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Pentagon more durablethan expected, study shows

The Pentagon is one tough office building, a newly released engineering study found.

BY TRAVIS DUNN | BALTIMORE | January 25, 2003

"We expected to see much worse devastation then there was."

—Dr. Paul K. Mlakar

The Pentagon is one tough office building, a newly released engineering study found.

Sponsored by the American Society of Civil Engineers, a team of six engineers investigated how well the Pentagon withstood the terrorist-piloted plane that struck it Sept. 11, 2001.

The leader of the study, Dr. Paul K. Mlakar, technical director for the U.S. Army Corps of Engineers, said the study's findings were "very surprising to the engineers who conducted the study."

The Pentagon, he pointed out, was not intended to serve as a hardened military installation when it was built in the early 1940s. Designed to serve as an office building, the Pentagon was expected to be used to store records.

The investigators were astounded to find the Pentagon was much more durable than they thought.

"We expected to see much worse devastation then there was," he said.

Mlakar said three major design features prevented a more generalized collapse after the plane impact.

First, the support columns, "which bore the brunt of the impact," he said, have spiral steel reinforcements that held up remarkably well considering the pressure applied. "This detail allows the column to resist that lateral pressure," Mlakar said.

Second, the resilience of the floors, designed to hold three times more weight than the floor of the average office building, also prevented "a general collapse." This "two-directional framing of the floor system" distributed the load of the area that had lost supporting columns to adjacent columns, keeping damage "limited to this isolated event."

Third, Mlakar said the continuity of the steel reinforcement, or rebar, in the concrete floors also did a good job resisting collapse.

The report suggests that architects keep these findings in mind when designing future buildings.

While Mlakar recognized that the type of terrorist attack is certainly not a normal occurrence, he said that the lessons drawn from this study show that the design features incorporated into the Pentagon can also be useful to other disasters, such as earthquakes or tornadoes, where the building is subjected to "a high lateral load."

"We strongly urge that the team's recommended measures be considered in future building designs," Mlakar said.

Current building codes do not take these features into account, he said. Mlakar said he did not intend this observation as a criticism, but as a recommendation for future construction.

"There's not a lot of documentation how to do this," he said, "but we have documented here from examples, and it really works."

Mlakar also investigated the collapse of the Murrah Federal Office Building in Oklahoma City.

The comparisons between the collapses of the two buildings were instructive, he said.

Although both buildings were constructed entirely out of reinforced concrete, floor collapse in the Murrah building extended far beyond the area of the explosion, something which did not happen at the Pentagon. The Murrah building, he noted, did not include the three key features his team found in the Pentagon.

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