That makes no sense, Royston. How far did the sections that were above the plane crashes fall? How much downward momentum did they have? How much force did they exert in their fall? How much upward thrust was provided by the rest of the building below the sites of the crashes? How much downward force were the cores of the buildings designed to take?

If a failure of the "angle braces" cause the collapse, how did this effect the core structure below the level of the plane crashes? If it was a failure of the angle braces, then the floor sections surrounding the cores would simply have fallen away from the core structures and would not have effected them at all. The floors inside the core structures were not integral with the floors of the area outside of the core structures. They were completely independent of them, so their falling would not have impacted the floors inside the core structures.

The scenario you describe, with the angle braces failing, does not support the hypothesis that the reinforced steel cage construction of the core could have been caused to fail as a consequence. If it could be supported by that, the people at NIST wouldn't have admitted that they can't explain why the buildings fell.