In August 2002, the U.S. National Institute of Standards and Technology (NIST) launched what would become a six-year investigation of the three building failures that occurred on September 11, 2001 (9/11): the well-known collapses of the World Trade Center (WTC) Twin Towers that morning and the lesser-known collapse late that afternoon of the 47-story World Trade Center Building 7, which was not struck by an airplane. NIST conducted its investigation based on the stated premise that the “WTC Towers and WTC 7 [were] the only known cases of total structural collapse in high-rise buildings where fires played a significant role.”
Indeed, neither before nor since 9/11 have fires caused the total collapse of a steel-framed high-rise—nor has any other natural event, with the exception of the 1985 Mexico City earthquake, which toppled a 21-story office building. Otherwise, the only phenomenon capable of
collapsing such buildings completely has been by way of a procedure known as controlled demolition, where by explosives or other devices are used to bring down a structure intentionally. Although NIST finally concluded after several years of investigation that all three collapses on 9/11 were due primarily to fires, fifteen years after the event a growing number of architects, engineers, and scientists are unconvinced by that explanation.
Preventing high-rise failures
Steel-framed high-rises have endured large fires without suffering total collapse for four main reasons:
1) Fires typically are not hot enough and do not last long enough in any single area to generate enough energy to heat the large structural members to the point where they fail (the temperature at which structural steel loses enough strength to fail is dependent on the factor of safety
used in the design. In the case of WTC 7, for example, the factor of safety was generally 3 or higher. Here, 67% of the strength would need to be lost for failure to ensue, which
would require the steel to be heated to about 660°C);
2) Most high-rises have fire suppression systems (water
sprinklers), which further prevent a fire from releasing
sufficient energy to heat the steel to a critical failure state;
3) Structural members are protected by fireproofing materials, which are designed to prevent them from reaching failure temperatures within specified time periods; and
4) Steel-framed high-rises are designed to be highly redundant structural systems. Thus, if a localized failure occurs, it does not result in a disproportionate collapse of the entire structure.