Sunday, July 6, 2008

Creeper Derricks

Both legs of the Arch acted as freestanding cantilevers before completion and were erected simultaneously without scaffolding. The first few triangular sections, up to a height of 72 feet were handled by crawler cranes operating from the ground. Above that height two creeper derricks, each weighing 100 tons, were used to raise the 12-feet high, 50-ton sections.

The derricks pulled themselves up the curved legs of the Arch; their adjustable supports kept them level regardless of the height and curvature of the legs. Because the height made it impracticable for workmen to climb to and from the work area, the derrick platforms (43 X 32 feet) were reached by a passenger elevator and were equipped with a tool shed for workmen, sanitary facilities, and communications equipment.

Two vertical tracks held the sled that supported the derrick and platform. These tracks, made from 12 WF steel beams with cover plates on both sides, were spaced 24 feet apart. Each track was about 2 feet from the extrados of the Arch leg and was attached to brackets held by four high-strength steel bolts of 1-1/4 inches in diameter.

Four high-strength steel pins of 5-3/4 inches in diameter connected the sled to the tracks. The telescoping steel legs that extended between the outer corners of the platform framing and the lower part of the sled had pin connections at both ends. As construction progressed, and the curvature of the Arch increased, the telescoping legs were shortened to keep the derrick platform level. Sections of track were added in about 48-feet lengths, and the entire derrick crept up after it had placed four sections of the Arch. Lifting an Arch section into place took only about a half hour.

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