Reports of the Federal Building and Fire
Investigation of the World Trade Center Disaster

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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.

 

• NIST NCSTAR 1-2: Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers
  • NIST NCSTAR 1-2A: Reference Structural Models and Baseline Performance Analysis of the World Trade Center Towers
  • NIST NCSTAR 1-2B: Analysis of Aircraft Impacts into the World Trade Center Towers (Chapters 1-8)
  • NIST NCSTAR 1-2B: Analysis of Aircraft Impacts into the World Trade Center Towers (Chapters 9-11)
    • 1-2B appendix

 

 

 

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Last updated:  November 2008