In cross section, each Arch leg is a double-walled equilateral triangle with hollow cores 40 feet wide at the base and tapering to 15 feet 6 inches at the top. The inner skin is made of A-7 carbon steel, 3/8 inches thick, except at the corners where it is 1 foot 3/4 inches thick to provide stiffness. HTe outside surface was fabricated from 900 tons of polished stainless steel in panels 14 inches thick, varing in sizes from 6 x 18 feet to 6 x5 1/2 feet. The outer and inner walls were fabricated in sections and bolted together at the Pittsburg and Warren, PA., plants of the Pittsburg-Des Moines Steel Company the steel fabricaotrs of the Arch.
MIG welding with an automatic electrode head, utilizing a sheilding mixture of 75 percent argon and 25 percent carbon dioxide, was selected for joining the stainless-steel plates on the polished side only.
Operators using templates welded rows of 5/16 inch stainless-steel studs of these plates. Then Z-bars were fastened to the studs with carbon-steel nuts tightend to 22-1/2 ft-lbs with a torque wrench.
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