Standing at a dizzying 630 feet, the St. Louis Gateway Arch is the tallest monument in the United States, but constructing Eero Saarinen’s iconic catenary curve in the 1960s posed a massive engineering nightmare. Ground-based cranes could only reach partway up, and the monument’s triangular legs leaned inward at constantly changing angles.
To solve this, engineers designed custom creeper cranes—specially built derrick cranes mounted on outer tracks attached directly to the Arch’s legs. As each double-walled steel section was welded and bolted into place, the creeper cranes used hydraulic jacks to hoist themselves higher along their tracks. Each 75-ton crane effectively acted as its own elevator and scaffolding, pulling up materials, workers, and equipment as the hollow legs rose toward the sky.
The ultimate test came on October 28, 1965, during the placement of the final eight-foot keystone piece at the apex. Because the sun heated the southern leg more than the northern one, thermal expansion caused the arch legs to misalign by several inches. While St.
Louis fire trucks sprayed thousands of gallons of cold water on the expanded steel to shrink it back into place, a massive hydraulic jack pushed the legs apart, and the creeper cranes precisely lowered the 10-ton final section into the gap. Once the keystone was secured, the creeper cranes were used to dismantle their own tracks as they slowly crept back down to the ground, leaving behind a seamless, shimmering masterpiece.