North Carolina's Dare Bridge fits right in

More than 3,000 locals celebrated the grand opening of the Virginia Dare Memorial Bridge on August 16, 2002, with a speech by North Carolina Gov. Michael F. Easley and a ceremonial first crossing in a motorcade led by actor and local resident Andy Griffith, driving his 1935 Packard convertible.

It was good cause for celebration. The Dare Bridge, which is two miles longer than any bridge in the state, bypasses downtown Manteo to cut 20 to 40 minutes off drivers' travel time. It also opens a gateway from the state's interior to the beaches, resorts and attractions that fuel the region's tourist economy. As part of the North Carolina Department of Transportation's (NCDOT) overall highway improvement program, the $90 million Dare Bridge was built without adverse impact on the sensitive Outer Banks wetlands.

The Outer Banks is a chain of barrier islands along the North Carolina coast, surrounded by 900 miles of shoreline and the world's largest estuary system. The Dare Bridge replaces the William B. Umstead Bridge, a 45-year-old two-lane structure on U.S. Highway 64-264, which connects the county seat of Manteo to the fishing hamlet of Manns Harbor. Raleigh lies about 197 miles further inland along this east-west highway.

During rush hours, when vacationers leave the island for the mainland, the Umstead Bridge caused big-city style traffic backups in Manteo. When two hurricanes in August 1996 forced evacuation of the island during peak tourist season, it was clear that another route was needed to get people off the Outer Banks in an emergency. Traffic backed up for several miles, from traffic signals in Manteo to a causeway linking Manteo and Nags Head, N.C.

The NCDOT had planned corridor improvements to make U.S. 64-264 a four-lane facility from Raleigh to Manteo, which would involve widening about 40 miles of roadway from two to four lanes. The need for a better hurricane evacuation route accelerated the project. Planning for a new four-lane bridge began in 1996 and, in January 1997, Wilbur Smith Associates (WSA) of Raleigh, N.C., was selected for the design. Balfour Beatty Construction of Atlanta was awarded the prime construction contract in April 1998, and HNTB Corp., Raleigh, N.C., was selected to perform construction engineering and inspection.

The Dare Bridge is a conventional AASHTO girder bridge carrying a four-lane highway divided by a center parapet. It rises 66 ft off the water at its highest point and clears a 200-ft-wide navigation channel. The concrete structure utilizes pre-cast piles and girders and cast-in-place pier caps and decks. WSA designed the structure for a 100-year life span, up to twice as long as its predecessor.

To increase the longevity, NCDOT specified an increase in the concrete cover on the epoxy-coated reinforcing steel and utilized a durability-based, high-performance concrete with calcium nitrate corrosion inhibitors. This was the first full-scale implementation of a durability-based, high-performance concrete in the state. The addition of silica fume and fly ash to the concrete increases the resistance to chlorides found in saltwater, which helps protect the reinforcing steel.

The bridge deck alone has 7,250 tons of reinforcement steel-enough to stretch 2,633 miles, or a round trip from Manteo, N.C., to Wichita, Kan.-and more than 43 acres of concrete riding surface, enough deck concrete to cover 278 basketball courts to a depth of one foot. Instead of traditional transverse grooving, the entire deck is diamond ground, yielding an exceptionally smooth ride. Traffic on the bridge can travel at speeds of up to 65 miles per hour with nary a bump.

For more on the story, read the May issue of ROADS & BRIDGES magazine.