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Design flaws did not lead to WTC collapse

WASHINGTON—In late October, the Commerce Department’s National Institute of Standards and Technology (NIST) released the latest findings from the building and fire safety investigation into the World Trade Center (WTC) disaster of Sept. 11, 2001. Those findings—while not yet final—basically rule out the possibility that a building design flaw was the main reason for the collapse.

"We always said [that] we had no preconceived notions, and that we would look at the failure information dispassionately," S. Shyman Sunder, Ph.D., the lead investigator at the institute told The New York Times. "The buildings performed as they should have considering the airplane impact and extreme fires to which they were subjected. There is nothing there that stands out as abnormal." The inquiry, which took two years and cost $16 million, is the most sophisticated building analysis in the nation’s history.

Following are some of the key findings not previously reported by NIST.

1) Hypotheses for the collapse of WTC 1 and WTC 2 were developed. They identify the chronological sequence of major collapse events for each tower and identify specific load redistribution paths and damage scenarios.

Previously, a single working hypothesis was defined for both towers without identification of the load redistribution paths and damage scenarios resulting from aircraft impact and the subsequent fires.

2) The time delay between the collapses of the WTC towers is explained. Although the WTC towers were attacked by virtually identical aircraft, WTC 1 stood for 103 minutes, while WTC 2 survived for 56 minutes. The time delay was primarily a result of the asymmetrical structural damage of the aircraft impact to WTC 2 compared to the aircraft damage to WTC 1; the time it took for heat to soften, buckle, and shorten core columns that had fireproofing dislodged by debris impact; the structure’s ability to redistribute loads as core columns shortened; the time it took for fires to travel from their initial location to the face of the towers where perimeter columns were bowing inward; and the time it took for heat to soften and buckle those columns.

3) The post-impact capabilities of the WTC towers were assessed. Demand-tocapacity ratios showed that for the floors affected by the aircraft impacts, the majority of the core and perimeter columns in both towers continued to carry their loads after the impact.

The loads from damaged or severed columns were carried by nearby undamaged columns.

Although the additional loads strained the load-bearing capabilities of the affected columns, the results show that the columns could have carried them. This proves that the towers withstood the initial aircraft impacts and that they would have remained standing indefinitely if not for another significant event such as the subsequent fires.

4) Fire-induced core column shortening was detected. As a result of the heat from fires following the aircraft impacts and subsequent buckling, there was a shortening of core columns seen in both towers on floors at or near the fire-affected impact sites. Shortening of the core columns caused the floor system to pull the perimeter columns inward—explaining the observed inward bowing that was seen minutes prior to the collapse of each tower.

5) The role of fireproofing was determined.

The structural components that became weakened because of the fires and eventually caused the towers to collapse had their fireproofing dislodged by debris from the aircraft impact. Had the fireproofing not been dislodged, the temperature rise of the structural components likely would have been insufficient to cause the global collapse of the towers.

6) The majority of the steel was found to have been stronger than minimum requirements.

Approximately 87 percent of the recovered WTC steel specimens that were tested exceeded the minimum yield strengths specified in the design criteria; 13 percent did not.

7) Full-building evacuation presented challenges for occupants. Based upon firstperson interviews, the occupants were challenged by the physical aspects of the evacuation and also were unprepared to encounter transfer hallways during the stairwell descent.

Movement in stairways also appeared to be an issue for first responders who had difficulty using the stairs due to crowding.

These findings still may be revised and additional findings still may be included in the team’s final report—which is scheduled for release either this month or in January 2005.

For more information, visit NIST’s website at wtc.nist.gov.