This page is devoted specifically to the collapse of building 7 ( WTC 7), also called Salomon Brothers building. The collapse of this building was an astonishing event in the history of modern architecture and structural engineering because never before had a tall steel framed building collapsed due to fire, though many have burned fiercely. That it happened suggested to some initial observers that explosives must have been used in a controlled demolition, but the media reported little about this building and did not discuss this possibility. The improbability of the collapse being due to fire became lost in the general turmoil that the tragic events of 9/11 created. Even now, few people are aware that a third building came down that day.
FEMA released its report World Trade Center Building Performance Study in May, 2002. This provides a description of the building and suggests several hypotheses for the mechanism of collapse, assumed to be due to fire. It is a surprise to read in this report that “…the best hypothesis has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue.”
One would think therefore that when NIST came to perform further investigations they would have widened the scope of their enquiries to include the possibility that explosives were used. They did not. This has to be regarded as one of the more sinister aspects of the NIST investigation, as the collapse has all the signs of an expertly performed controlled demolition: sudden onset, acceleration close to free fall, symmetry and totality.
There are also convincing reports that explosions were heard and felt. The testimony of Barry Jennings, who was trapped for some time in WTC 7, after an explosion damaged a staircase, is particularly revealing.
NIST apparently was not able to find a sufficiently plausible explanation for the collapse until 2008. In the NIST final report their earlier suggestion that the collapse was due to damage from falling debris and severe fire on the south side was abandoned. They had previously referred to severe fire on the south side because we have videos and photographs which show there was little fire on the north side. It is not surprising that NIST abandoned this idea as, on its face, it was ludicrous. For example in Controlled Demolition at the WTC we read “It is inescapable that if one side of a tall steel structure is heated to the point of failure, while the other side is not, the structure must lean toward the heated side. WTCÂ 7 did not lean however, it just came straight down…”
To replace the south side damage theory, NIST devised a theory where the damage initiated more centrally, and depended on “thermal expansion”. This was supposed to have caused the severing of attachments of a section of a floor on the east side to a support column, starting a domino effect on several floors, finally allowing a critical column to buckle. Numerous authors have found this explanation to be equally implausible, as may be found on the Papers page, for example The NIST WTC 7 Report: Bush Science reaches its Peak. The explanation appears to fail at the outset as this critical column still had support from floors running to the west and south and therefore could not buckle.
What really does happen when a tall steel framed building burns?
Beijing TVCC building on fire.
There have been many examples of fires in tall buildings. In some cases a portion of the building may collapse. If it does, this will be spread out over a period of hours, not minutes and certainly not a few seconds, as happened with all three buildings at the WTC. It is very difficult to properly direct water into a tall building and in most cases the fires eventually just burn out. Two types of tall building may be distinguished: steel frame and steel reinforced concrete. In the latter case the heat may cause the concrete to crack and fall away, in which case the relatively thin reinforcing steel may eventually collapse where it is exposed. True steel framed tall buildings apparently never collapse due to fire.
The most striking example of a steel framed building burning was provided in Beijing recently (February 2009).
The TVCC building was ignited by fireworks and the fire quickly spread throughout its entire height, as can be seen in this video. Apparently the building was designed in such a way that an updraft could develop. The contrast with the WTC buildings in this respect was extreme. In WTC 7 we see that several fires spread horizontally for some distance, and eventually most burned out, but it is hard to find evidence that there was any vertical spread.
After the fire.
After the fire burned itself out the frame remained standing. Though the fire was vastly hotter than the fire in WTC 7 the building did not collapse. An eye witness gives a clue to the temperature of the steel:
“The building was like an oven, red inside,” Hu Jing, a 26-year-old paralegal and witness to the disaster, told the Washington Post.
To make the contrast clear I will show a similar pair of photos of WTC 7. First, here is the video.
WTC 7, beginning of collapse.
During the fire, at the moment when the collapse has just begun, we are hard pressed to find any fire. Apparently most of the fires reported earlier in the day have burned out. Remembering that steel regains strength as it cools, this building should not be able to collapse straight down now. If there is severe fire on the far side the building should have leaned away, but it did not.
WTC 7, after collapse
After the collapse, which took about 6.5 seconds, we find the building is neatly stacked in a small area, hardly bigger than its footprint. As there were valuable buildings each side this is a remarkable achievement and the perpetrators are to be congratulated for minimizing the damage to neighbouring real estate. To place the collapse time into perspective we note that the roof would have reached the ground in 6 seconds in free fall in a vacuum.