supporting not only their share of the upper block, but also the share that had been shed by the core.
Fig 12-61 shows that this is absolutely not the case.
The vertical collapse happens between 14 & 16 seconds on this timeline. The horizontal progression of collapse happens between 16 & 21.5 seconds.
It is clear from this graph that the south outer wall's loads (black line) stay approximately constant, while the north wall's load (blue line) actually decreases.
The simple fact that neither of these walls' loads increase by approximately 50% (south wall from ~30 to ~45 MN & north wall from ~36 to ~54 MN) PROVES that the collapse progressed from the collapse floors to the roof AS IT HAPPENED.
One might also note comment 3 in Fig 12-61: "Unloading due to interior structure falling away from external columns." This is a clear give-away as to NIST's thoughts on the matter, although they are explicitly talking about "falling away from" the north & east wall in this comment. Regardless, the structure of the building was the same on the south & west walls as it was on the north & east walls. The structural response to internal collapse would be the same as well.
As a further note, it can be seen from this chart that NIST says that the east outer wall buckled partially first, then the north wall began to buckle slowly (phase I) about 2 seconds later. About one second after that, the south & west wall buckled together (the south wall completely & the southern portion of the west wall). About a second after this, the north wall started its global collapse (phase II) along with the remaining northern half of the west wall. Finally, the remaining portion of the east wall collapsed.
Here again, NIST explicitly shows that their models do NOT show all external walls collapsing simultaneously, but rather collapsing in a complex sequence, each releasing (in 1 or 2 steps) over a period of about 5 seconds.
One last thing to note: These graphs also show that NIST's structural modelers do NOT believe that the external walls went "into free fall". If they did, these load vs. time graphs would have an infinite (straight up) slope as the load reduced instantly to zero. The slopes are steep, but they are not anything close to vertical. Especially for the north wall.
These graphs show exactly what the accurate analysis of the fall data of the north wall show: they did not fall "at free fall", but rather at an acceleration substantially less than free fall.
The second proof is the drawings of the "collapse of the core", Figures 12-66 thru 12-69. In these drawings, one can watch the horizontal progression of the core collapse, at each interval progressively taking down with it the internal upper core above the collapse zone right to the roof.
These drawings confirm completely the conclusions reached above.
It should be noted that these images do NOT show the outer shell. This is proven by the various beams & columns that continue to hang motionless, in free space, to the right of the collapse front throughout the collapse. These graphs are derived from a "physics based" program. Beams, girders & columns do not hang motionless in space unless they are attached to something. They are attached to the outer frame of the building.
Examine this drawing carefully, and compare the circled elements in the sequence below to see components that are "hanging in air".
Core Collapse sequence:
In these images, the collapse of the entire structure, floor to ceiling, under the east penthouse is complete at 2.3 seconds. (Note that these times are offset from the times noted in the graphs above. The number in parentheses above each graph corresponds to the time scale above.)
The global collapse has not yet begun at 13.5 seconds. (At 15.5 seconds, it is "underway".) So there is a stretch of 13.5 - 2.3 = 11.2 seconds during which the interior core, right to the roof, is collapsing.
This IS "what NIST says".
Regardless of Chris7's, or anyone else's, attempts to quote mine a single sentence.
Chris Mohr, I hope that this is clear and understandable. If you need any elaboration on any point, let me know.
PS. Chris7, I note well that, as expected, you simple ran from the opportunity to back up your assertions. What a non-surprise.