The 9/11 Forum

I'm very busy with my new job (8 thickbooks waiting for me...) and still haven't seen the draft, but I guess that in order for a collapse hypothesis caused by fire the fireproofing must be stripped away ?

I thought I was pretty familiar with the WTC 7 videos available. Are there any videos that corroborate NIST's model of deformation of the lower half?

Major Tom, do you know of any videos where lower deformation can be discerned?

Max

Just what you have, Max. You'd think NIST would cite the data from which it draws it's deformation models.

The comments at both JREF and 9-11 Blogger are revealing but not surprising.

I don't want to look into, at least not in the near future, but the following raises the question of how NIST took into account the exact chemical composition of the steels used in their sims. Not saying that they did or didn't do this correctly, just that this is something that, as far as I can infer, needs to be checked.


The distribution of boron in stainless steels as revealed by a nuclear technique

Boron is also used to increase the creep properties of oxidation-resisting steels and stainless steels used at elevated temperatures.3 The addition of about 50 p.p.m. leads to an increase in the mean stress-to-rupture life by a factor of 3 or an increase in stress to failure in 10,000 hours of up to 25 per cent.

Hambone wrote:NIST’s fire simulation would have us believe that a very substantial heat release rate was sustained for over 2 hours over a floor area of about 500 m2 in building 7. Thus Figure 9-13 of NCSTAR 1-9 shows that a heat release rate of 200 MW was attained on floor 12 at about 3:00 p.m. on September 11th and remained above 200 MW until well after 5:00 p.m. But we need to ask: Is a 200 MW fire consistent with a fuel loading of 32 kg/m2 - the value used by NIST for its floor 12 fire simulations? The answer appears to be no. Thus a 200 MW heat release rate for 2 hours implies a total energy release of 1,440 GJ. If the combustible material on the 12th floor of WTC 7 is assumed to release 20 MJ/kg, we have to conclude that 72,000 kg of office material was combusted over an area of 500 m2, or there was a fuel loading in WTC 7 of 144 kg/m2 – a value over four times NIST’s assumed fuel loading.

Picking out one point in an extremely lengthy account, I have some concerns with the approach here. I've had trouble downloading the NIST report, so I wanted to check: Dr. G, is the figure of 500m2 quoted from the NIST report, or estimated from the areas of the diagrams that show the greatest heat output? Given that NIST modelled the power output as a function of time and position, and determined the total output, presumably, by integrating over the entire floor in both space and time, a discrepancy of a factor of four between the correct figure and a simple estimate might well arise. It's clear to me how NIST must have (or rather, ought to have if they were carrying out their calculations correctly) arrived at their figures, so for a valid comparison I'd like to know the details of how you arrived at yours.

Since NIST's source for the 200MW number is, presumably, their own fire modelling, and this modelling is based on their own fuel loading, it would be difficult to see how any discrepancy like this could arise accidentally. Are you accusing NIST of deliberately and fraudulently misrepresenting their own results?

Dave

metamars wrote:I don't want to look into, at least not in the near future, but the following raises the question of how NIST took into account the exact chemical composition of the steels used in their sims. Not saying that they did or didn't do this correctly, just that this is something that, as far as I can infer, needs to be checked.


The distribution of boron in stainless steels as revealed by a nuclear technique

Boron is also used to increase the creep properties of oxidation-resisting steels and stainless steels used at elevated temperatures.3 The addition of about 50 p.p.m. leads to an increase in the mean stress-to-rupture life by a factor of 3 or an increase in stress to failure in 10,000 hours of up to 25 per cent.

http://wtc.nist.gov/media/NIST_NCSTAR_1 ... omment.pdf

Page 538 - 543 it isnt that much to read.

and if you want i can get you more details about the LS-Dyna standard materials they used, end of next week, when i have acces to LS-Dyna.

OneWhiteEye:

Thanks for your comment on the scaling issue. I pretty much agree with your assessment of what is being displayed in NIST's collapse simulation images. NIST do appear to mix REAL (unscaled) deflections with SCALED deflections, but this only serves to confuse to reader don't you think! Especially when NIST claim that its simulations agree with observations.

Certainly Figure 4-54 of NCSTAR 1-9A drives me nuts, because it shows a major cave-in of the upper NE portion of the building that is surely meant to be to scale, but no such cave-in was seen in that part of the building in the real collapse of WTC 7.

I am still working on this issue and hope to produce a revised comment on this if I can ever sort things out.

Dave Rogers:

Am I "accusing NIST of deliberately and fraudulently misrepresenting their own results?" NOT AT ALL! I am simply asking NIST to explain what I perceive to be a discrepancy in their Draft Report, because I have no idea how such contradictory results could be obtained. As for the 500 m^2 figure, that is my own estimate and I think it could be revised to say 750 m^2, but there are still major problems with NIST's fuel load.

My starting point is Figure 9-13 which shows a 200 MW fire on floor 12 from 3:00 pm to past 5:00 pm. Then I look at NIST's calculated floor temperatures. And by the way, NIST's floor temperatures in no way correspond to the upper layer air temperatures shown in Figure 9-11.

I get the feeling that one group at NIST did the air temperatures and another did the floor temperatures. The data was then shipped to the FEA guys and they did their thing. Some other group did the visual evidence work and another group did some collapse time calculations. Then everything was sown together like grandmother's patchwork quilt. I am sure noone at NIST read the whole Draft Report and cross-checked the data!

for a simulation of a total collapse there is no reason to use deflection scales other than 1:1

OneWhiteEye wrote:

Dictator Cheney wrote:for a simulation of a total collapse there is no reason to use deflection scales other than 1:1

Hello, Dictator Cheney, and welcome. My feelings exactly, and I think I've implied some reasons it might be bad in my questions above. I gather you work with this sort of stuff; any comments you could give that would assist me in resolving some of my confusion?

Hi and thank you for the welcome.

im not an expert, i am a layman. Sometimes i have to do some FE sims of a single solid model in SolidWorks (Cosmos) wich requires only basic knowledge in FE.

But in regards to the WTC collapses i got myself acces to Ansys and LS-Dyna , via a friends company, but i am still very new to it because of very limited acces

and about the Figs 4-43 through 4-46.
i am not sure if i did understand you correctly, my english is not very good.
i think what is confusing is the labeling.
the label implies that red is 1 m deflection, actually it is 1m and all that is above 1m. and blue -1m and all what is above
the label describes the color range, not the visual deflection of the model,
they should have choosen a wider range and more colors.

Dictator Cheney wrote:
http://wtc.nist.gov/media/NIST_NCSTAR_1 ... omment.pdf

Page 538 - 543 it isnt that much to read.

and if you want i can get you more details about the LS-Dyna standard materials they used, end of next week, when i have acces to LS-Dyna.

On p. 539 (201 in the pdf reader), we read that

For the regions of WTC 7 subjected to heating by fires (between Floors 7 and 14), termperature-dependent material models were used for the framing. The material model used for the steel in the fire-affected floors was the Elastic-Viscoplastic Thermal (Type 106) model in LS-DYNA, which included thermal expansion and thermal degradation in material stiffness and strength. This model used the same parameters to define the nonlinear material behavior of steel at room temperature as the Type 24 model, but included additional parameters to define temperature dependence. The yield strength, elastic modulus, Poisson's ratio, and thermal expansion coefficient were all identified as a function of temperature. The temperature-dependent models used the same failure criterion as that applied at room temperature. The temperature-dependent material properties and constitutive model parameters for the steels and bolts used in WTC 7 are presented in Appendix E. Example of the stress-strain curves for the 50 ksi steel at various temperatures is shown in Figure 12-2.

However, when we look at E.3.1, p. 708, we read

The WTC investigation developed a methodology for estimating the creep properties of untested steels based on creep models of existing steels.....Although tensile strength scaling produced the best agreement, in many cases the agreement was not very good. Frequently the predicted strain differed from the actual by more than a factor of ten.

Well, that doesn't sound too good. However, the very next sentence says

Differences of this magnitude correspond to temperature offsets of around 35 deg C or stress offsets of 17 MPa.

Frankly, while I have no intuition on the 17 MPa offset figure, 35 deg C doesn't sound bad, at all. So, color me confused.

However, the last sentence is also mysterious to me, and also raises doubts about how accurate their modeling was:

Because no steel relevant to the creep modeling was recovered from WTC7, it is impossible to create more accurate models.

The problems I have with this are as follows:

1) If they have accurate knowledge about exact steels used, and good models for those same steels, then if their fire models were accurate and adequate (and if they had enough computer horsepower), steel specimens would not be essential, other than as sanity checks.
2) If they are missing some essentials I list in 1), and physical specimens could have been used to provide data needed to tweak the models to make them accurate, the fact that these were not recovered means that they can't even do that! So, while NIST is implying that their models are good enough, I don't see how they can have any confidence that this is so.

Getting back to my quote, even miniscule amounts of Boron added to a steel can have a big effect on it's behavior at high temperatures. So, I don't have have a warm fuzzy feeling about using models for "similar" steels, as, e.g., is mentioned on p. 716, where they used behavior based on "Austrailian AS149 steel, which is similar to ASTM A 36"

As if that's not enough, on p. 718, we read

The models developed were technically for the steels from which the bolts were made, rather than for the bolts. Bolt failure is complex at both room- and elevated-temperature, and no methodology exists for modeling the failure of bolts, as distinct from the steels form which they are made, at elevated temperature.

(emphasis mine)

While it may not even be possible to make an accurate model of such a complex, large system as WTC 7 + fires, should NIST be claiming explanations based on what amounts, apparently, to a massive guess, just because they made an effort? Did they give themselves an 'A', just for effort?

OneWhiteEye wrote:metamars, good quotes and good points.

Thanks.