The structural steel of the World Trade Center Towers

In September 2002, the National Institute of Standards and Technology (NIST) became the lead agency in the investigation of the World Trade Center (WTC) terrorist attacks of September 11, 2001. The investigation addresses many aspects of the building collapse, from occupant egress to factors affecting how long the towers stood after being hit by the airplanes, with the goal of gaining valuable information for the future. The complete plan for the NIST investigation is available at http://wtc.nist.gov. A major part of the investigation is the metallurgical analysis of structural steel from the World Trade Center (Fig. 1). The analysis includes characterization of mechanical properties, failure modes, and temperature excursions endured by the steel. This overview on the metallurgical investigation describes the structure of the towers, steel recovered from the site, and special issues faced in the analysis of the steel.