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Laser scanning technology helped a Minnesota bridge find its third home
One of 24 historic bridges chosen for preservation by the Minnesota Department of Transportation, Bridge 5721 is one of the state’s only remaining wrought iron bridge structures. The bridge was originally built to carry pedestrians over a river in Sauk Center, Minnesota, in 1870, before modern steel production methods had become available. In 1937, the bridge was disassembled and moved to span the Little Fork River near the town of Silverdale. But more than two years ago, the structure began its journey to a third incarnation, this time as an equestrian and pedestrian bridge for the Gateway Trail in the town of Stillwater, near Minneapolis. Because of the bridge’s provenance and the desire to keep its wrought iron parts intact, the Minnesota DOT worked with new owner Minnesota Department of Natural Resources and structural engineers at HNTB and Olson & Nesvold Engineers (O.N.E.) to collect crucial data for the rehabilitation using new 3-D laser scanning technology.
While the project’s main goal was to preserve the 162-foot-long bridge’s historic character, the team nevertheless recognized that certain parts would have to be replaced to ensure the structure’s safety and longevity. Because original plans for the span were unavailable, MN DOT surveyors used a Leica laser scanner to create a 3-D map of the structure before it was disassembled. The scanner fires a laser more than 50,000 times per second, collecting data from reflected light.
Placing the camera in nine different locations over the course of two days, the team collected more than 13,000 million data points with x, y, and z coordinates. After each truss member was removed, it was placed on a scanning table and fastener patterns were scanned. The geometric data created a point cloud of the bridge, allowing the team to isolate specific members or generate and view sections even after the span was dismantled and put into storage.
See a video of the laser scan below:
The team also made detailed drawings of two floor beams to be replaced at either end of the structure. These were given to steel fabricator White Oak Metals, who was able to create new beams that would fit into the structure with the original connections. Other updates included the replacement of roller nest bearings with elastomeric bearings and replacement of 10 steel stringers that had been added after the 1937 move. The 17-foot-wide wooden deck was replaced with lightweight concrete deck to minimize the structure’s dead load.
Photos taken more than 100 years ago show that the bridge’s portals originally had a clearance of 14 feet. These had been raised two feet after the move to Silverdale. Using their laser scans, the team determined the fastener patterns of the existing portals and used these to detail replacement portals that would return the bridge to its original clearance.
Once steel fabrication was completed last year, erection crews reassembled each truss on the ground in its new Stillwater location. Two cranes slid it into place on new concrete abutments, then the concrete deck, safety railing, and a new four-coat sealing paint system were added to ensure the structure’s continued longevity under its new title, Bridge 82524. See a video of the installation below: