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Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster (NIST NCSTAR 1-6) ***DRAFT for Public Comments***



John L. Gross, Therese P. McAllister


One of the four main objectives of the National Institute of Standards and Technology (NIST) investigation of the collapse of the World Trade Center (WTC) towers was to determine why and how the two towers collapsed. Events that played a significant role in the structural performance of the towers were the aircraft impact, the rapid ignition of fires on multiple floors, the growth and spread of fires and the structural weakening resulting from effects of high temperatures. The passive fire protection applied to the steel structural components in the WTC towers was investigated to provide information on the in-place condition of the fire protection before and after aircraft impact. Standard fire resistance tests were conducted to establish the appropriate classification (fire resistance rating) of the original design of the WTC floor system and to develop insight into the structural performance of the composite steel and concrete floor system under exposure to a standard fire. Results of simulations of the aircraft impacts were used to predict damage to the structure, fire protection, and partition walls in the path of the debris field. Characterization of the temperatures of the structural components, determined from simulated WTC fires, allowed the calculation of the performance of major subsystems constituting the structural system of the towers including the core framing, the exterior wall (columns and spandrels), and full tenant floors. Insights gained from these analyses were used, in turn, to formulate and execute nonlinear, temperature-dependent finite element analyses of global structural systems to predict the collapse sequence of each tower. The structural analyses were guided, and where possible validated, by observations made from the review of thousands of photographs and video recordings.
National Construction Safety Team Act Reports (NIST NCSTAR) - 1-6
Report Number


World Trade Center, high rise buildings, building collapse, disasters, fire safety, fire investigations, terrorists, terrorism, fire protection, fire resistance tests, aircraft impact, time, stability, structural analysis, structural damage, structural response
Created September 1, 2005, Updated February 19, 2017