Probable Cause for WTC Collapse Finalized

April 14, 2005
NIST concludes the most detailed examination of a building failure ever conducted
On April 5 at a press briefing, the National Institute of Standards and Technology (NIST) presented conclusions on how the World Trade Center (WTC) towers collapsed after the 9/11 terrorist attack, during which two commercial aircraft were flown into the buildings.According to Shyam Sunder, lead investigator for the agency’s building and fire safety investigation into the WTC disaster, “While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts. This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled.”The finalization of this sequence, confirms the hypotheses released by NIST last October. Extensive computer modeling and evidence held by NIST (including photos, video, recovered steel, eyewitness accounts, and emergency communication record) support the probably collapse sequence presented.As a result of this investigation and analysis, NIST will release recommendations for improvements to building and fire codes, standards, and practices for public comment in June. For further information, visit (

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