Stillwater Lift Bridge Pedestrian Conversion

Challenge

As the only connection point between Minnesota and Wisconsin for several miles, traffic on the Minnesota side of the City of Stillwater Lift Bridge was often backed up for many blocks down the City’s Main Street. This was compounded during times when the bridge was raised to accommodate boating traffic.

To combat traffic congestion, MnDOT initiated the St. Croix River Crossing project. The project called for a new, four-lane, high speed bridge less than a mile south of the historic Stillwater Lift Bridge. The St. Croix Crossing Bridge now connects Minnesota TH 36 and Wisconsin STH 64, bypassing the congested route through downtown Stillwater. After completion of the project, plans called for the original lift bridge to be closed and converted to exclusively a bicycle and pedestrian bridge as part of the 4.7 mile Loop Trail system connecting both sides of the St. Croix River.

The Stillwater Lift Bridge – connecting the City on the west banks of the St. Croix River to the City of Houlton, Wisconsin on the east – is one of only six vertical lift span bridges built in Minnesota and Wisconsin, and one of just three remaining today. The bridge was originally built in 1931 as a 10-span, two-lane highway connecting Minnesota and Wisconsin, carrying both vehicles and pedestrians. The current bridge has seven, 140 ft., steel, riveted, Parker truss spans, including the vertical lift span. There are two reinforced-concrete approach spans on the west and a rolled-beam span on the east. At the west approach to the bridge is a reinforced-concrete circular concourse, designed with a classical revival architectural treatment. The concourse is integrated with the west approach spans in materials and design, including a continuous and open-balustrade railing.

The bridge, including the concourse, is listed in the National Register of Historic Places (National Register). The concourse is included in the Stillwater Commercial Historic District (also listed in the National Register). The bridge and concourse together reside within the Stillwater Cultural Landscape District (determined eligible for the National Register).

Solution

The Stillwater Lift Bridge Pedestrian Conversion project included design and planning for repairs to the bridge’s steel trusses, concrete substructures, concrete railing and deck, mechanical elements, electrical elements, and the restoration of the concrete approach concourse. The project also converted the bridge from a vehicular bridge to a bike/pedestrian facility. The SEH team provided detailed analysis and design, bridge ratings, cost estimating, construction engineering, and plans and specifications.

Original color
When the bridge was originally built, its truss members were painted a natural green color. But, in 1942, the entire span was painted an industrial grey; the color it remained until the pedestrian conversion project. As part of the project, plans called for the bridge to be repainted back to its historic green color. The railings remain silver, as they were originally.

Mechanical repairs
Spans three and five of the bridge each include an 82 ft. tower to accommodate the lift span and counterweight system. The vertical lift span itself (span four) is a 140 ft., seven panel Parker through truss design. The lift mechanism design itself is known as a Waddell and Harrington vertical lift, named for the early 20th century engineering company of the same name. The span is raised and lowered by up-haul and down-haul steel ropes. To ensure easy movement, the span is counterweighted by concrete blocks that travel up and down within the tower framework. The span is connected to the counterweights with steel ropes carried on steel sheaves at the tops of the tower.

A gasoline engine originally supplied the power for the up- and down-haul ropes and was replaced by a 25 horsepower electric motor in 1980. it was subsequently replaced by a new motor in 2005. Much of these systems had to be repaired or upgraded by the SEH team as part of the project. These efforts included:

• Replacing of the lift span operating ropes and guides
• Replacing of the counterweight ropes and guides
• Replacing of the counterweight sheaves, shafts and bearings
• Upgrading of the lift span operating machinery

To keep vehicles and pedestrians from accessing the bridge when the lift span is raised, the bridge also included a gate system with warning lights and bells. There were two gate systems, one for vehicles and one for pedestrians. As part of the conversion process, the vehicular gates were removed and the pedestrian gates were upgraded. To replace the pedestrian gate, the SEH design team replicated the first safety gates originally installed in 1940.

The Minnesota side of the bridge includes a concrete-reinforced concourse. The materials and Classical Revival architectural treatment of the concourse is integrated into the bridge itself. The circular concourse is approximately 94 ft. in diameter and includes four reinforced-concrete retaining wall segments around the outside edge. The concourse was reconstructed to match its original form. The wall segments are topped with open-balustrade railings, sidewalks and curbs. Eight concrete railing posts carry ornamental metal lights that replicate the lights depicted in the original 1930 plans. The bridge’s south railing includes the installation of replica lights.

Overall, the completed project has provided Minnesota and Wisconsin pedestrian residents and visitors alike a unique with a picturesque way to experience the St. Croix River Valley. After three years of restoration work, the historic bridge officially opened to pedestrians and bicyclists on June 1, 2020.