Softening of Steel
NIST theory relies on dramatic softening of steel
to achieve "column instability".
NIST determined that there was no evidence that any of the samples
had reached temperatures above 600 ºC.
In the mid-1990s British Steel and the Building Research Establishment
performed a series of six experiments at Cardington to investigate the
behavior of steel frame buildings.
These experiments were conducted in a simulated, eight-story building.
Secondary steel beams were not protected.
Despite the temperature of the steel beams reaching 800-900 C
in three of the tests
(well above the traditionally assumed critical temperature of 600 C
(1,100 F), no collapse was observed in any of the six experiments).