Sunday, July 6, 2008

Foundation Depth

Foundation: 60 Feet Deep

Reinforced concrete foundations sunk 60 feet into the ground extend 30 feet into bedrock and contribute substantially to the structural strength of the Arch. Structural engineers Severud, Elstad, Krueger and Associates report that under a wind load of 55 psf (equivalent to a 150-mph wind), the Arch will deflect at the top only 18 inches in the east-west direction. Its legs are oriented on a north-south line. To prepare the site for the Arch foundations, the visitor center and museum, MacDonald Construction Company, general contractor for the project, excavated 300,000 cubic feet of earth and rock. Alloy-steel tensioning bars or tendons, 252 of them for each leg, extend 34 feet below the top of the foundations to anchor the structure securely to its base. At ground level, only the two outside corners of each triangular base are pre-stressed by two groups of 63 steel bars in each corner. To facilitate the fabrication, shipping, positioning, and erection of the 35 feet-long steel bars, the fabricators, Stress Steel Corp., chose to pre-assemble them into groups of 21 each, which could be shipped and erected as a unit. Each unit has three rows of seven bars each; the bars in the middle row starting 10 feet above the bottom. Specially designed jigs were used in the shop to fabricate and assemble the steel bars, grout tubes, anchors and bracing. The bars, 1-3/4 inches in diameter, are made of alloy steel with an ultimate strength of 145,000 psi (pounds per square inch). The ends of the bars were threaded into steel end plates, 1-3/4 inches thick. Rigid welded steel tubing of 1-1/2 inches inside diameter, tack welded to the end plates, served as grout tubes around the steel bars and acted as stiffeners for the 21-bar assemblies during shipping and concreting. Steel angle braces were attached to each group of bars for added rigidity during handling and erection, as well as steel cross-plates for extra support. A completed bar assembly weighed less than 3 tons and could be handled quickly and easily. Six such assemblies (126 bars) made a truckload for shipment to the job site. Three bar assemblies (63 bars each) were placed on each side of the two exterior corners of each triangular Arch leg.

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