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What caused the fires in WTC 7?
Debris from the collapse of WTC 1, which was 370 feet to the south, ignited fires on at least 10 floors in the building at its south and west faces. However, only the fires on some of the lower floors—7 through 9 and 11 through 13—burned out of control. These lower-floor fires—which spread and grew because the water supply to the automatic sprinkler system for these floors had failed—were similar to building fires experienced in other tall buildings. The primary and backup water supply to the sprinkler systems for the lower floors relied on the city’s water supply, whose lines were damaged by the collapse of WTC 1 and WTC 2. These uncontrolled lower-floor fires eventually spread to the northeast part of WTC 7, where the building’s collapse began.
How did the fires cause WTC 7 to collapse?
The heat from the uncontrolled fires caused steel floor beams and girders to thermally expand, leading to a chain of events that caused a key structural column to fail. The failure of this structural column then initiated a fire-induced progressive collapse of the entire building.
According to the report’s probable collapse sequence, heat from the uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, damaging the floor framing on multiple floors.
Eventually, a girder on Floor 13 lost its connection to a critical column, Column 79, that provided support for the long floor spans on the east side of the building (see Diagram 1). The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the 5th floor. Many of these floors had already been at least partially weakened by the fires in the vicinity of Column 79. This collapse of floors left Column 79 insufficiently supported in the east-west direction over nine stories.
The unsupported Column 79 then buckled and triggered an upward progression of floor system failures that reached the building’s east penthouse. What followed in rapid succession was a series of structural failures. Failure first occurred all the way to the roof line—involving all three interior columns on the easternmost side of the building (79, 80, 81). Then, progressing from east to west across WTC 7, all of the columns failed in the core of the building (58 through 78). Finally, the entire façade collapsed.
Did fuel oil systems in WTC 7 contribute to its collapse?
No. The building had three separate emergency power systems, all of which ran on diesel fuel. The worst-case scenarios associated with fires being fed by ruptured fuel lines—or from fuel stored in day tanks on the lower floors—could not have been sustained long enough, could not have generated sufficient heat to weaken critical interior columns, and/or would have produced large amounts of visible smoke from the lower floors, which were not observed.
As background information, the three systems contained two 12,000 gallon fuel tanks, and two 6,000 gallon tanks beneath the building’s loading docks, and a single 6,000 gallon tank on the 1st floor. In addition one system used a 275 gallon tank on the 5th floor, a 275 gallon tank on the 8th floor, and a 50 gallon tank on the 9th floor. Another system used a 275 gallon day tank on the 7th floor.
Several months after the WTC 7 collapse, a contractor recovered an estimated 23,000 gallons of fuel from these tanks. NIST estimated that the unaccounted fuel totaled 1,000 ±1,000 gallons of fuel (in other words, somewhere between 0 and 2,000 gallons, with 1,000 gallons the most likely figure). The fate of the fuel in the day tanks was unknown, so NIST assumed the worst-case scenario, namely that they were full on Sept. 11, 2001. The fate of the fuel of two 6,000 gallon tanks was also unknown. Therefore, NIST also assumed the worst-case scenario for these tanks, namely that all of the fuel would have been available to feed fires either at ground level or on the 5th floor.
Failure between steel and concrete. Long span steel beams. Girder on floor 13 failed, causing floor to collapse. Cascading failures. Column 79 failed.
Originally posted by PplVSNWO
Thanks for the video links.
Why does their computer model of the collapse not resemble the actual collapse videos at all? There model shows the colums failing at one end and progressing across the building eventually pulling the sides inward as it starts global collapse.
Actual videos reveal that the center of the building starts to kink and then the entire structure falls vertically, there is no signs of the ends being pulled in by the failing floors and columns.
[edit on 21-8-2008 by PplVSNWO]
Originally posted by ThroatYogurt
reply to post by dariousg
I do not work for NIST. I am only showing you what was brought up today.
You don't beleive it? Come up with your own hypothisis.
Originally posted by Pootie
Am I right to say that they are now claiming a common office fire heated the structural steel to a point where it expanded enough to cause column 79 to fail leading to a global failure?
No more diesel fires?
No more "column damage" from the collapse of WTC 1 or 2?