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

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 World Trade Center Website, 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 World Trade Center 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 under way, using detailed three-dimensional 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 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 as part of an insurance litigation involving the World Trade Center towers. NIST did not perform the tests. 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.

Kudos for Workers

Your editorial, "Working with Hands and Tools is a Noble Activity," is a noble move in its own right (12/29/03 p. 68). American construction workers are truly an underappreciated lot. They rarely get the opportunity to perform their tasks in the comfort of a controlled, safe environment. It takes guts and stamina to crawl out of bed, travel to ever-changing jobsite locations, and perform skilled tasks in the stifling humidity of summer or the bitter cold winds of winter.

In the various trade publications to which I subscribe, there are far too many pictures and kudos to the "building team" of executives and consequently too little praise for the foot soldiers who actually got the job done. Your cover story is an appropriate tribute to the men and women who have chosen these professions. Why not take a couple of pages of each weekly issue of ENR for such a photographic tribute to these professionals?