- Mar 29, 2011
The new Wakota Bridge spanning the Mississippi River near South St. Paul connects Washington and Dakota Counties, giving the bridge its name. To meet a variety of challenges, the structures was designed with cast-in-place, segmental box girders with an internal center web.
The new side-by-side structures carry Interstate 494 over the river as well as railroad tracks, a city street and a bike trail. At 122 ft and 111 feet wide at the tapered end spans, to carry 10 lanes of traffic between them, the twin bridges are the widest in the state. The westbound bridge, which was constructed first, also includes a pedestrian/bicycle trail with two scenic overlooks.
The design required variable-width roadway and bridge geometry to accommodate merging entrance and exit ramps at both ends of the bridges. Each structure features parabolically haunched spans for efficiency and to gain clearance over the navigation channel. The slope of the outer webs was held constant, creating a dramatic width variation in the bottom slab along the haunched spans.
A balanced layout was used, creating five spans (266, 328, 466, 466 and 353 feet) for each bridge. The span lengths were achieved with variable-depth, twin-cell box girders ranging in height from 24 feet at the pier tables to 12 feet at mid-spans.
The concrete design will reduce long-term maintenance costs for the bridge, which is anticipated to be in service in excess of 75 years. A major factor in increasing durability was the use of a top slab that is bi-axially post-tensioned to achieve the established design criteria of zero tension for service-level stresses after all losses.
A dense concrete overlay was applied for the wearing surface to increase the corrosion resistance of the deck. All reinforcing bars above ground consisted of epoxy-coated steel to increase durability and extend the service life. The bridges’ design also will benefit future projects, thanks to the embedment of sensors that will provide information to evaluate thermal loading in the piers and girders.