Timmons Group served as the prime designer for roadway and bridge design as part of the Mist Lake Water Management Facility development in Durham, North Carolina. A key component of the project was an access road that required a stream crossing, resulting in the design of a 90-foot prestressed concrete girder bridge, chosen as the most cost-effective and low-maintenance solution for the site.
Originally envisioned as a longer, multi-span curved structure on a steep vertical grade, Timmons Group’s structural team worked with the owner, hydraulics, and roadway engineers to simplify the structure. The alignment was modified to support a straight bridge, significantly simplifying the design and reducing construction costs.
The jointless bridge design features integral abutments to minimize long-term maintenance. Prestressed concrete girders efficiently span the required length while reducing upkeep, and abutment piles were designed for expected scour conditions. The bridge accommodates vehicular and pedestrian traffic with a dedicated barrier-separated walkway. Aesthetic enhancements such as concrete formliners and staining give the bridge a natural stone appearance on the parapets, abutments, and retaining walls.
Timmons Group conducted a full hydrologic and hydraulic analysis of the stream crossing, including the CLOMR/LOMR process through FEMA, to account for changes to the 100-year floodplain. Scour analysis was also performed to evaluate potential erosion around the bridge foundation, ensuring the stability and safety of the bridge. Initial scour estimates exceeded expectations, prompting close coordination between structural, hydraulic, and roadway engineers. Using updated terrain data and revised wing wall designs, the team optimized hydraulic conditions, reduced water velocities, and ultimately minimized scour impacts.
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