Figure 4.20 Velocity comparison between Chandler measurement (green plotted line) and UAF simulation (red plotted line). Bold green trend line illustrates free fall. It should be noted that we conducted two separate simulations involving the failure of the core columns and exterior columns over 8 stories: One was the failure of all columns from Floor 12 to Floor 19; the second was the failure of all columns from Floor 6 to Floor 13. The two simulations were identical in terms of the downward velocity and acceleration of the northwest corner at the top of building. We therefore found that the collapse could have started at various floors. Based on our subsequent review of video footage, we found that Floor 16 is the uppermost floor where the collapse could have initiated, because the video shows Floor 17 to Floor 47 falling uniformly as a unit (see Figure 4.21 below). Although some floors below Floor 17 are somewhat visible in the video, it cannot be determined with confidence that they are falling as a unit with the floors above.
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