NIST updates technical findings on World Trade Center collapses

Twenty-five years after the September 11 attacks, the National Institute of Standards and Technology (NIST) has updated its technical documentation regarding the collapse of the World Trade Center towers. The update reaffirms the findings of the 2005 investigation, which concluded that the collapses were caused by the loss of fire protection on the steel structures due to debris and fuel impacts, followed by intense multi-floor fires.

Technical simulations show that while temperatures reached approximately 1,000 degrees Celsius, the steel did not reach its melting point of 1,500 degrees. However, at 1,000 degrees, the steel retained only about ten percent of its room-temperature strength. This led to floor sagging, which pulled exterior columns inward, causing structural instability.

The investigation, supported by the National Construction Safety Team Act of 2002, resulted in 31 recommendations. These led to over forty modifications to the International Code Council codes and fifteen changes to NFPA standards, impacting building safety, fire protection, and elevator design.