The preliminary design for the Arch called for guy cables at about the 530-foot mark to stabilize the structure. Computations indicated that such stabilization would require one 6-inch or two 3-inch cables on each leg - resulting inconsiderable cost, as reuse of the cables would not be practicable. This, and some additional problems with the cables, led designers to the idea of using a stabilizing strut or truss at the 530-foot level.
When this level was reached, the two creeper derricks working together raised and positioned a steel stabilizing truss 255 feet long, which connected the ends of the two legs and braced them against each other while the remaining sections were put in place to complete the Arch. The bridge-like truss was shop fabricated of small wide-flange beams and tubular members made from high-strength constructional alloy steel. It was assembled at the site with 325 high-strength bolts and lifted into place as a unit.
The truss was 40 feet wide, 15 feet deep and weighed about 60 tons. It was connected to the Arch by means of a steel harness fitted around each leg so that the loads due to the increased weight and wind force on each leg would be transferred directly to the foundations. Two 45-ton horizontal jacks were used at each end, at the points of contact of the strut and the harness, to fit the brace against the legs.
After the stabilizing truss was erected, 21 sections were added to each leg. The next step was to put the final "keystone" section into place to complete the Arch. A jacking frame was used to exert pressure against the two legs so that this section could be set in. With the Arch completed, the stabilizing strut was lowered by the creeper derricks before they started their trip down the legs, dismantling and lowering their tracks as they progressed. At the same time the bolt holes in the stainless-steel surface were plugged and ground smooth.
An important phase of the project was the method of survey control. Triangulation was used with the aid of four monuments, which formed a rectangle 300 x 720 feet around both center lines of the Arch. By using the two north monuments for triangulation on the south leg, and the two south monuments for triangulation on the north leg, all three corners of each leg could be seen. All survey work was done at night to eliminate distortion caused by the sun's rays striking one side of the Arch while the other two sides were in shadow.
