Fire Structure Interface and Thermal Response of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.
Fire Structure Interface and Thermal Response of the
World Trade Center Towers. Federal Building and Fire
Safety Investigation of the World Trade Center Disaster.
(24171 K)
Prasad, K. R.; Baum, H. R.
NIST NCSTAR 1-5G; 338 p. September 2005.
Keywords:
World Trade Center; high rise buildings; building
collapse; disasters; fire safety; fire investigations;
terrorists; terrorism; structures; thermal response;
flameproofing; radiative heat transfer; thickness;
columns; beams; concrete slabs
Abstract:
The collapse of the World Trade Center (WTC) towers on
September 11, 2001, resulting from a combination of
aircraft impact damage and subsequent fires, was studied
as part of the Federal Building and Fire Safety
Investigation of the WTC Disaster. This report documents
the evolving thermal state of the structure on the focus
floors of each tower. A methodology was developed to
couple the thermal response of the towers to the fire
dynamic simulations. Heat transfer to sub-grid scale
structural elements was computed using a simple
radiative transport model that assumes the compartment
can be locally divided into a hot, sooty upper layer and
a cool relatively clear lower layer. Properties of the
two layers were extracted from temporal averages of the
results of the fire simulations. The model predictions
were found to compare favorably with measurements from a
series of large-scale experiments. Exploratory studies
were conducted to estimate the role of fireproofing
thickness and sensitivity of the results to input
parameters. The methodology was subsequently used
extensively in National Institute of Standards and
Technology's (NIST's) WTC Investigation to predict the
thermally induced structural response to spatially and
temporally developing fires. Finite element models were
constructed for the steel structural elements and the
fireproofing that covers the floor trusses, core beams,
perimeter and core columns as well as the concrete slab
on floors 92-99 of the North Tower and floors 78-83 of
the South Tower. Structural and fireproofing damage due
to aircraft impact on the various floors of each tower
was incorporated into the models. The three dimensional
time dependent thermal response of the structural
elements was generated in a format that is consistent
with the structural models and the finite element
analysis software. Four global simulations, two each for
WTC 1 and WTC 2 are reported for prediction of thermally
induced structural response and collapse of the towers.
Based on the results of the global simulations,
fireproofing thickness and fireproofing damage due to
aircraft impact was identified as the single most
important parameter that had the largest effect on steel
temperature and on the thermally induced structural
response.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899