The Gateway Arch is strikingly simple in design — a sweeping curve of stainless steel rising 630 feet above the ground. Its 142 welded pieces are equilateral triangles, one of nature’s most durable forms.

But there was nothing simple about building it.

The Arch is embedded deep into limestone bedrock and held in place by foundations made of 26,000 tons of concrete, more than 2,000 truckloads. The engineers had to be precise in measurements and calculations, from their drafting boards to fitting the last piece. Much was at stake — a “miss” of the two legs at the top would be a mortifying and expensive embarrassment, to say the least.

The triangles, known to the workers as “cans,” were double-walled structures of carbon steel inside and stainless steel exterior skin. For the first 312 feet, workers poured concrete between the walls and ran continuous reinforcement rods. Above that height, welds held everything together.

The engineers and iron workers knew their stuff. Each time a can was installed, engineers would measure the tips to a tiny fraction of a degree. Then the iron workers would grind, shim and weld the next can to keep the legs true. When the final piece was installed on Oct 28, 1965, the legs were only three-eighths of an inch off, making for an easy fit.

By Tim O'Neil

St. Louis Post-Dispatch