The time it took the Towers to fall may be one of the most important pieces of evidence in determining their mode of destruction.
It is widely accepted that both Towers completely fell (nearly everything but the dust reached the ground) in around ten seconds. This estimate appears to be based mainly on seismic data. However, video evidence of the North Tower collapse suggests that it took close to 15 seconds for the destruction to reach the ground. Establishing a precise time of duration for each fall may not be possible, but there are measurements that can be made. Video records show that each Tower’s top began its fall precipitously, and show the falling tops for a few seconds before they disappeared into the exploding dust clouds. It is also possible to track other features of the waves of destruction that traveled down each Tower. In both collapses dust clouds, exhibiting the behavior of pyroclastic flows associated with volcanoes, rapidly grew as they fell. Each cloud consumed its Tower’s top in a few seconds, then continued to descend, remaining centered around the Tower’s axis. Each cloud had a fairly well-defined top and bottom, whose descent can be timed using video records.
Despite the availability of detailed studies of collapse times based on the compositing of video and photographic evidence, and in-depth analysis of the seismic records, many commentors have incorrectly treated the durations of the largest seismic signals as synonymous with total collapse times. Statements that the Towers fell in eight and ten seconds have been repeated by both proponents and critics of the official explanation.
Meaning of the Seismic Records:
Seismic records of the Twin Tower collapses show a large signal for each collapse lasting just under 10 seconds. The durations of the large signals are widely equated with the durations of the collapses themselves. However, the signals may correspond to only parts of the collapse events, such as the rubble reaching the ground.
Consider the seismic records of the closest seismic recording station, at Palisades, NY (PAL). They show a very similar pattern for the leveling of WTC 1 and 2. In both cases there is about five seconds of high-amplitude movement, followed by about three seconds of movement at less than half that amplitude, and then by about 15 seconds of much weaker movement. In addition there is some still weaker movement starting about 12 seconds before the onsets of the high-amplitude movement. The main difference is that for WTC 1 the initial high-amplitude phase builds in intensity to a much higher spike than any seen for WTC 2.
The fact that the largest movement is followed by smaller movement has been cited as evidence that bombs, detonated at the starts of the collapses, generated the large movement, and that the debris impacting the ground contributed to the smaller subsequent movement. However, bombs, if detonated underground, would have generated strong P waves in addition to S waves. The fact that only strong S waves were reported is consistent with the theory that the largest movement was caused by building remains hitting the ground.
Tracking the Tops:
The time of onset of collapse is clear in the North Tower, which initiates its telescoping collapse in an instant. The case of the South Tower is complicated by the fact that the top leans for a few seconds before beginning its vertical descent. Although determining the onset of vertical collapse in the South Tower is more difficult, its top is visible for longer and offers a longer span through which to time the rate of fall.
The South Tower:
The following analysis is based on a NBC video taken from east of the Towers.
For the South Tower we define onset of collapse as the moment downward movement of the highest point of the roof starts. Unfortunately this time is difficult to determine since the roofline is obscured by smoke when the fall begins. The fall is preceded by a leaning movement that starts at about the beginning of the video clip, and accelerates for about three seconds. At somewhere between two and three seconds, the top starts to fall. Once the top starts to lean, the highest point of the roof is the northwest edge. At about 2.5 seconds, the top starts to fall. A good marker for this is a small white squib that emerges from the level of the impact zone about three-quarters of the way back on the right face. That immediately precedes the first large ejections from the southeast face.
Using that marker it is possible to time the fall of the South Tower’s top up to the moment it gets swallowed up by the dust cloud. At that point the dust cloud rises only slightly above the level of the 78th-floor sky lobby visible as a two-story band on the adjacent North Tower. A small extrapolation gives a good estimate of the time of fall of the South Tower’s roof to the level of the 78th floor of five seconds. That distance is about 384 feet (12 feet per story times 32 stories).
The North Tower:
The top of the North Tower began to suddenly telescope about a fourth of a second after the radio Tower started to fall. In views from the north the top is swallowed up in about two seconds. The CNN live video clip shows the mushrooming dust cloud reaching the ground at about 13 seconds. As the descending pyroclastic dust cloud drops below Building 7, the rising smoke plume shifts to the east revealing the empty space except for the short-lived spire. The first glimpse of empty space where the building stood is at about 13 seconds.
Detailed analysis of the motion of the North Tower’s top is provided by physics teacher David Chandler, who measured the top and concluded that, the rapid descent of the top, though slightly slower than free-fall, means that the force it exerts on the intact portion is actually less then when the building is motionless.
–The roofline of WTC1 (The North Tower) begins dropping with sudden onset and accelerates uniformly downward at about 64% of the acceleration of gravity (g) until it disappears into the dust. This means it is meeting resistance equal to about 36% of its weight. The implication of this, however, is that the force it is exerting on the lower section of the building is also only 36% of the weight of the falling section. This is much less than the force it would exert if it were at rest. The acceleration data thus prove that the falling top section of the building cannot be responsible for the destruction of the lower section of the building.–
A Timeline for the North Tower:
It would be useful if collapse events evident in videos could be associated with seismic signal features. Since some news broadcasts have real-time clocks on their banners, it may be possible to match visual events with features of the seismic signals.
Consider the North Tower, whose entire collapse was recorded by the above-mentioned CNN live feed, which has a clock on its banner. That clock does not have a second counter, but its minute counter flips to 10:29 37 seconds after collapse starts, which places the collapse start, according to the CNN clock, at 10:28:23.
Various pages on columbia.edu put the origin time of the signal at the source at 10:28:31, plus or minus one second. This is based on an estimate of 2 km/s travel speed for the S waves, which, given the PAL station’s distance of 34 KM from the WTC, gives a travel time of 17 seconds.
The CNN video suggests that it takes about ten seconds for the bottom of the mushrooming dust cloud to reach the ground, and another seven or so for the top to reach the ground. The following composite timeline combines timing estimates of collapse events from the CNN video and the PAL seismic record. It assumes rubble hitting the ground caused the large ground movement, and thus that the crumbling of the Tower prior to that caused only minor ground movement. Given that, the times from these pieces of evidence match up remarkably well.
10:28:23 A.M.* Top of the North Tower starts to break apart
10:28:31 A.M.* Rubble starts to hit the ground (start of big signal)
10:28:36 A.M.* The heaviest rubble hits the ground (peak of big signal)
10:28:39 A.M.* Most heavy rubble has reached the ground (end of big signal)