© Joe Szurszewski, Courtesy of HNTB
Rehabilitation of the Third Avenue Bridge, which MnDOT initiated in 2017 to address the structure’s deteriorated conditions and extend its service life for 50 more years, required the updated structure to comply with Section 106 of the National Historic Preservation Act of 1966. The HNTB team was selected as engineer of record to study the bridge’s history, inspect, load rate, design, and assist in its reconstruction under a construction manager/general contractor (CMGC) model. That methodology brought the contractor on board during preliminary design to provide design review, collaboration, and pricing with the goal of reducing project risk. Innovative Solutions to Complex Challenges Described as a “reserved, orderly and imposing structure” when it opened in 1918, the Third Avenue Bridge can still be described this way, with its two steel beam spans on the south, five open-spandrel column arch rib spans, two open-spandrel wall barrel arch spans and two concrete beam spans on the north. It crosses both the Upper St. Anthony Falls lock and dam in downtown Minneapolis and a smaller, lower falls. The 40 years since the last rehabilitation had taken their toll on the bridge. That 1980 rehabilitation replaced all original deck, spandrel cap beam, and rail pilaster elements, erasing much of the bridge’s historical architecture. Only the 1939 ornamental railing remained. Historic concrete was extensively deteriorated. Some spandrel caps and spandrel columns were sheared off. Piers were damaged from years of water draining from the deck above, and the top of Pier 8 had moved about an inch. Partially
© Joe Szurszewski, Courtesy of HNTB
historic retaining walls at the northern abutment had deteriorated so badly that they were starting to lean outward. When the recent rehabilitation project was initiated, it wasn’t clear how it would be accomplished. The first phase of the contract simply aimed to identify the scope of work needed. To achieve that goal, HNTB developed a full, as-built and as-inspected load rating to understand the bridge’s capacity. WJE, a subcontractor to HNTB, completed full bridge inspection and testing. With that information, HNTB finished preliminary design in phase two, resulting in taking the bridge down to its arch ribs and building it back up. Final design, which included the
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December 2023 csengineermag.com
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