Previous Analyses
Previous analyses were conducted by NIST as part of the WTC Investigation. The work will be made available to the Contractor as background information. The work of the Contractor shall not be limited by these models, hypothesis, or analysis results. A working collapse hypothesis is described in the June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster (Volume 1, pg 17), as follows:

• An initial local failure at the lower floors (below floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event), which supported a large span floor bay with an area of about 2,000 ft2,
• Vertical progression of the initial local failure up to the east penthouse, as large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse; and
• Horizontal progression of the failure across the lower floors (in the region of floors 5 and 7 that were much thicker and more heavily reinforced than the rest of the floors), triggered by damage due to the vertical failure, resulting in disproportionate collapse of the entire structure.

This hypothesis may be supported or modified, or new hypotheses may be developed through the course of this study.

Previous models and analyses included:
• Identification of events and data from photographs, videos, and witness accounts.
• Studies of fire spread and growth on Floors 5, 7, and 8.
• Studies of heat transfer to Core Columns 79, 80, and 81 on Floors 5, 7, and 8.
• SAP2000 linear global model of WTC 7, which was based on structural drawings, that included beam elements for columns and floor beams, and representative shell elements for reinforced concrete floor slabs. The model was evaluated for design gravity and wind loads, service gravity loads, and stability for damage conditions caused by debris impact.
• Studies of Core Column 79 (see Appendix A) response to elevated temperature profiles for collapse initiation, using Mathcad calculations and an ANSYS shell element model.
• SAP2000 model of a single floor model representative of Floors 8 to 46. The floor model was more detailed than the floors in the global model; plastic hinges were added at specific locations to beams and columns. Core columns support was removed (i.e. Columns 76 to 81 were removed individually in separate analyses) to determine the mode of floor failure as part of the analysis of the vertical progression of failure.
• SAP2000 model for the horizontal progression analyses of failure across the lower core columns. The model was extracted from the global model and plastic hinges were added at specific locations to beams and columns. The horizontal progression was analyzed by removal of components or application of an action resulting from a failure of Floor 5 or Floor 7.
• SAP2000 kinematic model (frame elements with all beam-columns connections assumed to be pinned) was used to evaluate the effect of assumed column failures and determine the resulting deformed shape of the structure.

The models will be made available to the Contractor for use as appropriate. The input files have been updated to SAP2000 version 10.

The objective of this solicitation is (1) to conduct analyses that support the determination of the location and cause of the initiating event and the probable collapse sequence, in conjunction with parallel NIST analyses, and (2) to validate the results with observations from video and photographic records and other evidence.

Scope of Work
In this scope of work, the determination of the location and cause of the initiating event is given primary importance. The sequence of failures following the initiating event that led to global collapse, while also important, is dependent upon the proper identification of the initiating event.

While the sequence of failures following the initiating event may be adequately addressed with less detailed analyses, the analyses must be of sufficient rigor to support the identification of the probable sequence of failures.

For all tasks included in this solicitation, the models, assumptions, and analyses will be subject to review and approval by NIST. NIST will arrange for third parties to conduct independent reviews before final approval. NIST plans to retain a third party expert in structural system behavior, structural stability, and failure criteria for members and system failure. The third party expert will provide expert technical assistance to guide and assist the Contractor’s work, but it is the Contractor’s responsibility to conduct the work described in this SOW. Third party experts will also review Contractor reports for: (1) appropriateness of the models for their intended uses, including modeling assumptions, level of detail, model geometry and material properties, and
verification and validation procedures; and (2) appropriateness of the analyses and accuracy of results.