NO. 7 BRIDGE: U.S. 21 over Harbor River Bridge Replacement

Dec. 2, 2021

New bridge spans environmentally sensitive salt marshes and a tidal river

(This feature published as "Challenging Design" in November/December 2021 issue)

The design team overcame numerous challenges to complete the final design and plans for the U.S. 21 over Harbor River Bridge project.

The project design criteria specified a 65-ft minimum vertical clearance from the mean higher high-tide elevation to the low chord of the bridge within the limits of the defined 90-ft navigational channel. The layout of the new bridge was established to satisfy the required vertical clearance over the navigational channel and tie back down to grade using a maximum grade of 4%.

The bridge design also had to accommodate the environmentally sensitive nature of the salt marshes and address the challenges involved with constructing substructure units within the tidal river. The team selected a superstructure type that could accommodate long spans (modified Florida BT-78 Beams spanning 167.5 ft) to minimize the number of substructure elements required. The team elected to use prestressed beams to eliminate long-term maintenance costs associated with structural steel in a marine environment.

The design criteria required a seismic “pushover” analysis and the substructure was also required to resist vessel impact. The team created multiple models of the entire bridge using CSiBridge software to ensure all seismic design requirements were satisfied. They also used FB-MultiPier software to accurately distribute the massive loads to adjacent bents and fully capture the effects of soil-structure interaction below the groundline.

The project criteria specified that drilled shafts having a diameter of 6 ft or greater were to be considered “mass concrete” elements. Nine interior bents located within the limits of the channel required two 8-ft-diam. drilled shafts each. Cooling tubes were placed in these drilled shafts and a special concrete mix. Computer simulations of the curing process were developed using ConcreteWorks software.

Project: U.S. 21 over Harbor River Bridge Replacement

Location: Beaufort County, South Carolina

Owners: South Carolina Department of Transportation

Designer: Infrastructure Consulting & Engineering PLLC

Contractor: United Infrastructure Group

Cost: $54.7 million

Length: 3,340 ft

Completion Date: April 26, 2021