The National Institute of Standards and Technology (NIST) press release (with links to the report) states:
The fall of the 47-story World Trade Center building 7 (WTC 7) in New York City late in the afternoon of Sept. 11, 2001, was primarily due to fires.... This was the first known instance of fire causing the total collapse of a tall building..... Citing its one new recommendation (the other 12 are reiterated from the previously completed investigation of the World Trade Center towers, WTC 1 and 2), the NIST investigation team said that “while the partial or total collapse of a tall building due to fires is a rare event, we strongly urge building owners, operators and designers to evaluate buildings to ensure the adequate fire performance of the structural system. Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: long-span floor systems, connections not designed for thermal effects, asymmetric floor framing and/or composite floor systems.” Engineers, the team said, should be able to design cost-effective fixes to address any areas of concern identified by such evaluations.
Speaking at a press conference on the issue, Dr. Shyam Sunder, lead investigator and director of the NIST Building & Fire Research Laboratory, said that another issue was the fact that sprinklers did not work, because both the primary and secondary water source for the lower floors was the city main. That was also the secondary source, after overhead tanks, for the upper floors.
Sunder noted that the collapse resembled a controlled demolition because, as the thermal expansion caused the failure of three critical internal columns, the collapse started internally and progressed to the exterior, just as in a controlled demolition.
This phenomenon of thermal expansion has not previously caused buildings to collapse. It reveals a potential fire-safety issue for other similar high-rise buildings.
Sunder notes that the problem arises because structural engineers have a job of making sure structural loads are adequate for dealing with conditions such as wind, while architects have the job of making sure fire proofing is specified. What is missing is the connection between these disciplines. No one actually evaluates the response of the structural system during a fire, he says.
The NIST press release further notes:
Sunder identified several existing, emerging or even anticipated capabilities that could have helped prevent WTC 7’s collapse. He cautioned that the degree to which these capabilities improve performance remains to be evaluated. Possible options for developing cost-effective fixes include:
More robust connections and framing systems to better resist effects of thermal expansion on the structural system.
Structural systems expressly designed to prevent progressive collapse, which is the spread of local damage from a single initiating event, from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it. Current model building codes do not require that buildings be designed to resist progressive collapse.
Better thermal insulation (i.e., reduced conductivity and/or increased thickness) to limit heating of structural steel and to minimize both thermal expansion and weakening effects. Insulation has been used to protect steel strength, but it could be used to maintain a lower temperature in the steel framing to limit thermal expansion.
Improved compartmentation in tenant areas to limit the spread of fires.
Thermally resistant window assemblies to limit breakage, reduce air supply and retard fire growth.
The draft report is open for comment through September 15, 2008. Comments (instructions for submission are available at http://wtc.nist.gov) may be submitted via:
- e-mail to firstname.lastname@example.org;
- fax to (301) 869-6275; or
- surface mail to WTC Technical Information Repository, Attn: Stephen Cauffman, NIST, 100 Bureau Dr., Stop 8611, Gaithersburg, Md. 20899-8610.