MDT MT-81 Bridges
The Montana Department of Transportation (MDT) hired Morrison-Maierle and Frontier West to replace three bridges along Montana Highway 81 (MT-81) between Fort Benton and Lewistown. The original structures were constructed in 1934 and 1948 and had begun to deteriorate.
The two 1934 structures were multiple span timber bridges and the 1948 Judith River bridge was a multiple span steel girder structure with piers in the river. The Judith River Bridge was considered the most significant scour risk in Montana and susceptible to catastrophic failure.
The new structures were designed and replaced with one phased concrete box culvert, one phased single span bulb-tee structure, and one single span steel girder bridge with lateral slide construction. All were completed in one construction season.
To minimize traffic disruption and its accompanying economic hardships, the design team opted to construct the two major components of the new Judith River Bridge separately. The team developed a detailed plan to build the new bridge’s superstructure immediately upstream of the existing bridge for temporary use while the old bridge was removed and new abutments built, and then slide the new bridge into place.
Once both major bridge components were completed, the bridge was moved laterally 37 feet back onto the road alignment. For the slide, the team lifted the superstructure onto tracks with hydraulic jacks. Using winches and pulley
systems connected to the bridge ends, crew members on both banks of the river hand-cranked the winches in concert with each other to slowly pull the 1.2-millionpound bridge into place and then secured to the new abutments.
This impressive process combined high-tech engineering with good old-fashioned human power and perseverance. The new bridge will not only securely carry travelers over the Judith River but will stand as a piece of history for many years to come as the first new bridge built using slide construction in Montana.