Thomas E. Tallman and Thomas M. Harris, Wilbur Smith Associates
The Virginia Dare Bridge, the longest bridge in North Carolina, is located on US 64/264 over the Croatan Sound at Manteo. The bridge connects the mainland to Roanoke Island at the midpoint of the Outer Banks and is on a hurricane evacuation route. With a target service life of 100 years, the most significant design challenges of the bridge project included the highly corrosive coastal environment, high level navigable clearances, vessel impact forces, coastal storm surge and scour characteristics, and environmentally sensitive high quality wetlands.
Early in the design process, a study of bridge types was performed in order to determine economical alternates for the bridge. The bridge was segmented into regions based on soil type, vessel impact loads, and bridge profile. For the superstructure, six different structural member types were analyzed with multiple variations in span lengths, girder spacings, and material strengths. The substructure analysis included three different structural frame systems and two foundation types. A precast segmental superstructure and a conventional precast, prestressed concrete girder superstructure were offered as alternates for bid. The bid was awarded to the conventional alternate.
The main three-span unit across the navigation channel has span lengths of 137.75, 229.6, and 137.75 ft (42.0, 70.0, and 42.0 m) and provides clearances of 65 ft (19.8 m) vertically and 180 ft (54.9 m) horizontally. The superstructure utilizes precast, prestressed concrete modified bulb-tee girders. Girder section depths are 6.5 ft (1.98 m) in the positive moment regions transitioning to 11.0 ft (3.35 m) in the negative moment regions. The girder lines consist of five segments post-tensioned in three phases during construction of the superstructure. Girder sections were initially supported by temporary towers and strongbacks and then posttensioned to form a continuous unit.
The substructure units are designed to withstand the vessel impact loads while providing flexibility to redistribute the impact loads through the superstructure. The midto high-level units use hammerhead piers supported by two columns on table-top footings and 30-in. (760-mm) square precast, prestressed concrete piles. The pile embedment depth in the soil is 100 ft (30.5 m), pile lengths approach 120 ft (36.6 m), and estimated scour depths approach 75 ft (22.9 m).
High Performance Concrete
The Croatan Sound has variable chloride content in the water ranging up to 13,000 ppm. As a result, high performance concrete (HPC) was utilized throughout the approximately 190,000 cu yd (145,000 cu m) of concrete in the structure. The different types of structural elements were evaluated independently with the goal of achieving a service life of 100 years before any member would need repair as a result of corrosion. Different dosages of calcium nitrite, chloride concentrations, and concrete permeabilities were considered using Fick’s Second Law of Diffusion. The most cost-effective scheme for each element was selected. Constructibility issues were also examined prior to final selection of corrosion inhibiting measures.
In the superstructure elements, calcium nitrite at a dosage of 2.0 gal/cu yd (9.9 L/cu m) was used in order to elevate the corrosion threshold of all members. For the substructure elements, the amount of calcium nitrite was increased to 3.0 gal/cu yd (14.9 L/cu m). Five percent silica fume was also used in the substructure elements in order to achieve low permeability at an early age. Class F fly ash at 20 percent of the total cementitious materials was utilized to reduce the permeability in both the superstructure and substructure with 30 percent fly ash being used for the pile caps in order to reduce the heat of hydration in these mass concrete elements. The specified maximum water-cementitious materials ratios were 0.40 for the precast, prestressed concrete and 0.43 for the castin-place concrete. The specified minimum cementitious materials content varied from 560 to 640 lb/cu yd (332 to 380 kg/cu m).
Epoxy-coated reinforcement was used in both the superstructure and substructure. The typical concrete cover was 3 in. (75 mm) with an increase to 4 in. (100 mm) for the main reinforcing steel in the substructure elements. As an additional corrosion protection measure, the precast, prestressed concrete members were designed for zero tensile stress under service loads. Specified concrete compressive strengths at 28 days were 4500 psi (31 MPa) for the bridge deck, railings, and the cast-in-place substructure; 8000 psi (55 MPa) for the precast, prestressed concrete girders; and 6000 psi (41 MPa) for the precast, prestressed concrete piles.
The Virginia Dare Bridge is an aesthetically pleasing crossing of the Croatan Sound, implementing design features respectful of the environment while providing safe, efficient travel for vehicle and marine traffic for the next 100 years. It was opened to traffic in August 2002.
Further Information
For additional information on research on HPC by the North Carolina Department of Transportation, visit www.ncdot.org/planning/development/research/research_str.html.
Acknowledgement
The authors thank Rodger D. Rochelle, of the North Carolina Department of Transportation for his contribution to this article.