An untested design failure.The NIST study was supposed to determine why little resistance was provided. Not merely declare that there was little resistance. This must be studied before building recommendations can be made. That is why a budget was allocated.
In other words, the momentum (which equals mass times velocity) of the 12 to 28 stories (WTC 1 and WTC2, respectively) falling on the supporting structure below (which was designed to support only the static weight of the floors above and not any dynamic effects due to the downward momentum) so greatly exceeded the strength capacity of the structure below that it (the structure below) was unable to stop or even to slow the falling mass.
This is simply wrong in so many ways. The towers were designed to support the static weight of the structure multplied by the safety factor. NIST are ignoring the safety factor, asking us to believe that the towers could carry only the static weight. The question must be asked that if NIST's collapse theory relies on there being no safety factor in order to initiate and progress the collapse, what removed that safety factor prior to initiation.
Conservation of Momentum dictates that the upper section must slow in order to accelerate any part of the lower section.
Conservation of Energy dictates that the upper section must slow in order to be able to cause the damage caused to the floors, core and perimeter structure.