© Joe Szurszewski, Courtesy of HNTB
columns, cap beams, deck and pilasters, concrete surface repairs, cost estimating, and staging, was completed in phase three. “Geometry was an especially complicated part of the design,” said HNTB Project Manager Daniel Enser. “The spandrels were straight, but we were working with a curved deck on top. The ends of the cap beams all varied. When the bridge was built, the spandrel columns and cap beams were the same width, but the 1980 rehabilitation flattened and squared off the cap beams. To meet the project’s historic requirements, the cap beams had to match the column width. The geometry to make that happen had to be as perfect as possible.” By placing LiDAR scanners on drones, on a boat and on a car, the design team was able to understand the size of the elements and develop a real-world bridge model that was accurate to within a couple of millimeters. That process allowed the cap beams to be restored to their original widths and pilaster to be located to match the 1939 ornamental railing lengths. A particular concern was Piers 3 and 4, which are in the lower pool of St. Anthony Falls, an inherently difficult area to access. The piers needed foundation repairs to the bottom of the footings, requiring a dewatered condition to remove 12 inches of existing concrete, install new dowels and form and pour the repairs. The project team took advantage of unusual, very low-flow river conditions in fall 2020, allowing for placement of giant sandbags, combined with the use of clean fill, to redirect the river, to access the piers and make repairs.
© Joe Szurszewski, Courtesy of HNTB
A leading cause of deterioration on the bridge was its large number of joints, which allowed chloride-laden drainage to reach the lower concrete elements. Spandrel columns were very lightly reinforced and cracked from joints that restrained the bridge from thermal movement. To confront this challenge and address thermal forces, HNTB designed a replacement deck, cap beams and spandrel columns and reduced the number of expansion joints from 40 to 14 so the bridge has less area for drainage to ingress. The team accomplished this by
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csengineermag.com
December 2023
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