Many local agencies in Minnesota lack funding to construct and maintain all the bridges in their roadway network. One way to lower costs is to reduce the number of bridges.
In Minnesota, some township bridges are on roads with low usage that have alternative accesses for nearby residents, but local officials are reluctant to remove the bridges.
To identify possible changes to how redundant and low-use bridges are identified and removed in Minnesota, the Local Road Research Board conducted a transportation research synthesis, “Local Bridge Removal Policies and Programs,” that explores how other states make bridge removal decisions.
Fifteen state DOTs responded to a survey about their processes, with varying levels of state oversight identified for bridge removal decisions. Researchers also examined funding and incentives offered by some DOTs to local agencies for bridge removal, as well as criteria for considering bridge removal.
A literature search of bridge design manuals, inspection manuals and bridge programs was also conducted to identify related policies and programs.
Read the TRS to learn more about the various bridge removal policies and procedures in place in Minnesota and other states.
By analyzing vibration data from the I-35W St. Anthony Falls Bridge, MnDOT is working to develop monitoring systems that could detect structural defects early on and ultimately allow engineers to improve bridge designs.
“With data spanning several years, the I-35W St. Anthony Falls Bridge offers a unique opportunity for investigating the environmental effects on a new concrete bridge in a location with weather extremes,” said Lauren Linderman, Assistant Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering. Linderman served as the research project’s principal investigator.
“This project gets MnDOT closer to using bridge monitoring systems in combination with visual inspection to help detect structural problems before they affect safety or require expensive repairs,” said Benjamin Jilk, Principal Engineer, MnDOT Bridge Office. Jilk served as the research project’s technical liaison.
What Was the Need?
In September 2008, the I-35W St. Anthony Falls Bridge was constructed to include a “smart bridge” electronic monitoring system. This system includes more than 500 sensors that continuously provide data on how the concrete structure bends and deforms in response to traffic loads, wind and temperature changes. Transportation agencies are increasingly interested in such systems. As a complement to regular inspections, they can help detect problems early on, before the problems require expensive repairs or lead to catastrophic failure. Smart bridge systems can also help engineers improve future bridge designs.
The smart bridge system on the I-35W St. Anthony Falls Bridge includes accelerometers, which provide data on the way the bridge vibrates in response to various stimuli, including structural damage. Vibration-based monitoring has the advantage of allowing damage to be detected at any location within the bridge rather than only at the specific locations where measuring devices have been placed.
However, it can be difficult to use vibration monitoring to detect damage when vibration is masked by the bridge’s natural response to traffic loads, wind, temperature changes and other environmental conditions. A crack in a bridge girder, for example, can produce a vibration signature similar to one produced by a change in beam length due to variations in temperature or other causes. Consequently, since 2008 MnDOT has conducted a series of projects using data from the St. Anthony Falls Bridge to establish a way to distinguish anomalous data indicating a structural defect or damage from background “noise” associated with other causes.
What Was Our Goal?
This project sought to develop a method for analyzing accelerometer data from the I-35W St. Anthony Falls Bridge that would show how the bridge naturally vibrates due to traffic, wind and other environmental conditions. With this fingerprint of the bridge’s natural vibration, engineers would have a baseline against which to measure anomalies in the data that might indicate structural damage.
What Did We Do?
A large amount of data has been collected from the bridge since its construction. To establish the vibratory fingerprint for the bridge, researchers examined the frequencies and shapes (or modes) of bridge vibration waves. The method they used to identify the data segments needed for the fingerprint was to evaluate the peak amplitude of bridge vibration waves and their root mean square (RMS), a measure of the intensity of free vibration.
The researchers applied this method to the vibration data collected on the I-35W St. Anthony Falls Bridge between April 2010 and July 2015, calculating the average frequencies for four wave modes and determining how they varied with the bridge’s temperature. They also calculated the way frequencies changed with the bridge’s thermal gradients, or variations in temperature between parts of the structure.
What Did We Learn?
The methods developed in this project were successful in establishing a fingerprint for the way the I-35W St. Anthony Falls Bridge vibrates due to environmental conditions, and a way to evaluate changes in vibration over time indicative of structural damage or other factors.
Researchers found that the ratio of peak signal amplitude to RMS in bridge vibrations was a strong indicator of data that should be analyzed, and was evidence of a large excitation followed by free vibration. By themselves, peak amplitude and RMS cannot distinguish between ambient free vibration and forced vibration.
Researchers were able to use this method to successfully analyze 29,333 data segments from the I-35W St. Anthony Falls Bridge. This analysis revealed that as temperature increases, the natural frequency of vibration tends to decrease. The magnitude of this change, they concluded, must be related not just to the elasticity of the bridge but also to other factors such as humidity. However, temperature gradients within the bridge did not appear to have a significant effect on the natural frequencies of the structure.
MnDOT will continue to collect data from the bridge as it ages to further understand its behavior. This will provide an opportunity to determine how anomalies in vibration data correspond to cracking and other forms of structural distress. Ultimately, MnDOT hopes to use this bridge monitoring system in combination with visual inspection both to detect problems in bridges earlier and to develop better bridge designs. Researchers are also currently working on a follow-up project, Displacement Monitoring of I-35W Bridge with Current Vibration-Based System, to determine the effects of temperature on the bridge’s dynamic and long-term vertical displacements, which can be used to monitor the bridge’s stiffness, connections and foundations.
MnDOT is exploring the development of freeway “lids” at key locations on I-94 in the Twin Cities. To analyze the potential for private-sector investment and determine what steps might be needed to make lid projects a reality, MnDOT invited the Urban Land Institute (ULI) MN to conduct a Technical Assistance Panel with real estate experts and other specialists. The U’s Metropolitan Design Center (MDC) provided background and research for the panel.
A lid, also known as a cap or land bridge, is a structure built over a freeway trench to connect areas on either side. Lids may also support green space and development above the roadway and along adjacent embankments. Although lidding is not a new concept, it is gaining national attention as a way to restore communities damaged when freeways were first built in the 1960s.
According to MnDOT, roughly half of the 145 bridges on I-94 between the east side of Saint Paul and the north side of Minneapolis need work within the next 15 years. A shorter window applies in the area around the capitol to as far west as MN-280. In anticipation of the effort to rebuild so much infrastructure, the department wanted a deeper understanding of how attractive freeway lids and their surrounding areas would be to private developers and whether the investment they would attract would generate sufficient revenue to pay for them.
The three-day panel session was designed to consider the I-94 corridor and study three specific areas: the I-35W/Minneapolis Central Business District, historic Rondo Avenue in Saint Paul, and Fairview Park in North Minneapolis. It also included a “lightning round” for high-level observations of five other sites.
Mic Johnson, senior fellow with MDC, provided background about lidding and shared successful examples from around the country at the panel kick-off dinner. MDC has analyzed a wide range of freeway lid structures and identified seven basic lid typologies. “These typologies provide broad thematic guidance for thinking about what features best serve a location,” Johnson says.
The briefing book provided to panelists included detailed research by MDC about the economic opportunities of the area’s freeway lids. MDC also created four appendices (projects, case studies, prototypical lid diagrams, and health and economic value) for the panel final report.
MDC has been involved in lid-related activities for several years. Students participating in an Urban Design Studio course in fall 2013 taught by Johnson conducted an extensive analysis of the I-35W/Minneapolis area and created an architectural model of a lid connecting the U of M’s West Bank to Downtown East. Their model was displayed at the IDS Center.
MnDOT Commissioner Charlie Zelle requested that ULI MN convene the panel as part of the larger “Rethinking I-94” project, which is developing a vision for the corridor through a comprehensive public involvement process. “Lid projects are one way being considered that could reconnect neighborhoods such as Rondo that were divided by freeways in the 1960s,” Zelle says. The Rondo neighborhood was also featured in the USDOT’s Every Place Counts Design Challenge in July.
As part of its report to MnDOT, the panel concluded that private-sector development would not pay for the lids directly, but lids would create development interest that could generate significant long-term revenue to pay for lid maintenance, programming, and other amenities.
To build momentum and create an identity for lid projects, the panel also recommended that the area’s lids be considered as a whole under a single banner, not as separate projects, as part of a rebranded vision called the Healthy Communities Initiative. The final report is available on the ULI MN website.
In the video, Jennifer Zink, MnDOT state bridge inspection engineer, explains the project, along with Tara Kalar, MnDOT associate legal counsel; Cassandra Isackson, director of MnDOT Aeronautics; and Bruce Holdhusen, MnDOT Research program engineer.
The initial drone project drew significant media coverage and a lot of attention from other state departments of transportation from all over the country.
As the flying electronic devices became easier to use and less expensive, all sorts of individuals, businesses, nonprofit groups and government organizations – including the Minnesota Department of Transportation (MnDOT) – are exploring ways to use them.
This past summer, MnDOT began researching how to employ these unmanned aerial vehicles, or UAVs, to someday help inspect the state’s many bridges.
“That day may still be far off, but our initial project was an encouraging first step,” said Jennifer Zink, MnDOT bridge inspection engineer. “Phase 2 of the project will better provide details as to methods, criteria and cost effectiveness for how to apply drone technology best to bridge inspection.”
The research team tested drones this past summer while inspecting four Minnesota bridges (in Chisago County, Olmsted County, Morrison County and near Stillwater) specifically selected for the study after an extensive evaluation and FAA approval.
Zink and her colleagues wanted to investigate whether drones could help MnDOT decrease the rising costs of bridge inspections and collect more detailed information. Drones could also minimize the risks for bridge inspectors, who currently use rope systems and special inspection vehicles to access hard-to-reach areas. Using a drone to gather images could keep inspectors out of harm’s way and inspection vehicles out of active traffic lanes.
“The goal of the project was to study the effectiveness and possibilities of using UAVs to aid in bridge inspection work, typically in gathering images without the use of an under-bridge inspection vehicle and in areas where access is difficult or not safe for an inspector,” Zink said. “There is no substantive guidance in existence for this application of this evolving technology. This initial effort was to gain a better understanding of potential capabilities, processes and planning best practices.”
Before simply launching drones and collecting bridge data, the research team reviewed current FAA rules and applied for the necessary exemptions. Approval was granted, but only for the use of an Aeryon Skyranger drone. Even though exemptions for several models were submitted to the FAA, none were approved in time for the field study.
Once in the air, the drone suitably performed a variety of inspection functions that didn’t require a hands-on physical inspection. Researchers tested the drone’s ability to gather high-quality still images and video footage of bridges. They also collected data from infrared cameras. In addition, the drone provided the ability to capture data needed to construct maps of bridge areas and 3D models of bridge elements.
“The images, including infrared images to detect deck trouble spots, obtained from the drone correlate to the findings in the bridge inspection reports for specific bridge elements,” Zink said.
Missing from the research were images of the underside of bridges. The drone model used in the study wasn’t able to shoot images upward from beneath a bridge, and inspectors identified that as a key feature along with the ability to operate without a GPS signal.
“The drone we used in this project was not completely ideal for an entire gathering of imagery for all bridge inspection elements as it was limited to GPS signal capability,” Zink said. “However, it did give us an idea of what a drone could provide, what the limitations were, and what features we would like to see on newly available UAV models. Unfortunately, our hands were tied with obtaining FAA exemptions only for the particular model used in this project within the funding timeframe.”
Conclusions and recommendations
The project’s final report listed several conclusions, including that drones can be used safely during bridge inspections and that risk to both the inspectors and public is minimal.
“Due to the successful outcome of the initial project, we have a better understanding of the drone capabilities we would like to use during an actual scheduled bridge inspection,” Zink said. “The drone that will be used in Phase 2 is specifically designed for inspection of structures. Several goals exist for the Phase 2 research project, and if we can accomplish them, they will decrease MnDOT’s costs and increase bridge inspection abilities. It could improve inspection data collection for local agencies as well.”
The researchers recently were notified that they received funding for Phase 2 of their project, which is expected to start later this fall.
Roadways for humans can sometimes create roadblocks for fish, but researchers hope to establish a set of culvert design practices to help aquatic creatures get where they’re going.
Many fish depend on mobility along a river for feeding and spawning. Where roads meet rivers, however, culverts can block fish and other aquatic organisms that can’t navigate changes in current, lighting and other factors.
Waterway barriers threaten an already endangered species of minnow known as the Topeka shiner (pictured above). It can also be a big problem for economically important fish such as trout or northern pike. That’s why the Minnesota Department of Natural Resources prefers building bridges to culverts.
However, bridges are not always economically feasible, and so MnDOT is working closely with the DNR to develop culverts that protect both public safety and the environment.
Recent research suggests that installing boxed culverts differently could greatly improve fish passage.
Culverts are typically placed a little below the streambed with the expectation that the stream flow will naturally fill them with sediment. Researchers tested that assumption and found it to not always be accurate.
“We found that pre-filling the culvert with sediment that replicates the streambed as part of the installation process helped prevent upstream erosion and the development of vertical drops that can become barriers to aquatic movement,” said Jessica Kozarek, a University of Minnesota research associate. “In addition, pre-filling the culvert helped ensure the sediment remained inside the culvert flows were high and water moved quickly during rainstorms.”
MnDOT has been working with the DNR to identify the conditions that determine whether a newly installed culvert will naturally fill with sediment, replicating surrounding streambed conditions, or whether a stream’s water flow will transport sediment out of a culvert.
Using an experimental flume at the University of Minnesota’s St. Anthony Falls Laboratory, researchers tested MnDOT’s standard box culvert design under a variety of stream conditions.
Laboratory simulations suggest that filling a culvert with sediment at installation, rather than allowing it to fill over time is, with some exceptions, generally the best approach for low- and moderate-grade streams. Additionally, steep, fast-moving waters require a filled culvert with structures such as larger rocks to keep sediment in place. These structures also create steps, pools and riffles that enable fish to rest as they move upstream.
MnDOT will use this latest research, along with conclusions from other recent studies, to create a guide for fish-friendly culvert designs.
“Of all the things we’ve studied, there are maybe three or four research projects. This manual will pull it all together,” said Petra DeWall, state waterway engineer at the Minnesota Department of Transportation.
Further research is underway to determine whether aquatic organisms are deterred by low light conditions in long, dark culverts. Researchers are also looking into whether mussel spat rope could be used to create a rough bottom to reduce water speed in culverts with no sediment.
Bridges over Minnesota waterways need to be protected from currents by a field of interlocking angular rocks called riprap. Without these rocks along the abutment, moving water could wear away the soil that supports a bridge’s foundation. The faster the water, the larger the riprap must be to provide adequate protection.
While some parts of Minnesota have quarries rich with angular rock, other parts don’t – particularly the northwest and western regions. Bridge projects in those areas sometimes resort to the expensive practice of trucking in stones. Other times field stones are used, but they are less effective and must be replaced more often.
There soon could be a better option thanks to research coordinated by the Minnesota Department of Transportation and funded by the Minnesota Local Road Research Board.
At a few test sites around the state, researchers have used a grout mixture to cement smaller, rounded rocks together at a bridge abutment. Once applied to the rocks, the mixture forms what is called “matrix riprap.” The concept is in use in Europe for many bridge piers, but MnDOT was more interested in learning how it could be used on bridge abutments.
Matrix riprap is currently in use in Minnesota at the following bridges:
Highway 23 over the Rum River in Milaca
Highway 8 over Lake Lindstrom channel in Lindstrom
Prairie Road over Coon Creek in Andover
In May 2012, matrix riprap was placed at the Milaca bridge, which sits alongside a high school. Researchers hoped the use of matrix riprap would prevent vandals from removing the riprap rock and throwing it into the river. According to Nicki Bartelt, a MnDOT assistant waterway engineer, the matrix riprap has proven to be extremely strong and effective.
“Not only is matrix riprap significantly stronger than regular riprap, but it helps prevent vandalism as well,” Bartelt said. “The Milaca installation has been in place for three years now. It looks pretty good and it’s weathering well.”
In the lab, matrix riprap held up extremely well on mechanical pull tests and hydraulic flume tests. In fact, researchers were unable to determine the matrix riprap fail point on many tests, even after applying 10 times the shear stress that regular riprap can withstand. Matrix riprap was tested with both angular and round rock with no change in performance.
A new matrix riprap installation recently went in on the Highway 95 bridge over the Rum River in Cambridge. Later this summer, plans call for an installation on the Highway 60 bridge over the north fork of the Zumbro River in Mazeppa.
“The Highway 60 bridge is being replaced, and the river there has extremely high velocities, so we’re using the matrix riprap instead of regular riprap just because of the size of rocks that would be needed,” Bartelt said.
At least two more installations are planned for 2016. In the future, researchers plan to determine the fail point for matrix riprap. They also hope to study potential environmental effects the grout may have underwater.
MnDOT has also worked with local governments that have tried matrix riprap for themselves. One municipality is trying it as a heavy duty erosion control measure. The concept is catching on outside Minnesota as well.
“We have gotten a lot of inquiries from other states, and we have lent out the spec a lot,” Bartelt said. “Iowa, New Hampshire, Maine, Indiana, Wisconsin and Illinois are among the states to express interest. We have talked to a lot of people about it, so they tend to use our research.”
Just how long will it be before a bridge deck needs to be rehabilitated? Why not look to history to find out?
Researchers have put several decades of MnDOT bridge inspection records to good use by analyzing old bridge deck condition reports to calculate how quickly similar bridge decks will deteriorate.
MnDOT inspects bridges regularly, but had never used this historical data to help determine the rate of bridge deck deterioration and what factors influence it.
“We’re always trying to improve the timing of bridge deck repair projects and improve our understanding of what contributors affect the way our bridge decks deteriorate,” said Dustin Thomas, MnDOT’s South Region Bridge Construction Engineer.
From their analysis, researchers created deterioration tables that can be used to better predict the timing and costs of repairs and maintenance.
Researchers looked at the inspection history and construction details of 2,601 bridges to determine the impact of factors such as type of deck reinforcement, depth of reinforcement below the driving surface, traffic levels and bridge location.
Using the inspection data, researchers developed curves that show how long a bridge deck is likely to stay at a given condition before dropping to the next. They developed separate curves for each variable that had a significant impact on deck deterioration rates.
What They Found
Several factors were found to have a notable impact on how quickly bridge decks deteriorate:
Decks without epoxy-coated bars built between 1975 and 1989 deteriorate more quickly than other bridge decks.
Bridges with less traffic showed slightly slower rates of deterioration than highly traveled bridges.
Metro area bridges drop to a condition code of 7 (good) more quickly than bridges in other parts of the state. This may be due to increased chemical deicer usage or because maintenance activities like crack-sealing are more likely to be delayed on larger metro bridges because of the difficulty accessing middle lanes.
When a new deck is installed on an existing bridge, the deck performs like a brand-new bridge and so MnDOT should use the deterioration table for the re-decking year, rather than the year the bridge was originally constructed.
MnDOT plans to incorporate future bridge inspections into the dataset to enhance the predictive value of the deterioration tables.
A leading cause of bridge failure is bridge scour, which occurs when rapidly moving water erodes riverbed soil around abutments or piers.
Monitoring bridge scour with traditional inspection methods can be dangerous and difficult, so MnDOT has been working with researchers from the University of Minnesota’s St. Anthony Falls Laboratory to develop a continuous monitoring system to test certain bridges more safely and efficiently.
MnDOT currently monitors 45 scour-critical bridges — and local Minnesota agencies monitor 360 more — using visual inspections or water data websites during flooding events. Once a predetermined threshold is exceeded, portable scour monitoring equipment is deployed to measure scour depth. If scour has undermined the foundations of a bridge, inspectors close it for repair.
But portable scour monitoring systems can be difficult and dangerous to deploy from the bridge deck or boat in fast-moving water. It can also be difficult to get inspectors to sites quickly enough in areas subject to flash flooding.
A better alternative for such situations are fixed scour monitoring devices that continuously monitor scour and send data wirelessly to bridge personnel, alerting them when scour reaches a dangerous level.
MnDOT has not historically made use of fixed scour monitoring equipment, but as advances in technology have made these devices more affordable and reliable, the agency became interested in exploring the use of fixed monitoring equipment at locations where the use of portable equipment is problematic. ( A major concern for fixed scour monitoring is damage from debris and ice.)
Researchers have installed fixed remote monitoring stations on four such bridges.
Stations on the first two bridges (Highway 14 over the Minnesota River in Mankato and Highway 43 at the Mississippi River in Winona, pictured at top) ran successfully for three years, with outages due to primarily to power and communication issues.
Researchers learned valuable lessons from these bridges and have now installed monitoring equipment on two more: The Old Hastings Bridge (Highway 61 over the Mississippi River), on which float-outs were installed; and the Dresbach Bridge (Interstate-90 over the Mississippi River), which had a tilt meter and underwater sonar device installed.
“The less familiar personnel are with technical equipment, the less they tend to use it,” said Andrea Hendrickson, State Hydraulics Engineer, MnDOT Office of Bridges and Structures. “This research project gave us the familiarity and technical information we need to be comfortable using fixed scour monitoring equipment on bridges that warrant it.”