US 17 Johnnie Dodds Boulevard

Mt. Pleasant, South Carolina

KCI provided structural design of two bridges as part of a design-build contract for roadway improvements along three miles of a heavily-traveled corridor. The improvements increased capacity by widening the road and alleviating congestion with a new overpass, and also included replacement of a second bridge that had become functionally obsolete.

The grade separation is accomplished by a 161-foot single-span steel plate girder bridge with an MSE wall system to build the embankment to finished grade. The approaches to the bridge are supported by MSE walls on both sides due to the extremely confined roadway footprint. The MSE wall panels and the back of the barrier walls feature architectural designs. The abutments support a pilaster on each end with ornamental lights.

It was a great opportunity to take the lead in managing the bridge design, quality control, and extensive aesthetics for a signature structure.

Adam E. Lansing, PEProject Engineer

Adam E. Lansing, PE

The challenges faced in the design of this bridge included extensive existing utility accommodations, aesthetic requirements, environmental specifications, geotechnical and seismic design requirements for accommodating the interaction between the bridge and the MSE wall, strict seismic design criteria in the Charleston region, and accommodating high traffic volumes during construction. In addition, a 90-foot flat slab bridge was designed as part of the Bowman Road widening project.

Seismic design was extensive and complicated by liquefiable soils, unpredictable behavior of the Cooper Marl Formation the piles were driven into, and the corrosive environment of the tidal stream the bridge crosses. These issues were addressed by careful detailing, increased concrete cover for reinforcement, a full seismic pushover analysis using expected material properties and in-situ soil behavior, and utilizing lightweight concrete to reduce structure dead loads and lateral earthquake forces while providing enhanced durability of the concrete flat slab.