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BrIM Bridges the Gap Over the Mississippi

Ivan Liu on June 5, 2023 - in Articles, Feature, Featured

The Third Avenue Bridge crosses the Mississippi River near Minneapolis. 



MnDOT Renovates Historic Third Avenue Bridge

To learn more about this project from the engineers’ perspective, Todd Danielson, Informed Infrastructure’s editorial director, interviewed Keith Molnau, P.E., major bridge projects engineer with MnDOT Bridge Office; and Chris Hoberg, P.E., construction engineer with MnDOT Bridge Office, via webcam.

To watch the full interview, visit bit.ly/3owhrgV 



By Ivan Liu, P.E., and Jerry Pfuntner, P.E., S.e., P.Eng.

The Third Avenue Bridge in Minneapolis is an iconic early 1900s construction that provides a means for road traffic to cross the Mississippi River and the upper area of Saint Anthony Falls. It’s recognized for being one of the final major arch bridges built using the Melan reinforcing system: steel arch ribs set in concrete. As such, it’s one of 24 bridges of prominent historic and architectural significance the Minnesota Department of Transportation (MnDOT) selected for long-term preservation.

“There are a couple of engineering reasons why it’s an important structure historically,” notes Chris Hoberg, construction engineer with MnDOT. “It’s one of the first examples of reinforced concrete bridge in the state and the last to use the Melan arch reinforcement. Furthermore, the alignment to the roadway on the bridge, the ‘s-curve,’ is another engineering reason why it’s historically significant.”

At 102 years old, the bridge was beginning to show signs of concrete distress, concrete cracking and corrosion damage. As such, in 2019 a restoration project was initiated to repair the arches and re-deck the bridge. The final design extends the bridge’s useful design life by another 50 years while improving safety and accessibility features as well as enhancing the bridge’s historic and visual features.

Revolutionizing Construction with BrIM Technology

The project team opted to use bridge information modeling (BrIM) to ensure successful restoration. BrIM is an application of building information modeling used for the design, construction and maintenance of bridges. It provides a digital representation of the bridge as well as all contained associated information such as dimensions, materials used, design specifications and maintenance requirements. In short, it allows users to interrogate the design of a bridge in 3D to help better understand how different elements integrate. It’s particularly useful for translating complex 2D designs into intelligible 3D imagery.

A photo shows the construction and refurbishment of the Third Avenue Bridge.

In the case of Third Avenue Bridge, as with most 100-year-old infrastructure, the original construction plans no longer reflected reality. However, the present-day site surveys and LiDAR scans also didn’t reflect the bridge in its entirety and didn’t assist in the visualization of the existing structure. BrIM was instrumental in bringing disparate datasets together that—once combined—filled in these project data gaps and presented a new perspective on the project requirements for construction.

This was particularly useful for accounting for some more-unusual historic details; when the bridge was originally constructed, steel angles were used to hold the formwork for the concrete. Through time, they have come to act as a reinforcement for the bridge and created a complication for post-installed elements on the existing Melan arches during construction.

“Rather than reinforced concrete, it has a system of angles that are riveted together that become composite with the concrete,” explains Keith Molnau, major bridge projects engineer with the MnDOT Bridge Office. “It’s unusual, because there isn’t any other reinforcement.”

By combining the datasets together, the project team gained a more-comprehensive representation of the bridge and could ensure the steel angles wouldn’t be accidentally damaged during restoration.

Finding a Solution to the Lack of Access

A major challenge for the project team was overcoming access constraints at the project site. The bridge is located upstream of a lock and dam that was permanently closed, limiting the size of barges to those that could be deployed from upstream. In addition, the limestone bedrock adjacent to the bridge had previously collapsed in the 1860s, so drilling into or loading up the bedrock with heavy stationary cranes adjacent to the bridge was a concern.

Additional deterrents to setting up large cranes adjacent to the bridge included fast-rushing water in the lower pool, overhead power-transmission lines and adjacent local paver streets in the St. Anthony Falls Historic District that could be damaged from hauling heavy loads over them. With all risks considered, tower cranes and top-down construction was ultimately selected.

Use of tower cranes positioned into the hollow piers at the ends of the arch spans also helped keep loads balanced on the arches during deconstruction and reconstruction of the arch spans using the top-down construction method. This meant almost all the project’s construction works would need to be executed from the existing superstructure, with careful consideration given to the bridge’s structural integrity as well as the impact of loading and unloading as existing concrete sections were removed and repaired.

This photo of The Third Avenue Bridge appeared in the Minneapolis Sunday Tribune on July 18, 1915. (1920’s Hennepin County Library)

The movement and location of construction vehicles as well as materials arriving and being removed from the site also needed to be carefully sequenced to maintain structural integrity. Using BrIM, the project’s engineers simulated various restoration scenarios to resolve potential conflicts and arrive at the most-effective schedule.

“The whole access issue on our project was solved by getting the contractor engaged during the design,” notes Molnau. “The BrIM modeling was key for the design teams to use.”

The ability to quickly consider alternative schedules became even more useful with the need to monitor and adapt to the real-time structural integrity of the bridge as the restoration progressed. Being able to track arch movement in relation to the weather and load on the bridge meant the project team remained one step ahead on safety and was able to adapt the construction method and understand the implications of those adaptions with ease.

Falsework was built from timber as the mold for pouring concrete. The first upstream rib was poured on July 8, 1915, and the last upstream rib was poured on Aug. 5, 1915. The falsework for the first rib was then struck and moved section by section into place under the center rib. Initially, it took the crews one day to move the falsework, but by the last downstream rib on the project, it took only 2 hours and 40 minutes to move. (MnDOT/Hennepin County Library, 1914)

The final schedule saw a staged removal of the deck and a temporary bridge constructed to allow the contractor to maintain access to all sections of the bridge while construction was in progress. Given the project’s complexity, visuals were essential to aid those not intimately familiar with the project’s details. A 3D BrIM model enhanced visualization, which improved step-by-step comprehension of the restoration as the work moved between spans.

As well as ensuring project teams were aligned, BrIM also was useful for providing reassurance to the local utility company that a key water line located underneath the bridge had been properly considered as part of the work.

Single Source of Truth

A further BrIM benefit of the Third Avenue Bridge project was the creation of a single source of project truth for the construction’s stakeholders. Any changes to the project were reflected in the model, so stakeholders worked from the same data, eliminating data silos and the potential for decisions to be made from outdated data. With stakeholders working from one centralized project source, collaboration and communication was more effective and led to better project outcomes. As a result, there were very few instances of rework or schedule delays due to unmitigated risk, which was an important performance indicator for the asset owner, who was working to a tight two-year time limit for the restoration project.

 

A stylized photograph (Top) highlights particular components of the bridge, including the railing, deck, overlook, spandrel, pier and arch rib. (MnDOT)

 

“Once we were clear about what it is we were looking to accomplish, the team really coalesced around it, and everybody pulled the rope in the same direction,” says Hoberg. “A big part of the success was the fact that we all had a unified vision of what we were trying to do out here—what the important aspects were.”

BrIM now is widely used by COWI on projects across Europe and India, proving to be a highly effective route to enhance communication and collaboration and improve project outcomes. But the potential is yet greater still. Although notable projects such as the Fargo Morehead Bridge in North Dakota and the Houston Ship Channel Bridge in Texas used BrIM, there are plenty of North American owners and contractors who would benefit from moving to 3D modeling technologies to support the delivery of projects on time and to budget. 


Ivan Liu, P.E., is senior bridge engineer, COWI; email: [email protected]. Jerry Pfuntner, P.E., S.E. P.Eng., is southeast region technical director, COWI; email: [email protected].

 

About Ivan Liu

Ivan Liu, P.E., is senior bridge engineer, COWI; email: [email protected]. Jerry Pfuntner, P.E., S.E. P.Eng., is southeast region technical director, COWI; email: [email protected].

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