Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.
Baseline Structural Performance and Aircraft Impact
Damage Analysis of the World Trade Center Towers.
Federal Building and Fire Safety Investigation of the
World Trade Center Disaster.
(60195 K)
Sadek, F.
NIST NCSTAR 1-2; 462 p. September 2005.
Keywords:
World Trade Center; high rise buildings; building
collapse; disasters; terrorists; terrorism; aircraft
impact; damage; floors; wind effects; wind velocity;
impact; gravity; fire dynamics; methodology; fire
safety; fire investigations; wind tunnels
Abstract:
The baseline structural performance and aircraft impact
damage analysis of the National Institute of Standards
and Technology (NIST) Investigation of the World Trade
Center (WTC) disaster had two primary tasks: (1) to
develop reference structural models of the WTC towers
and use these models to establish the baseline
performance of each of the towers under gravity and wind
loads, and (2) to estimate the damage to the towers due
to aircraft impacts and establish the initial conditions
for the fire dynamics modeling and the
thermal-structural response and collapse initiation
analysis. This report provides the technical approach.
methodology, and results related to both tasks. For the
first task, the baseline performance of the WTC towers
under gravity and wind loads was established in order to
assess the towers' ability to withstand those loads
safely and to evaluate the reserve capacity of the
towers to withstand unanticipated events. The baseline
performance study provides a measure of the behavior of
the towers under design loading conditions,
specifically: (1) total and inter-story drift (the sway
of the building under design wind loads). (2) floor
deflections under gravity loads, (3) the stress
demand-to-capacity ratio for primary structural
components of the towers such as exterior walls, core
columns, and floor framing, (4) performance of exterior
walls under wind loading, including distribution of
axial stresses and presence of tensile forces, (5)
performance of connections between exterior columns, and
(6) resistance of the towers to shear sliding and
overturning at the foundation level. Wind loads were a
governing factor in the design of the structural
components that made up the frame-lube steel framing
system. Wind load capacity was also a key factor in
determining the overall strength of the towers and was
important in determining not only the ability of the
towers to withstand winds but also the reserve capacity
of the towers to withstand unanticipated events such as
major fire or impact damage. Accurate estimation of the
wind load on tall buildings is a challenging task, given
that wind engineering is still an evolving technology.
For example, estimates of the wind-induced response
presented in two recent independent studies of the WTC
towers differed from each other by about 40 percent. In
this study, NIST developed refined estimates of wind
effects by critically assessing information obtained
from the Cermak Peterka Peterson. Inc. (CPP) and Rowan
Williams Davis and Irwin, Inc. (RWDI) reports and by
bringing to bear state-of-the-art considerations.
Furthermore, the available prescriptive codes specify
wind loads on tall buildings that are significantly
lower than wind tunnel-based loads. This case study
provided an opportunity to assess effectively current
design practices and various code provisions on wind
loads. For the purpose of establishing the baseline
performance of the towers, various wind loads were
considered in this study, including wind loads used in
the original WTC design, wind loads based on two recent
wind tunnel studies conducted in 2002 by CPP and RWDI
for insurance litigation concerning the towers, and
refined wind load estimates developed by NIST.
