Therese McAllister, Ph.D., P.E.
Building and Fire Research Laboratory
National Institute of Standards and Technology
U.S. Department of Commerce
Therese.McAllister@nist.gov
Federal Building and Fire Safety Investigation
of the World Trade Center Disaster
WTC 7 Technical Approach and Status
Summary
December 12, 2006
Status of Investigation
More comprehensive approach for all technical issues
Progress to date
No findings or conclusions will be presented as the analysis is ongoing
Status Summary Status
When NIST initiated the WTC investigation, it made a decision not to hire new
staff to support the investigation.
After the June 2004 progress report on the WTC investigation was issued, the
NIST investigation team stopped working on WTC 7 and was assigned fulltime
through the fall of 2005 to complete the investigation of the WTC
towers.
With the release and dissemination of the report on the WTC towers in October 2005, the investigation of the WTC 7 collapse resumed.
Considerable progress has been made since that time, including:
review of nearly 80 boxes of new documents related to WTC 7
development of detailed technical approaches for modeling and analyzing various collapse hypotheses
selection of a contractor to assist NIST staff in carrying out the analyses.
It is anticipated that a draft report will be released by Spring 2007.
Working Collapse Hypothesis Working Collapse
The current NIST working collapse hypothesis for WTC 7 is described in the June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster (Volume 1, page 17, as well as Appendix L), as follows:
An initial local failure occurred 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 square feet;
Vertical progression of the initial local failure occurred up to the east
penthouse, and as the large floor bays became unable to redistribute the
loads, it brought down the interior structure below the east penthouse; and
Triggered by damage due to the vertical failure, 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) resulted in a disproportionate collapse of the entire structure.
This hypothesis may be supported or modified, or new hypotheses may be developed, through the course of the continuing investigation.
Hypothetical Blast Analysis
NIST is analyzing scenarios for the event that initiated the collapse of WTC 7. As a part of this work, NIST is considering whether hypothetical blast events could have played a role in initiating the collapse. While NIST has found no evidence of a blast or controlled demolition event, NIST will estimate the magnitude of hypothetical blast scenarios that could have led to the structural failure of one or more critical elements as a result of blast.
Phase I Identify hypothetical blast scenarios and materials, based on
analysis and/or experience, for failing specified columns by direct
attachment methods. Preliminary section cutting shall be considered.
Compare estimated overpressures for each scenario against window
strength.
Phase II For blast scenarios with overpressures that clearly would not have broken windows, the worst case scenario(s) will be analyzed using SHAMRC software to determine overpressures at windows.
Phase III If Phase II overpressures did not clearly fail windows, 3 blast scenarios will be selected to determine the sound levels that would be
transmitted outside the building through intact windows.
ANSYS Analysis of Initiating Events
The response of WTC 7 to debris impact damage and fire events is being
analyzed to identify initiating events of component and subsystem failures
leading to global collapse.
The analysis will use an ANSYS model of the lower 16 floors, as significant
fires were observed between floors 7 and 13.
The ANSYS model includes steel framing, concrete slabs with smeared
properties for steel decks and reinforcement, material properties for elastic,
plastic, and creep behavior at elevated temperatures, break elements for
connection capacity and service gravity loads.
Analyses of up to 12 damage and fire scenarios will be based on:
two structural damage states based on available evidence
assumed fireproofing conditions
temperature histories that may be up to 7 h long.
The fireproofing condition was not directly observed, and its condition is based on analysis that considers the FP bond strength, accelerations imparted by the debris impact, and observations from the Banker’s Trust building.
LS-DYNA Analysis of Global Response to Initiating Events
The LS-DYNA global analyses will determine if global collapse will result from
initiating events (due to fire or blast) which will be input as initial conditions.
Task 1. Floor Component and Subsystem Analyses
Determine floor response and failure modes for up to 20 initiating event
scenarios
Develop an equivalent floor model mesh appropriate for global analyses
Task 2. Global Analyses
Determine global structural stability for two damage states
Conduct a sensitivity study where up to 20 initiating events are
analyzed, through member removal or other appropriate approach, to
determine structural stability following the loss of support and the
sequence of member failures that result from a given scenario
Conduct final analyses that simulate the initiating event, whether due to
fire or other effects, and the subsequent failure sequence up to global
collapse (including the vertical and horizontal progression of failures up
to the point of global instability)
