Finally, we examine the variation in NIST’s impact cases. For both WTC 1 and WTC 2, NIST ran three different impact cases – a baseline, a less severe case, and a more severe
case. However, this was not done, as Dr. Griffin claims, to fudge the results. This is
because the inputs to the model are imprecisely known. Tables 7-3 and 7-8 in
NCSTAR1-2 show the model inputs, including the speed and angle of the aircraft, and
the material strength of the aircraft and the building interior. Aircraft data values are
derived from analysis of videos, since both Flight Data Recorders were completely
destroyed, and this method is subject to significant uncertainty. In like fashion, the
building contents and aircraft materials all vary in composition, and there is some
uncertainty in estimating an average factor for all of the materials. NIST ran three cases
because it needed to see how sensitive its models were to the input conditions. It is
important to note that at this stage of the investigation,
all of these inputs are completely
reasonable – the “more severe” case is effectively the one-sigma upper bound, while the
“less severe” case is the one-sigma lower bound, meaning these inputs are all within the
accuracy of measurements.
NIST did not, as Dr. Griffin states, select the “more severe” cases because those and only
those led to collapse. This is totally false. Each simulation produced a number of
outputs, some of which – like the damage to core columns – could not be estimated from
photographs and videos of the event, but others could be compared to additional evidence
directly. NIST describes its selection criteria in brief in NCSTAR1-2, page
lxxiii:
The less severe damage case did not meet two key observables: (1) no aircraft debris was
calculated to exit the side opposite to impact and most of the debris was stopped prior to reaching
that side, in contradiction to what was observed in photographs and videos of the impact event and
(2) The subsequent structural response analyses of the damaged towers indicated that the towers
would not have collapsed had the less severe damage results been used.
Dr. Griffin is fixating on the second criterion, but completely ignores the first. He also
ignores the more detailed selection criteria presented on pages 267-291 of NCSTAR1-2:
The observable evidence available to help validate the global impact analyses included the following:
Damage to the building exterior documented by photographic evidence
Floor damage visible from the building exterior documented by photographic evidence
Aircraft debris external to the towers as documented by photographic evidence
Eyewitness accounts from survivors who were inside portions of the building.
Another observable was that each tower remained standing after sustaining the impact-induced
structural damage. Analyses of the structural response of the damaged towers immediately after
impact, presented in NIST NCSTAR1-6, showed that this observable was met for both towers.
In short, Dr. Griffin’s claim, that NIST selected the most severe case solely in order to
guarantee a collapse, is wrong. The base case also would have led to a collapse.
Furthermore, while we do not have photographic or eyewitness evidence of the state of
the core columns for either impact, we have a litany of other evidence that was found to
best match the “more severe” cases.
Additionally, it is not entirely clear that, even had the less severe impact conditions
applied, the structures would have survived. NIST did a preliminary assessment that
reached this conclusion, as remarked above, but a competing analysis by Dr. Usmani
et
al. at the University of Edinburgh [35], suggests that even if the impact damage was
negligible, the fires would have destroyed the Towers