Discussion of contents of NIST Letter of Rejection

Well, one does not necessarily equate to the other, Ultima. The problem is, we don't have a legitimate report to look at to say one way or the other whether the fires were sufficient.

[edit on 11-3-2007 by Valhall]

Originally posted by Valhall
Well, one does not necessarily equate to the other, Ultima. The problem is, we don't have a legitimate report to look at to say one way or the other whether the fires were sufficient.

I agree. So why do so many people still want to state that they know the fires brought the towers down when they do not have the evidence to support it?



[edit on 3-11-2007 by ULTIMA1]

reply to post by Valhall


1. This is making an assumption that the core columns needed to be heated and/or weakened from fire for the collapse to initiate. I don't believe this has been claimed by NIST. Given the length of the report I could be wrong. but again, I don't believe they have claimed the core columns failed due to heat and I believe they say just the opposite. The cause of failure was splice connection failures and fractures and possible direct damage from the plane impact (in WTC 2).

2. None of which were core columns.

3. Why do you say this? How many do they need? I think this argument is again based on the notion that the collapse required the core columns to be weakened from heat in order for a collapse to occur.

4. That cannot be said with any accuracy. It's not as simple as the hotter stuff was at the bottom and the cooler at the top. While that is true over all, it can't really be reliably calculated or depended upon.

5. I think the statement is being misunderstood as temperatures of 250c instead of the actual statement of "in excess of 250c". Plus the samples alone cannot be indicative of all of the steel. Just because the samples may have only been found to be a certain temperature does not mean other portions did not reach higher temperatures. Thus justifying testing under higher conditions to account for that possibility.

And even then, they still cannot determine if those temperatures were reached before or after the collapse. So it's important to test both. But only one resulted in a collapse, so it's pretty safe to say based on that that higher temperatures had to have been reached.

Photographic evidence shows steel reaching temperatures up to 600c. And again, it doesn't have to be the core columns, due to the nature of the design. It can clearly be seen that from this weakening that there were gross deformations. Inward bowing could clearly be seen. Photos compared from 30 minutes before vs 5 minutes before can show the movement of the bowing (from none to 10 inches, to 20 inches) . That of course is connected to the cores via the trusses.

6. again, in "excess" of 250c. And capped at 600c. And that still does not mean some areas got hotter. But what can't be questioned is that the outer columns were heated enough to weaken them and cause them to bow inwards. This is going to have a dramatic effect on the core columns which are connected through the trusses.

I think the big misunderstanding here is the claim that the maximum temp reached was 250c and that the core columns have to be weakened by heat for a collapse to initiate. Just to be safe, here is a quote from the report:

"..locations had a positive result indicating they may have reached temperatures in excess of 250c (note that exposure could have occurred pre or post collapse)"

I have a question for you engineer types.

If the core columns were blown would the damage witnessed be consistent with what you might expect?

It occured to me that "inward bowing" could be a result of the core column failure. The stress or pulling of the core columns causing the observed deformation and collapse.

I realize it is speculation but I would be curious to hear your opinion.

From NIST NCSTAR-1-6C:

wtc.nist.gov...

This document details the modeling structure of the analysis.


They ran the model at temperatures to at least 950 C. They did single column testing at two points - ambient and 700 C. They modeled floor trusses at at least 950 C. They, at times, refuse to give the upper limit temperature they employed in certain modeling efforts and fall back on the ambiguous term "elevated temperatures".

They used mechanical properties for all materials at elevated temperatures (at least 700 C) and when they could, employed "creep effects" if it didn't make their model unstable. Creep cannot be employed with any sanity at temperatures below 250 C, so this application must be viewed as either being applied in a rational manner at more elevated temperatures, or chaos run riot where all engineering practices have been thrown out the window. I'm assuming the former.

They increased the gravity force acting on floor trusses by a factpr of 3.4 to model "debris loading", but never state the underlying reasoning behind this 300% increase other than to state they stopped increasing it when "... the analysis failed to converge at room temperature".

They do indeed conclude core column yielding "when their temperatures exceed 600 C".

They conclude the floor trusses "walked off" the exterior floor trusses. But they also admit -

The maximum displacements at 40 min were 44 in. for the model with creep and 26 in. for the model without creep. After 40 min., the model with creep walked off the exterior truss seat, while the model without creep did not walk off the exterior truss seat..." "The maximum pull-in forces were 14 kip for the model with creep and 8 kip for the model without creep.

Again, they went with the model employing creep which took substantially higher temperatures...since they have previously stated they disregarded lesser damage models due to their failure to initiate collapse in the buildings, I'm assuming they rejected the non-creep model for the same reason. I also wonder if another letter challenging the rejecting of the non-creep model would result in more sanitizing of the report.

Lastly, in response to your statement about the sagging floors pulling in the exterior columns, NIST states in their final summary

Although the floor sagging was captured by the floor models in the heated area, the pull-in force on the exterior columns was not captured in most of the full floor model analyses. To accurately calculate pull-in forces between the floors and the exterior columns in the full floor model, much more detailed modeling will be required.

From NIST NCSTAR 1-6D:

wtc.nist.gov...

This document presents the results and final statements on how the building components/substructures failed in the modeling efforts described in 6C.

They state that their FEA models show that

...the key structural responses that led to the collapse of the towers were as follows: 1) floor sagging caused by the failure of thermally-weakened truss members, resulting in pull-in forces between the floors and the exterior wall [NOTE: EVEN THOUGH THEY HAVE ADMITTED THEY COULD SUFFICIENTLY MODEL THIS], disconnection of the floor from the exterior wall [NOTE: THIS MEANS THE CREEP MODEL AT THE HIGH TEMPERATURES BECAUSE THEY ALREADY STATED THEY COULDN'T MAKE THIS HAPPEN WITHOUT CREEP], 2) downward displacement of the core due to aircraft impact damage and shortening of the remaining core columns from increased load, plasticity, creep of steel at high temperatures [NOTE: Since the columns did not get overloaded into the plastic deformation region these two statements "plasticity, and creep of steel" are redundant. The plasticity resulted in them employing creep to achieve plastic deformation below the material's yield point. In order to employ creep they had to go to the extreme temperatures far in excess of their collected test data.]; 3) bowing and buckling of exterior walls caused by the pull-in forces and loss of lateral support from the sagged floors, and floor/wall disconnections at high temperatures; NOTE: There's the creep again and the statement they are using "pull-in forces" they already said they couldn't model.; and 4) redistribution of gravity loads among the columns locally, among the exterior walls, and between the exterior walls and the core, resulting from [THE SAME THING STATED IN THE PREVIOUS 3 FACTORS].

Once again, to make sure I drive home that not only did they state the core columns failed due to temperature-degradation of strength, but they repeatedly state due to "creep effects" which mandates they run the model at the high-end temperatures:

The subsequent fire-induced high temperatures caused the core to displace downward from plasticity and high creep strains in high stress and high temperatures.

Unloading of Core. Temperatures in the core area rose quickly, and thermal expansion of the core was greater than the thermal expansion of the exterior walls in early stages of the fire. This increased the gravity loads in the core columns until 10 min after impact. The additional gravity loads from adjacent severed columns and high temperatures caused high plastic and creep straing to develop in the core columns in the early stages of the fire. More columns buckled inelastically due to high temperatures. Creep strain continued to increase to the point of collapse. by 30 min, the plastic-plus-creep straings exceed thermal expansion strains. Due to high plastic and creep strains and inelastic buckling of core columns, the core columns shortened, and the core displaced downward.

That should put to rest any questions about what the NIST states happened to the core columns.

Again I will state what my contention is: It matters not to me what range of temperatures NIST chose to run in their model. What matters to me is that they did not publish the model that meets their actual data - temperatures that did not exceed 250 C. What matters to me is the repetitive instances in these documents of unsupported assumptions or worse yet assumptions in conflict with real data.


[edit on 11-4-2007 by Valhall]

reply to post by Leo Strauss


That's fairly close to my thinking.

Originally posted by Valhall
From NIST NCSTAR-1-6C:

That should put to rest any questions about what the NIST states happened to the core columns.

Again I will state what my contention is: It matters not to me what range of temperatures NIST chose to run in their model. What matters to me is that they did not publish the model that meets their actual data - temperatures that did not exceed 250 C. What matters to me is the repetitive instances in these documents of unsupported assumptions or worse yet assumptions in conflict with real data.

I pretty much follow what you are saying but I am not clear what your ultimate conclusion is about the towers' collapses. Are you just criticizing NIST's methods and that they have not adequately justified their conclusion on the causes of the collapses, or that the data that does exist supports a different conclusion altogether?

Both.

I contend they did a negligent job on the investigation and that the data they present in their report ranges from not supporting their conlusions to contradicting it.

Originally posted by Valhall

6. The weakening of the steel - the decrease in the yield strength at 250 C (650 F) is less than 20% of its original yield strength. With the design safety factors employed, that decrease isn't going to make a column fail.

Does this apply to the bolts and welds holding the steel together too, or just to the columns?

Is there anything published showing the yeild strength of the bolts and welds?

Yes, you can find the information about the bolts in NIST NCSTAR 1-1 wtc.nist.gov... and NIST NCSTAR 1-3 wtc.nist.gov... The bolts were much higher strength material (as is typical) than the structural steel making up the columns and floor trusses. P.S. I believe the welds are addrsesed in those sections as well, but also 1-6 talks about the welds.

[edit on 11-7-2007 by Valhall]

reply to post by craig732


Bolts and welds (connections) are designed to be stronger than the members they connect. Same as what Valhall was saying.

Does anyone care to elaborate on what we saw on 9/11? Was the collapse closer to controlled demo and Nist is trying to prove otherwise? I guess what im asking is what is it more of a textbook case of demo or collapse? Does that make sense?

I don't think that's relevant to the discussion.

What's relevant is that the report forced the models (while outright ignoring data) to a collapse caused by plane impact damage and fire. So it force the exclusion of ANY additional factors leading to global collapse.

That's what is important for this discussion.

Opinions on what model the collapse more closely matches would be better taken to separate discussions.