Final Reports of the Federal Building and Fire
Investigation of the World Trade Center Disaster
NIST NCSTAR 1-6: Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
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.
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