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NIST Presents Latest Findings in World Trade Center Building and Fire Safety Investigation

Outcomes to Shape Federal Recommendations for Building Code Changes across the Country

New York, Apr 06, 2005

How would decisions made more than thirty years ago affect the outcome of the events of September 11, 2001? This question, among many others, was addressed yesterday as the National Institute of Standards and Technology (NIST) presented its analysis of how the World Trade Center (WTC) collapsed on September 11, 2001 after two aircraft were flown into the buildings by terrorists.

“Like most building collapses, these events were the result of a combination of factors,” said Shyam Sunder, lead investigator for NIST’s building and fire safety investigation into the WTC disaster.

In the detailed collapse sequences presented on Tuesday by Sunder, the floors played an important role in the collapse of the structures. Once the aircraft severed the perimeter columns of the buildings, they not only damaged the interior core columns, but knocked off the fireproofing from the steel flooring. Subsequently, the fires initiated by the aircraft’s jet fuel spread rapidly and weakened the unprotected floors which caused them to sag, pulling the perimeter columns inward to bow and buckle, ultimately causing the collapse of the buildings.

A previous report from NIST indicated that series of four fire resistance tests conducted in 2004 showed that the test structures were able to withstand fire conditions for between 45 minutes and two hours. Because fire conditions in the towers on September 11 far exceeded those found in a traditional office fire, the tests were not able to determine the actual performance of the floor systems in the collapse of the WTC towers. However, the tests, which used the standard procedure ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, were able to provide valuable information about the impact of the floor system’s performance in the collapse of the towers.

During his presentation, Sunder discussed the NIST WTC investigation’s analysis of building and fire codes and practices; occupant behavior, egress and emergency communications; and fire service technologies and guidelines. During the design of the trade center, the Port Authority’s chief planning engineer took advantage of the more lenient provisions regarding exit stairs. Following a new version of the city building code at the time, the towers were designed and constructed with only three staircases instead of six, and without a specially reinforced fire escape, as was indicated in the earlier version.

While the Port Authority as an interstate agency is not bound by local building codes, they did pledge to “meet or exceed” the city code in building the Trade Center. NIST’s findings determined that the Port Authority had not supplied enough staircases. For every man and woman on the upper floors of the towers, the lack of intact staircases meant it was impossible for escape after the planes struck.

There is a far greater knowledge now of how fires influence structures than there was in the 1960’s, and this shortcoming in the design of the towers raises the question about necessity of current building code changes across the country. As noted in NIST’s findings, while documents from the Port Authority indicate that the impact of a commercial aircraft crashing into the upper floors of the towers was analyzed during the design of the WTC, the effect of the subsequent fires was not considered. Recommendations for improvements to building and fire codes, standards and practices derived from these various projects in the WTC investigation will be released for public comment in June, along with the draft of the final investigation report and drafts of 27 reports from the remaining five projects.

Using the evidence that supports the sequences, which includes extensive computer modeling, photographs and videos, recovered steel and emergency communication records, NIST has also identified a number of future practices and technologies that could have potentially enhanced building performance and life safety capabilities, including the ability of occupants to escape prior to the collapses, had they been available at the time for implementation. This information will be considered for inclusion in NIST’s upcoming recommendations.

Please visit to view Sunder’s presentation and for comprehensive information on the NIST WTC investigation.

Related stories:

Latest Findings from NIST World Trade Center Investigation Released

On Eve of 9-11 Anniversary, More Details Emerge on Structural and Fire Elements of WTC Disaster

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