The Air Force Memorial recently unveiled in Washington, D.C. ranks as one of the world’s largest structural applications of stainless steel along with the Dublin Spire in Ireland and America’s largest memorial, the Gateway Arch.
Consisting of three stainless steel spires reaching 64 metres into the air, the new memorial honours the millions of men and women who have contributed to the United States Air Force and its predecessors over the years, including 54,000 who died in combat.
Each spire has a ¾ inch (19 mm) skin of low sulfer (0.005% max) Type 316 stainless steel covering a core of reinforced concrete. The total weight of the spires is 7,200 tonnes, including 345 tonnes of type 316 plate.
Air Force Memorial
Engineers involved in the design chose type 316 to prevent corrosion and allow the structure’s appearance to be retained over decades without the need for manual cleaning. Though Washington is not coastal, nor particularly polluted, the memorial is surrounded by three highways that use de-icing salt that could threaten a lesser material.
Type 316 also provides structural integrity to help withstand the tendency for the spires, which are curved, to sway in windy conditions.
“From a structural standpoint, the Air Force Memorial was very challenging,” says Catherine Houska, senior market development manager for TMR Stainless, the consulting firm chosen to provide advise on materials for the project. “When you have that kind of a curve unsupported, except at the base, it is going to tend to move even with the slightest breeze. There are very elaborate damping systems to prevent it from shaking apart.”
The “ball-in-box” damping system contains 13 20-inch-diameter lead balls, weighing 2000 pounds each, encased in stainless steel shells. The balls roll freely within boxes that are lined with synthetic damper pads. As the balls hit the pads, energy is dissipated and structural movement constrained.
The memorial was designed by the late James Freed, the internationally-renowned architect from Pei Cobb Freed, while global engineering firm Ove Arup developed the spire structure. The spires are meant to evoke the bomb –burst flying formation made famous by the United States Air Force Thunderbirds.
Construction commenced at the beginning of 2005 and was completed in the autumn of 2006. The total cost for construction was more than US$30 million.