A lifeline for residents of North Carolina’s barrier islands for nearly 60 years, the Herbert C. Bonner Bridge over Oregon Inlet had become vulnerable to scour and  the corrosive effects of the saltwater environment. Those concerns are now a memory, thanks to a $252-million, 2.8-mile-long high-rise replacement structure.

Named for a former state legislator, the Marc Basnight Bridge features variable depth, post-tensioned segmental concrete box-girder spans that feature a trapezoidal shape, with tapered overhangs and a parabolic haunch. The superstructure is supported by box columns with uncluttered vertical lines integrated with the superstructure’s streamlined arcs.

The team tailored specific construction activities to land, deep water and environmentally sensitive submerged aquatic vegetation (SAV) areas. Ground-based equipment was used in the south approach spans, while deepwater construction progressed simultaneously in two directions using barge-mounted ringer cranes and balanced-cantilever superstructure erection. In the SAV region, a 1,600-ft-long work trestle acted as a “leap frog,” moving spans from the tail to the leading edge to minimize environmental impacts.

Because Oregon Inlet’s deep scour and high lateral forces place significant demands on bridge foundations, the project team based the new structure’s scour profiles on sophisticated hindcast simulations of more than 100,000 storms dating back 160 years. Each bent was subjected to a unique multi-pier soil-structure interaction analysis, with multiple models to capture zero-, full- and partial-depth scour conditions. Optimizing the foundations design provided a path toward the most economical and constructible design for each bent.

Similarly detailed calculations optimized the nominal driving resistance for each pile installation, ensuring that the new bridge can resist scour up to 84 ft deep as well as from 105-mph winds and vessel impacts.

Complementing the new bridge’s ruggedness is its ease of maintenance, with extensive use of precast and stainless-steel components that will withstand the harsh saltwater environment for at least a century.

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