Sunday, July 6, 2008

Pre-stressing the Foundation

Each group of tensioning bars required careful positioning because the bars had to be inclined in two directions - to fit the curvature of the Arch as well as the tapering cross-section of the legs. With the steelwork in place, concreting proceeded in lifts of about 5 feet each. The bars were post-tensioned after the 5,000-psi (pounds per square inch) concrete reached a strength of 4,000 psi - usually after 7 to 10 days. The stressing was done by a hydraulic jack, which applied a load of 71 tons on each bar, or a total of 18,000 tons for each leg. Bars were tensioned in a strict sequence specified by the engineers. The full load was applied to each bar in one operation with a center-hole hydraulic jack of 100-ton capacity, which reacted against a steel jacking plate 1-3/4 inches thick, embedded in the top of the concrete. Welded steel tubing around each 1-1/4 inch diameter bar had an inside diameter of 1-1/2 inches. This left little space between the bar and the inside wall of the tubing, so that relatively high pressures were needed during grouting. To simplify this operation, pairs of grout tubes on adjacent tensioning bars were connected at their lower ends by specially fabricated steel grout pipes 1 inch in diameter. Grouting was done by a worm-type, high-pressure pump that forced the grout down one tube and up the adjacent tube. This procedure enabled the contractor to take advantage of the hydraulic head in the first tube to grout the second tube and eliminated the need for special grout pipes and bleed openings. The top of the second tube served as a bleed opening to ensure complete expulsion of air. When steel erection on the south leg had reached an elevation of about 120 feet, it was noticed that the bars would not elongate properly under post-tensioning because of jamming in the grout tubes. The post-tensioning tendons were coupled on each station line about 12 feet apart. A "coupler shield," essentially a piece of tubing with increased diameter, surrounded each coupler. When concrete seeped into the tubing, the space above the coupler (inside the shields) was filled up; it was decided to add 12 tendons to each leg from that elevation to the 300 foot mark, where concreting ended. Also, additional steel was used in the upper sections of the Arch. When the keystone sections were placed, a crown thrust was applied by hydraulic jacks to increase the compressive bending stress in the extrados of the Arch legs to offset tensile stress under wind load. Following the trouble with the tendons, it was decided to apply an additional 10 percent of crown thrust so that the increased compressive bending stress in the extrados of each Arch leg could offset the pre-stress loss caused by the jammed tendons. The extra steel in the upper region of the Arch resists additional axial and bending stresses due to the increased crown thrust. In addition to the vertical, or near vertical, pre-stressing of the concrete foundation, horizontal post-tensioning was utilized to pre-stress that part of the foundation slab designed as a cover or bridge above the access tunnel to the Arch. Forty-two bars of 1-1/4 inches diameter, in flexible steel grout tubes, were used for this operation. They were pre-stressed the same way as the vertical bars. Above the foundation, concrete in the walls of the Arch was pre-stressed up to a height of 300 feet Above this level the space between the inner and outer walls is hollow and steel bracing joins the inner and outer skins. This design reduces sway because the bulk of the weight is in the concrete-filled lower sections of the Arch. Massive foundations and filled walls are typical of a weighted catenary arch, structurally the soundest of all arches, for the thrust passes through the legs and is absorbed in the foundations. In other arch shapes, pressure tends to force the legs apart.

1 comment:

Dan said...

amazing. thanks for posting!