In a cross section, each Arch leg is a double-walled equilateral triangle with a hollow core 40 feet wide at the base, tapering to 15-1/2 feet at the top. The inner skin is of A-7 carbon steel, 3/8 inches thick, except at the corners where it is 1-3/4 inches thick to provide greater stiffness. The outside surface was fabricated from 900 tons of polished stainless steel in panels 1/4 inch thick, varying in size from 6 x 18 feet to 6 x 5 1/2 feet. The outer and inner walls were fabricated in sections and bolted together at the Pittsburgh and Warren, Pennsylvania, plants of the Pittsburgh-Des Moines Steel Company, steel fabricators and erectors for the Arch.
In the shop, the fabricator designed and built two house-size welding fixtures, one for butt welding the inner shell plates of carbon steel and filleting angles to them, and the other - the larger one - for butt welding the stainless steel.
MIG (metal inert gas) welding with an automatic electrode head, utilizing a shielding mixture of 75 percent argon and 25 percent carbon dioxide, was selected for joining the stainless-steel plates on the polished side only. Nondestructive testing with spot X-ray was used to check the welding both in the shop and in the field. Electrolytic cleaning removed the halos on the stainless steel caused by heat from welding.
After the stainless-steel plates for the sections up to the 300-foot level were welded together, they were turned over and moved to a worktable where they were cut to size. Operators using templates welded rows of 5/16-inches stainless-steel studs to these plates. Then Z-bars were fastened to the studs with carbon-steel nuts tightened to 22-1/2 feet -lbs. with a torque wrench.
For the Arch sections up to the 300-foot level, high-strength steel bolts were attached to the Z-bars. These bolts passed through holes in the inner skin of carbon steel and were held in place by nuts that applied a squeezing force to the concrete core of the wall "sandwich," creating a friction bond. To resist local bending, the outer and inner skins act as the top and bottom flanges of a beam, providing a stressed-skin action comparable to that employed in modern aircraft design.
From the top of the composite wall section to the crown of the Arch, all direct compressive loads are carried by the outer stainless steel skin and the inner carbon-steel skin. Vertical steel diaphragms, spaced 2 feet on centers, connect the two skins and serve as stiffeners to prevent buckling of the inner skin. Steel angles spaced halfway between each diaphragm stiffen the outer skin. The stiffeners and diaphragms are interrupted at each field splice and do not contribute to Arch actions.
Spot welding was chosen for attaching the carbon-steel stiffeners to the stainless-steel skin to eliminate the warping that would be caused by heat if arc welding were used. Except for a few seconds of localized heat, spot welding is practically heatless.
The completed Arch sections were shipped to St. Louis on gondola cars. Two "sandwich" wall sections rode side by side, their stainless sides facing each other but held apart by steel uprights covered with wood and neoprene. Steel rods welded to the carbon-steel plates and to the steel sides of the cars secured each section.
Fabricating the Steel