This paper presents an evaluation of rock anchors installed to support the suspension cables on the Royal Gorge bridge in 1929 and the subsequent design and construction of replacement anchors. Corrosion was observed on several of the suspension cables in 1982 and subsequent evaluation indicated some of the cable wires required replacement. An investigation was conducted to determine the subsurface conditions beneath the new anchor locations and to design new anchors. Precambrian gneiss was encountered at the rock anchor locations. Several vertical joint sets were identified though the joints were widely spaced. Post-tensioned grouted anchors were subsequently designed and constructed to transfer the load from the existing suspension cable to a new anchor system. Multiple anchors were installed at each cable location. Testing of the new rock anchors indicated allowable loads exceeding 200 kips per anchor. The suspension cable load was transferred to the new anchors by systematically splicing the cable tendons to the rock anchors.
1 INTRODUCTION AND HISTORY
The Royal Gorge Suspension Bridge was constructed in 1929 by the Royal Gorge Bridge and Amusement Company. The bridge was built as a tourist attraction to view the Royal Gorge. The bridge has the unique distinction of being the highest suspension bridge above a point - being 1,053 feet above the Arkansas River. The bridge has a total length of 1,200 feet with a main suspended span of 880 feet. An aerial view of the bridge is shown on Figure 1. The bridge is located approximately six miles west of Canon City, Colorado. The bridge was designed for a concentrated live load of 10 tons, a uniform live load of 900 pounds per lineal foot and a dead load of 650 pounds per lineal foot. Each suspension cable contained 2,100 No. 9 cold drawn galvanized steel wires spun in parallel strands. Each wire strand was tied into a unique anchorage system at each abutment. The original anchor system layout is shown on Figure 2. The anchorage of each of the wires involves a series of pipes which have been grouted into a trench in the rock beyond each abutment. The sequence for construction of the original anchorage consisted of excavating a 75-foot long trench at the same inclination as the suspension cable beyond the abutment to a depth of approximately 25 feet. A series of 100, 2-inch diameter pipes 36 inches in length were grouted into the base of the trench 18 inches deep. Twenty-one wires were then attached to each pipe with a field or shop hitch. After attaching twenty-one wires to each pipe, a 3/4-inch diameter reinforcing rod was inserted and grouted in place. The reinforcing rods were allowed to extend to the top of the trench. To complete the construction, the trench was backfilled with concrete and a small masonry structure constructed where each suspension cable entered the ground (Cole, 1930).
Figure I - Aerial View of Royal Gorge(available in full paper)