Description: B-3263 - WATAUGA COUNTY Location: Guy Ford Road.
Item(s) of Interest: 3 span cored slab with a Nebraska flow-through rail. End Bents are on drilled shafts. This structure is a low water structure with an annual frequency of overtopping.
The normal water surface (NWS) is about even with the bottom of the cap, placing the bottom of cored slab unit at about 3.5 feet above the NWS.
The bridge was originally built with an asphalt wearing surface that delaminated and peeled off during a storm event three weeks after opening to traffic. The approach slabs also washed out. After the approach washed out, Ralph Whitehead designed one of the approach slabs (under End bent closest to Route 321) with 33 piles spaced at 9’-6" under a 70 ft long approach slab. The approach slab is overlayed with asphalt to match the grade.
Bridge Maintenance rebuilt approaches and replaced the asphalt wearing surface with a concrete overlay with welded wire fabric reinforcement. On the rail exterior, between the rail posts, a galvanized plate was attached to keep water from getting under the concrete overlay.
Observations and Points of Discussion:
At this site, the lack of a flood plain for stream relief during storm events exacerbates overtopping on this bridge.
For sites similar to this, a permanent solution would require a multispan curved girder bridge.
Cost for such a large bridge cannot be justified for the current 210 ADT, which is projected to be 500 in 25 years.
Also, in the mountains, approvals for long detours are difficult to obtain.
FEMA studies do not provide conveyance credit for openings in the rail. As such, there is not much hydraulic design benefit in using the Nebraska barrier rail.
For similar sites, additional measures to mitigate debris accumulation should be considered.
Shallower bent caps with closer pile spacing, and
End treatments for guardrail attachment to bridge rail.
NC DOT
NC DOH
NC GOV
