On the morning of September 11, 2001, Americans and people around the world were shocked by the destruction of the World Trade Center in New York City and the devastation of the Pentagon near Washington, D.C., after large aircraft were flown into the buildings, and the crash of an aircraft in a Pennsylvania field that averted further tragedy.

Published stories of the survivor's of the World Trade Center (WTC) attacks were collected to document the event and as background material to guide the development of the National Institute of Standards and Technology (NIST) Investigation on occupant behavior during the evacuation of the WTC.

This is the final report of the National Institute of Standards and Technology (NIST) investigation of the collapse of the World Trade Center (WTC) towers, conducted under the National Construction Safety Team Act.

The Nat. Institute of Standards and Technology (NIST) announced its 24-month building and fire safety investigation of the World Trade Center (WTC) disaster on Aug. 21, 2002. At the same time, NIST also released the final plan for its investigation. In addition, NIST provided a public update on the progress of the investigation on Dec. 9, 2002. This volume, a public update issued in May 2003, summarizes the progress made by NIST and the cooperation it has received from a variety of organizations since the December update. Chapters: Implementation of Legislative Authorities; Progress on WTC Investigation; Update on WTC R&D Program; Update on WTC Dissemination and Technical Assistance Program. 8 Appendices. Illustrated.

This report describes how the fires that followed the impact of debris from the collapse of WTC 1 (the north tower) led to the collapse of WTC 7; an evaluation of the building evacuation and emergency response procedures; what procedures and practices were used in the design, construction, operation, and maintenance of the building; and areas in current building and fire codes, standards, and practices that warrant revision. Also in this report is a summary of how NIST reached its conclusions. NIST complemented in-house expertise with private sector technical experts; accumulated copious documents, photographs, and videos of the disaster; conducted first-person interviews of building occupants and emergency responders; analyzed the evacuation and emergency response operations in and around WTC 7; performed computer simulations of the behavior of WTC 7 on September 11, 2001; and combined the knowledge gained into a probable collapse sequence. The report concludes with a list of 13 recommendations for action in the areas of increased structural integrity, enhanced fire endurance of structures, new methods for fire resistant design of structures, enhanced active fire protection, improved emergency response, improved procedures and practices, and education and training. One of these is new; the other 12 are reiterated from the investigation into the collapse of the towers. Each of the 13 is relevant to WTC 7.

This report analyzes the structural steel available from World Trade Center (WTC) 1, 2, and 7 to determine the metallurgical and mechanical properties and quality of the metal, weldments, and connections and to provide these data for other analyses in the National Institute of Standards and Technology (NIST) Investigation.

This report analyzes the global response of WTC 7 to fireinduced structural failures and the resulting sequence of component and subsystem failures to determine the events that led to the global collapse of the building. The analysis involved a finite element model of the entire building, which was developed in LS-DYNA. Preliminary work involved twostory and 14 story subassemblies. The 47 story global model was based on the original structural drawings as well as construction fabrication shop drawings for connections. The model included the primary structural componentsthroughout the building, including exterior and core columns, floor framing beams and girders, floor slabs, column transfer truss structures, wind bracing truss members, and penthouse structures.