May 01, 1988
American Bridge erected a 1,904, three-span continuous Pratt Truss over a deep gorge, with a simply supported plate girder approach span at each end under subcontract to PCL Civil. The interior supports for the truss are two main piers of monumental proportion. The taller of the two rises 350 above its foundation on the west slope of the gorge. Each of the main pier shafts is shaped to meet the geometric requirements of the truss and to maximize efficiency throughout its height. The transverse and longitudinal pier faces have curves that blend to produce an hourglass shape when viewed from an oblique angle. The truss ends are supported on 20 tall steel rocker bents. Beyond each bent, the superstructure extends to a stub abutment by means of a simply supported, multi girder steel span. The entire bridge is founded on 3 diameter reinforced concrete caissons drilled into rock. The steel truss carries the floor system and consists of a center span of 784 and side spans of 560. The truss provides a maximum depth of 110 at the main piers and 40 and 45 at the ends of the side spans and the center of the main span, respectively. Construction of the bridge was accomplished using a 2,879 span (tower to tower) cableway with backspans of 640 and 546. Cableway towers were 270 high, which allowed for a vertical clearance (load cable) at center span of approximately 50 under a 50 ton load. The cableway would luff 37 right or left to accomplish steel erection across the roadway width. The load cable consisted of 2 by 3 cables carrying a trolley that was moved horizontally using a 1-1/8 (29mm) wire rope inhaul/outhaul line. The load falls was rigged to handle a 50 ton load using a 7/8 wire rope loadline. The cableway was anchored into the soil using large concrete anchor tiebacks secured into the rock with 55 long rock anchors. The projects design engineer was URS.