NFPA Session Highlights FEMA 9/11 Report

Following the cataclysmic events of Sept. 11, it is not surprising to find that numerous professional organizations are exploring what can be learned from the disaster—and what can be done in the future to safeguard buildings.

Among those concerned parties, the National Fire Protection Association hosted its own session on the topic May 14 at its annual conference in Minneapolis. With the release of a Federal Emergency Management Agency report on the World Trade Center only two weeks before, the session was well attended. Several NFPA members, including a staff engineer, were part of FEMA's building performance assessment team (BPAT), and a special panel of these members commented on their experiences creating the report.

Initially, the panel cautioned that the FEMA report is preliminary and cursory, but added that it certainly offers insight on the structural performance of the towers and surrounding structures following the impact of the terrorist-piloted jets.

Overall, the panel commended the towers' design, which allowed the structures to survive the initial impact. At the same time, they lamented the fact that when the upper portions of the buildings eventually fell, they took the entire structures with them and critically impacted much of the surrounding area.

In recalling the assessment team's investigation, panelists argued that calculations confirmed that the aircraft impact alone did not cause the catastrophic collapse of the towers. But when combined with the resulting fires, the impact served to cause a collapse that might not otherwise have occurred.

While fire was a major contributor to the towers' structural failure, the panel was quick to dispel some of the initial hypotheses floated by building experts immediately following the event, such as the idea that jet fuel burned intensely enough to melt the steel structure of the towers. According to the NFPA presentation, the jets' fuel would have been consumed in the first three to five minutes following impact. In effect, the panel suggested that the jet fuel actually ignited common office materials, which were the source of the fires that continued to burn until the towers collapsed. Accordingly, the fires in the towers would not have sustained the temperatures necessary to melt steel.

As for what exactly led to the nearly unthinkable collapse of the towers, panelists said the report offers a few theories, but leaves much to explore. Follow-up studies, they said, will explore possibilities such as the floors collapsing or the failure of the outrigger trusses.

In fact, one distinct possibility, as noted in the report, is that each tower collapsed in a different manner, due to the disparate angles and locations of airplane impact, in addition to other factors.

One possible facet, according to panelist James Milke, P.E., professor of fire protection at the University of Maryland, Falls Church, Va., was the performance of fireproofing. It has been determined that much of the spray-on fireproofing in the areas where the aircraft collided was immediately blown off by the impact. Milke also noted that WTC 1, the north tower, had recently doubled the amount of fireproofing in the building to a thickness of 1.5 in., while the south tower, WTC 2, still had the original 0.75 in. of fireproofing installed. The south tower, of course, collapsed first—despite being struck second.

In addition to the two towers, presenters noted that damage to the surrounding facilities may offer the most important lessons for the fire-protection and life-safety industry.

One of the most intriguing mysteries that remains, according to the panel, is the fate of WTC 7, the 47-story steel office building across the street from the towers, which collapsed later in the day after being ignited by debris from the falling towers. The complete collapse of WTC 7—and a partial collapse of WTC 5—represent the only known instances where steel structures collapsed from the effects of fire damage. While both structures were damaged by debris from the two towers, it has been judged that the buildings were structurally sound, and that resulting fires caused the buildings' collapse.

As presented by the panel, the study also took stock of the evacuation and rescue efforts, offering valuable information for future crisis planning. Overall, the presenters lauded these efforts, noting that 99% of the occupants in the building below the floors of impact survived. Conversely, only four people above the impact floors made it out alive. Their possible progress, the panel noted, was most likely impeded by emergency exits and stairwells that were pretty much wiped out on the floors where the planes hit.

In addition, panelist Robert Solomon, P.E., NFPA, noted a positive side effect of the cooperative study itself—in which the Structural Institute of the American Society of Civil Engineers also participated—is a growing understanding of the complementary disciplines of fire protection and structural engineering as they relate to life safety.

While the presentation at the NFPA conference focused on the most catastrophic events—the impact, fires and collapse of WTC 1, 2 and 7—the full FEMA report offers detailed information on a number of surrounding structures, including a detailed account of the quantitative facts that could be found at the site, and recommendations on what future studies should explore.

To find out more about the BPAT findings, find the World Trade Center Building Performance Study online at the FEMA web site: http://www.fema.gov.