“The concrete bridge design was chosen for initial cost savings over a steel girder alternative as well as the long-term maintenance cost savings.”

Thai Trinh

 

Best Bridge with Spans over 150 ft (46 m)

SC 802 Bridge over the Beaufort River/Intracoastal Waterway in Port Royal/Lady’s Island, S.C.

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On a cool spring evening in 2007, a crane boom attached to a large river barge slammed into the J. E. McTeer Bridge over the Beaufort River in South Carolina. Two beams on the southernmost portion of the bridge deck were destroyed, and a third beam was seriously damaged (http://www.lowcountrynewspapers.net/archive/node/25682#storylink=cpy).

It was one of only two bridges connecting the town of Port Royal to Lady’s Island, and the crash created months of traffic problems in the area.

“When the existing bridge was struck by a barge and temporarily shut down, it showed how vital the bridges were to the community,” says Thai Trinh, PE, engineer for Collins Engineers Inc. in Charleston, S.C.

The bridge owners needed to rebuild the structure, but they also saw it as an opportunity, Trinh says. “Through community outreach meetings, it was clear the community wanted accommodations for bicycle and pedestrian traffic as well.”

In response, Collins designed a 4211-foot-long (1284 m) high-level bridge that would stretch over the Intracoastal Waterway and adjacent salt marsh, running parallel to the original span. The new SC Highway 802 bridge would not only lessen traffic problems, Trinh says. It would also provide multimodal access with a pedestrian walkway protected by a concrete barrier and shared bike lanes.

“It felt good to design the new parallel bridge that provided enhanced connectivity to the islands,” he says.

At the outset of the project in 2009, Collins chose a precast concrete design because they knew it would enable them to meet all of the cost, schedule, durability, and aesthetic goals of the project.
Because the project was funded through the Beaufort County Sales Tax Referendum, rather than state and federal funds, construction cost was a significant hurdle. Time, aesthetics, and durability were also top concerns. For all of these reasons, Collins chose a high-performance precast concrete design.

“Prestressed concrete beams were chosen to accelerate construction time and reduce long-term life-cycle costs,” Trinh says.

Collins worked with Standard Concrete Products to brainstorm ways to increase the span beyond the capability of standard cross sections, says Richard Potts, vice president of engineering for Standard Concrete Products. Potts’ team suggested using a specialty cross section that had previously been used on a splice girder project. “This 96 in. (2400 mm) prestressed concrete bulb-tee girder section provided the span range and beam spacing needed to deliver an economical concrete solution,” Potts says.

The bulb tees allowed the profile of the bridge to be fairly symmetrical with the existing bridge, creating an appealing twin bridge appearance while also utilizing a conventional and readily available construction method that did not require specialty subcontractors to erect the beams.
It also made the 170 ft (52 m) main span one of the longest conventional prestressed concrete bridges in South Carolina, Trinh says. “The use of 10 ksi (69 MPa) concrete and 96 in. (2400 mm) deep bulb-tee beams really extended the limits of conventionally prestressed concrete bridges.”

Adding further time and cost savings, the 116 ton girder was locally produced and shipped via the Intracoastal Waterway, saving on transport costs and special overland heavy load permits.

Safety and durability were also provided through the use of precast concrete. The site required a multihazard extreme event design methodology to account for possible storm surges, scour, seismic events, and vessel collisions. The prestressed concrete pile bents, isolated reinforced concrete drilled shaft bents, and shaft groups supporting waterline footings and hammerhead piers met all seismic design requirements.

The combination delivered an aesthetically pleasing bridge that will withstand the worst that nature—or rogue river barges—might deliver.

PHOTO CREDIT

United Contractors, Collins Engineers

PROJECT CREDITS

Owner: South Carolina Department of Transportation/Beaufort County, Beaufort, S.C.
Engineer of Record: Collins Engineers, Inc., Charleston, S.C.
Precaster: Standard Concrete Products, Savannah, Ga.
Contractor: United Contractors, Great Falls, S.C.
Project Cost: $34.6 million
Bridge Length: 4211 ft (1284 m)