28 Kingdom,
I understand you have some trouble working your way through the NIST report. To make things easier for you, I have created a simplified series of diagrams that display the progression of the WTC collapses.
1 - Here we have a tower, fully intact. Note the floor trusses, exterior columns, and core columns.
2 - This is just after impact. Red indicates fire, and grey smoke. Notice how the fire spreads rapidly across the impact zone.
3 - The fire spreads rapidly through the upper floors. Notice localised floor collapses, as documented by 9-1-1 calls from people trapped inside. By this stage the fire is causing floor trusses inside the impact zone to sag (well documented) and this in turn is pulling in the exterior columns (also well documented).
4 - Things are really bad now. The bowing of the exterior columns has reached critical point, and the exterior columns have failed, collapsing inwards. The floor trusses, with nothing holding them at the exterior, fall downwards.
5 - The core in the impact zone was weakened by impact and fires - as per testimony from victims and the NYPD aviation unit. With the exterior columns now failed, the core in the impact zone is trying to hold the entire weight of the upper floors. Meanwhile the failed exterior columns and floor trusses are falling onto lower floors, which in turn are stripped from their mounts by the force of impact.
6 - The core is unable to hold the weight of the building, so, unsurprisingly, it fails in the impact zone. The upper floors smash down into the carnage left as the floor trusses continue to fall through more floors. As the top falls inside the exterior columns it pushes them outwards.
7 - The more intact core below the impact zone offers more resistance, and the upper floors begin to disintegrate under the enormous colision forces. Exterior columns shear away from the structure and peel outwards, crashing into neighbouring buildings. The floors and their contents continue downwards, forcing debris through windows.
8 - By now the smoke is obscuring much of the building. The upper floors have all but disintegrated. The floors are crashing down still, and the exterior columns continue to sheer away and fall to earth. The only part of the structure still intact is the lower core, though it has suffered damage.
9 - The majority of the collapse has finished, and the last of the debris falls into a large pile. Only the core remains standing, though it is severly damaged.
10 - The core is unable to support itself in its devastated state, and it too fails.
11 - The tower has completely collapsed, leaving nothing but a large pile of debris. Note the buried fires, which will continue to burn amidst the fuel-rich rubble for weeks.
Any questions?
-Gumboot
