PROJECT PROFILES : ANGELS CREST HIGHWAY

2 mi

5 km

The project was located in a remote mountain area where a slide had closed the the main highway linking a resort town with a major metropolitan area. The only other route into the area required a detour of approximately 60 miles.

The DOT wrestled with various solutions that were complicated by the active nature of the slide. The side of the mountain continued to slump over 500’ to the valley floor. Efforts to stabilize the mass of rock proved futile and therefore it became evident that the only logical solution was to place abutments on stable soil comfortably outside the slide limits.

This decision meant that the bridge span would reach to over 200’. A cast in place structure was quickly eliminated since falsework could not be placed on the sliding mass of rock and soil. Steel was dropped as an alternative due to its cost and the 6 month delivery schedule. After careful consideration and planning with industry representatives an 8’-0” spliced precast bulb-tee girder was selected.

Due to the narrow mountain roads, it was quickly evident that the girder would have to fabricated in pieces short enough to maneuver the two lane road that lead to the site. The 208’ long girder was divided into three pieces, two 70’ end sections which housed the post-tensioning anchorages and one 90’ center section.

The bridge geometry further complicated the design due to the significant grade and the tight radius curve that the highway profile originally had taken. Designers took the opportunity to increase the curve radius by choosing to go with a slightly wider deck than the two lane roadway would require. The center of the road curved as it proceeded across the bridge and allowed the traffic to better maintain their speed while negotiating the steep grade through a curve that now was independent of the existing mountainside terrain.

Special bearing seats were built into the ends of the girders to reduce the bearing pad thickness. This design feature also allowed the bridge abutment to remain in a horizontal plane at minimal cost to the overall project.

A hikers parking area was identified as a staging area about three quarters of a mile to the east of the slide. This area was large enough and relatively flat to allow the girders to be assembled into one piece.

The transport of the one piece girder the three quarters of a mile from the staging area was complicated by the tight radius curves which also had significant amounts of superelevation. Girders of this size and length can become laterally unstable when subjected to cross slopes that place the center of mass of the girder offset from the support.

To compensate for this, special hauling rigs called platform trailers were selected. These trailers were equipped with hydraulics that would allow the support on which the girder sat to be automatically adjusted for the superelevation and maintain the girder in a vertical position.

When the girders arrived at the bridge site, a portion of the uphill side of the slope needed to be excavated to allow enough room for the girder to situated where the two cranes to lift the unit off of the platform trailers. To do this part of the final abutment had to be covered with temporary road base.

This meant that the girders had to be initially stacked flange to flange on the down slope or cliff side of the abutment. After all of the girders had been set, the uphill side of the abutment, that was buried, was excavated and the girders were unstacked and set in their final configuration.

With the bridge site at over 6500’, several design features were considered to protect the bridge from corrosion and the freeze thaw cycles. The required design strength for the girders was 8500 psi at twenty eight days. This coupled with the requirement for 6% air content, required the development and testing of special concrete mix designs capable of meeting this criteria.

Another feature that was dictated by the harsh mountain environment was the use of epoxy coated rebar. All of the rebar in the deck, including the girder stirrups were treated with epoxy to improve the corrosion resistance of the completed bridge.

The post-tensioning was applied in two stages, the initial stage saw the introduction of two thirds of the force being applied to the girder at the staging area prior to shuttling the girders to the bridge site. The second stage was applied after the deck was placed and cured. This methodology resolved two of the designer’s major concerns. The first was to improve the long term durability of the deck by introducing some compression from the post-tensioning into it. The second was to improve the girder stability during transportation by not having the full prestressing force applied to the girder which would have the net affect of amplifying any lateral deflections or sweep.