When it opened to traffic in 1923, the Franklin Avenue Bridge over the Mississippi River in Minneapolis was the world’s longest concrete arch bridge at 1,055 feet. Added to the National Register of Historic Places in 1978, its ornamented cap beam ends, deck fascia, and railing beautifully illustrate the transition from classical revival to art deco architectural design.
Many flourishes were lost when the bridge was widened and other improvements were made. Owner Hennepin County recently restored these historically significant details by incorporating them into a comprehensive reconditioning that included precast cap beams, deck panels, and railing. To minimize inconvenience to the traveling public, the county used accelerated bridge construction (ABC) to finish the project in four months instead of two to three years.
Three precast fabricators spent a year planning and preparing to ensure their product arrived as scheduled and easily connected to other bridge components.
The deck: 30 panels per week
The 14-inch-thick deck panels are 8 feet, 9 inches wide and up to 27 feet, 11.5 inches long depending on whether they’re an expansion joint, sliding, or fixed panel. Sides facing outward recess slightly so the ends of the new cap beams would protrude from directly below each railing post.
Like a puzzle, these components had to fit seamlessly together at the jobsite. Kraemer North America was general contractor and precast the deck panels. The cap beam ends had been flattened and widened over the years; Forterra Pipe & Precast in Elk River, Minn., provided new cap beams with curved ends that match the original classical revival design.
The two began collaborating a year ahead of time. When construction began, Kraemer set up a temporary yard with five casting beds and a steam-curing system near the river so panels could be floated downstream on a barge and hoisted into position by waterborne cranes. The contractor then surveyed the substructure to confirm the geometry of each spandrel column – in the space between the top of the arches and the bottom of the deck – before beginning production. Of 350 panels fabricated, 186 were shifted and 20 were modified to fit before arriving at the jobsite.
Visualizing historical details
With its pilasters (slightly protruding rectangles that look like supporting columns) and elongated-hexagon openings, the classically‐inspired railing presented the most challenges. The openings had to be within 1% of vertical, but the deck’s grade ranged from 2.4% to 3.5%. To help team members wrap their heads around the geometry, the project historian molded a clay model onto a piece of wood.
The job of supplying the railing systems and pillar caps went to American Artstone in New Ulm, Minn., which specializes in historically accurate architectural precast concrete. The company produced 163 precast ornamental railing panels according to Cast Stone Institute Section 04-72-00, “Standard Specifications for Architectural Cast Stone.” Updated most recently in September 2017, it provides basic requirements for cast stone, a refined architectural concrete building unit manufactured to simulate natural cut stone, used in the Construction Specifications Institute’s MasterFormat Division 04 – Masonry applications.
The moral of the story? Historical restoration is harder to get right than new construction, but very satisfying when it all comes together.
