Historic Minneapolis Stone Arch Bridge Gets Repaired

Digital twins by Bentley are incorporated into the bridge world in new and exciting ways.

Bentley Systems has sponsored this post.

The Minneapolis Stone Arch Bridge, originally intended for rail traffic, is currently in use for pedestrians and cyclists—and at least one wedding proposal. (Image courtesy of Brioart.com.)

The Minneapolis Stone Arch Bridge, originally intended for rail traffic, is currently in use for pedestrians and cyclists—and at least one wedding proposal. (Image courtesy of Brioart.com.)

Historic bridges are among the most iconic structures in America, but with that history comes the tall order of maintaining them and keeping them safe for public use. This isn’t an easy task, especially when considering that many of these historic bridges were not designed to modern standards, have significant deterioration and defects or lack existing plans, making it difficult to evaluate their load-carrying capacity or foundation type. To ensure that historic bridges remain safe and maintenance projects can be completed smoothly, engineers are often forced to get creative.

That’s exactly what the team of bridge designers at Collins Engineers did to complete their rehabilitation of the historic Stone Arch Bridge in downtown Minneapolis, Minnesota.

Completed in 1883 as a railroad bridge for the Great Northern Railroad, Minneapolis’s Historic Stone Arch Bridge is a 23-span masonry arch structure. The bridge is 2,100 feet long and is the most recognizable bridge—and arguably, it is the one in Minnesota with the most history. During its heyday as a railroad bridge, it carried over 80 passenger trains into Minneapolis per day. Over time, the demand for passenger rail travel waned and the bridge fell into disuse. The rail company tried to sell the bridge to the city for a single dollar in 1980, but the offer was rejected over concerns about the long-term costs of maintenance and repairs.

 (Image courtesy of Friends of the Mississippi River.)

(Image courtesy of Friends of the Mississippi River.)

Ultimately, the rail company found a taker for the aging bridge and agreed to sell it to Hennepin County in 1989 for $1,001 with hopes of developing a light-rail network. Those plans never panned out and the bridge was transferred yet again. This time, it was to the state of Minnesota in 1993 and the bridge was turned into a pedestrian and bicycle path through the heart of a park system on both banks of the Mississippi River and Hennepin Island. Today, the bridge offers panoramic views of downtown Minneapolis and St. Anthony Falls, and connects residents on the north bank of the river to the heart of downtown with its trendy shopping and restaurants.

Nearly 140 years after its construction, having been passed through several owners with multiple extended periods of disuse, maintenance of the Stone Arch Bridge has always been difficult. The nature of masonry arch construction left thousands of masonry joints in various degrees of deterioration. Stones were missing and there were bulging areas or deformations not found in modern steel or concrete superstructures, abutments and piers. For bridge engineers and inspectors, quantifying and delineating every defect requiring repair on an aging masonry structure of this length could take months.

(Image courtesy of Bentley.)

(Image courtesy of Bentley.)

The bridge engineers at Collins Engineers were tasked with developing detailed rehab plans. They used a relatively new technology to complete their comprehensive evaluation of the condition of the Stone Arch Bridge: digital twins.

“Original plans for the bridge and original information were not always complete since the bridge is almost 140 years old. Detailed inspections of masonry bridges are time-consuming by nature. There’s so much detail to capture in an inspection. And this is such a large bridge with a significant masonry area that doing a detailed inspection can take a significant amount of time,” explained Barritt Lovelace of Collins Engineers.

Digital twins may be considered the evolution of BIM. A digital twin is exactly what it sounds like—a digital representation of a physical object that captures every aspect and minute detail down to the millimeter. A digital twin of a half-mile long bridge can be dropped right into the middle of a conference room, allowing the engineers to interact with it as if they were hands-on in the field. The models are created through a scan of a physical object, typically done with drones or LiDAR, and enhanced with geospatial data. Sensors and trackers on the physical twin make it possible to receive real-time updates from the field, which differentiates digital twins from a static BIM model that does not offer real-time processing of data.

“It is a combination of different sets of data, including reality data such as aerial and mobile captured LiDAR images, images taken by drones, images taken by mobile devices during an inspection,” explained Lomme Devriendt, Senior Manager of OEM Business Development for Bentley Systems. “You want to combine all this with databases that you have available to make decisions and do your job. Anyone involved in the project should have access to this data.”

Transportation agencies are still learning how to develop approved practices for using digital twins, and are in the process of building out more complete models of their entire networks. The long-term potential for digital twins remains huge and largely unrealized. Infrastructure could potentially be monitored constantly without sending engineers into the field, maintenance could be planned years in advance with predictive analytics and proposed construction can be modeled on a system-wide level to evaluate its impacts fully.

For their Stone Arch Bridge rehabilitation project, Collins and Minnesota DOT piloted Bentley’s digital twin platform to accelerate their inspection process and develop more accurate repair quantities and details. The scope of the rehabilitation project encompassed repointing aging and deteriorating mortar and replacing missing or cracked stones. Additional minor repairs will be made based on the results of the inspection.

To create the digital twin, Collins first had to capture the necessary data. This was done by flying around the project site with drones while engineers worked on the ground. The team captured over 9,000 images and processed them through ContextCapture, then uploaded the images to Bentley’s ProjectWise Cloud collaboration platform. Once the model was functional, bridge inspectors were able to interact with it directly and add their inspection notes and callouts right on the digital twin in the exact location of the defect. Ultimately, the model will become part of the construction plans and bid package.

“You can easily see the scale and weathered concrete, the exposed rebar and the condition of the riverbed. On the upstream side, we can see the riprap countermeasures and on the downstream side, we can see the aggregation of sand. Traditionally these conditions would be communicated in a text format with sample pictures. But the 3D digital twin is a much more efficient way to communicate the condition and configuration of the bridge,” explained Lovelace.

In addition to working with Bentley on the digital twin platform, Collins also worked with Microsoft on a mixed reality HoloLens capability to make digital bridge inspection possible. Collins had previously worked to develop this futuristic method for bridge inspection in conjunction with MnDOT and turning to digital twins to aid in a large design and construction project was the next logical step.

“The digital twin technology and HoloLens application definitely has a cool factor. But there is also a high return on investment that agencies like the Minnesota Department of Transportation are realizing. The first and easiest way to calculate these savings is by looking at the reduced hours for inspection, the need for inspection, expensive inspection, access equipment and traffic control,” said Lovelace.

“I would argue, however, that the even larger cost savings comes from the increase in the quality and quantity of data that is available to the project team. No longer do we have to make conservative design assumptions due to a lack of information. This translates to reduce risks to the designers, owners and the contractor,” he adds.

The implications and long-term value of digital twins for the bridge engineering industry cannot be understated: accelerate bridge inspections and keep workers and drivers safer by spending less time climbing at height or exposed to traffic, provide richer reporting data by tying field notes directly to its location on the bridge and enable inspectors to revisit the bridge instantly and conveniently to clarify their work or collaborate with a coworker, whether or not they were in the field during inspection.

For state transportation agencies, the benefits of digital twins include cost savings, receiving more accurate bids from contractors who have access to the model and better work on key projects. Led by Bentley, digital twins will usher in an exciting new way of working for bridge engineers, resulting in better outcomes for municipalities and government agencies—as well as the public who will use the bridge.

Visit the Bentley website to learn more