One must observe that we know about uniform vertical movement only during the time the upper part of the building is visible and before it disappears in a cloud of dust. Later on, large structural elements can be seen falling from inside the cloud. This may suggest a loss of symmetry in the event, but then the process might have been advanced enough by that point, so that accurate preservation of symmetry may not have mattered.

APPENDIX C. THE SEQUENCE OF THE INITIAL COLLAPSE

The event may be summarized as follows. On September 11, 2001, the North Tower was impacted by a Boeing 767 airliner, which literally flew into the building. There was an immediate explosion followed by prolonged fires, which were ignited and possibly continuously fed by an estimated 38,000 liters of fuel which had been on board the aircraft (there is not unanimous agreement as to how long the fuel would have been available to sustain the fires. The NIST report and some researchers postulate that it would have burned up within minutes of the aircraft impact and thus only served to ignite the fires). After 1 hr and 42 min had elapsed, from the time of impact, the entire building collapsed, in a progressive downward movement, beginning at the 98th floor.

From an engineering viewpoint the event had many fascinating aspects. Not the least of them was “the aircraft flying into the building” mentioned above. This means the aircraft structure cut the building structure on its way. The aircraft was built of an aluminum shell on the order of 2 mm thickness, which was additionally stiffened by longitudinal and lateral elements. At contact with the building the fuselage was pitched nose down by 10.6° and hit the building between the 95th and 96th floors. There was a 25° roll, so one wing impacted a higher part of the building than the other. The exterior columns in the area of impact had 6.9 mm thick hollow square sections of 356 mm side length. Yet, the thin aluminum wings cut through the much thicker steel. The aircraft would have certainly suffered severe unspecified fractures in the process. An analysis by NIST shows the wings would have largely been disintegrated while going through the exterior wall, but that the fuselage would continue along its path with enough speed to damage some core columns on its way between the 94th to 96th floor levels. The 98th floor, where the collapse initiated, was at the edge of the impact, and was only hit by a wing tip and would have suffered no core column damage.

The Sauret video [7] quoted before is the best visual record that we have of the early phase of the fall of the North Tower as seen from the north side. It allows us, along with an NBC video [18], showing the west side, to quantify visible downward motion of the northeast, northwest, and southwest roofline corners. Measurements show them falling within 0.25 to 0.50 seconds of each other in a nearly straight downward collapse of the upper part of the building. This continues for at least two stories, after which a tilt angle of about 8 degrees southward appears while the top part of the building continues its fall. During the measurable fall, the roof is moving downward at a constant acceleration, much larger than Pave of columns heated to failure would allow.

The NIST report claims that the south exterior wall failed, as a result of prolonged heating of the attached floor trusses on that side, and that this caused a propagation of failure across the 98th floor resulting in a vertical drop, but they do not analyze the spread of failure in either the horizontal or the vertical directions. They do provide the load redistribution from this south wall failure, but that does not generate loads big enough to fail the remaining columns across the floor. Bazant [2] does not explain the initial propagation of failure across the 98th floor in any of his analyses, other than to assume uniform heating and collapse. The reasons for the evenness of the initiation at the 98th floor and straight down fall in spite of asymmetric damage, the smooth motion history while falling through successive floors, and the promptness of collapse of the North Tower, remain yet to be explained.