Fire Model Validation Experiments and Fire Testing of the WTC Floor System
NIST is using a combination of analytical, experimental and numerical tools to analyze the various collapse scenarios hypothesized for the WTC buildings. The progress report details the key factors that NIST is considering in this analysis; the elements required to simulate the thermal and tenability (survivability) conditions in the buildings during the fires; and the experiments being conducted to provide data for and validation of computer models (including studies of the mechanical properties of recovered WTC steel, the thermal-insulating properties of WTC fireproofing materials, the dynamics of the fires in the buildings and their effect on the response of the various WTC structural components, and the fire endurance testing of a typical WTC floor system and individual steel members).
The WTC building fires will be modeled using NIST-developed software that has been used for nearly a decade and has been validated against experimental data for a variety of applications. A series of fire tests was conducted by NIST to assess the accuracy of the modeling software in predicting the thermal environment in a burning compartment, and provide a reference set of data to validate the software’s ability to predict temperature rise in steel components similar in geometry and cross-sectional dimensions to those used in the WTC towers. The progress report details these experiments and their preliminary results.
NIST plans fire endurance tests of a typical WTC steel-concrete composite floor system and individual steel members under the fire conditions prescribed by current standards (ASTM E 119).
Assessing the Most Probable Structural Collapse Sequence
A number of hypotheses have been publicly considered for the sequence of events that led to the collapses of WTC Buildings 1, 2 and 7. Based on an initial assessment of the expert studies conducted as part of recent litigation and other relevant data, NIST still considers it premature to exclude any of these theories.
Appendix 5 of the progress report released on May 7, 2003, details the integrated approach that NIST will use to identify the most probable of the technically possible collapse sequences. This approach combines three assessment methods in parallel-physics-based mathematical modeling, statistical and probabilistic-to evaluate and compare the plausible collapse scenarios.
Status of the Recovered WTC Steel and Its Analysis
NIST has nearly 250 pieces of steel recovered from the WTC site-a number adequate for the purposes of this investigation. Among the types of pieces in hand are columns, rectangular box beams, wide flange sections, truss sections, channels and bolts. The recovered steel-representing roughly 0.25 to 0.5 percent by weight of the 200,000 tons used in the WTC towers-includes:
- twenty-eight perimeter and 11 core columns for which locations have been identified;
- several columns from areas impacted by the hijacked aircraft; and
- samples of all 14 steel strengths used in the different components of the towers.
Numerous chemical and mechanical analyses have been conducted on the recovered steel. Preliminary results indicate that the yield strength (ability to resist stress) of the different steel types satisfied applicable specifications and that most of the structural elements were made from steel with higher-than-required yield strengths.
First-Person Data on Occupant Behavior, Evacuation and Emergency Response
At an April 8, 2003, public forum in New York City, NIST presented its plans for studying the WTC evacuation and emergency response by collecting first-person data from survivors (both WTC occupants and first responders), families of victims, and individuals with operational and command authority during the WTC disaster. The study will involve 750 face-to-face interviews, 800 telephone interviews and 15 focus groups.
NIST believes that it is possible to learn from the WTC disaster and to improve public safety through the collection and analysis of first-person accounts. This is an ambitious undertaking and will need the active participation of WTC employers, occupants, first responders and victims’ families.
Other Items in the Progress Report
The progress report also includes sections detailing the following items:
- the implementation of the National Construction Safety Team (NCST) Act under which the NIST WTC investigation is being conducted;
- the establishment and first meeting of the NCST Advisory Committee;
- funding status of the WTC investigation, as well as the parallel R&D and dissemination and technical assistance programs in NIST’s response plan for the WTC disaster; and
- a summary of contracts solicited and awarded for work on the investigation.
