The Twin Towers' Fires and Their Possible Effects
|The South Tower's fires burned hot enough at produce visible flames and light smoke (photograph) until the jet fuel burned off less than ten minutes after the crash. Thereafter the fires dwindled and the smoke darkened. When it collapsd 56 minutes after the crash, the invisible fires were emitting only a thin veil of black smoke.|
Much was made of the severity of the fires in the Twin Towers, since fires were invoked to explain failures they had never before caused. Some reports compared the heat produced by the fires to that of nuclear power plants. In fact the fires were not as severe as many other highrise fires, none of which caused the buildings to collapse. Furthermore, the fires became less severe over time, at least in the South Tower, whose smoke became thin and nearly black by the time its total destruction.
Fire-induced column failure collapse theories, such as Prof. Bazant's, assume scenarios in which fires consume entire floors and burn for extended periods at temperatures of over 800° C. There are several problems with such scenarios.
- 800°C is near the maximum flame temperature of hydrocarbons burning in air without pre-heating or pressurization of the air (estimates of which range from 900°C to 1250°C 1 ). Those temperatures are usually reached only with premixed (blue) flames, such as in gas stoves and blowtorches. Diffuse flames, such as in building fires, tend to be cooler. Although enclosures can elevate fire temperatures considerably by containing the fires' heat, tests that have recorded gas temperatures of over 800°C have involved ventilation and fuel supply characteristics arguably not present in the Twin Towers.
- Widespread fires reaching 700°C would have caused extensive window breakage over time. Although there are breaches in the perimeter wall glazing of parts of the Towers that appear to have ocurred after the plane crashes, such as in a fire zone on the 104th and 105th floor of the North Tower, descriptions of windows popping or falling on victims are not readily apparent in the eyewitness reports from that day. This contrasts with the prominence of reports of fire-induced window-popping in other highrise fires.
- Widespread fires reaching 700°C and would have made the steel glow red-hot. Visual records of the events, while showing fire damage to the aluminum cladding covering the perimeter columns, do not appear to show glowing steel. 2
- Fires would have to be very extensive to raise the temperatures of columns to near the fire temperatures, given the thermal sinks of the steel structures. Columns of the perimeter walls were thermally coupled to eachother by broad spandrel plates at each floor, and the core columns were part of a lattice of beams and columns which would have wicked heat from a hotspot in three dimensions. In order to soften columns, fires would have to exceed the capacity of the many tons of steel in and around the crash zones to draw away the heat -- a difficult feat in the 56 and 102 minutes that the fires burned.
- Fires apparently did not involve entire floors of either Tower at any one time. The South Tower shows no evidence of fires on its northwest side at any time. The North Tower at times shows fires spanning most of a face, but the fires are not even emergant, in contrast to the One Meridian Plaza or First Interstate Bank fires.
- Heating the external columns would be especially difficult because the columns were situated outside the interior volume, with only one of the four sides adjacent to the building's interior.
- Heating of core columns would be especially difficult given the apparently poor ventilation of the core regions, being further from any air supply.
- As fires consumed fuel supplies and became less severe, affected columns would have cooled and regained strength lost due to elevated temperatures.
Even if such hot and widespread fires existed, they would still be unlikely to cause failures of the columns in either of the Towers.
The incompatibility of the fire-triggered column-failure scenario with the observed characteristics of the fires created the need for the truss theory.
2. Metal Temperature by Color, processassociates.com, [cached]