The original Four Bears Bridge was completed in 1934 and located on native grounds of the Three Affiliated Tribes. Following severe flooding in 1943, Congress directed the U.S. Army Corps of Engineers to investigate flood control measures for the Missouri River. The approved plan included the construction of Garrison Dam, which would flood hundreds of acres on the Fort Berthold Indian Reservation to create Lake Sakakawea ("Sacagawea" to some outside of North Dakota). The 1,425-foot continuous through truss main span of the original bridge was moved to the current location when the tribes were resettled, thus, the bridge provided a spiritual connection to the tribes' ancestors. By the 1990s, the bridge, with its 20-foot roadway width, and increasing amounts of traffic, was classified as substandard in both roadway width and safety features. In 1993, the North Dakota Department of Transportation (NDDOT) undertook an engineering feasibility study to evaluate the possibility of widening the bridge to accommodate current and future traffic demands. The outcome dictated that the narrow bridge should be replaced, based on load capacity, lifespan of a new bridge, and economics.

In 1999, the NDDOT conducted a second study to evaluate bridge types for replacement. The department's options included truss, arch, cable stay, and box girder. Based on the depth of the lake (up to 85 feet), a long span length was preferred from an economical consideration to minimize the construction expense of numerous foundations. The NDDOT determined that a box girder design would be most appropriate, and the state elected to move forward with alternate designs in concrete and steel. A subsequent environmental assessment approved locating the bridge 100 feet north of the existing bridge, which minimized roadway changes and allowed the old bridge to remain in service while the new one was constructed.

The Design Team

The NDDOT selected a design team led by Kadrmas, Lee & Jackson (Prime and responsible for environmental and permitting approvals, tribal coordination, alternate designs, and removal of the existing bridge, in addition to teaming with Lichtenstein Engineers for the steel box girder design alternate and existing bridge demolition) and FIGG (responsible for the concrete segmental box girder alternate, aesthetics, and design charettes). Final plans for the competing designs were competitively bid in February 2003 and Fru- Con Construction (now known as Bilfinger Berger Civil, Inc.) offered the low bid at $55 million for the precast concrete segmental bridge, with a driven pipe pile foundation option. Precast concrete segmental technology provided a number of efficiencies for this particular site.

First, by casting the segments onsite, the mobilization of heavy construction equipment could be scheduled around the load restrictions common on most North Dakota highways in the springtime. Second, the desire to limit the foundation construction expense led to long spans (typically 316 feet). Through post-tensioning, typical segments - which were easily transported - were assembled into the desirable long spans. Finally, precasting operations allowed construction activities to continue through most of the winter.

Construction Begins

Segment erection; lifting precast segment from the barge with beam and winch. Photo courtesy of FIGG.

Construction of the new Four Bears Bridge began in April 2003 with mobilization of sectional barges, small tugs, cranes, piling, and materials to the construction site adjacent to the lake. Preparation of the casting yard and building, water access, and pile and foundation operations were started the first year, along with positioning four barge-mounted cranes on the water to work on foundation construction. Winter came early the first year, freezing the lake in mid-November, requiring that the tugs continually break the ice to demobilize.

North Dakota's extreme winter weather challenged the design team with large ice floes on Lake Sakakawea each spring, in addition to thick ice that forms early in the winter and lasts until late spring, limiting construction activities. The water level in the lake fluctuates significantly based on snowfall amounts and the need for water downstream. Thus, the bridge design had to take that into account. These challenges resulted in the design of a "floating" precast concrete lost cofferdam that provides a pile driving template, a footing at each pier, and the form for casting that footing. A truncated cone shape was selected for the 39-foot-diameter cofferdam, with the faces at 65 degrees from the horizontal, providing a shape that will resist the significant ice forces that will bear upon it. The force imparted to the footing is a combination of the force to fail the ice in bending and the frictional force required to push the ice up the face of the sloped surface of the cofferdam.

To reduce the frictional force on the outside surface of the precast cofferdam, two coats of an epoxy sealer were applied. A lateral ice force of 845 kips is imparted on the pile cap for the maximum ice loading design case. During construction, the cofferdams were floated into position, and then suspended with high-strength rods. Two hydraulic pile guides aligned the piles through a template in the base of each cofferdam for driving. Each cofferdam is supported by 13 or 14 piles in a circular arrangement, driven 90 feet to 160 feet in length into the alluvium layer. Once piles were driven, a tremie seal was placed in the base, the cofferdam dewatered, pipe piles cut off, and reinforced and filled with concrete.

A casting yard, containing four casting beds, was established on the shoreline and local residents were trained in precasting operations and employed by the contractor during construction. Two cast-in-place abutment segments, along with 482 precast segments were match cast, while construction of the foundations and cast-in-place piers proceeded. The 4,500-foot-long bridge has 13 typical, 316-foot spans and 196-foot end spans that were erected in balanced cantilever. High winds are common in the area and are especially accentuated on the lake. To minimize their impact on erection operations, the contractor set the pier segments with a barge-mounted crane and then used a beam and winch system for the subsequent segments. This allowed erection operations to continue until winds exceeded 40 mph, at which time operations were suspended.

Installing segment (just lifted into place) with P-T bar. Photo courtesy of FIGG.

Typically, erection operations were completed with two pair of segments per day, with four to seven days for the erection of the pier tables (the first four segments at each of the 14 piers). At full production, the contractor erected an entire, 316- foot cantilever in just nine days. The pier heights range from 44 feet 9 inches to 73 feet 2 inches, and the superstructure has a variable depth that ranges from 16 feet 7 inches at the piers to 7 feet 7 inches mid-span.

Since a minimal number of expansion joints would decrease both construction and maintenance requirements and associated expenses, the design specified just two expansion joints, one at each abutment of the 4,500-foot bridge. They allow for significant movement generated by temperature variations, along with creep and shrinkage of the structure over time. Extensive modeling and analyses were completed to determine the amount of movement necessary to accommodate the expansion joints. To oblige the bridge expansion and contraction, while providing adequate overall stability, the four piers in the center of the bridge use fixed pot bearings, while the others were designed with expansion pot bearings.

A total of 614 longitudinal post-tensioning tendons were used for the box girder superstructure, along with transverse deck tendons spaced at 1 foot 11 inches, to provide a durable driving surface. The current generation of standard concrete segmental bridge specifications requires bore scope inspections behind post tensioning anchorages to ensure complete grouting, thus details of the VSL anchorages were modified to permit the required inspections.

Collaborating With the Tribes

Given the cultural importance of the Four Bears Bridge, the design team and representatives from the Three Affiliated Tribes who reside on the Fort Berthold Indian Reservation met six times to discuss concepts. This provided tribal members with a steady stream of information. Interested parties were invited to participate on a Cultural Advisory Committee (CAC). The CAC voiced its desires to the design team for inclusion in the bridge design of symbols from each of the tribes to reflect their histories.

In April 2002, a design charette was held with selected representatives from the owner, the Federal Highway Administration, the CAC, county and city officials, and the North Dakota State Historical Society to determine the bridge's aesthetic elements, including pier shapes, lighting, girder profiles, colors, and textures. They determined that the CAC would be responsible for the final selection of tribal symbols to be utilized on the exterior faces of the pier segments and along the pedestrian walkway.

The 10-foot-wide pedestrian walkway on the north side of the bridge features a unique railing that incorporates monuments at the pier segments, linked together by a railing reflective of the American Indian culture. Each panel in a monument is four feet in diameter and fabricated from a color-safe material that will be durable in the severe weather of North Dakota. Each of the three tribes selected four images to reflect their individual cultures and histories. A single panel that features tribal leaders from each of the tribes is featured twice on the bridge, as a unifying image. The railing between the monuments is earth-colored and contains cutout images of animals important to the tribes. On the waterside of the pier monuments, a sculpted precast concrete design reflects the four directions of the compass within a circle. The surface of the walkway was cast with a design of color and texture that creates a pattern, further reflecting the tribe that is showcased in the associated panel. On the web walls, at each pier, a circular medallion features the silhouette of an animal also found in the railing. At night, feature lighting illuminates the web walls at the medallions and highlights the pier shapes.

Conclusion

The Four Bears Bridge demonstrates that concrete segmental technology can be applied to a wide variety of unique project sites and situations, including extreme weather conditions, through the safe and efficient use of conventional equipment. Even with severe weather conditions presented by the typical North Dakota climate, the bridge was completed as scheduled.

The Three Affiliated Tribes, upon whose history the bridge aesthetics are centered, are the Mandan, Arikara, and Hidatsa. Sakakawea, a Hidatsa for whom the man-made lake is named, was instrumental in leading the Lewis and Clark Expedition. The ongoing celebration of the 200th anniversary of the expedition includes 15 national, signature events being held along the path that Lewis and Clark took.

The new Four Bears Bridge opened to traffic in September 2005, followed by a three-day dedication ceremony held in early October. Tribal and civic leaders joined with local residents and those responsible for the design and construction of the bridge to welcome a new landmark to North Dakota. The bridge stands poised to serve the tribes, allowing for their growth for years to come, while telling the remarkable story of their history.

Authors

Jeff Lungrin is a project manger for VSL, Hanover, Md.

Russell D. Call, P.E., S.E., is a project manager for FIGG Engineering Group, Tallahassee, Fla.