Final Reports of the Federal Building and Fire
Investigation of the World Trade Center Disaster
NIST NCSTAR 1-2: Baseline Structural Performance and Aircraft
Impact Damage Analysis of the World Trade Center Towers
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 interstory
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 frametube
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.
In order to develop the reference models and conduct the baseline
performance analyses, the following
steps were undertaken:
- Develop structural databases for the primary structural components of the
WTC 1 and WTC 2
towers from the original computer printouts of the structural design
documents.
- Develop reference structural analysis models that captured the intended
behavior of each of
the two towers using the generated databases. These reference models were
used to establish
the baseline performance of the towers and also served as a reference for
more detailed
models for aircraft impact damage analysis and thermal-structural response
and collapse
initiation analysis. The models included: (1) two global models (one for
each tower) of the
major structural components and systems of the towers, and (2) floor models
of a typical
truss-framed floor and a typical beam-framed floor.
- Develop estimates of design gravity (dead and live loads) and wind loads
on each of the two
towers for implementation into the reference structural models. The
following three loading
cases were considered:
- Original WTC design loads case. Loads included dead and live loads as
in original
WTC design, in conjunction with original WTC design wind loads.
- State-of-the-practice case. Loads included dead loads; current New York
City Building
Code (NYCBC 2001) live loads; and wind loads from the RWDI wind tunnel
study,
scaled in accordance with NYCBC 2001 wind speed.
- Refined NIST estimate case. Loads included dead loads; live loads from the
American
Society of Civil Engineers (ASCE) 7-02 Standard (a national standard); and
refined wind
loads developed by NIST.
- Perform structural analyses to establish the baseline performance of each
of the two towers
under design gravity and wind loads.
For the second task related to aircraft impact, the aircraft impact
damage to the exterior of the
WTC towers could be visibly identified from the video and photographic
records. However, no visible
information could be obtained for the extent of damage to the interior of
the towers, including the
structural system (floors and core columns), partition walls, and interior
building contents. Such
information was needed for the subsequent fire dynamics simulations and
post-impact structural analyses.
In addition, for the fire dynamics modeling, the dispersion of the jet fuel
and the location of combustible
aircraft debris were required. The estimate of the extent of damage to the
fireproofing on the structural
steel in the towers due to impact was essential for the thermal and
structural analyses. The aircraft impact
damage analyses were the primary tool by which most of the information on
the tower damage could be
estimated.
The focus of the analysis was to analyze the aircraft impacts into each
of the WTC towers to provide the
following: (1) estimates of probable damage to structural systems, including
exterior walls, floor
systems, and interior core columns; (2) estimates of the aircraft fuel
dispersion during the impact; and (3)
estimates of debris damage to the building nonstructural contents, including
partitions and workstations.
The results were to be used to estimate the damage to fireproofing based on
the predicted path of the
debris field inside the towers. This analysis thus estimated the condition
of the two WTC towers
immediately following the aircraft impacts and established the initial
conditions for the fire dynamics
modeling and the thermal-structural response and collapse initiation
analysis. The impact analyses were
conducted at various levels of complexity including: (1) the component
level, (2) the subassembly level,
and (3) the global level to estimate the probable damage to the towers due
to aircraft impact.
In order to estimate the aircraft impact damage to the WTC towers, the
following steps were undertaken:
- Constitutive relationships were developed to describe the behavior and
failure of the
materials under the dynamic impact conditions of the aircraft. These
materials included the
various grades of steels used in the exterior walls, core columns, and floor
trusses of the
towers, weldment metal, bolts, reinforced concrete, aircraft materials, and
nonstructural
contents.
- Global impact models were developed for the towers and aircraft. The
tower models
included the primary structural components of the towers in the impact zone,
including
exterior walls, floor systems, core columns, and connections, along with
nonstructural
building contents. A refined finite element mesh was used for the areas in
the path of the
aircraft, and a coarser mesh was used elsewhere. The aircraft model included
the aircraft
engines, wings, fuselage, the empennage, and landing gear, as well as
nonstructural
components of the aircraft. The aircraft model also included a
representation of the fuel,
using the smooth particle hydrodynamics approach.
- Component and subassembly impact analyses were conducted to support the
development of
the global impact models. The primary objectives of these analyses were to
(1) develop an
understanding of the interactive failure phenomenon of the aircraft and
tower components,
and (2) develop the simulation techniques required for the global analysis
of the aircraft
impacts into the WTC towers, including variations in mesh density and
numerical tools for
modeling fluid-structure interaction for fuel impact and dispersion. The
component and
subassembly analyses were used to determine model simplifications for
reducing the overall
model size while maintaining fidelity in the global analyses.
- Initial conditions were estimated for the impact of the aircraft into the
WTC towers. These
included the aircraft speed at impact, aircraft orientation and trajectory,
and impact location
of the aircraft nose. The estimates also included the uncertainties
associated with these
parameters. This step utilized the videos and photographs that captured the
impact event and
subsequent damage to the exterior of the towers.
- Sensitivity analyses were conducted at the component and subassembly
levels to assess the
effect of uncertainties on the level of damage to the towers due to impact
and to determine the
most influential parameters that affect the damage estimates. The analyses
were used to
reduce the number of parameters that would be varied in the global impact
simulations.
- Analyses of aircraft impact into WTC 1 and WTC 2 were conducted using the
global tower
and aircraft models. The analysis results included the estimation of the
structural damage that
degraded their strength and the condition and position of nonstructural
contents such as
partitions, workstations, aircraft fuel, and other debris that influenced
the behavior of the
subsequent fires in the towers. The global analyses included, for each
tower, a “base case”
based on reasonable initial estimates of all input parameters. They also
provided a range of
damage estimates based on variations of the most influential parameters.
This range included
more severe and less severe damage cases.
- Approximate analyses were conducted to provide guidance to the
global finite element
impact analyses. These included: (1) analysis of the overall aircraft
impact forces and
assessment of the relative importance of the airframe strength and
weight distribution,
(2) evaluation of the potential effects of the energy in the rotating
engine components on the
calculated engine impact response, (3) influence of the static preloads
in the towers on the
calculated impact damage and residual strength predictions, and (4)
analysis of the load
characteristics required to damage core columns compared to the
potential loading from
impact of aircraft components.
ERRATUM
Baseline Structural Performance and Aircraft Impact Damage Analysis
of the World Trade Center Towers (NCSTAR 1-2)
Page 167, Paragraph 1, Line 8
The reports provided here are presented in .pdf format. To read these files, you can download
the
latest version of Adobe Acrobat Reader free. If you have difficulty
reading pdf files, you may need to update
your version of Acrobat reader to the latest version. These 5 pdf
files for Project 1-2 may also be downloaded as a group within a
WINZIP (.zip) file. (Note: this
file is very large - 130 MB)
