Appendix I: The University of Alaska Fairbanks study

Though not directly referenced in the question, this study is one of the strongest arguments used to claim that Building 7 was brought down by controlled demolition. Therefore, examination of this study is relevant to the question. The study concludes:

The principal conclusion of our study is that fire did not cause the collapse of WTC 7 on 9/11, contrary to the conclusions of NIST and private engineering firms that studied the collapse. The secondary conclusion of our study is that the collapse of WTC 7 was a global failure involving the near-simultaneous failure of every column in the building.

But the study has fatal flaws. Some of the greater examples are touched upon below.

Treating the building like infinitely strong rubber

To see how plausible NIST's progressive collapse model was, UAF ran a simulation with the NIST damage model.

From their figures 4.8 to 4.16, which shows the results of their analysis of the NIST scenario, we learn that they used a linear static analysis. What does this mean?

A linear static analysis is an analysis where a linear relation holds between applied forces and displacements. In practice, this is applicable to structural problems where stresses remain in the linear elastic range of the used material. In a linear static analysis the model’s stiffness matrix is constant, and the solving process is relatively short

Source

That means they neglected plasticity and fracture in a simulation of something being destroyed. The result: Their building was indestructible; they needed to remove columns manually at each step to keep the collapse going, as shown by the captions to their figures.

Ignoring momentum

From the above, we can see that they used a "linear static analysis". The "static" in there is particularly damning.

Linear static analysis has two main assumptions:....

• The loading is static.
  • Magnitude and direction do not change with time.
  • All loads are applied slowly and gradually until they reach their full magnitudes. Inertial and damping properties are ignored due to negligibly small accelerations and velocities.
  • Time variant loads that induce considerable inertial and/or damping forces may warrant dynamic analysis.

Source

By using this static model, they are ignoring all that momentum, which would play a pivotal role in a building collapse.

Here are some ways that ignoring momentum can invalidate their results

• Failing to predict the failure of components under sudden load redistribution. For example, the load increase on adjacent connections and columns when column a column fails could be magnified by up to two times due to the momentum of the structure displacing downward.

• Failing to predict component failure under impact loads. Momentum of falling debris can increase the force they apply several times.

The result of these errors can be seen in this video. Watch the east penthouse kink, spring back and come to a stop on the top floors of the building.

The above is not exhaustive. But it is enough to see that this "study" has no basis to conclude that "fire did not bring down WTC7".

WTC7, like most modern skyscrapers, relied on the walls to brace it against storms. Formed of columns tied with plates, these walls formed a shell around the building. The columns making up these walls in addition carried part of the building weight, along with columns located inside the building.

WTC7, like most modern skyscrapers, relied on the walls to brace it against storms. Formed of columns tied with I-beams("wind girders"), these walls formed a shell around the building. The columns making up these walls in addition carried part of the building weight, along with columns located inside the building.

Speed: Multistorey buckling

• The reaction force to the southward sway caused several lower floors to slide northward, completely undermining the building for multiple storeys.