As an exercise let's set aside all of the evidence about the actual severity of the Twin Towers' fires, and imagine that the fires were incredibly intense and widespread. Let's imagine that the jets were full tankers and spilled 80,000 gallons of fuel into each tower. Let's imagine that there was a strong wind giving the fires plenty of air. Let's imagine that the the fires engulfed over 10 floors in each tower, saturating the capacity of the steel buildings to draw away the heat. Let's imagine the fires burned intensely for hours, completely gutting several stories of each tower. Would that cause them to collapse? Not according to people who have studied steel structures subjected to such stresses. The following passage is from Appendix A of FEMA's World Trade Center Building Performance Study.

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 (1,500-1,700° F) 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).

This graph represents strength as a function of temperature, which is expressed in degrees Celsius (C).

At temperatures above 800° C structural steel loses 90 percent of its strength. ¹ Yet even when steel structures are heated to those temperatures, they never disintegrate into piles of rubble, as did the Twin Towers and Building 7. Why couldn't such dramatic reductions in the strength of the steel precipitate such total collapse events?

High-rise buildings are over-engineered to have strength many times greater than would needed to survive the most extreme conditions anticipated. It may take well over a ten-fold reduction in strength to cause a structural failure.
If a steel structure does experience a collapse due to extreme temperatures, the collapse tends to remain localized to the area that experienced the high temperatures.
The kind of low-carbon steel used in buildings and automobiles bends rather than shatters. If part of a structure is compromised by extreme temperatures, it may bend in that region, conceivably causing a large part of the structure to sag or even topple. However, there is no example of a steel structure crumbling into many pieces because of any combination of structural damage and heating, outside of the alleged cases of the Twin Towers and Building 7.

References

1. Effect of temperature profile, corusconstruction.com, [cached]
2. Fire Resistance of Steel Framed Car Parks, corusconstruction.com, [cached]

page last modified: 2007-09-30

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