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Reports of the Federal Building and Fire
Investigation of the World Trade Center Disaster
NIST NCSTAR 1-9: Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7 (two volumes)
This is the primary technical report on the investigation of the collapse of World Trade Center Building 7 (WTC 7), providing technical details of all aspects of the investigation, including:
- The procedures and practices used in the design, construction, operation, and maintenance of the building; structural features, interior layout and contents, and fire protection measures; and the emergency power systems (Chapters 2, 3, and 4);
- Photographic and videographic records that provided timed data on the impact damage from the collapse of WTC 1, the building fires, and the structural collapse (Chapter 5);
- The building evacuation and emergency response (Chapters 6 and 7);
- Structural evaluation of the visual evidence of the structural collapse, comparison of uncontrolled fires in other tall office buildings with the fires in WTC 7, the leading hypothesis for the collapse of WTC 7, and evaluation of other hypotheses (Chapter 8);
- Computer simulations of the spread and growth of the sustained fires of ordinary combustibles on the tenant floors and possible diesel fuel fires on the lower mechanical floors (Chapter 9);
- Computer simulations of the heating of the insulated structural components, based on the time-varying gas temperatures from the fire simulations (Chapter 10);
- Computer simulations of the structural response to the fires and debris impact damage, the propagation of structural damage from the initial failure event up to the global collapse of WTC 7 (Chapters 11 and 12);
- The probable collapse sequence for WTC 7, based on the evidence and the computer simulations (Chapter 13); and
- The findings of the NIST Investigation of WTC 7 (Chapter 14).
This report also contains five appendices, which provide supporting background information on the Con Edison substation in WTC 7, analysis of the September 11, 2001 seismogram data recorded near the WTC site, video analysis of WTC 7 building vibrations before its collapse, analyses of hypothetical blast scenarios, and plasticity and creep models for WTC 7 steels.
NIST NCSTAR 1-9A: Global Structural Analysis of the Response of World Trade Center Building 7 to Fires and Debris Impact Damage
This report presents analysis of the global response of WTC 7 to initial failure events due to fire and the resulting sequence of component and subsystem failures to determine the events that led to the global collapse. The WTC 7 finite element model used in the analyses was based on the structural and fabrication shop drawings. The model construction techniques and steel material models were adapted from the modeling of the WTC towers. New models were developed for the composite floor slabs and for the various types of connections in WTC 7. A model initialization sequence was developed to sequentially apply gravity loads, WTC 1 impact debris damage, and fire induced temperatures on the structure prior to model analyses. Development of the whole-building model was guided by the results from a two floor model and a 14 story subassembly model.
Using the 47 story model, global analyses were conducted of the progression from initial localized failures to global collapse. The sequence of events within the model were documented and compared to observable behavior from photographic evidence.
- NIST NCSTAR 1-9: Volume 1: Chapters 1 - 8 *
- NIST NCSTAR 1-9: Volume 2: Chapters 9 - Appendix E *
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Last updated: January 2009
