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Rehabilitation Extends Life, Restores Historical Aspects of Minneapolis’ Third Avenue Bridge

Rehabilitation Extends Life, Restores Historical Aspects of Minneapolis’ Third Avenue Bridge

© Joe Szurszewski, Courtesy of HNTB

By Suze Parker

Since the days when flour mills lined both sides of the Mississippi River in downtown Minneapolis, Minnesota, the Third Avenue Bridge has been a central connector in the area. It originally carried freight; today, it links retail and residential neighborhoods, carrying TH 65 across the river in the St. Anthony Falls Historic District.

The span, which is listed on the National Registry of Historic Places, is a classic example of a cast-in-place concrete arch-type bridge. When it opened in 1918, it ushered in a new chapter of the city’s growth.

More recently, a Minnesota Department of Transportation assessment of the state’s historic bridges resulted in the Third Avenue Bridge being included among 24 bridges selected for long-term preservation under MnDOT’s Statewide Historic Bridge Management Plan. The Third Avenue Bridge was selected for its four character-defining features:

  • Cast-in-place Melan Arch System construction, which encases steel trusses in concrete;
  • Classic Revival aesthetic treatment on piers, the projecting pedestrian bays and ornamental railing added in 1939;
  • Reverse S-curve alignment. Created to follow the shortest path over a break in the limestone that supports the bridge footings, the unusual horizontal alignment came to define the bridge as a gateway to downtown Minneapolis; and
  • St. Anthony Falls setting. The bridge is a contributing property to the St. Anthony Falls Historic District’s National Register of Historic Places designation.

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

© Joe Szurszewski, Courtesy of HNTB

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 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 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.”

© Joe Szurszewski, Courtesy of HNTB

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.

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 separating the deck from the cap beams and installing elastomeric bearing pads to allow improved deck movement. This necessitated more headroom between the deck and an existing 36-inch city water main running below the bridge, requiring the two north concrete beam spans to be raised.

Tower cranes, which limited loading on the bridge, allowed for a downstream access lane so the bridge could be constructed in phases. The final construction staging plan integrated tower cranes and temporary superstructures to enable concurrent construction in multiple areas.

Although a full closure was necessary for a period of time, the project team kept the bridge partially open for as long as possible, maintaining one open lane in each direction for six months. An additional dynamic, dedicated transit lane that allowed buses to carry passengers into downtown Minneapolis in the mornings was reversed in the evenings to permit bus passengers to exit downtown.

Historical Consistency

© Joe Szurszewski, Courtesy of HNTB

The rehabilitated Third Avenue Bridge includes changes that meet current safety and engineering criteria while preserving its important features, structural integrity, and historic significance.

Original design details were reinstated on the cap beams. Durable, updated lighting speaks to the past and respects the property’s historical aspects. The ornamental concrete balustrade railing placed on the bridge in 1939 and restored during the recent rehabilitation was enhanced to narrow the gaps between the rails. This change adheres to today’s safety standards while fitting aesthetically with the railing’s overall nature.

Collaboration with local and state agencies, historical experts, the contractor, a peer reviewer and teaming partners was key to delivering a revitalized Third Avenue Bridge, which reopened on October 28, 2023.

“The rehabilitated Third Avenue Bridge is an example of MnDOT’s stewardship of the state’s historic resources,” said MnDOT Project Engineer Christian Hoberg. “The team has worked to understand the roadway needs, made investments to support all users and ensured there was no change in the traffic loads the rehabilitated bridge could carry. Without creating any adverse impact to the span’s historic designation, the Third Avenue Bridge will continue to serve downtown Minneapolis as it has for the last 100 years.”

Suze Parker is a public relations consultant who frequently writes about transportation infrastructure projects.