November 1, 2004—Aircraft impact to the World Trade Centers structural components resulted in redistribution of column loads and damaged the floor framing, core columns, and fireproofing, eventually causing the towers to collapse, the National Institute for Standards and Technology found in an investigation of collapse of the WTC towers.
“Had the fireproofing not been dislodged, the temperature rise of the structural components would likely have been insufficient to cause the global collapse of the towers. Fireproofing dislodged by debris left the components more sensitive to heat than any areas where there was missing or think fireproofing before the aircraft impacts,” investigators noted in their October 19 update of findings related to the collapse of the towers on Sept. 11, 2001.
The team also found that the about 87 percent of the recovered WTC steel specimens tested exceeded the required minimum yield strengths specified in the building criteria. “However, the safety of the towers was most likely not affected by the small percentage of steel below the minimum. Building designs routinely allow structures to withstand greater loads than are expected by including significant factors of safety. Moreover, the structural loads on September 11, 2001, were well below this design level,” NIST reported.
The NIST WTC investigations goal is to recommend improvements in the way people design, construct, maintain, and use buildings, especially high-rises. NIST plans to release its draft report and recommendations for public comment in December 2004 or January 2005. Current findings, including those released on October 19, may still be revised prior to release of the final report later in 2005.
For more information on the latest findings from the World Trade Center investigation, visit NIST.
