One of several features of the total collapse of WTC Building 7 that is common in controlled demolitions is the rapid descent of the building. Several videos captured the descent of WTC 7's facade from the north. A video taken from West Street about 1000 feet from the building shows the north facade in considerable detail as it descends. The following montage was created using frames from that video separated by one-half second intervals.

	facade movement			free-fall
frame	seconds	displacement, floors	displacement, feet	seconds	displacement, feet	difference, feet
0	0.0	0	0.0	-0.65	0.0	0.0
1	0.5	0	0.0	-0.15	0.0	0.0
2	1.0	0.2	6.0	0.35	2.0	-4.0
3	1.5	1.0	12.0	0.85	11.6	-0.4
4	2.0	2.4	28.8	1.35	29.2	-0.4
5	2.5	4.5	54.0	1.85	54.8	+0.8
6	3.0	7.3	87.6	2.35	88.4	+0.8
7	3.5	10.7	128.4	2.85	130.0	+1.6
8	4.0	14.2	170.4	3.35	179.6	+9.2

The table above contains data on the downward displacement of the northwestern corner of WTC 7 over the period of four seconds shown in the image slices comprising the montage. The red rulers superimposed on the image slices demarcate floors. Displacements were measured by comparing the rulers to the roof of the white building in the foreground. The displacements in feet were computed from the displacements in floors using the estimated floor spacing of 12 feet. The error margin in placing and reading the rulers is about 3 feet.

The displacement of an obect in free-fall is shown in columns to the right of those showing the facade movement. The free-fall distances at the half-second intervals are based on the object being released 0.15 seconds after the time coordinate of the second image slice.

The rightmost column gives the free-fall distances minus the facade displacements. The differences are within the estimated margin of error for the facade displacement measurements for all but the first and final frames. This shows that the facade of WTC 7 accelerated downward at very close to the rate of free-fall in at least its first three seconds of descent,

page last modified: 2006-11-17