Trunk Highway 101 Bridge
The new Trunk Highway 101 Bridge connecting the cities of Chanhassen and Shakopee, Minnesota, required careful planning to ensure it overcame past issues with closures due to flood waters that covered the previous low-lying structure. To achieve this goal, engineers at the Minnesota Department of Transportation designed a 4,225-foot-long, 41-span structure featuring precast, prestressed concrete I-girders reinforced with epoxy-coated rebar.
With the original bridge, flooding often caused closures up to several weeks at a time, which was projected to cost travelers $1.675 million per day in 2030, according to an article written by Senior Engineer Paul Gronvall and Principal Engineer Benjamin Jilk at MnDOT for Aspire magazine.
“The final design concept needed to minimize the risk of flooding without causing an increase in the 100-year floodplain elevation,” they explained. The new design raised the bridge above the flood-plain elevation and eliminated the existing causeway. It consists of four 12-ft-wide lanes for traffic, 8-ft-wide shoulders, and a 10-ft-wide multiuse trail separated from traffic to connect regional trails.
“Unique design criteria were developed to address the long-term durability and serviceability of the structure and its components while minimizing future maintenance needs,” the two wrote. This applied to all portions of the bridge, from the piers to the beams.
Precast, prestressed concrete I-girders were chosen early in the process, they noted. “This girder type is a go-to solution in Minnesota for a low-cost and low-maintenance superstructure,” they wrote. “Multiple suppliers in the area, quick turnaround time, and ease of construction make them the preferred option in most cases.”
To span between the piers, 45-in.-deep MnDOT girders were selected. The typical bridge cross section consists of nine girders spaced at 8’ 11”, with the last three spans varying slightly because of a curve at the end of the bridge. The spans range from about 87 feet to about 105 feet in length. There are a total of 41 spans in 13 units separated by strip-seal joints. A unit consisted of three or four spans. The girders were designed as simply supported, but additional reinforcement was provided over the piers in the bridge deck where there was no joint.
Epoxy-coated reinforcement was used to extend the life of the bridge, Jilk told EIG. “Our bridges see significant exposure to salts, and the epoxy coating helps to protect the rebar. We use epoxy-coated reinforcing steel for all new bridges on portions that are above ground. It’s our standard rebar for above-ground rebar.”
MnDOT has been using epoxy-coated reinforcing steel for many years, he notes. The state also has been looking at high-grade options such as stainless-steel rebar and GFRP in select locations.
MnDOT often tries new materials, and this project offered a great opportunity, the two wrote in their article. “The bridge deck is 0.8 mile long, and it therefore provided a great opportunity to incorporate different experimental items to compare concrete placement methods and performance of the concrete deck,” he told EIG.
With the bridge deck separated into 13 units, each joint formed a physical separation to try different methods for deck placement. The standard method for placing decks in Minnesota was used as the basis of the comparison, and all the other deck sections had more stringent specifications aimed at improving deck performance, durability, and quality.
The variations included the wet cure time, the number and type of vibrators required for placement, reinforcing bar chair spacing, the inclusion of fibers in the concrete, and the maximum aggregate size in the concrete. Every permutation tried outside of the standard method proved to help reduce cracking. Epoxy-coated reinforcement was used in all concrete elements of the structure that were not completely under soil to increase durability and improve performance.