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Engineers find novel solution for Air Force memorial structural issues

The unique design of the Air Force Memorial presented the project’s engineers with some equally unique challenges.

“It’s not like a normal building where we’ve all done a hundred of them before,” said Leo Argiris, project manager for the memorial with the engineering firm Arup. “Nobody came into this project saying, ‘I know all of the answers.’”

The biggest challenge, Argiris said, was to make sure wind wouldn’t cause the spires — which are unusually tall and slender — to oscillate or vibrate excessively, causing catastrophic structural failure.

After a series of wind-tunnel tests using scale models, engineers determined that such failure was a possibility.

“Those [tests] indicated that we had pushed the envelope in terms of the slenderness of the structures a little too much,” Argiris said. “Something needed to be done.”

The solution the team came up with was simple but novel.

Inside each spire is a series of “ball-in-box” mechanisms that each contain a 20-inch, 1,700-pound lead ball inside a stainless steel box lined with a dampening material. The tallest spire has six of these boxes, the second tallest has four and the shortest has three.

When the wind blows and the spires begin to sway, the lead balls roll free inside their boxes and bang against the sides, absorbing energy and dampening the swaying enough to prevent instability.

Energy-dampening is often used in skyscrapers, but the typical solution is a hanging pendulum that swings and counteracts the building’s swaying.

That solution wouldn’t work in the memorial, Argiris said, because of the curved shape of the spires.

“The [solution] here was pretty unique,” he said.