Dr. Paul Mlakar, a technical director in the Geotechnical and Structures Laboratory at the U.S. Army Engineer Research and Development Center, talks Monday about the impact that the airliner crash had on the physical structure of the Pentagon on Sept. 11, 2001. (The Vicksburg Post/C. Todd Sherman)

[09/10/02]Architects and engineers who will design buildings in the future need to heed the lessons taught when an airliner taken over by terrorists crashed into the Pentagon Sept. 11, 2001, said Dr. Paul Mlakar.

A technical director in the Geotechnical and Structures Laboratory at the U.S. Army Engineering Research and Development Center in Vicksburg, Mlakar headed a six-person team from the American Society of Civil Engineers that studied how the structure of the Pentagon reacted when a 182,000-pound jetliner slammed into the building at 530 miles per hour. A total of 189 people died in the terror attack.

By coincidence, groundbreaking for construction of the Virginia building that is operational headquarters for America’s military began on Sept. 11, 1941, 60 years to the day before the attack.

Since Pearl Harbor followed on Dec. 7, 1941, the Pentagon was finished on a rapid schedule and went into service in 1943.

Portions, including where the plane hit underwent reconditioning and strengthening in recent years.

Mlakar prefaced the talk he made Monday to about two dozen ERDC staff members by saying it was a rehearsal for a talk he will give Wednesday as part of the ceremonies at the Pentagon for the first anniversary of the attacks on the Pentagon and the World Trade Center in New York.

“I’m going to talk to you today about some of the positive lessons learned from this tragic experience at the Pentagon,” he said.

One of the first things the team did was to examine the plans and specifications, Mlakar said. The examination of the plans and actual building showed a grid system of columns spaced no more than 20 feet apart and some as close as 10 feet. These were spanned by concrete slab floors and beams and girders.

“The framing is relatively simple and actually redundant and this becomes important in the response of the building,” he said.

The airliner crash represented an extreme structural loading, Mlakar said. Pointing to a photo of the crash site, he said the columns in an area up to 80 feet across were either destroyed or rendered incapable of holding loads.

But “the building is still standing,” he said.

The plane and its wreckage penetrated a distance of 320 feet into the Pentagon from the E ring through to the C ring and came to rest at the service drive called A-E Drive.

“In total, about 50 columns supporting the second and upper stories … were either destroyed or significantly impaired,” he said, adding, “it’s amazing more of the building didn’t collapse.”

An analysis of the way the Pentagon was built shows why it was able to withstand the devastating damage. One reason was the close spacing of the support columns allowed nearby columns to bear weight when some were lost.

“These redundant load paths, the original 1941 construction inherently possessed were responsible for the fact that we didn’t have a tremendous collapse,” Mlakar said.

Another reason the Pentagon was not damaged further was the steel reinforcing material in the columns. The reinforcing rods were configured in a spiral form to confine the concrete in the center of the column to increase its resistance to both vertical and horizontal loads.

“This was responsible for a very strong lateral resistance,” he said, adding this was another lesson for the engineering profession.

Although there was a collapse involving all five floors of the E ring, it did not happen for about 19 minutes following the crash, allowing people not severely injured or killed in the impact to escape.