Rehabilitation of the Third Avenue Bridge in downtown Minneapolis both restores the span to historic preservation standards and extends its service life for 50 years
DESIGNER
2024
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 opened in 1918 and continues to move freight and be a link to retail and residential neighborhoods, carrying TH 65 across the river in the St. Anthony Falls Historic District.
The span, which will be listed on the National Registry of Historic Places under Criterion A as a contributing element to the St. Anthony Falls Historic District and Criterion C for its engineering significance, 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.
In 2006, 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 and the projecting pedestrian overlooks as well as ornamental railing added in 1939;
- Reverse S-curve alignment. Created to avoid breaks in the limestone bedrock 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 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 beams and rail pilaster elements, erasing much of the bridge’s historical architecture. Only the 1939 ornamental railing remained.
Third Avenue Facts
- 1,888-foot bridge
- 5 Arch rib spans
- 2 Barrel arch spans
- S curve alignment
- Carries 18,000+ cars daily
- 2.2 million pounds of epoxy coated reinforcement
- 1 million pounds of stainless steel reinforcement in the deck
- 86,238 drilled in anchorages
- 19,000 cubic yards of concrete
- 17,837 square feet of 4-inch deep concrete surface repairs
- 22,093 square feet of 4-to 6-inch-deep concrete surface repairs
- 24,312 square feet of 6-to 8-inch-deep concrete surface repairs
- 9,254 square feet of 8- to 10-inch-deep concrete surface repairs
- 4,127 square feet of 10-to 12-inch-deep concrete surface repairs
Some spandrel columns exhibited severe shear cracking at their base. Pier bases were damaged from years of water draining from the deck above via ill-configured drain outlets, and the top of Pier 8 had moved about an inch severely cracking the large pier wall. Extensions added to the historic retaining walls at the northern abutment in 1980 to raise the grade were leaning outward. Historic concrete was extensively deteriorated throughout the bridge.
“The concrete mixes used previously on the bridge were different than those we use today,” said MnDOT Project Engineer Christian Hoberg. “That created design and construction challenges and demanded that we understand the structure’s capacity so that when we removed and replaced millions of pounds of concrete, we sequenced that work in a way that didn’t introduce new damage to the bridge.”
The first phase of the contract 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 replacing spandrel columns, spandrel cap beams, deck, railing pilasters, refurbishing the 1939 aluminum railing, 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 straight spandrels skewed to the supporting arch ribs, but we were working with a curved deck on top. The ends of the cap beams all varied on curved Spans 1, 5, 6 and 7.
Protecting the Arches
Minneapolis has grown up around the Third Avenue Bridge since the span was first constructed. Rehabilitation work occurred in space constrained by buildings – in one area, apartments were only 20 feet away – and the upper Mississippi River, where the bridge stands.
When rehabilitation required removal and replacement of 76 million pounds of concrete bridge deck, cap beams and spandrel columns, limited river access precluded deployment of a marine fleet with large equipment that could have operated from the waterway to move the concrete on and off the bridge. The team had to work from the bridge itself – but that presented a second challenge.
“Taking off a piece of bridge deck and placing it back down elsewhere on the bridge can create asymmetric loading,” said Christian Hoberg, MnDOT project engineer. “Arches are sensitive to unbalanced loading. We risked bending them and inducing cracking if the work wasn’t handled properly.”
To maneuver in the constrained space and avoid damaging the arches, the team used tower cranes, which allowed concrete removal and replacement to be sequenced, ensuring the arches were symmetrically loaded throughout the process.
Historical Consistency
The rehabilitated Third Avenue Bridge includes changes that meet current safety and engineering criteria while preserving its important features, structural integrity and historical 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 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 opened October 28, 2023.
“HNTB worked closely with MnDOT’s Cultural Resources Unit as the liaison between the design team and the State Historic Preservation Office,” said Daniel Enser, HNTB project manager. “The CRU had input on everything from how the Third Avenue Bridge should be repaired and how much original material would be removed to how the bridge should look, down to the concrete repair board forming and coating colors. This was a rewarding working relationship that ensured the finished product met the state’s historic requirements.”
When the bridge was built, the spandrel columns and cap beams were the same width, and the 1980 rehabilitation widened and squared off ends of the cap beams. To meet the project’s historic requirements, the cap beam width had to match the column width making reinforcement detailing uncommon. The ornamental cap beam ends were recreated from existing plans in coordination with project historians. The geometry to make that happen had to be as perfect as possible.”
By placing a LiDAR scanner on a drone, a boat and a car, the design team was able to understand the size of the elements and develop a real-world bridge model that was an accurate representation of the existing bridge. The LiDAR information allowed the cap beams to be restored to their original locations on the arch rib and pilasters to be located to match the original 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 repair from their top where the arches framed in 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 that reduced the number of expansion joints from 40 to 14 so the bridge has less points for drainage to ingress. The team re-articulated the deck by installing elastomeric bearing pads between the deck and the cap beams to provide 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 existing concrete beam spans to be raised.
“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.”
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.
“The CMGC model served MnDOT well,” Hoberg said. “HNTB as our primary consultant and the contractor alongside them, directly contracted to the state, paid dividends for the project. We got a good understanding of the delivery schedule. We got a good understanding of the access needs and sequencing work. That model ensured we had thought through this project well and were making the right decisions.”
He added that the rehabilitated Third Avenue Bridge is an example of MnDOT’s stewardship of the state’s historic resources.
“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,” Hoberg said. “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.”
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