NIST's WTC 7 working collapse hypothesis,
progress of and projections for the investigation

NIST WTC 7 working hypothesis restated, December, 2007(hasn't changed since 2004):
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.

The working hypothesis for the initiating event sequence that characterizes the initial local failure is based on fire induced failures initiating in the tenant floors:

Floor beams, slabs, and connections heat more quickly and to higher temperatures than the columns.

Elevated temperatures in the floor elements lead to thermal expansion, sagging, and weakening that result in failure of floor connections and/or buckling of floor beams.

Sufficient floor component failures (connections and/or beams) result in at least one long unsupported column at the lower floors, which leads to the initiation of global collapse.

• While NIST has found no evidence of a blast or controlled demolition event, it is evaluating the magnitude of hypothetical blast scenarios that could have led tothe structural failure of one or more
critical elements.

• The working hypothesis is based on an initial local failure caused by normal building fires, not fires from leaking pressurized fuel lines or fuel from day tanks.

• This hypothesis may be supported or modified, or new hypotheses may be developed, through the course of the continuing investigation.

Projected Investigation Schedule

1/08 Complete analysis of initiating event.
3/08 Complete analysis of global building response to initiating event.
4/08 Identify leading collapse hypothesis.
5/08 Complete draft reports for NIST Team review.
6/08 Revised draft reports transmitted for NIST level and NCST Advisory Committee Review.
7/08 Release draft reports for public comment.
8/08 Release final reports on WTC 7 Investigation.

Followup questions to S. Shyam Sunder from National Construction Safety Team Advisory Committee:
Q: What do you mean by normal building fires?

A: These are fires in spaces where the combustibles are normal building contents, ventilation is the normal building ventilation, and there are no exceptional combustibles such as diesel fuel in day tanks or in large tanks at the base of the building. In the case of the towers, the jet fuel was unusual, but even there we talked about normal building fires since the jet fuel burned within a matter of a few minutes. What burned over the next hour to hour-and-a-half were normal fires where the combustibles were building contents plus the airplane contents.

Q: But they were ventilated fires?

A: In both cases the ventilation was probably somewhat limited. Typically, when flames extend out from windows, there is excess fuel looking for air with which to react.

Q: What will be the magnitude of the final report? Will there be a sizeable amount of documentation as with the towers?

A: In the case of the towers, the final reports totaled about 10,000 pages. In the case of WTC 7, we anticipate the final reports will total about 1000 pages or less. We anticipate releasing three reports, but this has not been finalized.

See also Therese McAllister: NIST WTC 7 Technical Approach and Status Summary (pdf)