Sunday, July 6, 2008

Assembly and Erection of the Gateway Arch


At the job site, cranes equipped with a tubular steel spreader-bar placed the sandwich panels upright in a specially constructed storage area. Initially, the sides of the triangular sections were prefabricated at the plant and sent to the site as three legs of a triangle. Assembling these sections at the site required extremely rigid controls for positioning and welding the corners, but any additional pre-assembly in the shop was impossible because the resulting sections would have been too large for shipment. Triangular sections for higher positions in the Arch had smaller dimensions and could be fabricated and shipped as three dog-leg-shaped pieces, each with a short side and a long side. Thus the corner connections could be made in the shop, leaving the simpler and faster task of welding along the sides for the field crews.

At the site the triangular wall sections were assembled on a 56 x 125 foot concrete welding pad. To shield the welding operations, the pad was equipped with a steel-framed shelter 56 x 60 feet, 20 feet high and roofed with corrugated galvanized steel. Canvas tarpaulins that could be raised and lowered protected the ends. The entire structure rode on wheels and moved back and forth over the pad as successive sections of the Arch were assembled.

A crane lifted a completed section from the welding pad onto a specially designed railroad car with a 42 x 52 foot deck composed of 24 WF beams. The car was equipped with an outrigger that rode on rubber-tired wheels and supported one corner of the triangular Arch section. A tractor pushed the railroad car along special tracks to a position near the Arch leg.

Specially designed creeper cranes climbing the Arch legs lifted a completed section into position 4 inches above the previously placed section and set it on three 35-ton screw jacks. The jacks positioned the section accurately, leaving a gap between the sections just wide enough for the welds. The first section on each leg was attached to the foundation by anchor bolts 5/8 inches and 1 inch in diameter. These were tightened with a manual wrench. To stiffen the triangular section during moving and concreting, the corners were braced with adjustable steel pipe struts of 5 inches in diameter and 12 WF steel-beam wales.

The inner plates of carbon steel were welded with low hydrogen electrodes using a normal welding procedure. Semiautomatic gas-shielded welding was used to join the outside stainless-steel surfaces. After welding, concrete was placed in the walls and post-tensioned. Concreting was stopped at the 300-foot level; above this point the triangular sections were fabricated with diaphragms between their outer and inner skins.

Welding sections for the assembly of a triangle caused some deformation due to heat shrinkage of the welds. Later, these sections were forced into the proper position for welding on the legs, causing a slight buckling of the stainless-steel surface. After a study of the situation and of the welding operations, the steel erector decided to camber the walls of each triangular section about 1-1/2 inches in 35 feet, so that the walls would be in the desired straight line after the welding shrinkage occurred.

It should be pointed out that all the welding on the Arch was done by expert welders who not only successfully completed welding tests but also showed exceptional welding skills.

Interior installations, including prefabricated steel stairs, tracks for the passenger-train conveyor system, utility pipes and electrical wiring were put in each Arch leg both from the bottom entrance and from the top, depending on the level of the construction stage.

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