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Three stainless steel spires soar skyward above the greater Washington, D.C. area, evocative of the “bomb-burst” flying formation performed by U.S. Air Force jets. The design is truly representative of flight, with the tallest of the spires reaching the height of 82 meters (270 feet).
The stainless sculpture is the most visible feature of the U.S. Air Force Memorial, dedicated on October 14, 2006. The memorial was designed by the internationally renowned architect, the late James Ingo Freed. The stainless steel plate for the skin of the spires was provided by Outokumpu’s plate mill in New Castle, Indiana.
The memorial ranks among the world’s largest stainless steel structures in terms of both height and in tonnage. “The freestanding curving shape of the spires may make this one of the most challenging stainless steel structural applications to date,” says stainless steel project consultant Catherine Houska of TMR Consulting. The project’s structural engineer ARUP spent well over a year developing the design for an elaborate damping system. |
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Obtaining the desired surface finish was also demanding. “The spires needed to have just the right level of reflectivity,” Ms. Houska explains, “low enough in the daytime to avoid blinding pilots on their way to nearby airports and high enough to provide night-time illumination.” To achieve the uniform rough texture chosen by the architect, the surface was polished, pickled and glass-bead blasted. A high level of plate flatness was required to ensure a high-quality result. Outokumpu was able to provide the plate to one-fourth of commercial flatness despite its 19 mm (3/4 inch) thickness. Another requirement for the desired finish, as well as for improved corrosion resistance, was low sulfur content (no higher than 0.001%).
Grade EN 1.4404 (ASTM 316L) stainless steel was selected. Ms. Houska explains that this was because, first, the memorial will be subjected to deicing salts from the adjacent highways and, second, the roughness of the surface finish. A rough finish will retain more salt, dust and pollutants, necessitating a higher level of corrosion resistance. Furthermore, because the spires must look seamless and the welds are also structurally critical, it was important to use a grade that allows for easy welding and handling.
Outokumpu was selected as the stainless plate supplier for reasons of both technical competence and level of service. First, Outokumpu’s New Castle plate mill could provide the necessary high flatness over the desired width and length, coupled with a low sulfur content. Second, Outokumpu spent two years providing technical assistance to the designers, including samples and answering questions about flatness.
“Outokumpu was the only plate supplier that was able to meet the technical requirements and willing to provide a high level of technical assistance to the designers,” Ms. Houska says. |