By DOMENIC COLETTI, P.E., M.ASCE, ELIZABETH HOWEY, P.E., P.G., M.ASCE, JOHN JAMISON, P.W.S., R. DOMINICK AMICO, P.E., M.ASCE, MOHIT GARG, P.E., NICHOLAS BURD ETTE, P.E., AND PHIL DOMPE, P.E., M.ASCE
The Marc Basnight Bridge, completed this year, replaces the existing Herbert C. Bonner Bridge, a 2.4 mi long structure that carried North Carolina Highway 12 over the Oregon Inlet. The new bridge, which is slightly longer at 2.8 mi, features extensive use of precast concrete for durability, economy, and constructability; a first-of-its-kind driven pile foundation verification method; and innovative, environmentally sensitive construction approaches.
The Outer Banks consist of a series of barrier islands off the coast of North Carolina; the area is essentially a 200 mi long series of sandbars several miles offshore. Featuring beautiful beaches, historic lighthouses, and abundant natural flora and fauna, the Outer Banks have been a recreational destination for decades. Isolated from the mainland by Pamlico Sound, the Outer Banks were originally reachable only by boat. By the early 20th century, ferries facilitated automobile access. In the mid- to late-20th century, the North Carolina Highway Department (now the North Carolina Department of Transportation [NCDOT]) constructed several viaduct bridges to improve access to the many popular tourist destinations in the area.
One of these was the Herbert C. Bonner Bridge. Completed in 1963, the bridge carried North Carolina Highway 12 (NC 12) across the Oregon Inlet. The 2.4 mi long, two-lane bridge comprised 201 prestressed-concrete girder approach spans that led up to a high-level, three-span, steel plate-girder unit over the designated navigation channel. This iconic structure connected Hatteras Island to the south to Bodie Island to the north, thus providing highway access to communities on the Outer Banks, including Rodanthe, Avon, Buxton, and Hatteras. Connection to Ocracoke Island can then be made via ferry.
However, within a few years of opening, the bridge began to show deterioration and damage. Repairs and retrofits became routine occurrences as NCDOT repeatedly patched spalls on the superstructure and substructure and constructed a variety of "crutch bents"-new and deeper piles to reinforce foundations scoured to nearly zero embedment into the channel bottom. During a severe nor'easter in 1990, the dredge
broke free of its moorings and crashed into the bridge, causing the collapse of six spans. An emergency repair project reconstructed the damaged bridge, but by this time it was clear the bridge needed to be replaced by a more durable structure.
NCDOT undertook a planning study to evaluate alternatives for replacement. However, the problem was more complicated than simply replacing the bridge. NC 12 south of the bridge runs through a national wildlife refuge, and in this area Hatteras Island has proved susceptible to breaches (locations where a storm blows an opening in an island, forming a new channel and splitting what was once one island into two) and washouts during storm events. For this reason, the entire NC 12 corridor needed to be studied. Eventually, in 2008, NCDOT advertised a design/build project to replace the existing bridge as the first step in what was termed the "phased alternative approach" for the overall NC 12 corridor.
Because of various obstacles to the completion of the environmental studies and the record of decision, the procurement process was delayed. However, by August 2011, a design/build team (DBT) led by the PCL Civil Constructors office in Raleigh, North Carolina, with HDR, headquartered in Omaha, Nebraska, as the lead design firm, won the contract to replace the Bonner Bridge. The contract called for the DBT to have ready a final design, permit applications, and agency coordination adequate for permit approvals within approximately 12 months of award, including coordination with other regulatory and resource agencies. Therefore, the majority of the design and permitting activities was completed by early 2013, a process that was aided by North Carolina's National Environmental Policy Act (NEPA)/404 Merger process, which garners agency reviews and concurrence throughout the planning and design processes.
However, the start of construction was delayed until litigation among outside parties was resolved in 2015. Mobilization and preconstruction activities began later that summer, and PCL was on site by January 2016. The official groundbreaking occurred two months later.
Although beautiful, the Oregon Inlet is not conducive to the construction and maintenance of large infrastructure. Often cited as one of the most dynamic and dangerous inlets on the Atlantic Coast, it is subject to frequent hurricane and nor'easter storm activity. The inlet's bathymetry changes constantly as tides and waves move the loosely deposited sand and shift the size, shape, and location of the natural channel daily and sometimes hourly. To maintain navigation under the single navigation span of the existing bridge, the U.S. Army Corps of Engineers had to dredge a channel that would be navigable nonstop and year-round.
Additionally, because the site is in the Atlantic Ocean, any structure in the Oregon Inlet is subject to damage and deterioration. In the case of the existing bridge, severe scour undermined the prestressed-concrete piles, and salt spray contributed to the corrosion of the steel, which led to spalling and deterioration of the concrete bents and prestressed-concrete girders.
These conditions contributed to unprecedented design criteria for the replacement of the bridge. The foundations had to be designed and constructed to experience anywhere from 0 to 84 ft of scour in some regions, combined with flow velocities of up to 12.4 fps, wind velocities of up to 105 mph (measured as the fastest mile of wind), and vessel impact forces of up to 2,151 kips. Because the natural channel shifts location frequently, the Corps and the U.S. Coast Guard requested that the bridge be designed to accommodate a "navigation zone" with a minimum width of 2,400 ft; all spans within this zone were to provide a minimum of 70 ft of vertical and 200 ft of horizontal navigation clearance so that the marked navigation channel could be easily relocated to minimize the need for dredging. To avoid the continuous and expensive maintenance and repair activities needed to keep the bridge operational, NCDOT specified prescriptive durability criteria associated with achieving a 100-year service life.
Construction at the site was constrained and challenged by the same conditions that beleaguered the bridge. High winds, rapid tidal flows, and frequent storms hampered construction, particularly in late fall, winter, and early spring. In addition, the majority of the project site was considered environmentally sensitive. The south end of the bridge lands within the Pea Island National Wildlife Refuge, which is operated by the U.S. Fish and Wildlife Service. The construction easement within the refuge was extremely narrow, and the length of the project site to the north was within the National Park Service's Cape Hatteras National Seashore, which also had a tightly constrained easement.
Exacerbating the challenges caused by a limited work area was the proximity of the existing bridge, which had to remain in service until completion of the new bridge. Much of the shallow aquatic environment is also important submerged vegetation habitat; in those areas neither dredging nor causeways were permitted, forcing the use of temporary work trestles. The project site was also remote, with limited access to a concrete batch plant and only a two-lane highway available for overland material deliveries.
The DBT recognized early in the prebid engineering phase that replacing the bridge would not be a typical bridge job; it was actually a large marine foundation project with a bridge on top. This realization meant that the team could focus on foundation design and construction as being the keys to the project's success. (See "Laying the Foundations" on page 51.)
The structural and geotechnical engineering teams created a color-coded longitudinal plot of the project that illustrated the subsurface conditions, scour profile, vessel collision zones, and navigation clearance zones. The longitudinal plot showed that the project could be partitioned into five regions: north approach spans, north transition spans, navigation unit, south transition spans, and south approach spans. These regions corresponded to the scour profile (which also features five regions, each with a prescribed minimum design scour depth) and the vertical geometry profile of the bridge. Coincidentally, the subsurface soil conditions, which were consistent throughout the length of the project, primarily fell into two regions: the south approach and transition spans and the navigation unit are located in a region where the dense sands are a bit shallower, while the north approach and transition spans are located in a region where the dense sands are a bit deeper. Vessel collision forces-a primary loading consideration for the deep foundations of the bridge-also vary along its length, with smaller forces in the north and south approach spans, larger forces in the north and south transition spans, and the largest forces in the navigation unit.
Considering all the parameters, each region lent itself to a tailored design approach that allowed for widespread use of repetitive construction elements. The saltwater environment and the request for proposal (RFP) emphasized the need for durability, corrosion resistance, and a 100-year service life. These requirements indicated building a concrete structure, while the remote location of the project site suggested the broad use of prefabricated elements and modular construction. All indicators pointed to the use of precast concrete as the optimal design solution.
The extensive use of precast concrete elements offered multiple advantages. First among these were durability and quality. Precasting Florida I beam (FIB) girders, box girder segments, bent caps, columns, and piles in an off-site precasting yard under controlled conditions resulted in the production of high-quality, durable concrete elements. These levels of quality and durability would have been difficult to achieve in the harsh marine environment of the Oregon Inlet. Second, precast elements were economical; fabrication off-site was less costly than trying to deliver concrete to the remote site and then casting it in place. Minimizing field construction work from barges and work trestles also led to faster and safer construction. Last, minimizing field construction work, construction duration, and the placement of cast-in-place (CIP) concrete on-site were environmentally friendly measures, reducing the duration and extent of temporary environmental impacts.
The north and south approach spans represent approximately half the total bridge length and were designed for 100-year return-period scour elevations from -22 to -34 ft. Foundations in this region are precast concrete pile bents, with three or four vertical, 54 in. diameter cylinder piles for each bent. Typical cylinder piles are about 135 ft long, and the total length of the cylinder piles is more than 3.4 mi. The piles were made with 8 ksi concrete and have 6 in. walls, with a 2.5 in. minimum concrete cover. Prestressing was applied with 32 0.6 in. diameter, grade 270 strands. The piles were cast monolithically and not spun cast in segments as is sometimes done with cylinder piles. Using spun-cast piles would have required greater wall thickness for posttensioning the ducts and strands, which was not desirable because of the considerable weight of these piles. Additionally, saltwater corrosion of any joint would have been a concern.
The cylinder piles connect to innovative precast bent caps via reinforced CIP concrete infills, which extend 30 ft into the hollow piles to provide stiffness in cases of severe scour and also transfer the moment and axial loads to each pile. The infills also convey strength locally to prevent damage to the pile wall in the event of a vessel strike. The precast bent caps were designed with voids to reduce their weight-a concern during shipment-and were nominally prestressed to address transportation and handling stresses. The voids were filled with CIP concrete when the caps were connected to the cylinder piles. After connection of the caps with the piles through the CIP infills, CIP pedestals were used to accommodate construction tolerances in the vertical locations of the precast bent caps, providing bearing seats at the correct elevations to support the precast superstructure girders. In accordance with project requirements, all the CIP concrete was reinforced with stainless steel.
The north and south transition spans represent approximately one quarter of the total bridge length and were designed for 100-year return-period scour elevations from -71 to -84 ft. Foundations in this region include as few as six and as many as 16 36 in. sq precast concrete piles, with a 4.5 ksi CIP waterline pile cap. All piles were battered on a 2:12 slope for greater stability, particularly for cases of extreme scour. The remainder of the substructure comprised precast, posttensioned, two-column bents supporting precast pier caps. Some of these pier caps include voids that reduced their shipping weight for truck delivery by land, but most are solid and were delivered by barge.
The typical 36 in. sq piles are about 130 ft long, and more than 12 mi of these piles were used on the project. They were made with 8 ksi concrete and have a 2.5 in. minimum cov-er to the reinforcement. The piles include a central 21 in. circular void away from the ends of the pile to reduce weight. Prestressing was applied with 36 0.6 in. diameter, grade 270 strands. The piles were embedded 4 ft into the CIP pile caps to develop a full moment connection at the pile head. Pile layouts and batter configurations were optimized to resist bent-specific transverse and longitudinal loadings.
Above the waterline pile cap, substructures in the transition spans include posttensioned two-column bents with column heights of up to 50 ft. These 8 ksi columns were made from 5 or 6 ft sq, solid, match-cast, precast concrete segments posttensioned together. One and threequarter in. diameter 150 ksi posttensioning bars were used for the columns; the bars end at various segments based on the moment demands. The column segments were typically 12 ft tall, but each bent includes a single unique segment to achieve the correct bearing elevations.
To meet the project's durability requirements, stainlesssteel posttensioning bars were used for all bars below the splash zone, which was defined as 12 ft above mean high water level. A precast bent cap was posttensioned to the top of two columns, which were seated on a 2 in. grout bed before bar tensioning. CIP concrete pedestals were used to accommodate construction tolerances in the vertical locations of the precast bent caps, providing bearing seats at the correct elevations to support the precast superstructure girders.
Throughout the project, column posttensioning bars were anchored in the CIP pile caps, just above the bottom mat of mild reinforcement. A dead-end anchorage assembly having a stainless-steel nut and anchor plate was placed above the reinforcement, followed by a standard plastic posttensioning duct sealed to the bar with a grout inlet pipe. The duct extends the full height of the column and cap, and standard duct couplers were used for joints between segments. For locations at which the bar was coupled, short lengths of oversized duct were used to accommodate the couplers. Coupling of the bars was staggered; no more than 50 percent of the posttensioning bars were coupled at a given elevation. At the top termination of each bar, a grout cap seals the top anchor plate and nut.
The 11-span navigation unit extends 3,550 ft, includes 12 substructure bents, and is designed for 100-year return-period scour elevations from -71 to -84 ft. Foundations in this region include 18-30 36 in. sq precast concrete piles with a 4.5 ksi CIP waterline pile cap. As with the transition spans, all piles in the navigation unit were battered on a 2:12 slope. On top of the pile cap, precast, posttensioned, hollow box columns support a rectangular precast column cap.
The foundations in the navigation unit are similar to those in the transition spans, the difference being that they have more piles and larger pile caps. The single columns are 16 by 11 ft and have heavily reinforced CIP column bases that extend 17 ft above mean water level. Above this CIP base, typical 12 ft hollow, precast, box-column segments were assembled, topped by a rectangular precast column cap. These 8 ksi precast sections were match-cast and posttensioned together with 2.5 in. diameter, 150 ksi posttensioning bars that ended at various segments based on the design moment demand. The column segment walls are 1.5 ft thick. Like the transition columns, stainless-steel posttensioning bars were used for all bars below the splash zone.
The design of the navigation unit featured individual soil-structure interaction models of each bent, built using FB-MultiPier (Bridge Software Institute, Gainesville, Florida), and a global 3-D finite element model, built using LARSA 4D (LARSA Inc., Melville, New York), of the entire 3,550 ft navigation unit, including the superstructure and substructure. The model permitted staged analysis to capture time-dependent effects from superstructure creep and shrinkage and posttensioning relaxation. Because of the length of the continuous unit, displacement-driven loads-such as temperature, creep, and shrinkage-were significant and could have applied large forces to the piers with fixed bearings. All piers needed to be designed to accommodate maximum scour and redeposited sand up to the pile cap. When the latter occurs, it results in an extremely stiff substructure, so fixed bearings are provided at only the two center bents in order to keep displacement-driven forces low. Sliding disk bearings were used for the remaining 10 bents, limiting their superstructure longitudinal loads to static friction forces only.
Design of the substructure and superstructure required multiple iterations as changes to the pile foundation layout altered the boundary conditions of the global 3-D LARSA model. Close coordination was particularly important in developing the jacking force and displacement requirements for span 24 between the fixed bents. Before placing the central concrete closure in this span, jacking forces were applied to push bents 23 and 24 apart, a process that was optimized to offset half the longterm superstructure creep and shrinkage. (See page 49.) The stiffest possible foundation condition (redeposited sand) was used in establishing the maximum applied jacking force.
For the majority of the bridge-the north and south approach spans and the north and south transition spans, representing 2.1 mi of the total 2.8 mi bridge length-the superstructure comprises a conventionally formed, CIP, lightweight concrete deck supported by precast, prestressed concrete FIB girders. Approximately 8.75 mi of FIB girders were used. The deck is a simple, conventionally reinforced design with stainless-steel reinforcing. The majority of the bridge has a roadway cross section comprising two 12 ft lanes and two 8 ft shoulders with a total out-to-out deck width of 42 ft, 7 in.
The most common span configuration features a fourgirder cross section and a span length of 160 ft, 10 in. A number of the south transition spans feature six-girder cross sections and span up to 182 ft. Typical continuous units consist of up to six spans, with unit lengths of 965 ft between expansion joints. The FIB girder spans do not have permanent intermediate diaphragms, but full-depth end diaphragms were used at the expansion bents, and full-depth continuity diaphragms were used at the interior bents. All FIB girder spans are supported on steel-laminated elastomeric bearing pads that have stainless-steel sole plates and anchor bolts. In some cases, when it was desirable to redistribute the vessel collision loads to adjacent bents, reinforced-concrete shear keys were used.
The FIB girders were designed as simple spans for dead and live loads but were detailed as continuous for live load, using typical NCDOT diaphragms at the interior piers. For the two spans at each end of the bridge, 45 in. deep FIB girders were used; 96 in. deep FIB girders were used for the remaining spans.
The navigation unit comprises 11 spans constructed as a single continuous unit, with a typical span length of 350 ft and an end span length of 200 ft, continuing the 42 ft, 7 in. superstructure width of the adjoining prestressed girder regions. The navigation unit features a posttensioned concrete segmental structure comprising 264 single-cell precast box girder segments supported on posttensioned precast concrete columns. The minimum vertical clearance is 70 ft above mean high water for nine of the 11 spans. Considering the vertical curvature with the variable-depth superstructure, the bridge length meeting the aforementioned navigation zone vertical and horizontal clearance definitions is 3,090 ft, exceeding the requests by the Corps and Coast Guard for a 2,400 ft accommodation.
The variable-depth superstructure is an aesthetically pleasing and economical solution, and it supports the application of the balanced cantilever construction method that facilitated construction of the long 350 ft spans. In this approach, the superstructure segments were erected in upstation and downstation pairs on either side of the pier. The segments were cast off-site and barged 80 nautical mi to the site. They were hoisted into position using deck-mounted lifters. Their joints were prepared with an epoxy and then stressed to the previously erected segments with temporary posttensioning bars. After the epoxy was set for each upstation and downstation segment pair, permanent cantilever tendons were installed-anchoring at both cantilever tips-and the next segment was prepared for installation. After completion of two adjacent cantilevers, the structures were joined at the midspan by casting a concrete closure segment between the cantilever tips. Bottom and top slab tendons were installed within the span to achieve continuity of the structure.
Longitudinal posttensioning consisted of 18- or 22-strand cantilever and 12-, 16-, or 20-strand continuity tendons encased in plastic ducts and high-strength grout. One pair of contingency ducts was provided for each cantilever. Transverse posttensioning in the segments used four-strand tendons spaced at 3 ft, 6 in. encased in flat 1 by 3 in. inner diameter ducts.
The ratio of the top slab overhang length to the interior top slab width is 0.44, resulting in an efficient configuration for transverse loadings on segmental boxes. This was complemented by top-slab haunches having a maximum thickness of 2 ft, 2 in. on either side of the webs. While this is relatively deep, the haunches enabled sufficient clearance to anchor two cantilever tendons per web on the joint face of select segments.
The superstructure segments range in height from 9 ft at the midspan to 19 ft at the interior piers. In addition to the variable depth, the bottom slab thickness varies along the first five segments of each cantilever from 2 ft at the piers to 10 in. toward the midspan. Because the segments were delivered by barge, their lengths were not limited by roadway or truck restrictions, and a longer 14 ft length was used to minimize erection operations while staying within reasonable pick weights for the segment lifters and cranes.
The superstructure is supported by two disc bearings at each pier, and the navigation unit is longitudinally restrained at the two middle piers. The bearings at the other piers were designed to slide longitudinally to relieve thermal, creep, and shrinkage stresses in the long navigation unit. Sliding bear-ings were temporarily pinned to allow for cantilever erection and were unlocked once structural continuity was attained after casting each successive closure pour and stressing of continuity posttensioning.
The interior of the superstructure is accessible for detailed inspection of the box girders and is equipped with lighting and electrical outlets that are powered by a portable generator. Access is gained through an entry door in the diaphragm opening of each end segment as well as 14 bottom slab access openings distributed among the 11 spans. Additional openings in the bottom slab at each pier segment provide access to the hollow precast columns below, which have embedded anchors to facilitate arms-reach inspections of the column interiors through a combination of rappelling and rope-to-rope transfers.
The bridge is a monumental structure built in a challenging marine environment and designed for a 100-year service life with minimal maintenance. Through the flexibility of the design/build process, the DBT developed an economical, durable, constructible design, and it fostered a spirit of partnership with NCDOT and other agencies, resulting in construction approaches that minimized temporary and permanent environmental impacts in this pristine coastal location.
The bridge opened to traffic in February. On April 2, the new bridge was formally christened as the Marc Basnight Bridge. (Basnight, a former state senator, was a staunch supporter of the project.) The existing bridge is being removed span by span and deposited offshore to improve fish habitats at four artificial reef sites. About 1,000 ft of the existing structure will be retained to be used as a training structure and fishing pier, which will still bear the name of Herbert C. Bonner. Demolition is expected to be complete by early 2020.
The Marc Basnight Bridge has received several accolades. In May, the project won the American Road and Transportation Builders' Globe Award in the $100 million or greater projects category, in recognition of doing "an outstanding job in protecting and/or enhancing the natural environment in the planning, design and construction of U.S. transportation infrastructure projects." In June, the project won the Deep Foundations Institute's Outstanding Project Award for innovative foundation design.
Domenic Coletti, P.E., M.ASCE, is a principal professional associate in HDR's Raleigh, North Carolina, office. He was the design manager and engineer of record for the project. Elizabeth Howey, P.E., P.G., M.ASCE, is a professional associate in HDR's Raleigh office. She was the project's lead geotechnical engineer. John Jamison, P.W.S., is currently the western environmental policy lead with North Carolina Department of Transportation in Raleigh but was previously an environmental permitting specialist with HDR. He was the project's permitting lead. R. Dominick Amico, P.E., M.ASCE, is a professional associate in HDR's Charlotte, North Carolina, office. He provided construction services support for the project. Mohit Garg, P.E., is HDR's Tampa bridge group manager. He was a designer for the segmental box girder superstructure. Nicholas Burdette, P.E., is a senior bridge engineer in HDR's Pittsburgh office. He was the substructure design lead for the project. Phil Dompe, P.E., M.ASCE, is a hydraulic engineering specialist with INTERA in Gainesville, Florida. He was the hydraulic modeling and scour lead for the project. This paper was originally presented at the 35th International Bridge Conference at National Harbor in Oxon Hill, Maryland, June 11-14, 2018. Next year's conference will take place June 8-11, 2020, at the David L. Lawrence Convention Center in Pittsburgh.
PROJECT CREDITS Owner North Carolina Department of Transportation Design/build team PCL Civil Constructors, Raleigh, North Carolina Lead designer HDR, Omaha, Nebraska Engineer of record Domenic Coletti, P.E., M.ASCE Geotechnical engineering HDR (design) and Froehling & Robertson, Richmond, Virginia (subsurface investigations) Structural engineering HDR Construction engineering Corven Engineering, Tallahassee, Florida Hydraulic and scour analysis INTERA, Austin, Texas Environmental permitting services HDR Precaster Coastal Precast Systems, Chesapeake, Virginia
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