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.
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?