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The Virginia Dare Bridge
The North Carolina Department of Transportation and the Federal Highway Administration proposed widening and relocating US64/264 over the Croatan Sound. The 5.2 mile bridge connects the mainland to Roanoke Island in Dare County providing access at the mid-point of the Outer Banks and is the longest bridge in North Carolina. The previous US64/264 route consisted primarily of the two lane bridge and roadway section through downtown Manteo, North Carolina. The relocation of US 64/264 and the Virginia Dare Bridge provides multi-lane free flow access across the Croatan Sound thereby improving traffic flow and enhanced safety, especially emergency hurricane evacuations which occur during the peak of tourist season. The hurricane season was extremely active in 1996 thereby placing a high priority on the US64/264 project thus requiring an accelerated design schedule.

The North Caroline Department of Transportation (NCDOT) selected Wilbur Smith Associates (WSA) to lead a design team for the "conventional" bridge type. The WSA team was also responsible for the Vessel Impact, Storm Surge and Scour analysis for both alternates.

The six most significant aspects of the bridge project included the highly corrosive coastal environment, the high level navigable clearances and vessel impact forces, the coastal storm surge and scour characteristics, environmental sensitivity and high quality wetlands, a total crossing length of 5.2 miles, and the implementation of a new bridge type in North Carolina with all project objectives of the design scope being met on a highly accelerated schedule.

NCDOT selected the Wilbur Smith Associates Team and negotiated a fee in January 1997 with notice to proceed in February 1997. The Bridge Type Study and Vessel Impact Analysis were completed in March and the Preliminary Bridge Plans submitted in April 1997. The Final Bridge Design and Review Plans were developed from May to September 1997. Review comments were received in October and Final Sealed Tracings submitted in November 1997. The project, from selection of the WSA Team to delivery of Final Bid Documents was accomplished in an amazing ten (10) month time frame.

High Performance Concrete was utilized throughout the structure to enhance the durability and life expectancy (100 years) of the bridge in the highly corrosive coastal environment. A continuous three span unit (137.75' - 229.6' - 137.75') spans the navigation channel providing clearances of 65' vertical and 180' horizontal. The superstructure utilizes precast/prestressed modified bulb tee sections. Girder sections were supported by temporary towers and strongbacks and then post-tensioned to form a continuous unit. Girder section depths are 6.5 feet in the positive moment region haunched to 11.0 feet in the negative moment region.

The substructure units are designed to withstand the vessel impact loads while providing flexibility to redistribute the impact loads through the superstructure. The mid to high-level units are two column hammerheads supported on table top footings and 30 inch square precast/prestressed piles. The pile embedment depth in the soil is 100 feet, pile lengths approach 120 feet and estimated scour depths approach 75 feet.

A horizontal curve is incorporated and bridge length extended to minimize the impact on high quality wetlands and fisheries primary nursery areas. The bridge segments spanning the wetland also incorporate a closed drainage system.

The Virginia Dare Bridge is an aesthetically pleasing 5.2 mile crossing of the Croatan Sound implementing design features respectful of the environment, providing safe efficient travel of vehicular traffic as well as marine vessels. The structure is of the highest economic efficiency at a total construction bid of $89,000,000 ($48/square foot) and designed to an anticipated life span of 100 years.

Wilbur Smith Associates won the North Carolina State ACEC Structural Award and recognition at the National ACEC level. WSA has also won one of eight National PCA (Portland Cement Association) Awards for design and construction of concrete bridge.

This article appeared in the June-July 2003 issue of "the Professional Engineer" magazine.

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