The Herbert C. Bonner Bridge creates a link between the northern and southern barrier islands of the Outer Banks

LOCATED OFF THE EAST COAST OF NORTH CAROLINA.
The existing bridge, completed in 1963, provides the only highway access to the communities to its south. The bridge spans the Oregon Inlet, one of the most treacherous navigable inlets on the East Coast with a dynamic and constantly shifting navigation channel. Due to the inhospitable location, the existing bridge has suffered deterioration over time and damage from scour from the aggressive currents.

In 2011, The North Carolina Department of Transportation awarded a design-build contract for the replacement of the Bonner Bridge to the team of PCL Civil Constructors and HDR, Inc. Design and permitting activities were largely completed by early 2012, but the start of construction was delayed until resolution of litigation in the summer of 2015. Planning and preconstruction activities started in late 2015. PCL mobilized on site in January 2016, and the official groundbreaking occurred in March of that year.

OVERVIEW OF PROJECT AND DESIGN For the new structure, NCDOT specified numerous durability criteria aimed at achieving a 100-year service life due to the corrosive ocean environment of the project. Two significant criteria were the use of stainless steel reinforcing in all cast-in-place concrete and stainless steel post-tensioning materials up to 12ft above the water. Because of these criteria, limited access to the remote project site, and an aggressive construction schedule, the D-B Team selected to precast as many of the bridge components as possible to improve constructability, quality and durability. The Bonner Bridge project has become NCDOT’s largest use of stainless steel reinforcing, the majority of which is in the cast-inplace marine footings and cast-in-place bridge decks of the approach structures.

The new bridge consists of a 2.8-mile long, two-lane structure. Due to the varying conditions across the length of the project site, the design team divided the bridge into five ‘regions’, with the design being customized to its subsurface and scour conditions for each region. Overall, the new bridge can be grouped into two distinct structures; approach structures totaling 11,000ft of precast girder with cast-in-place bridge decks and a 3,550-footlong, 11-span, segmental concrete box girder bridge, centered within the approach structures. Demolition of the existing bridge is included as part of the project scope and will take place after traffic is switched to the new bridge at the end of the year.

APPROACH STRUCTURES
The approach spans used an extensive amount of precast members; prestressed piles (36" square and 54" hollow cylinder), bent caps, posttensioned columns and prestressed Florida I-Beam girders up to 185ft long. The precast members were fabricated by Coastal Precast Systems of Chesapeake, Va. Extensive use of precast concrete also helped address site access challenges, with the only land access to the bridge from the two-lane NC Highway 12. Consistent, timely deliveries of large quantities of concrete for cast-in-place operations would have been challenging, especially during busy summer months with high tourism traffic. Transporting already fabricated precast elements proved to be economical and streamlined the project schedule.

The main navigation structure crossing the Oregon Inlet is a precast segmental bridge erected by the balanced cantilever method.

SEGMENTAL NAVIGATION STRUCTURE
The main navigation structure crossing the Oregon Inlet is a precast segmental bridge erected by the balanced cantilever method. What is unique to the Oregon Inlet is the ever-changing location of the navigation channel. To accommodate these variations, the bridge included nine possible navigation spans, each having a vertical clearance of 70ft and a horizontal span length of 350ft. Should the location of the channel change, the channel navigation lights on the bridge may be moved to any one of the nine spans with inserts already cast into multiple segments to allow relocation.

Working at water level in a very dynamic and fast-changing waterway with significant swells rolling in directly from the Atlantic Ocean made construction of foundations a constant challenge. The foundations consisted of driven 36" square prestressed concrete piles with a cast-in-place footing caps. Because of the aggressive scour at the site all of the piles were installed on a 2:12 batter with lengths up to 140ft to account for up to 70ft of potential future scour erosion. Footing caps covered pile clusters of 18 to 30, the largest of which was 640cy. Concrete placement was complete by crane and bucket, delivered on the existing bridge.

The substructure and superstructure segments were also fabricated by Coastal Precast Systems of Chesapeake, VA. All elements for the segmental portion of the project were shipped by barge on an inland tow. The superstructure segments were match-cast using three sets of forms. One for pier and end segments, one for the first five variable-depth segments, and one for the constant depth segments; six through twelve.

The substructure consisted of single hollow-box column segments that were match-cast in the vertical position. Typical sizes were 11 x 16ft, 12ft in height high weighing 65tons. The column segments were erected by a barge mounted 4100 Manitowoc Ringer crane. The pier caps were solid precast concrete the largest of which was 200tons, erected with a barge mounted 4600 Manitowoc Ringer crane. Fall hazard protection was engineered into the erection of the segments with temporary access platforms installed prior to erection. Post-tensioning bar couplers were used at each column joint to allow bars to be pre-installed into the column before erection. Due to the durability criteria 2½" stainless steel bars were used in the lower segments up to 12ft above the water.

Each cantilever consisted of 26 segments: 2-split pier segments and 24 balanced-cantilever segments ranging in height from 9 to 19ft, 42ft 7in wide and 14ft long, weighing up to 140tons. Superstructure segment erection started with the Pier Table supported on a falsework tower, placed on the footing, which also served to resist the out-of-balance loads during construction. The pier table consisted of six segments; the 2-split pier segments, and 4 variable depth box girder segments (the nos. 1 and 2 segments). The crane used for pier segment erection was a 4600 Manitowoc Ringer with an adjustable manipulator frame (from the hook), so the segments were set in their final position. An erection frame helped guide the 19ft tall, 100ton segments into location and secured them in place. It also provided access to stress the pier segments together. After the erection of the sixpier table segments, the floating operation was moved to the next pier.

The balanced cantilever segment erection was completed by segment lifters that were anchored to the previously erected segment with post-tensioning bars. The segment erectors were self-launching and were selected for their stability while erecting from the permanent structure independent of changing marine conditions. However, segments still had to be delivered under the lifter by barge which certainly had its challenges given the extreme weather of the Outer Banks and strong currents and swells moving through the Oregon Inlet.

Closures segments, 4ft in length, were constructed using conventional methods with light-weight wood form panels for ease of installation and stripping by hand. Once the closure was formed and locked into place with alignment beams, concrete was placed and stressing of the continuity tendons was completed. The center closure segment located between two fixed bearings was jacked apart prior to concrete placement. The last closure segment was cast late August 2018.

The extensive use of precast concrete members, both conventional and segmental box girder, greatly enhanced the quality and durability of the structure.

CONCLUSION
The 2.8-mile long replacement of the Herbert C. Bonner Bridge at the North Carolina Outer Banks is a monumental structure built in a challenging marine environment, designed for a 100-year service life. Through the spirit of partnership with NCDOT and various other agencies this complex project has come to a successful completion. The extensive use of precast concrete members, both conventional and segmental box girder, greatly enhanced the quality and durability of the structure while facilitating safer and more economical construction and streamline schedule.