WTC7: Is AE911's (and NIST's) Focus on A2001 Justified if it Was Not "Key" in NIST's Global Model?

[Mick West]

Source: https://www.youtube.com/watch?v=u1CZmtR8gno&feature=youtu.be&t=21m21s

(Skip to 21:21 if not auto-started there)

This first post is the transcript, with minor asides omitted and some slides added for context. Discussion to follow.

From The JB Podcast Episode 22 Mick West & Tony Szamboti Debate What happened to WTC 7 DEBATE, 21:21

Mick: The NIST study, the ANSYS simulation came up with a set of damage that the fires did to the building, and you can see here these little black dots are what they refer to as being "Full Connection Damage" and these beams here, the red beams, are beams that have suffered significant damage themselves. So this is Column 79 over here. ... There's damage that is shown throughout the building.

And when they took this pattern of damage from this simulation here, the ANSYS simulation, and then they applied it to this full height model, it caused a collapse.

Mick: But it didn't actually use that particular connection that you just talked about [A2001 to C79] in this simulation.

Tony: Oh sure, sure it did, sure it did. It had to.

Mick: No, it didn't

Tony: It had to Mick.

Mick: But it didn't though.

Tony: Well what did they use? Tell me that they used then Mick, because they say in Chapter 12 they use what they got from the ANSYS model.

Mick: They did, they used that damage pattern. But that damage pattern didn't have that particular ... column connection failure, A2001 to column 79, That wasn't the first thing that actually failed. There was actually a number of simultaneous collapses.

Mick: Which you can see here, there's collapses over here and over here [indicates the two circled areas above]. This say the "ANSYS-based damage application is resulting in floor structure failures around Column 79 to 81", which as you know is all the interior East side of the building, so they didn't actually..."

Tony: Well where is that, ... they haven't told us that in the report. [Mick note: this IS from the report]

Mick: It's quite apparent though, when the girder falls,

Tony: you can't just show ... we want an analysis ... this is not an analysis

Mick: If you look at this particular image here, which people are familiar with, the girder in question, A2001, I think is somewhere over here, it didn't actually fall down through the other floors.

Tony: You need to justify, when you give a presentation like this, and you're making a claim, you have to justify it, you can't just say this is what we found, ...

Mick: You were saying you found that some particular thing didn't happen earlier, I think if I did the same ... But this isn't really something that's actually up for debate really. We know that the ANSYS damage contained other connection failures.

Tony: No, we don't know! They haven't released their data.

Mick: well, look at this slide here:

Mick: There's two areas in which collapse is happening simultaneously. Now this collapse over here toward the center of the building is completely separate from this collapse over here.

Tony: Mick, if they left off the stiffeners which prevented the failure I showed earlier...

Mick: No, I agree with that Tony...

Tony: .. who's to say they might not have left off something here?

Mick: Yes, they may well have done, but my point here is that you are focussing just on this one connection. And your analysis of that one connection is pretty good. Because it's quite comprehensive, and you did identify a number of things, like the stiffener plates that were missing, and the differences in the width of the plate, and other things like the amount of thermal expansion, things like that, it's all valid. And that's something that in that limited case does actually make a difference. But what we've got to look at is the actual global case. And when NIST did it they didn't use just that one collapse of that one girder. They used a large number of other collapses of other girders.

Tony: I hear what you are saying. I saw you say this on the internet, okay, recently. But you have to be specific. You can't tell people we found some other areas where it collapses - well how did that happen? You have to show. They are not showing that.

Mick: That's what came out of their simulation.

Tony: Well, I want to see the results, the simulation is going to show the stresses ...

[J.B. asks about missing data]

Tony: I'm questioning why Mick supports it, if he doesn't have a basis of justification for supporting it.

Mick: Right, well what I'm trying to explain to people is your criticism of the NIST report hinges around this on connection, the connection of girder A2001 to column 79, and the fact that you think it couldn't get off the seat, and if it did then it wouldn't damage the floors below. However, that's not what NIST are claiming in the global model. They claim that, they describe that as a possible initiating event, or a probably initiating event, because they did some simulations of just that one thing because it seemed like a likely thing.

Tony: It's the main hypothesis Mick.

Mick: Now, you've identified some problem with the simulation, and that's perfectly valid, that we should look at whether that's actually possible, if that girder did actually go off its seat or not. And it's possible that it did not. That is something that should be looked into. However that doesn't invalidate the entire study because the actual simulation that they used did not that as a necessary component.

 

[Mick West]

So I was a bit surprised at this bit of the conversation with @Tony Szamboti

Mick: But it didn't actually use that particular connection that you just talked about [A2001 to C79] in this simulation.

Tony: Oh sure, sure it did, sure it did. It had to.

Mick: No, it didn't

Tony: It had to Mick.

Mick: But it didn't though.

Tony: Well what did they use? Tell me that they used then Mick, because they say in Chapter 12 they use what they got from the ANSYS model.

I think there's a general misconception that in NIST's modeling this:

Fed directly into this:

and then into:

But it didn't, in fact in the 16 floor ANSYS model that Tony refers to, that connection did not actually initially fail.

This has been discussed at some length years ago, but I think it would be useful to get a more definitive description of what's actually going on. Here's the old discussion, prompted mid-thread by @gerrycan posting a video tracing A2001 as it fell in the global model.
https://www.metabunk.org/posts/69512/

Source: https://www.youtube.com/watch?v=VQkylMIuH-g


In subsequent posts I start out thinking NIST never said A2001/C79 failure was an initiating event, but then discovered some places where they did (obviously). However in large sections of the various NCSTAR reports the initiating event is discussed just in general terms as failure of the floors around C79-C81. Like in this post:

"Eventually falls". I'd agree with that. But the question here is what is the initiating event for the building collapse? Your video seems to show that girder did not have much to do with it.

NIST says the initiating event was the buckling of column 79. They list a great many things leading up to that, but basically the collapse of the floors around it. Now going by just the written report, and the simulations, what made the floors collapse?

I must admit here I had assumed it was the 79-44 girder falling, and I thought that was the initiating event. But upon reading the relevant sections of the report in depth, and viewing your helpfully highlighted video, that really does not seem to be the case. Working backwards from the actual initiating event (C79 buckling), we have the "Initial Local Failure", which is:

1-9 572 (pdf 638).

Initial Local Failure for Collapse Initiation

The global collapse analysis calculated a sequence of events that resulted in the buckling of critical
Column 79.

The floor framing structure was thermally weakened at Floors 8 to 14, with the most substantial fireinduced
damage occurring in the east region of Floors 12, 13, and 14. Even though each floor had been
weakened over hours of exposure to separate and independent fires, it was not until there was substantial
damage to the long span floors in the northeast region of Floor 13 that the initial failure event, i.e.,
buckling of Column 79, was triggered.

After the fire-induced ANSYS damage was applied, floor sections surrounding Columns 79 to 81 on
Floors 13 and 14 collapsed to the floors below, as shown in Figure 12–42. The LS-DYNA analysis
calculated the dynamic response of the structure to the floor failures and resulting debris impact loads on
the surrounding structure. The thermally weakened floors below Floors 13 and 14 could not withstand
the impact from the collapsing floors, resulting in sequential floor collapses. The floor systems
progressively failed down to Floor 5, where the debris accumulated, as shown in Figure 12–43.

What was the ANSYS damage?
1-9 505 (pdf 571)

Summary. After 4.0 h of heating, Columns 79, 80, and 81 had lost lateral support in the north-south
direction at Floor 13, due to failure of the girders between the columns. The girders between Columns 80
and 81 had buckled and the girders between Columns 79 and 44 and Columns 26 and 81 had walked off
the bearing seat at Column 79 and 81, respectively. In addition, all of the bolts had sheared at Column 79
on Floor 14, and two to three bolts had sheared on Floor 12. Approximately one-half to three-quarters of
the east floor beams had a connection damage index of 0.75-0.99 on Floors 11, 12, and 14 and all of the
east floor beams had failed on Floor 13. After 4.0 h of heating, there was substantially more damage and
failures in the WTC 7 structural system than after 3.5 h of heating. Columns 79, 80, and 81 had lost
lateral support at one or more floors.

The impact of a floor section falling on the floor below was not analyzed in the 16 story ANSYS model,
but was simulated in the 47 story LS-DYNA model (Chapter 12).

Perhaps I'm missing something here? But the eventual falling of the girder seems like a consequence of the flooring system collapse, and not the cause.

This all feels like partially covered old ground, and I'm likely missing something. But it's Saturday evening, and I shall have to continue later.

 

[Christopher 7]

I gave you the NIST quote than confirms the walk-off of the girder at column 79 was the event that triggered the cascade of floor failures.
https://www.metabunk.org/posts/211464/
Here it is again.

The simple shear connection between Column 79 and the girder that spanned the distance to the north face (to Column 44) failed on Floor 13. The connection failed due to shearing of erection bolts, caused by lateral thermal expansion of floor beams supporting the northeast floor system and, to a lesser extent, by the thermal expansion of the girder connecting Columns 79 and 44. Further thermal expansion of the floor beams pushed the girder off its seat, which led to the failure of the floor system surrounding Column 79 on Floor 13. The collapse of Floor 13 onto the floors below—some of which were already weakened by fires—triggered a cascade of floor failures in the northeast region. This, in turn, led to loss of lateral support to Column 79 in the east-west direction over nine stories (between Floors 5 and 14). The increase in unsupported length led to the buckling failure of Column 79, which was the collapse initiation event.

NCSTAR 1-9 Vol.2 pg 611 [PDF p. 677]
There were other failures but they did not trigger a progressive collapse. Without the walk-off of that girder between columns 79 and 44, the cascade of floor failures does not begin.

mod add link http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=861611

 

[deirdre]

I gave you

you've been a member for 3 months. Mick is talking about realizing this in 2013. I think you are missing the point of his question.


the global model shows the floor could have collapsed without the 79-44 walk off. (although when I picture the entire structure and read EVERYTHING nist says about what was happening prior, I don't think Tony's claim that it couldn't have walked off is correct anyway, but that's me.)

 

[Christopher 7]

you've been a member for 3 months. Mick is talking about realizing this in 2013. I think you are missing the point of his question.


the global model shows the floor could have collapsed without the 79-44 walk off. (although when I picture the entire structure and read EVERYTHING nist says about what was happening prior, I don't think Tony's claim that it couldn't have walked off is correct anyway, but that's me.)

Mick made a statement, he did not ask a question.

Mick: But it didn't actually use that particular connection that you just talked about [A2001 to C79] in this simulation.

That's the same claim he made in September and it is not true. The NIST quote I posted clearly says that the walk-off of the girder at column 79 is what triggered the cascade of floor failures that led to the buckling of column 79. They offer no other explanation as only one is necessary. The collapse never would have started without that girder falling and starting the cascade of floor failures around column 79.

Tony pointed out several reasons why the NIST scenario is impossible. The most damning is the blatant fraud of leaving out the web stiffeners that would have prevented the bottom flange from folding.

 

[deirdre]

Mick made a statement, he did not ask a question.

the thread title is the question.

 

[Mick West]

Trying to find exactly where in the NCSTAR 1-9 report this connection acquired such significance. Here it is discussed in the context of the damage from the ANSYS model in section 11.4.1 Discussion of Results for Case B Temperatures. There's quite a large list of failures.

11.4.1 Discussion of Results for Case B Temperatures
The fires on Floors 11, 12, and 13 generated significantly more heat than the fires on Floors 7, 8, and 9.
This was in large part due to the higher fuel load and the larger concurrent burning area on Floors 11, 12,
and 13.
After 3.5 h of heating, the fire had damaged the floor beams, girders and floor slab sections on Floors 8,
12, 13, and 14. The girders bracing interior Columns 79, 80, and 81 were still intact, except for the girder
between exterior Column 26 and interior Column 81, which had buckled and walked off of the bearing
seat on Floors 13 and 14. At the seated connection at Column 79, all the bolts had sheared on Floor 13
and two to three bolts had sheared on Floors 12 and 14. Approximately one-half to three-quarters of the
east floor beams had a connection damage level of 0.75-0.99 on Floors 11, 12, 13, and 14.
By 4.0 h of heating, there was substantially more damage in the WTC 7 structural system, particularly the
loss of lateral support to Column 79 after the failure of girder connections at Floors 10, 11, 12, and 13.
Floor beams, girders and floor slab sections at Floors 8, 10, 11, 12, 13, and 14 were damaged in the
vicinity of Column 79, primarily from the effects of thermal expansion within the structural system. All
the north-south girders framing into Columns 79, 80, and 81 on Floor 13, and the east floor beams, had
been damaged. The girder between Columns 44 and 79 had walked off the bearing seat at Column 79 on
Floor 13, and all 4 bolts had failed on Floor 14 and two to three bolts had failed on Floor 12 at this seated
connection. The girder between Columns 79 and 80 and Columns 80 and 81 had buckled on Floor 13.
The girder between Columns 81 and 26 had buckled and walked off the bearing seat on Floor 12.
Approximately one-half to three-quarters of the east floor beams had a connection damage level of 0.75-
0.99 on Floors 11, 12, and 14.

Thermal Effects on Columns

None of the columns reached temperatures over 300 °C and, therefore, did not buckle due to fire-induced
thermal weakening (thermal weakening occurs at temperatures greater than about 500 °C). The interior
columns were not thermally restrained, so they could not develop increased loads due to thermal
expansion. The exterior columns had some restraint to thermal expansion, due to the moment frame
construction of the exterior framing. However, the exterior columns tended to have lower temperatures
than the interior columns as they were only heated on one side and the heat was dissipated to the outside.
Thermal expansion of either interior or exterior columns had little or no effect on the failure mechanisms
that occurred in the floor systems.

Thermal Expansion Effects on Shear Studs

Before the floor beams could exert an axial force on the interior girders, the shear studs had to have
failed. Shear stud failures in WTC 7 were found to be primarily due to differential thermal expansion
effects as the floor beams heated more quickly than the concrete slab. The capacity of a shear stud
connector in a concrete slab was found to be 19.4 kip (Section 11.2.3). For 28 shear studs on each floor
beam, the shear force required to fail all of the shear studs was approximately 545 kip.

The interior girders connecting Columns 44, 79, 80, 81, and 26 were non-composite and floor beams
framed into one side of these girders. The less stiff interior girders allowed the floor beams to thermally
expand toward the girders, but the concrete slab was restrained by the surrounding floor slab. When a
northeast corner floor beam was heated to an average temperature of 100 °C, the floor beam exerted an
axial force of up to 950 kip on its shear studs, significantly grater than the capacity of 545 kip.

Shear stud failures also occurred when fires caused local thermal expansion in the concrete slab (the floor
surface where the fire was burning) such that compressive failure by concrete crushing occurred in the slab,
causing the shear studs to became ineffective. This type of failure occurred in the southwest floor slab of
Floor 11 where fires burned on the slab surface. There were fin connection failures along the girder between
Columns 26 and 81 and a buckled floor beam, even though there were no fires on Floor 10.
Once the shear studs failed, the floor beams were no longer laterally restrained. Clearly, this was the case
if the shear stud connector failed at its weld. If a shear connector failed through crushing of the concrete
surrounding the stud, there would also be little resistance to lateral movement as the cone of failed
concrete would pull out of the slab when the beam rotated about its flange tip.

Review of the literature did not find much data that documented shear stud failure in composite floor
assemblies subjected to fire. Fire tests of composite floor assemblies typically have the same, highly
restrained boundary conditions for the floor slab and the steel floor beams. It is not likely that shear stud
failures would be observed in Standard Fire Tests of floor assemblies since the beam is wedged tightly to
the reaction frame and the concrete slab is cast against the frame, resulting in the condition where neither
the beam nor slab can expand. Thus, there is no differential displacement between the beam and the slab,
and no shear transfer.

If such tests had been conducted where the concrete slab was laterally restrained (representing the
restraint of the surrounding slab) but the floor beams were allowed to elongate thermally with minimal
restraint (representing the weak axis of the wide flange girder), the shear studs would likely have failed.
The behaviour of composite floor assemblies when exposed to fire is strongly dependent on the boundary
conditions and degree of thermal restraint imposed by the member connections and surrounding structure.

Thermal Effects on Floor Beams and Girders

Figure 11–46 shows temperatures in the steel framing for Floor 13 at 3.0 h, 3.5 h, and 4.0 h (graphics are
from Chapter 10). Figure 11–47 and Figure 11–48 show the floor beams and girders that had failed at
different points in time as the temperatures were applied. Beams and girders subject to fires were loaded
and/or displaced by the combined effects of gravity loads from the floor slab and compressive axial forces
from restrained thermal expansion. At temperatures below approximately 400 °C (when averaged over
the beam length), thermal expansion effects caused two types of failures in the floor beams and girders:
the buckling of beams and girders and the failure of end connections (discussed in the next section). At
temperatures greater than 400 °C, the strength and stiffness of the steel floor beams, girders, and their
connections began to degrade.

Figure 11–47 shows the east floor beam failures, due to buckling and connection failures, that occurred
between 3.25 h and 3.5 h on Floor 13. Comparison of Figure 11–46 and Figure 11–47 shows that floor
beams had failed prior to the beam temperature reaching 400 °C.

Buckling in the floor beams was due to the combined effects of (1) loss of lateral restraint, (2) increased
axial loads due to thermal expansion effects, and (3) gravity loads from the floor slab. Floor beams lost
lateral restraint when the majority of their shear stud connections failed, either by differential thermal
expansion between the steel beams and the concrete slab, or by local concrete failure due to fires on the
floor slab.

Primarily for the east tenant floor, when a floor beam thermally expanded, the beam displaced the girder
at the interior end of the floor beam but did not displace the exterior frame at the other end of the floor
beam. The exterior frame underwent minimal lateral displacement when floor beams thermally
expanded, since the exterior framing with moment connections was much stiffer than the interior girders.
In comparison, the girders were simply supported, laterally restrained by the floor beams only as they did
not have shear stud connections to the slab, and laterally loaded by the floor beams in their weak axis
(Section 8.7.4). Many of the east floor beams on Floors 12, 13, and 14 failed by buckling, as shown in
Figure 11–27 and Figure 11–35.

The girder between Columns 26 and 81 buckled and walked off the bearing seat between 3.25 h and 3.5 h.
In a similar fashion, the girders between Columns 79 and 80 and Columns 80 and 81 buckled and the
girder between Columns 44 and 79 buckled and walked off the bearing seat between 3.7 h and 4.0 h.
Girder buckling was due to the combined effects of (1) gravity loads from the floor beams, (2) lateral
westward displacement due to the thermal expansion in the east floor beams, and (3) increased axial loads
due to thermal expansion in the girder.

Thermal Effects on Connections for Floor Beams and Girders

Thermal expansion of beams and girders also caused connection failures. Restrained thermal expansion
of steel beams and girder within the structural system resulted in (1) bolt shear due to increased axial
forces, (2) walk off of seated connections after bolts had sheared, and (3) failure of connection welds in
shear.

Shear failure of all the bolts in fin and knife connections, or failure of the weld at the beam and girder
web in header connections, resulted in a loss of horizontal and vertical support to the beam or girder. In
seated connections (SWC, STP, and STC), the shear failure of bolts at the bearing seat and top clip or
plate, caused loss of horizontal support but not vertical support. When four bolts at the seated connection
were sheared due to thermal expansion effects in the east floor beams and girders, there was a loss of
horizontal support at the connection. Loss of vertical support occurred when the beam or girder walked
off the bearing seat or when the bearing seat weld failed.

Walk off occurred when beams that framed into the girders from one side thermally expanded and the
resulting compressive forces in the beams pushed laterally on the girder from one side, sheared the bolts
at the seated connection, and then continued to push the girder laterally until it walked off the bearing
seat. A girder was considered to have lost vertical support when its web was no longer supported by the
bearing seat. The bearing seat at Column 79 was 11 in. wide. Thus, when the girder end at Column 79
had been pushed laterally at least 5.5 in., it was no longer supported by the bearing seat. Additional
factors that contributed to this failure were the absence of shear studs on the girders that would have
provided lateral restraint and the one-sided framing of the northeast corner floor beams that allowed the
floor beams to push laterally on the girder due to thermal expansion.

On Floors 10, 11, and 12, tensile failure of knife connections occurred in the girder between Columns 76
and 79. The temperature of the girder between Columns 76 and 79 on Floor 13 was sufficient to displace
Column 76 to the west and Column 79 to the east. The forced displacements at Floors 10, 11, and 12
created a tensile load in the girder knife connections to the columns, and failed the connection fillet weld
to the column.

Thermal Effects on Concrete Floors

Thermal expansion of concrete floors was restrained by the surrounding unheated slab sections, the
interior and exterior columns, and shear studs at the floor beams and exterior spandrel beams. Restraint
of thermal expansion led to compressive failure of the concrete slab through crushing, which softened the
slab in tenant floor areas, and also led to loss of composite action with the floor beams. This failure
mechanism usually occurred at slab surface locations where fires were burning, which led to much higher
slab temperatures in a localized area.
Slab tensile failures were related to the response of floor beams and girders to thermal effects. When
beams and girders failed at their connections, affected slab section cracked under tensile stresses where
reverse curvature in the slab occurred over adjacent intact girders.

Then in Section 13.1 we have:

11. Initial Local Failure for Collapse Initiation. The simple shear connection between Column 79
and the girder that spanned the distance to the north face (to Column 44) failed on Floor 13. The
connection failed due to shearing of erection bolts, caused by lateral thermal expansion of floor
beams supporting the northeast floor system and, to a lesser extent, by the thermal expansion of
the girder connecting Columns 79 and 44. Further thermal expansion of the floor beams pushed
the girder off its seat, which led to the failure of the floor system surrounding Column 79 on
Floor 13. The collapse of Floor 13 onto the floors below—some of which were already
weakened by fires—triggered a cascade of floor failures in the northeast region. This, in turn, led
to loss of lateral support to Column 79 in the east-west direction over nine stories (between Floors
5 and 14). The increase in unsupported length led to the buckling failure of Column 79, which
was the collapse initiation event.

• The failure of the girder-to-column connections was caused primarily by the thermal
expansion of the large span-length northeast floor beams. Additional factors that contributed
to the failure were the absence of girder shear studs that would have provided lateral restraint
and the one-sided lateral support to the girder provided by the northeast corner floor beams.
• In addition to the failure of the connection to Column 79 for the girder spanning to Column
44 on Floor 13, which had the most severe fire condition, the same connection on Floors 8 to
12 were partially damaged due to the failure of some or all of the four bolts. Complete failure
of the connection required girder walk-off in addition to the failure of all four bolts

And yet this is not reflected in the LS-DYNA model which is supposedly initiated with the Case-B 4.0h ANSYS damage. Here's a more detailed tracking of A2001 (green) in the LS-DYNA full height model.

Source: https://youtu.be/cqh1Ye3Mt9s

 

[Mick West]

The progression of damage in the LS-DYNA global model is described in NCSTAR 1-9, Chapter 12, section 12.4.4 (page 572, pdf 638)

Initial Local Failure for Collapse Initiation

The global collapse analysis calculated a sequence of events that resulted in the buckling of critical
Column 79.

The floor framing structure was thermally weakened at Floors 8 to 14, with the most substantial fireinduced
damage occurring in the east region of Floors 12, 13, and 14. Even though each floor had been
weakened over hours of exposure to separate and independent fires, it was not until there was substantial
damage to the long span floors in the northeast region of Floor 13 that the initial failure event, i.e.,
buckling of Column 79, was triggered.

After the fire-induced ANSYS damage was applied, floor sections surrounding Columns 79 to 81 on
Floors 13 and 14 collapsed to the floors below, as shown in Figure 12–42. The LS-DYNA analysis
calculated the dynamic response of the structure to the floor failures and resulting debris impact loads on
the surrounding structure. The thermally weakened floors below Floors 13 and 14 could not withstand
the impact from the collapsing floors, resulting in sequential floor collapses. The floor systems
progressively failed down to Floor 5, where the debris accumulated, as shown in Figure 12–43.

Figure 12–43 shows a cutaway view that illustrates the structural condition surrounding Column 79 as it
buckled to the east. Figure 12–43 includes resultant lateral displacements and column axial stress
histories for Columns 79 to 81. The plots show that Column 79 buckled 1.3 s before the kink was
observed in the east penthouse roof framing (which occurred at 0 s (16.0 s) in the analysis). The rapid
increase in lateral displacement indicates column buckling. Likewise, the axial column stresses indicated
a rapid loss of stress at the time, signifying column buckling. Column 79 was laterally unsupported in the
east-west and south directions between Floors 5 and 14. There was still some lateral support in the north
direction at Floors 8 to 12 and Floor 14, as the erection bolts in the seated connections had all failed at
these girder ends, but the girders had not walked off the bearing seats. Thus, these girder connections
possibly prevented lateral displacement in the north direction, but did not resist lateral displacements in
the east, west, or south direction. At Floor 14, the girder to the west of Column 79 (between Column 79
and 76) was still attached and provided lateral restraint from the west. Figure 12–44 illustrates the girderto-
column connection status at the time of buckling of Column 79.
...
The buckling failure of Column 79 was the initial failure event that led to the global collapse of WTC 7,
as will be shown in the subsequent sections.

This matches what we see in the animation.

 

[Mick West]

To be clear here, in the close up animation the exterior walls and columns are not show. A2001 goes from 79 to the exterior wall column 44, which is (from this view) to the right. You can only first see it here, which I unfortunately missed when coloring it in.
View attachment 31054

Then you see more in the next frames
View attachment 31055


View attachment 31056

At this point it's still attached to C79 and C44, just sagging
View attachment 31057

There's some valid criticism of NIST here. Saying that A2001 initiated the collapse, but not actually showing it in the their global model. Instead they seem to be dumping a bunch of accumulated damage in at once, and the collapse happens without A2001 being the cause.

Note when they refer to thing on floor 13, that means on the floor of that floor. The girder on floor 14 from C79 to C 44 does not fall until the columns buckle:

 

[Oystein]

Hehe I opened a thread on this very topic at ISF (formerly JREF) a bit over two years ago - but it didn't get anywhere:
http://www.internationalskeptics.com/forums/showthread.php?t=304531

I raised the problem that NIST incurs all of the ANSYS connection failures at the exact same instance in the LS-DYNA model, when in fact they would have occurred at different points in time. This changes the dynamics of the collapse - significantly, I'd guess.

 

[Mick West]

Hehe I opened a thread on this very topic at ISF (formerly JREF) a bit over two years ago - but it didn't get anywhere:
http://www.internationalskeptics.com/forums/showthread.php?t=304531

I think it's all a bit greek to most people.

I raised the problem that NIST incurs all of the ANSYS connection failures at the exact same instance in the LS-DYNA model, when in fact they would have occurred at different points in time. This changes the dynamics of the collapse - significantly, I'd guess.

Quite significantly. As it is you get a simultaneous buckling failure of SIX girders, none of which is A2001 (79-44).

There's also a failure of 81-80 on floor 12, but that does not seem to buckle.

So the LS-DYNA model does not really work well as a continuation of the ANSYS model. The sudden dumping of damage is not realistic. But those are the types of shortcuts when the model takes 8 weeks to run a full 25 second simulation.

It demonstrate that progressive collapse of those floors leads to global collapse. But does not gel with the narrative of A2001 being a "key girder".

I'm surprised that Tony was not familiar with this, and that it's not been raised more often. When I've raised it with @Gerry can in the past he's just said things along the lines of "that's how crap the model is".

It's odd, as it seems like a big deal, and clearly people have noticed it before. Maybe it's just too difficult to explain to people.

There's no detailed NIST analysis specific to A2001 walk off, is there? It's just a product of the 16-floor ANSYS model?

 

[Mick West]

Not only does the LS-DYNA model seem to show A2001 not failing, the graphical representation of the output of the ANSYS model does not seem to show it either:

Section 11.3

A damage index was computed for the horizontal and vertical direction (shear and axial loads) for each
connection. Failure of fin, knife, and header connections resulted in a loss of support for both horizontal
and vertical directions, whereas, loss of horizontal and vertical support in the seated connections (SWC,
STC, and STP) occurred separately. For example, if all the bolts in a fin connection were sheared, the
connection would be fully damaged (failed with a damage level of 1.0) in both the horizontal and vertical
directions. However, if all the bolts of a seated connection were sheared, the connection would be fully
damaged in the horizontal direction, but not in the vertical direction as the beam or girder would still be
resting on the seat. Failure in the vertical direction would occur when either the member supported by the
connection walked off the seat or the weld at the seat failed.

The damage state of connections, floor beams, and girders for each floor is illustrated in Sections 11.3.2
and 11.3.3. Connection damage is shown separately for the horizontal and vertical directions. For
example, in Figure 11–22 (first graphic in Section 11.3.2), tan dots represent connections without any
damage and black dots represent connections with full damage for a given direction. The other colors
indicate the degree of damage to the connection at a given time in the analysis. For example, a blue dot,
with the degree of damage labeled as "0.5 to 0.74," represents a 6-bolt fin connection with 4 failed bolts,
which gives a damage index of 0.66. Connection damage was typically gradual, with bolts and/or welds
failing sequentially over time. In addition to showing connection damage, these graphics also present the
floor beam and girder damage, where buckled beams are pink lines and failed end connections are red
lines.

(This is the diagram from the draw report which used nicer vector graphics, but the data is the same)

Notice in the horizontal damage it's marked as "Full Connection Damage"

But in the vertical, it is marked as "No connection damage".

So it looks like according to the 16 floor ANSYS simulation, A2001 did not walk off its seat.

So why did NIST say that it did?

In addition, all the girders along 79-81 are marked as Buckled/Failed.

 

[Mick West]

I said the draft graphics above were the same as the final graphics. but it turns out not. Here's the final report's version:

This is a rasterized version of the vector graphics, done so the pdf displays faster, but it makes the dots look jagged here because they did not use anti-aliasing.

However that allows us to see that they manually changed the vertical connection status at C81 and C79. As they were added with a circular brush in a program like Photoshop they have anti-aliased edges, meaning soft grey pixels.

However, the Summary description is unchanged in both versions, describing two girders as having walked off the bearing seat.

Summary. After 4.0 h of heating, Columns 79, 80, and 81 had lost lateral support in the north-south
direction at Floor 13, due to failure of the girders between the columns. The girders between Columns 80
and 81 had buckled and the girders between Columns 79 and 44 and Columns 26 and 81 had walked off
the bearing seat at Column 79 and 81, respectively. In addition, all of the bolts had sheared at Column 79
on Floor 14, and two to three bolts had sheared on Floor 12. Approximately one-half to three-quarters of
the east floor beams had a connection damage index of 0.75-0.99 on Floors 11, 12, and 14 and all of the
east floor beams had failed on Floor 13. After 4.0 h of heating, there was substantially more damage and
failures in the WTC 7 structural system than after 3.5 h of heating. Columns 79, 80, and 81 had lost
lateral support at one or more floors.

This piece of consistency is then spoilt by the LS-DYNA model, where those two connections remain firmly attached.

All seems rather messy.

 

[Mick West]

A manual change was also made to the C81 connection (girder from C26). This appears to be the only other one.

The Case C damage state diagram seem unchanged.

 

[Mick West]

I've added "(and NIST's)" to the title of this thread, as it's not really clear if A2001@C79 failed in the ANSYS OR the LS-DYNA simulations.

 

[Mick West]

One must also not forget the other simulation NIST did involving A2001, section 8.8, simulation that results in the girder NOT walking off, but failing via buckling.

This one was described as:

The predicted response of the system is summarized in Table 8–2. The first failures observed were of the
shear studs, which were produced by axial expansion of the floor beams, and which began to occur at
fairly low beam temperature of 103 °C. Axial expansion of the girder then led to shear failure of the bolts
at the connection to Column 79; and, at a girder temperature of 164 °C, all four erection bolts had failed,
leaving that end of the girder essentially unrestrained against rotation. Continued axial expansion of the
floor beams pushed the girder laterally at Column 79, as shown in Figure 8–26, in which failed shear
studs and bolts were evident. When the beam temperatures had reached 300 °C, all but three shear studs
in the model had failed due to axial expansion of the beams, leaving the top flanges of the beams
essentially unrestrained laterally. Continued axial expansion of the girder caused it to bear against the face
of Column 79, generating large axial forces that led to failure of the bolts connecting the girder to Column
44. When the girder temperature had reached 398 °C, all four erection bolts at Column 44 had
failed, leaving the girder essentially unrestrained against rotation at both ends. After failure of the erection
bolts in the seat at Column 44, continued axial expansion of the floor beams pushed the girder laterally,
where it came to bear against the inside of the column flange. Axial compression then increased in the
floor beams, and at a beam temperature of 436 °C, the northmost beam began to buckle laterally.
Buckling of other floor beams followed as shown in Figure 8–27 (a), leading to collapse of the floor
system, and rocking of the girder off its seat at Column 79 as shown in Figure 8–27 (b). The collapse
process took time to occur in the LS-DYNA analysis, during which the temperatures had ramped up to
their maximum values in the simulation.

This analysis is generally rejected by AE911 as the W24x55 beam next to col44 is missing some connecting beams that would have inhibited buckling. That's one of the points made by Hulsey and repeated by Tony in the "debate" with me.

However this is not something that got into the ANSYS model or the LS-DYNA.

This raises a somewhat discomforting possibility, that someone at NIST mistakenly thought that this study actually proved the push-off theory, and that mistakenly got shoehorned into the report.

This seems unlikely though, as there were three changed to the ANSYS -> LS-DYNA data, not just the one that might arise from such a mistake.

A more likely explanation is that different runs of the ANSYS model indicated the three walk-offs.

It's certainly not clear though.

 

[Nada Truther]

I think it's all a bit greek to most people.

For me it is a bit like Greek, translated into Spanish, written in Japanese, and spoken back in Klingon sign language.

Carry on, I'll just watch from here.

 

[Jeffrey Orling]

All of these models are

WHAT IF...

there is no "hard" data... some perhaps derived from videos and "historical" understanding of fire... and steel structures, statics and so on on.

Ergo it is totally absurd to take these models are representing anything other than assumptions made to create them.

Add to this that the fire event was incredibly complex and the system under stress were also extremely complex.

This amounts to debating how many angels can dance on the heat of a pin.

 

[Mick West]

All of these models are

WHAT IF...

there is no "hard" data... some perhaps derived from videos and "historical" understanding of fire... and steel structures, statics and so on on.

Ergo it is totally absurd to take these models are representing anything other than assumptions made to create them.

Add to this that the fire event was incredibly complex and the system under stress were also extremely complex.

This amounts to debating how many angels can dance on the heat of a pin.

That's not the point. The point is the high level interpretations on the model (by both AE911, and NIST themselves) don't match the models. How well they match history is another aspect entirely.

AE911 go on and on about how A2001 on floor 13 could not have walked off the C79 seat. It's a big part of their $300K study. But in the draft diagrams of the ANSYS model results, and in the actual detailed results of the Global model it did not walk off (fail vertically). Nor did it walk of in the local model of just that corner, although it did fail vertically off the other side.

NIST, in a few bits of the report describe A2001 as a key girder, and say it did walk off. The diagrams were hand edited to indicate it (and two other girders) walked. But the bulk of the reports just refer to general floor collapse.

In NIST's presentation back in Nov 2008 we have:

So there's a slide (#33) showing the girder was pushed off its seat, right next to slides (#31, #34, #35 & #36,)based on simulations in which that does not happen.

 

[deirdre]

so... Hulsey and his students wasted 2 years trying to show something that the NIST models already show? That's messed up.

and the NIST global model clearly shows that a2001 doesn't have to have fallen first for progressive collapse to have happened anyway. My poor little brain is like "huh?".

 

[John85]

That's not the point. The point is the high level interpretations on the model (by both AE911, and NIST themselves) don't match the models. How well they match history is another aspect entirely.

AE911 go on and on about how A2001 on floor 13 could not have walked off the C79 seat. It's a big part of their $300K study. But in the draft diagrams of the ANSYS model results, and in the actual detailed results of the Global model it did not walk off (fail vertically). Nor did it walk of in the local model of just that corner, although it did fail vertically off the other side.

NIST, in a few bits of the report describe A2001 as a key girder, and say it did walk off. The diagrams were hand edited to indicate it (and two other girders) walked. But the bulk of the reports just refer to general floor collapse.

So there's a slide (#33) showing the girder was pushed off its seat, right next to slides (#31, #34, #35 & #36,)based on simulations in which that does not happen.

The NIST reports do then describe several different and slightly incompatible sequences to account for the loss of support around column 79, but the buckling of this column is nonetheless meant to be what turned a series of somewhat undetermined local failures into a global, progressive collapse. It's definitely right to study column 79, and I think we can all agree that if NIST proposed a mechanism, even if only one of several, that only works if a series of errors are inputted, some significant doubt is cast on NIST's work. Are we able to dismiss this doubt by looking at other possible initiation mechanisms? Well no, because they have ignored requests to release their model, and refuse to correct their work for errors already identified.

 

[Mick West]

The NIST reports do then describe several different and slightly incompatible sequences to account for the loss of support around column 79, but the buckling of this column is nonetheless meant to be what turned a series of somewhat undetermined local failures into a global, progressive collapse. It's definitely right to study column 79, and I think we can all agree that if NIST proposed a mechanism, even if only one of several, that only works if a series of errors are inputted, some significant doubt is cast on NIST's work. Are we able to dismiss this doubt by looking at other possible initiation mechanisms? Well no, because they have ignored requests to release their model, and refuse to correct their work for errors already identified.

Their global model collapses, but it it does not use the "series of errors".

 

[John85]

Their global model collapses, but it it does not use the "series of errors".

By what mechanism? Is there any girder walk-off supported by data?

 

[Mick West]

By what mechanism? Is there any girder walk-off supported by data?

Perhaps you should re-read the thread?

NIST say that the ANSYS model showed walk-off of three girders, two on floor 13 and one on 14. This was not indicated on the damage diagram in the early draft, and was manually added to the diagram in the final report.

The global models do not have any girder walk off. All the initial failures come from buckling, which is the mechanism of the initial collapse in the global model.

 

[Christopher 7]

To be clear here, in the close up animation the exterior walls and columns are not show. A2001 goes from 79 to the exterior wall column 44, which is (from this view) to the right. You can only first see it here, which I unfortunately missed when coloring it in.
View attachment 31054

Then you see more in the next frames
View attachment 31055


View attachment 31056

At this point it's still attached to C79 and C44, just sagging
View attachment 31057

There's some valid criticism of NIST here. Saying that A2001 initiated the collapse, but not actually showing it in the their global model. Instead they seem to be dumping a bunch of accumulated damage in at once, and the collapse happens without A2001 being the cause.

Note when they refer to thing on floor 13, that means on the floor of that floor. The girder on floor 14 from C79 to C 44 does not fall until the columns buckle:
View attachment 31058

Mick, You missed something. The girder does fall about 1/10th of a second after your 3rd screen capture from the LS-DYNA simulation. (lower image)

 

[Mick West]

Mick, You missed something. The girder does fall about 1/10th of a second after your 3rd screen capture from the LS-DYNA simulation. (lower image)

I know it falls, I don't think I missed anything. It appears to be a buckling failure.

 

[Christopher 7]

One must also not forget the other simulation NIST did involving A2001, section 8.8, simulation that results in the girder NOT walking off, but failing via buckling.

This one was described as:

The predicted response of the system is summarized in Table 8–2. The first failures observed were of the
shear studs, which were produced by axial expansion of the floor beams, and which began to occur at
fairly low beam temperature of 103 °C. Axial expansion of the girder then led to shear failure of the bolts
at the connection to Column 79; and, at a girder temperature of 164 °C, all four erection bolts had failed,
leaving that end of the girder essentially unrestrained against rotation. Continued axial expansion of the
floor beams pushed the girder laterally at Column 79, as shown in Figure 8–26, in which failed shear
studs and bolts were evident. When the beam temperatures had reached 300 °C, all but three shear studs
in the model had failed due to axial expansion of the beams, leaving the top flanges of the beams
essentially unrestrained laterally. Continued axial expansion of the girder caused it to bear against the face
of Column 79, generating large axial forces that led to failure of the bolts connecting the girder to Column
44. When the girder temperature had reached 398 °C, all four erection bolts at Column 44 had
failed, leaving the girder essentially unrestrained against rotation at both ends. After failure of the erection
bolts in the seat at Column 44, continued axial expansion of the floor beams pushed the girder laterally,
where it came to bear against the inside of the column flange. Axial compression then increased in the
floor beams, and at a beam temperature of 436 °C, the northmost beam began to buckle laterally.
Buckling of other floor beams followed as shown in Figure 8–27 (a), leading to collapse of the floor
system, and rocking of the girder off its seat at Column 79 as shown in Figure 8–27 (b). The collapse
process took time to occur in the LS-DYNA analysis, during which the temperatures had ramped up to
their maximum values in the simulation.

This analysis is generally rejected by AE911 as the W24x55 beam next to col44 is missing some connecting beams that would have inhibited buckling. That's one of the points made by Hulsey and repeated by Tony in the "debate" with me.

However this is not something that got into the ANSYS model or the LS-DYNA.

This raises a somewhat discomforting possibility, that someone at NIST mistakenly thought that this study actually proved the push-off theory, and that mistakenly got shoehorned into the report.

This seems unlikely though, as there were three changed to the ANSYS -> LS-DYNA data, not just the one that might arise from such a mistake.

A more likely explanation is that different runs of the ANSYS model indicated the three walk-offs.

It's certainly not clear though.

A lot of people, including myself, were confused by the "rock off to the west" analysis. As it turns out, the "rock-off" analysis was an interim analysis. NIST carried the shear stud failures forward to the 16 story ANSYS and 47 story LS-DYNA analyses but not the "rock-off" part.

NCSTAR 1-9 p. 349 [PDF p. 393]
8.8 FINITE ELEMENT ANALYSIS OF THE NORTHEAST FLOOR FRAMING SYSTEM

A finite element analysis of the northeast corner floor system was conducted to evaluate its response to elevated temperatures and to confirm which failure modes needed to be accounted for in the 16-story ANSYS model. A finite element model of the northeast corner was developed using the LS-DYNA software that included the design details described in the previous section such as shear studs on the beams and seat connections at the girder ends and exterior ends of the beams.

…
p. 353 [PDF p. 397]

This analysis demonstrated possible failure mechanisms that were used to develop the leading collapse hypothesis further. The failure modes in this model were incorporated into the 16 story ANSYS and 47 story LS-DYNA analyses.

 

[Christopher 7]

I said the draft graphics above were the same as the final graphics. but it turns out not. Here's the final report's version:
View attachment 31063

This is a rasterized version of the vector graphics, done so the pdf displays faster, but it makes the dots look jagged here because they did not use anti-aliasing.

However that allows us to see that they manually changed the vertical connection status at C81 and C79. As they were added with a circular brush in a program like Photoshop they have anti-aliased edges, meaning soft grey pixels.
View attachment 31064

However, the Summary description is unchanged in both versions, describing two girders as having walked off the bearing seat.

Summary. After 4.0 h of heating, Columns 79, 80, and 81 had lost lateral support in the north-south
direction at Floor 13, due to failure of the girders between the columns. The girders between Columns 80
and 81 had buckled and the girders between Columns 79 and 44 and Columns 26 and 81 had walked off
the bearing seat at Column 79 and 81, respectively. In addition, all of the bolts had sheared at Column 79
on Floor 14, and two to three bolts had sheared on Floor 12. Approximately one-half to three-quarters of
the east floor beams had a connection damage index of 0.75-0.99 on Floors 11, 12, and 14 and all of the
east floor beams had failed on Floor 13. After 4.0 h of heating, there was substantially more damage and
failures in the WTC 7 structural system than after 3.5 h of heating. Columns 79, 80, and 81 had lost
lateral support at one or more floors.

This piece of consistency is then spoilt by the LS-DYNA model, where those two connections remain firmly attached.

All seems rather messy.

I'm not one to make excuses for NIST but it appears that NIST noticed or was informed of the error and corrected it.

 

[Mick West]

I'm not one to make excuses for NIST but it appears that NIST noticed or was informed of the error and corrected it.

What error? They used the vertically undamaged connections in the global model. What are you suggesting happened? That they had them undamaged in the diagrams by mistake, then corrected the diagrams, but not the data transferred to the global model.

 

[Christopher 7]

What error? They used the vertically undamaged connections in the global model. What are you suggesting happened? That they had them undamaged in the diagrams by mistake, then corrected the diagrams, but not the data transferred to the global model.

As I noted in post #25, they did use the vertical damage on the global model. As you noted,

At this point it's still attached to C79 and C44, just sagging

 

[Mick West]

As I noted in post #25, they did use the vertical damage on the global model. As you noted,

No they don't. If they did then it would have immediately fallen and not sagged.

 

[Christopher 7]

No they don't. If they did then it would have immediately fallen and not sagged.

??? You said that it was just sagging but still attached.
It was sagging as it was being pushed off the seat but it did not fall due to the sagging or it would have fallen before it came into view in the simulation.

 

[Christopher 7]

??? You said that it was just sagging but still attached.
It was sagging as it was being pushed off the seat but it did not fall due to the sagging or it would have fallen before it came into view in the simulation.

Furthermore, it is unthinkable that they would not include the trigger that started the collapse. That was a critical part of their presentation as you noted above.

Here is the video of the August 21, 2008 press conference where NIST presented its findings on the collapse of WTC 7.
Source: https://vimeo.com/11955064


Starting at 9:43, Shyam Sunder explains the NIST collapse hypothesis.

The critical part starts at 12:55 when Sunder explains the NIST hypothesis of the failure of the girder between columns 79 and 44 on floor 13 being the failure that led to the total collapse of the building.

 

[Mick West]

??? You said that it was just sagging but still attached.
It was sagging as it was being pushed off the seat but it did not fall due to the sagging or it would have fallen before it came into view in the simulation.

It's sagging in the middle. It's attached at the ends. It seems to fail because the beams pull it down. It has not failed in the vertical direction at the start of that simulation.

 

[benthamitemetric]

It is interesting that the ultimate failure mode for A2001 that seems to be shown in the LS-DYNA simulation is similar to the failure mode identified by ARUP (and later confirmed by WAI).

This thread definitely raises some good questions about the internal consistency of NIST's approach, in any event, and I think Mick has done a good job parsing through the various descriptive narratives. Given the volume of the NIST reports and how it was published in an unsearchable format, such parsing is certainly a chore.

 

[Mick West]

Furthermore, it is unthinkable that they would not include the trigger that started the collapse. That was a critical part of their presentation as you noted above.

Why is it unthinkable? Regardless, it very clearly happened. SIX other girders collapsed in the global model. A2001 only fell later.

In the frame above A2001 on Floor 13 has not yet visible moved.

I've attached a 5fps full size version of the girder tracking with is easier to frame advance.

 

[Jeffrey Orling]

If the sequence was anything like what is shown in this animation... it's absurd even to consider a single node as THE straw that broke the camel's back. It appears that there were many failures all interacting leading to the collapse. And this looks reasonable. But it could be different and ALSO look reasonable.

So where is the CD "version" of this showing how the frame came undone and the motion we saw? Was this collapse engineered? If it was then why can't truthers engineer a CD now with years to work on it???

 

[Oystein]

Haha great finds, Mick! The photoshopped connection failure dots - excellent! Makes one wonder why none of the 2964 architects and engineers who have a problem with the NIST reports ever found these problems!

My hunch is this: Backwards-engineering, they needed a failure at col 79 first, to match the observed collapse sequence. Perhaps ANSYS didn't give them enough of this, so they looked manually into the state of the key connections within ANSYS, and found some failure even though ANSYS' criteria were not triggered. That would actually be a smart way to use FEA results, though it's sensitive to bias.

At the same time, there seems to be some difficulties with editing the entire report. I'd guess that different teams worked on different chores and chapters, some of the work having been done asynchronous with the chapter sequence.

Chapter 8 has evidence for the plausibility of several failure modes.
Chapter 11 shows that multiple connection failures around columns 79-81 are to be expected from the wandering fires on many floors.
Chapter 12 shows that multiple beam failures around columns 79-81 can progress to global collapse.

The safety recommendations adress systemic failure modes and progression proneness. Those would be valid even if NIST doesn't nail the actual details of collaps initiation.

I think it is a good idea at this point to take NIST's detailed hypothesis exactly as the marketed it: Their most probable scenario at the time (almost 10 years ago), though not very probable in absolute terms.

We know there was fire
We know there was no CD
We know there was a collapse.
We thus can be sure that fires caused the global collapse - and NIST shed valuable light on how such a thing can occur, even if the ultimate cause, the straw that broke WTC7's neck, has not been demonstrated beyond doubt.

 

[Mick West]

My hunch is this: Backwards-engineering, they needed a failure at col 79 first, to match the observed collapse sequence. Perhaps ANSYS didn't give them enough of this, so they looked manually into the state of the key connections within ANSYS, and found some failure even though ANSYS' criteria were not triggered. That would actually be a smart way to use FEA results, though it's sensitive to bias.

But in the global analysis C79 still buckled first, even though the vertical connection failures were not there.

 

[Oystein]

But in the global analysis C79 still buckled first, even though the vertical connection failures were not there.

That's the global, asynchronous editing then - perhaps LS-DYNA work was already too advanced (and had eaten too much of the budget) by the time they decided to foucs on A2001?