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iv NIST NCSTAR 1-2, WTC Investigation • 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.

Baseline structural performance and aircraft impact damage analysis of the World Trade Center towers