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NIST Response to September 11 Advocates’ Statement

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PostPosted: Wed Oct 01, 2008 8:18 pm    Post subject: NIST Response to September 11 Advocates’ Statement
From Wed Oct 01, 2008 3:00 am to Sat Oct 04, 2008 2:59 am (included)
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NIST Response to September 11 Advocates’ Statement

Oct. 1, 2008

We read with interest the statement that the September 11 Advocates (see below) issued on Friday (9/26/08) regarding the findings of our World Trade Center Building 7 investigation. We hope the following response clarifies the issues and concerns your statement raises.

While we noted—and strongly recommended—that building owners, operators, and designers evaluate buildings to ensure the adequate fire performance of the structural system in the light of our findings on WTC 7, your statement's characterization of our findings that “any existing building is prone to a progressive collapse if a fire should start and the sprinkler system fails” is not accurate and misinterprets our results.

In those rare cases in which sprinkler systems do not operate properly during a fire and the resulting uncontrolled fires involve multiple floors, we have concluded there is a particular risk of fire-induced progressive collapse for those buildings that have one or more of four features that were outlined in the WTC 7 report:

· buildings with long span floor systems (40 feet or more in length, which heat will expand by greater amounts than smaller span systems);

· irregular floor framing (which can result in imbalanced forces in the components of the building structure);

· composite floor systems (such as those made of steel and concrete, which thermally expand by different amounts when heated since steel may heat up more quickly than concrete); and

· connections not designed to resist the effects of heat (we call for all of these components from now on to be designed with thermal effects in mind).

In the case of WTC 7, these four features combined in a specific way to cause a fire-induced progressive collapse in the building. Therefore, it would not be technically accurate for us to suggest that all buildings are at risk of progressive collapse during uncontrolled fires. Rather it is only a small subset of buildings with one or more of these or other design features that are especially at risk.

While the September 11 Advocates suggest that our report does not address what to do in buildings that are already in place, Recommendation B clearly applies to both new and existing buildings. The report strongly urges building owners, operators, and designers to act upon the new recommendation (B) to evaluate their buildings for potential problems. In the NIST WTC 7 press conference opening statement, NIST’s WTC Lead Investigator Shyam Sunder repeated this call to action: "We strongly recommend that building owners, operators, and designers evaluate buildings to ensure the adequate fire performance of the structural system."

In general, tall buildings are very safe. The partial or total collapse of a building due to fires is an infrequent event. We have decades upon decades of real-life experience to prove this. Major fires in commercial buildings are rare, and uncontrolled fires are even rarer. The fires in WTC 7 were particularly severe since they spanned multiple floors, with fires burning out of control on six floors. Although these fires were similar to those that have occurred previously in several tall buildings, the other buildings did not succumb to their fires and collapse because they did not possess the combination of design features found in WTC 7.

Sprinkler systems that protect occupants from fire work almost all of the time, but our findings as a result of the WTC 7 investigation led us to emphasize additional cautionary steps. With regard to sprinklers for buildings in earthquake-prone areas, as was mentioned in your statement, the piping in sprinkler systems is often designed to be protected from damage in regions of high seismic activity.

As we mentioned in the WTC press conference and in our continuing outreach efforts to the building community, possible retrofits for existing buildings may include strengthening connections, strengthening floor framing, increasing structural redundancy, and adding additional fireproofing to vulnerable areas. We believe there are ways to address any concerns that arise from building evaluations conducted by professionals based on our recommendations, and that each building structure should be analyzed by owners, operators, and designers on a case-by-case basis. For the longer term, we also have asked for changes in codes and standards, which influence building design, construction, and major renovations.

We share the goal of the September 11 Advocates to increase the safety of tall buildings and the emergency responders who fight fires in them. That's why it is important for our findings and recommendations to be understood and clarified as much as possible. We hope this addresses your concerns and we welcome your continued input for increasing understanding and awareness of the findings in our report.

Mat Heyman
Chief of Staff
National Institute of Standards and Technology
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