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Fires in the Twin Towers

The jet collisions injected atomized jet fuel into the Twin Towers at the points of impact, instantly setting extensive fires. Detailed assessment of the extents of the fires over time is impossible given the killing of firefighters in both towers by their collapses, and the destruction of the steel remains without forensic examination. However, much can be learned from surviving photographs and from a recording of radio communications by firefighters who reached the collision zone in the South Tower. NIST's report documented the extents of fires near the windows based on photographs for two points in time for the North Tower and one point in time for the South Tower.

Just the Facts

The severity and extent of the fires in the two towers were dramatically different, a fact missed by many commenters. The North Tower, whose impact was centered, captured much more of its jetliner's fuel than did the South Tower, whose offset impact spilled most of the fuel outside the building. Hence we would expect the North Tower to have more serious fires. That is borne out by all of the major pieces of evidence about the fires.

  • Flames. In the North tower, flames were visible for the duration of the 102 minutes between its jet impact and its explosive collapse. In the South Tower, flames diminished rapidly and were not visible at the time of its collapse.
  • Smoke. The North Tower's fires produced prodigious quantities of smoke. The northeasterly wind carried the North Tower's smoke plume over the South Tower, obscuring the much sparser smoke from the latter. By the time of its collapse 56 minutes after its jet impact, the South Tower was emitting only a thin veil of smoke. The color of smoke is also telling. Both towers emitted light smoke for the first few minutes after their impacts, and thereafter emitted darkening smoke, once the jet fuel had burned off. Whereas subsequent changes in the color of the North Tower's smoke are difficult to ascertain, it is clear that the color of the South Tower's smoke continued to darken, and was almost black by the time of its collapse.
  • Survivors. At least 18 people are known to have escaped from above the South Tower's impact zone, and many more might have if not for confusion that hampered the evacuation of the building. In contrast, no one is thought to have escaped from above the North Tower's impact zone. Instead onlookers witnessed the horrifying spectacle of victims trapped above the North Tower's fires jumping to their deaths.

Fires and Building Collapses

Fires had never before caused steel-framed buildings to collapse, but they were blamed primarily for the collapses of the Twin Towers, both collapses having occurred long after the jet impacts. In media reports and quotes from engineers, jet fuel suddenly had the ability to melt steel. One of the first people to point out the absurdity of the idea that building fires with hydrocarbon fuels could melt steel was J. McMichael who published "Muslims Suspend Laws of Physics!" on October 21, 2001.

e x c e r p t
title: Muslims Suspend Laws of Physics!
authors: J. McMichael

...

Using jet fuel to melt steel is an amazing discovery, really. It is also amazing that until now, no one had been able to get it to work, and that proves the terrorists were not stupid people. Ironworkers fool with acetylene torches, bottled oxygen, electric arcs from generators, electric furnaces, and other elaborate tricks, but what did these brilliant terrorists use? Jet fuel

...

I try to forget that heating steel is like pouring syrup onto a plate: you can't get it to stack up. The heat just flows out to the colder parts of the steel, cooling off the part you are trying to warm up. If you pour it on hard enough and fast enough, you can get the syrup to stack up a little bit. And with very high heat brought on very fast, you can heat up one part of a steel object, but the heat will quickly spread out and the hot part will cool off soon after you stop.

...

I try not to wonder how the fire reached temperatures that only bottled oxygen or forced air can produce. And I try not to think about all the steel that was in that building -- 200,000 tons of it.

The steel-melting jet fuel fires remained a popular theme, but the official explanations required only that the steel be softened to cause the collapse-precipitating structural failure of the columns for instance. McMichael addressed the column failure theory with his second article, "Muslims Suspend Laws of Physics! Part II".

e x c e r p t
title: Muslims Suspend Laws of Physics! Part II
authors: J. McMichael

...

Table 3 lists the maximum temperatures reached in open sided car park tests in four countries. These can be compared with the characteristic failure temperatures for beams carrying insulating floor slabs and columns of 620 [degrees] C and 550 [degrees] C respectively."

Note that the description does not limit the duration of the fire. From this it does not appear to matter whether the fire burned all week or just for two hours. No mention is made, as some people have suggested (from erroneous interpretation of other graphs involving time), that prolonged heat brings about progressive weakening of steel.

Here is the data from Corus' Table 3 (beams are horizontal members, columns are vertical):

Full scale fire tests Maximum measured steel temperature
Country Beam Column
UK 275 C (527 F) 360 C (680 F)
Japan 245 C (473 F) 242 C (467 F)
USA 226 C (438 F) -
Australia 340 C (644 F) 320 C (608 F)

A fire in a steel car park is a very imprecise event, and the heating of the steel supports varied widely in the tests. The temperature of (horizontal) beams varied from 226 C in the USA to 340 C in Australia; and the temperature of (vertical) columns varied from 242 C in Japan to 360 C in the UK. None of the steel was protected with the thermal insulation that is commonly used in office buildings, including the WTC.

To my mind, this is definitive answer: the maximum temperature in the unprotected steel supports in those test fires was 360 degrees C (680 F), and that is a long way from the first critical threshold in structural steel, 550 degrees C (1022 F).

Analysis of fire temperatures and steel strength is only relevant to whether fires could cause some kind of structural failure. The explosive collapses suffered by the Twin Towers are an entirely different matter than such a failure, and require explanations beyond newly destructive kinds of fires, such as the progressive collapse theory.
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The fires in the towers shortly after the South Tower impact
Smoke from the fires seen from the northeast. This photograph was apparently taken shortly after the South Tower impact, as it shows some flames and light-colored smoke.
Towers from the north