Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers. Chapter 7-Appendix B. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.
Structural Fire Response and Probable Collapse Sequence
of the World Trade Center Towers. Chapter 7-Appendix B.
Federal Building and Fire Safety Investigation of the
World Trade Center Disaster.
Gross, J. L.; McAllister, T. P.
NIST NCSTAR 1-6; 204 p. September 2005.
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
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. This report covers the
characterization of the conditions of the WTC towers
before the attacks, their weakening due to the aircraft
impacts, the response of the structural systems to the
subsequent growth and spread of fires, and the
progression of local failures that led ultimately to the
total collapse of both towers.