1.   As the roof fell, the tensioning ring that had for so long held up the arches was being used to pull down the exterior columns.

2.   Attached to the top of each exterior column would be four highly polished steel plates that extend into the column slightly and jut substantially outside of it.

3.   Second, the interior columns act as springs, forcing the exterior columns to right themselves.

4.   Seconds later, near the heavily damaged southeasterly portion of this same floor, close to where the aircraft had entered, exterior columns began to buckle.

5.   So the exterior columns, for example, were braced laterally by their connections to the floor trusses themselves.

6.   The roof was a clear arching span suspended atop exterior columns.

7.   Another suggests that the exterior columns pulled inward, giving in to floor joists weakened by raging fires.

8.   Another theory suggests that the exterior columns pulled inward, giving in to floor joists weakened by raging fires.