Additional unusual behaviors observed for WTC2 included the correlated smoke puffs observed over large areas of the fa?ade and over the roughly one-minute periods during which the smoke flows increased dramatically, often accompanied by external flames. These events were usually most evident on the 79th and 80th floors of the east face, but could often be detected on other floors and faces as well. At least 65 occurrences of smoke puffs were documented along with 7 times when the one-minute long smoke releases took place. |
The evidence suggests that these smoke puffs resulted from pressure pulses generated within the tower and transmitted to other locations. It is considered likely while these pressure pulses were of sufficient magnitude to affect smoke flows over multiple floors, they were much too small to affect the tower's structural components. For most of the smoke puffs, there was no visual indication of the event that generated the pressure pulse or its location. In a few cases, such as when molten metal poured from the tower, circumstantial evidence indicated that the puffs were associated with specific observed events.
- NIST NCSTAR 1-5, WTC Investigation (p.p.36-38)
The video shot from the WTC plaza captured an intriguing event at 9:37:04 a.m. A jet of air, dust, and a large piece of debris was ejected from a window, 77-355, on the 77th floor at an extremely high velocity. Longer distance videos show that puffs of smoke and/or dust appeared simultaneously on the east face from several open windows near the center of the 78th floor and from open windows on the north side of the 79th floor. Interestingly, the smoke flow from the windows on the west sides of the 79th and 80th floors, which had decreased markedly by this time, did not increase. Within 14s of this release, a large fire either grew or became visible near the center of the east face on the 82nd floor.
A long distance video shot from the south showed that fire and smoke were pushed from multiple locations on the south face at the same time the strong jet occurred on the 77th floor of the east face.
-NIST NCSTAR 1-5A, WTC Investigation (p. 346)
(extracted from public draft)
It has been reported in the FEMA report (McAllister 2002) as well as in the media that what appeared to be molten metal was observed pouring from the north face near the northeast corner prior to the collapse of WTC2. This is the area where the sustained fires on the 81st and 82nd had been burning since the aircraft impact. The likely explanation for the observation of pouring liquid is that the material had originally pooled on the floor slab above, i.e., the 81st floor, and that it was allowed to pour out of the building when the floor slab in the immediate vicinity either pulled away from the spandrel or sank down to the point where the window was exposed. The puff of smoke and/or dust just prior to the release of the material occurred suddenly, in the process creating a pressure pulse that forced smoke and/or dust out of open windows over three floors.
The composition of the flowing material can only be the subject of speculation, but its behavior suggests it could have been molten aluminum. Visual evidence already discussed shows that significant wreckage from the aircraft passed through the building and came to rest in the northeast corner of the tower on the 81st floor, i.e., at the location where the molten material apparently originated. Much of the structure of the Boeing 767 is formed from two aluminum alloys that have been identified as 2024 and 7075 (NISTSTAR 1-3). The melting points for these alloys vary as the material melts. The Aluminum Association handbook (The Aluminum Association 2003) lists the melting point ranges for the alloys as roughly 500C to 638C and 475C to 635C for alloys 2024 and 7075, respectively. These temperatures are well below those characteristic of fully developed fires (ca. 1000C), and any aluminum present is likely to have been at least partially melted by the intense fires that had been in the area for nearly 48 min.
There was another release of smoke and/or dust from near the east edge of the north face at 9:52:38 a.m. Immediately afterward, the spot near the top of window 80-255 brightened considerably. At 9:52:47 a.m. a series of three much larger pressure pulses, which took place over 35s, pushed smoke and/or dust from several locations on multiple floors of the north face. These locations included those described above, the two areas on the north side of the 83rd floor where fires were burning, the vicinity of the pile of debris near the center of the 79th floor, an area of newly observed fire on the 79th floor near windows 79-209 to 79-213, and the opening of the northeast corner of the 81st floor. The fire burning near the center of the 79th floor flared up noticeably. Immediately following one of the pressure pulses (at 9:52:48 a.m.) intense flames suddenly reappeared in windows 91-301 and 81-302 on the east face just to the south of the northeast corner of the 81st floor. At roughly the same time, part of the debris lodged at the northeast corner of the 81st floor fell out of the opening. Relatively small amounts of molten material poured from window 80-255 near the start and the end of the series of pressure pulses. Videos show that smoke was also expelled from windows near the north edges of the 79th and 80th floors of the east face during these pressure pulses.
A high-resolution photograph of the north face shot at 9:52:51 a.m., i.e., just 4s after the start of the first of three pressure pulses, is shown in Figure 9-71. The white cloud in the photograph is the rising dust and/or smoke that was pushed from windows on multiple floors during the pressure pulse. Blow-ups of the area around 80-255, which is the location on the 80th floor where the molten material poured from, and the cold spot are shown in Figure 9-72 and Figure 9-73, respectively.
First, consider Figure 9-72. Videos show that a small amount of molten material poured from window 80-255 within a second or two after this photograph was taken. In the photograph the bright material is visible inside this window at the top. The area seems to be surrounded by a large number of glowing drops, which are likely associated with the liquid that will pour out shortly. Dull, deep-red glowing areas are just visible at the tops of windows 80-252 and 80-254. These might be an indication that the floor slab has sunken below the spandrel at the base of this floor.
NISTNCSTAR 1-5A Chapter 9 Appendix C
p.p. 375-376 (pdf p.p. 79-89)
Of particular relevance is the bright emission from the top of window 80-255, which was the source of the molten flow on the 80th floor in window 84-223.
At 9:53:41 a.m. and 9:53:46 a.m. two pressure pulses forced additional smoke from windows on the north face. Both of these occurrences were accompanied by short flows of molten material from the same window, 80-255, on the 80th floor observed earlier. The largest flow occurred during the second release. Figure 9-75 shows a view of the northeast corner of WTC2 taken from a video at 9:53:51 a.m. The bright molten stream flowing from the top of windows 80-255 is prominent. Unlike earlier images (e.g., see figure 9-71 and figure 9-74), both of which were taken within a minute of 9:53:51 a.m.) showing this area of the tower, the intense fire burning on the 81st floor at the eastern edge of the north face has substantially died down, and it is now possible to see clearly the 82nd and 83rd floors above for the first time. Videos suggest that the fire intensity at this location decreased around the time of the first pressure pulse at 9:53:41 a.m.
p. 380-381 (pdf p. 84-85)
An image of the north face taken at 9:55:18 a.m. is shown in Figure 9-76. The image was shot 1 min 45s after the one shown in Figure 9-74. The fire distributions are very similar in the two photographs, with the exception of the fire on the 82nd floor to the right of the cold spot in Figure 9-76. As discussed above, the fire was first observed at 9:53:47 a.m. burning in windows 82-234 to 82-337. At 9:55:18 a.m. flames were visible in windows 82-231 to 82-236, and flames were coming out of windows 82-233. The bright spot at the top of window 80-255 is visible on the 80th floor, but there is no obvious molten liquid flow at this time. Unlike in the earlier photograph, fire is now visible in the adjacent window, 80-256.
During the next several minutes, following the two pressure pulses around 9:53:45 a.m., numerous pressure pulses were noted that pushed smoke from open windows on the north and east faces of WTC2.
A particularly strong pressure pulse started at 9:56:11 a.m. It lasted nearly 10s. This particular pulse was strong enough to push flames from the tower at many locations. Following a pressure pulse at 9:56:31 a.m., the flames present on the 81st floor in the northeast corner opening and nearby flames on the north face abruptly died down. The large flame in window 81-301 on the east face also declined within 2s. All of these flames reappeared after a few seconds.
At 9:57:21 a.m., shortly after another pressure pulse, the bright light reappeared at the top window, 80-255, on the 80th floor from which the flow of molten metal had been observed earlier. Almost immediately, it appeared to jump one window to the east, i.e., to window 80-256. Five seconds later a light flow of molten metal began pouring out of window 80-256. The flow of material from this window would now be nearly continuous until the tower collapsed. At 9:57:32 a.m. there was a fairly intense pressure pulse within the tower. The flow rate of the molten metal increased dramatically at this time.
p. 383 (pdf p. 87)
Figure 9-77 is a frame from a video that was shot at 9:57:45 a.m. It shows the falling metal as it was approaching the ground. The large amount of falling material is evident. Most of the molten metal seems to have broken up into small "droplets," but there are a couple of large pieces falling as well. Apparently, the material flowing out of window 80-256 also included pieces of debris that had not fully melted. There was another heavy flow of molten metal at 9:58:35 a.m.
Figure 9-78 shows a photograph of the north face of WTC2 taken at 9:58:37 a.m. The bright source of the recent molten metal flows stands out at the top of window 80-256 on the 80th floor. Some metal is visible flowing from the window at this time.
p.384 (pdf p. 88)
Numerous visual observations suggest that important changes, which might have had structural relevance, were taking place in WTC2 in the period following aircraft impact until collapse. These observations include hanging objects, some of which, based on appearance, may have been locally dislodged floor slabs that had settled down to locations below the spandrel, at several locations on the north and east faces, changes in positions of the hanging objects during the period, the occurrence of numerous pressure pulses identified by smoke and/or dust puffs generated over multiple windows and floors, the appearance of molten metal pouring from the tops of open windows, and bowing of outer steel framework.
The sudden release of molten material (possibly aluminum) that began pouring from window 80-255 on the north side of the 80th floor at 9:51:51 a.m. provides evidence for the extensive heating that had taken place from the fire that had been burning in the area for nearly 50 min. The melting point range for the relevant aluminum alloys varies from 475C to 635C, and a great deal of heat would have been required to melt the large volume of liquid metal observed pouring from the tower. The sudden appearance of the flow at the top of the window was likely the result of the formation of a pathway from the 81st floor, where the aluminum possibly had pooled on top of the floor slab as it melted. This, in turn, suggests that the 81st floor slab possibly sank down or pulled away from the spandrel at this time.
During the 7 min between when the flow of molten metal was first observed and the tower collapsed, the amount of material flowing from the 80th floor increased and decreased repeatedly. At one point the flow shifted from window 80-255 to window 80-256. The change in the source window for the liquid suggests that the lowest local point with pooled aluminum somehow moved to the east. These observations suggest that the 81st floor slab in the immediate vicinity was possibly shifting almost continuously during this time, and in the process, spilling more and more of the pooled liquid.
A similar release of liquid occurred from window 78-238 on the 78th floor.(Figure 9-32) It is possible that this material came from the pile of debris immediately above on the 79th floor. Since this flow was observed for only a few seconds, it is not appropriate to speculate further concerning its source.
p.p. 412-413 (pdf p. 116-117)