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No Detail Too Small: Engineer Recalls Vital Work Following 9/11

U.S. Army Engineer Research and Development Center (ERDC)
Published Sept. 10, 2021
As Dr. Robert Hall walked around the spacious hangar, he came across components and materials used in his more than 40 years of research at the Waterways Experiment Station, the predecessor to the U.S. Army Engineer Research and Development Center (ERDC).

As Dr. Robert Hall walked around the spacious hangar, he came across components and materials used in his more than 40 years of research at the Waterways Experiment Station, the predecessor to the U.S. Army Engineer Research and Development Center (ERDC).

As Dr. Robert Hall walked around the spacious hangar, he came across components and materials used in his more than 40 years of research at the Waterways Experiment Station, the predecessor to the U.S. Army Engineer Research and Development Center (ERDC).

As Dr. Robert Hall walked around the spacious hangar, he came across components and materials used in his more than 40 years of research at the Waterways Experiment Station, the predecessor to the U.S. Army Engineer Research and Development Center (ERDC).

As Dr. Robert Hall walked around the spacious hangar, he came across components and materials used in his more than 40 years of research at the Waterways Experiment Station, the predecessor to the U.S. Army Engineer Research and Development Center (ERDC).

What many would consider relics created out of steel and concrete, Hall saw in each a story, an answer to a question and an engineering solution that no doubt resulted in Warfighter lives being saved.

As the nation – and the world – mark the 20th anniversary of Sept. 11, 2001, when nearly 3,000 people were killed in the horrific terrorist attacks on New York, Pennsylvania and the Pentagon in Washington D.C., Hall, who was among the engineers who provided life-saving research ahead of and after the attack, can vividly remember the emotions of the day.

“Every time we have a day of remembrance, reach a milestone like the 20th, it just brings back all those memories,” Hall said, standing next to scaled models of walls twisted by testing in one of ERDC’s blast load simulators. “You see the television screens, the pictures of the aircraft, and the emotions you felt that day are all rekindled.”

Hall, who began work with the U.S. Army Corps of Engineers in 1971, retired as a supervisory research civil engineer in 2009. His final role was Chief, Geosciences and Structures Division, in ERDC’s Geotechnical and Structures Laboratory.

On the morning of Sept. 11, 2001, American Airlines Flight 77 took off from Washington’s Dulles Airport bound for Los Angeles. Moments after takeoff, terrorists overwhelmed the crew and took control of the plane, turning it back toward the nation’s capital.

Moments later, at 9:37 a.m. EST, the terrorists crashed the plane – carrying nearly 10,000 gallons of fuel – into the Pentagon’s western wall at a speed of nearly 345 miles per hour.

In all, 184 people were killed in the attack, including 125 in the Pentagon.

The death toll was tragic, but it could have been far worse.

The section of the Pentagon struck by the plane – Wedge 1 – had undergone major renovations in which new techniques and technologies had been placed within the walls. Columns in the section had been strengthened, ballistic membranes had been placed on the interior of the brick walls, and the windows had been replaced with complete window systems designed to withstand a severe blast and shockwaves.

In 1990, Congress had approved a massive overhaul and renovation project for the Pentagon. In 1998, renovation of Wedge 1 began and by September 2001, the wedge was mostly reoccupied.

The blast-resistant windows, support systems that housed each window, the reinforced columns and ballistic membranes were each based on research conducted years earlier by ERDC, some of which involved Hall.

According to a report commissioned by the Department of Defense documenting the history of that day, “after the nose of the plane hit the Pentagon, a huge fireball burst upward and rose 200 feet above the roof … no building could have absorbed the energy of such a crash without suffering structural damage and, if occupied, casualties. Nevertheless, the Pentagon fared better than less sturdy buildings would have.”

Following the attacks, Hall was among a group of engineers who worked closely with Pentagon leadership who wanted immediate “off-the-shelf” engineering solutions to strengthen the Pentagon as it was rebuilt and renovated in the months and years following the attack.

Hall’s team evaluated every component of the attack and the building’s performance during the attack to determine what worked, what didn’t, and what could be improved. The building’s construction and the performance of retrofitting elements that had already been placed in the building during an ongoing renovation project were thoroughly scrutinized.

“My main activity after 9/11 was putting together the team of researchers to provide the Pentagon with the latest technology off the shelf they could use that day; everything from air conditioning, fire control, safe havens for personnel – the whole gamut of research that could be applied,” he said.

Hall said even placing emergency exit signage just above floor level was suggested because following the attack, smoke filled the hallways, rising near the ceilings and blocking the view of existing signage. No detail was too small.

He and others also reviewed the then 60-year-old structure’s original drawings, making sure the construction of the building, which ironically began on Sept. 11, 1941, matched the drawings. “Were we missing something?” Hall asked.

For engineers, the difference in damage between the area of Wedge 1 impacted by the airplane and Wedge 2, an area that had not been renovated, was drastic and provided a treasure trove of evidence that research conducted years before had worked and saved lives.

“I could not remove my engineering hat from the emotion because I was really interested, because Wedge 1 provided so much more protection, even though it was designed for blast not aircraft, but just because the structure provided so much better protection and resiliency; and not just from blast, but it also carried on to fire and impact,” Hall said.

Among the elements most credited with adding resilience to the structure were the blast-resistant windows, the way they were installed, the brick walls lined with a ballistic membrane and how those elements, and others, Hall said, worked together and not just as individual pieces.

“Seeing it first-hand gave us the information on how the whole structure worked together as a composite structure instead of as pieces of windows and walls,” he said.  “That made us really focus on the whole structure and getting it to function as a unit and make sure we understood where the weakest point was.

“You need to understand how those pieces individually work, but better yet, how they all work together as a unit.”

In the years following 9/11, even more advanced – and some simpler – techniques were included in the final retrofit and renovation of the Pentagon. Hall said much of his focus turned to improving the analytical capabilities, spending “more than five years validating that our numerical models could predict the ultimate capacity of these and other walls from blasts.”

While the past two decades have done little to dampen the emotion Hall felt following the attacks on 9/11, he does have warm memories of the work he and others were able to do.

“Anniversaries such as this bring back memories of the team of people I worked with and how well they worked together to provide solutions to those tough questions,” Hall said. “It was a real blessing to be part of that team and see them get together.

“It was never a competition of who could have the best ideas or the most ideas. It was really about what they could present that would, in the end, save lives.”


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