Local engineer analyzes Pentagon’s durability in crash

Published 12:00 am Wednesday, January 29, 2003

Civilian as well as military building designers now have in hand the analysis of a team led by Vicksburg’s Dr. Paul Mlakar on the Pentagon’s resistance to collapse under attack.

The findings by Mlakar of the U.S. Army Corps of Engineers and five other of the nation’s top structural, fire-protection and forensic engineers were published last week by the American Society of Civil Engineers.

“What we want to happen as engineers is when we have localized limited damage to part of the structure, that it doesn’t lead to a progressive collapse of the entire structure,” Mlakar said, explaining that the value of the study will be in designing new construction.

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The Pentagon, the Defense Department’s 34-acre Arlington, Va., headquarters, was struck on Sept. 11, 2001, by a hijacked airliner. The plane penetrated the five-story building about 310 feet between its first and second floors.

The crash, one of four that day, killed 125 military and civilian workers in the building and all 64 people aboard the American Airlines Boeing 757.

Shortly after the crash, Mlakar and his team began their study, which took seven months. Though complete for nearly nine months, release of the 80-plus-page report was delayed pending a Defense classification review.

The Pentagon held up well, Mlakar said. “Yes, there was a portion that collapsed,” he said, “but in comparison to the part that was engulfed in the impact, it was relatively small.”

The seven months of work on the report began with about a month of information-gathering from the “perishable” parts of the crash site.

“There was an intense search and rescue for survivors,” he said. “Things were moved around and changed and buried. We had to do (the immediate information-gathering) as soon as we possibly could.” The Pentagon’s reconstruction schedule was a quick 11 months, Mlakar added.

Mlakar said he spent about half of that first month on-site, in the Washington, D.C., area, but that he was able to do practically all the rest of the work on the report from Vicksburg.

More deliberate information-gathering took about another three months, Mlakar said. Scientific testing continued for three months, he said.

Mlakar said engineers had not considered the effects of the kind of blows the airliners delivered on the Pentagon and New York’s World Trade Center on the same day.

“It has come as a complete surprise to everyone here as well as the engineering profession in general,” Mlakar said.

One reason the Pentagon held up well was the extra-strong framing system of its floors, which in its 1943 construction were built in anticipation of using much of the building for records storage.

“There was continuity of the reinforcement in that floor system over the supporting columns,” Mlakar said.

Also, part of the Pentagon crash site involved a section of the building that was renovated in 1991 using a damage-limiting material developed by Vicksburg engineers, Mlakar said. A “geotextile” fabric, applied to the inside of brick walls, helped control potential shattering of the masonry that could have injured more people, he said. Blast-resistant windows were also part of that renovation, he added.

Not all buildings need to be designed to Pentagon standards, but the information in the engineers’ report can be valuable for a variety of design situations, Mlakar said.

“For lots of unforeseen reasons we may have damage to a small part of the structural frame of a building,” Mlakar said.