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Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers. Appendices A-E. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.


pdf icon Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers. Appendices A-E. Federal Building and Fire Safety Investigation of the World Trade Center Disaster. (60195 K)
Sadek, F.

NIST NCSTAR 1-2; APPENDICES A-E; 41 p. September 2005.

Keywords:

World Trade Center; high rise buildings; building collapse; disasters; fire safety; fire investigtions; terrorism; terrorists; aircraft impact; damage; structural design; wind tunnels; wind velocity; wind effects

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.