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Structure of Pentagon helped contain 9/11 damage, engineers say

Several structural features, including steel-reinforced columns, helped the 60-year-old Pentagon withstand the Sept. 11 attacks relatively well, a team of engineers said Thursday.

The attack on the Pentagon claimed the lives of 125 Pentagon employees and 64 passengers on American Airlines Flight 77, but it is "remarkable" that there were not more casualties, said Paul Mlakar, an engineer who led the six-member Pentagon Building Performance Study team, which was made up of engineers from both government and industry. The American Society of Civil Engineers released a report on the team's findings Thursday.

On Sept. 11, construction crews were wrapping up the first phase of a decade-long Pentagon renovation when the hijacked airliner slammed into the west wall of the building at 531 miles per hour, engulfing the area in a giant fireball. The crash destroyed much of the newly renovated portion of the facility.

Pentagon security camera pictures confirmed that the plane was flying nearly level to the building, and so low to the ground that it reportedly clipped a vehicle antenna and severed light posts, when it hit just below the second floor of the facility's west side. On impact, the plane penetrated through three outer rings of the building, traveling 310 feet in less than one second. It destroyed about 50 structural columns on the first floor and burst into a fire that weakened the building and caused a small area above the point of impact to collapse about 20 minutes after the crash.

But the Pentagon's resilient structural system "clearly mitigated the casualties and damage that resulted from the impact and fire," the engineers concluded in their report, which was based on site inspections shortly after the attack and a seven-month review of aircraft data, eyewitness accounts and casualty records.

The building held up better than would be expected because its structural design helped it absorb energy from the impact and resist collapse, the report said. Spiral steel reinforcements prevented columns outside the impact area from breaking and allowed some columns to bear more weight when others collapsed. These are the same type of columns that protect buildings from high wind or earthquakes, Mlakar said.

Sturdy floors, held up by cross-directional beams and a grid of closely spaced girders, helped contain the damage from the impact, according to the team's report. The floor slabs contained "continuous" steel reinforcements, meaning the reinforcements continued in the areas where the floor intersected with support columns. Some of the reinforced slabs did not fall even when the columns supporting them collapsed.

Fire damage resembled that of a typical office building fire, the report said. But the engineers noted that the blast-resistant windows installed as part of the Pentagon Renovation Program held up well, withstanding the initial impact and pressure caused by heat from the ensuing fire.

These design features would help make all federal buildings more stable, said Mlakar, who also worked on impact reports after the 1995 bombing of the Alfred P. Murrah Federal Office building in Oklahoma City, Okla. The Murrah building was not as resilient as the Pentagon, and much of the damage from that explosion could have been prevented had the building been built the same way as the Pentagon.

The Pentagon's strong structural features would be especially easy, and relatively cheap, to include in construction plans for new buildings, Mlakar said. They are not only important in the event of a catastrophic terrorist attack, he added, but would also help prevent damage in a less serious scenario, such as a car crashing into the outer wall of a building.

Mlakar's engineering team helped plan the reconstruction of the damaged portion of the Pentagon by the one-year anniversary of the attack. Pentagon officials and the engineers declined to elaborate, for security reasons, on many of the new design features, but said that the building study, along with eyewitness testimony, helped them come up with improvements to the already strong Pentagon structure.

For instance, the newly rebuilt area contains exit arrows along the floor, which will illuminate in the event of a power outage, said Michael Sullivan, acting manager of the Pentagon Renovation Program. The renovation team decided to include the arrows after interviewing survivors of the Pentagon attack, who said they had to crawl out of the building through smoke, on their hands and knees, and found it nearly impossible to see the red exit signs above the doors.

The Pentagon Building Performance Study team is one of several groups that have assessed how the building withstood the attacks. Other teams also concluded that the building held up well, especially considering the magnitude of the attack.

For more information about the Pentagon Building Performance Report, click here: http://www.asce.org