The release of the revised report on the original Seven World Trade Center in New York City marks the end of the $16-million investigation into the World Trade Center. The investigation was triggered by the destruction of the complex on Sept. 11, 2001, by terrorists.

Based on feedback, Gaithersburg, Md.-based National Institute of Standards and Technology made several changes to the draft final report on the 47-story steel-framed tower, which collapsed on Sept. 11 after burning unattended for some seven hours. The report was released for public comment on Aug. 21. The revisions did not alter the investigation team’s major findings. According to NIST, the fire heated floor beams and girders, causing a critical column to fail.

In response to comments, NIST conducted an additional computer analysis to see if the loss of column 79, identified as the trigger for a progressive collapse, would still have led to a complete loss of the building if fire or damage from the falling debris of the nearby 110-story One WTC tower were not factors. The investigation team concluded the column’s failure under any circumstance would have initiated the destructive sequence of events.

Other revisions to the final WTC 7 report include an expanded discussion of firestopping, material which was placed between floors to prevent floor-to-floor fire spread; a clarification of the description of thermal expansion as it related to shear studs and floor beams; and a more detailed explanation of the computer-modeling approach used to define where and when the fire started and the extent of window breakage as a result of fire.

The report can be accessed at http://wtc.nist.gov