The St. Croix Crossing: DYWIDAG Bar Reinforcement Saves Time and Money
Opened in 1931, Stillwater Bridge is a lift bridge that crosses the St. Croix River between Stillwater, Minnesota and Houlton, Wisconsin. Within the last few years, the historic bridge became a bottleneck for traffic because of its out-dated lifting mechanism. Furthermore, it was beginning to show structural deficits.
Consequently, work has begun on a new, four-lane bridge approx. 1.6km downstream of the Stillwater Bridge. After completion of the new structure, the historic bridge will only be used by pedestrians and cyclists.
The new bridge has an approx. 1.6km long main span consisting of precast segments supported by two abutments and 13 piers. It is the second bridge in the USA to be built as an extradosed bridge.
The soft river bed was a major challenge for the participating companies. At the deepest point, a nearly 26m thick clay layer had to be drilled through for the bridge foundation in order to reach the loadbearing limestone of the river bed located at a depth of 30-40m.
The General Contractor, Edward Kraemer & Sons, drilled five pile foundations that were needed for the 40 caissons in diameters of approx. 2.7m. Steel cased drilled shafts were installed through the water and river bed. They bear on competent limestone material at their tips.
Afterwards, the mud was removed from the caissons, and large reinforcing cages were set into the caissons, which were subsequently filled with concrete. The interior reinforcing cages were tied using vertical, high-strength DYWIDAG THREADBAR.
The drilled shafts were originally designed with an inner reinforcing cage consisting of 16 Ø 43mm, Grade 60 vertical reinforcing bars and an outer cage consisting of 32 Ø 43mm, Grade 60 vertical reinforcing bars inside the steel casing.
DSI proposed a more economical alternative using only 22 high strength, Ø 57mm, Grade 75 DYWIDAG Bars with a yield strength of 264kN per vertical bar for the reinforcing cages. The owner, Minnesota Department of Transportation, accepted DSI’s proposal in March 2013. A total of 30,050m (98,590ft) of DYWIDAG THREADBAR were supplied to the jobsite together with 1,470 EA Couplers.
Thanks to the higher strength of the DYWIDAG THREADBAR, only one cage was necessary per drilled shaft, so the contractor was able to save on labor costs as well as construction time.