Statement of Clarification

NIST sent the following letter to the Editor of Engineering News-Record on January 15, 2004:

Let me make a few clarifications on your article "Research May Never Pinpoint Sequence of Events on 9/11" (ENR 1/19 p. 12) regarding the federal building and fire safety investigation of the World Trade Center disaster being conducted by the National Institute of Standards and Technology.

First, NIST seeks to determine the most probable structural collapse sequence for each tower, from the several scenarios under consideration, using established statistical and probabilistic analysis methods. The methodology is described in Appendix 5, p. 93, of the May 2003 progress report that is available on the NIST WTC Web site (http://wtc.nist.gov). While we recognize that it is possible there may not be a unique collapse sequence, we are not far enough along in our analyses to make that determination at this time.

Second, the preliminary NIST finding that the WTC tower perimeter column would yield first is from an analysis of a scenario that considers only uniform thermal expansion of a floor. Work is still underway, using detailed 3D computer models of the fires and aircraft impact, to analyze this and other leading scenarios or hypotheses and to determine the sequence of events that led to the initiation of collapse of each of the WTC tower structures. NIST will report on the results of those analyses in future updates.

Third, the two sets of wind tunnel tests performed in 2002 were conducted for the parties to an insurance litigation involving the WTC towers and not by NIST. They were performed by two independent firms that routinely conduct such tests for use in design practice. Reports from those tests were made available to the NIST investigation by the parties to the litigation.

S. Shyam Sunder
Lead Investigator
National Institute of Standards and Technology
Gaithersburg, Md.

Created: 1/26/2004
Updated: January 26, 2004