Interim WTC Investigation Findings

The National Institute of Standards and Technology (NIST) has issued its second major progress report and interim findings related to the WTC disaster of 9/11/2001. The report discusses, in part:

• Working hypotheses for the collapses of the towers, as well as for the 47-story WTC 7 building.
• Factors contributing to the building collapses including the impacting aircraft, building structural systems, fireproofing, active fire protection systems, structural materials, and failure mechanisms.
• Emergency response, command and control, and communications during the disaster
• Applicable building codes at the time of the buildings' construction
• Design practices related to fire- and structural-safety, performance-based fire safety design, and the roles of building standards, codes, and regulations in such design
• Issues meriting further consideration

A few of the more interesting findings in the report include the following:

NIST's working hypothesis is that the towers failed due to column instability caused by the airplane impacts and subsequent fires. One question concerns why WTC 1 stood for nearly twice as long after plane impact as WTC 2. Though the report draws no inference, it notes separately that affected floors in WTC 1 had significantly upgraded amounts of fireproofing in comparison to affected floors in WTC 2.

The role of spray-on fireproofing in protecting the building frames from the heat of fire continues to receive considerable attention. Fireproofing thicknesses applied to the long-span floor trusses ranged from 1/2-inch to 1 1/2-inches. Separately, an unrelated 2001 model code report recommended 2-inch thickness for a similar floor system. These large discrepancies indicate a lack of technical basis for the fireproofing thicknesses used in the towers.

More than 99 percent of the tower occupants below the impact floors were able to evacuate safely prior to building collapse. Significant drops in evacuation rates prior to the collapses suggest that there was sufficient egress capacity to accommodate these building occupants. However, on the day of the attacks, both buildings were each occupied at approximately only 1/3 of full capacity. Estimates indicate that evacuation of either tower at full capacity would have required significantly more time, approximately 4 hours.

The towers were not constructed to meet requirements of the building code in effect at the time that they were built, the 1938 New York City Building Code. Rather, the Port Authority opted to conform to requirements of the upcoming 1968 Code, even though this new code was three years away from going into effect. As a result, significant reductions in life-safety requirements were applied, such as reduction in number of required stairwells, reduction in levels of fire-resistance ratings, and reduction in partition structural load requirements.

The Institute's Key Findings document is especially recommended for further reading.

More Info
NIST News Release
NIST Progress Report
Columns Likely Failed First in Terrorist-Triggered WTC Fires, ENR, June 28, 2004