Using drones to inspect bridges allows MnDOT to collect very high quality data while exposing fewer inspectors to far less risk and reducing inconvenience to the public. A new fleet of UAS will be used to conduct inspections statewide.
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New Project: Phase 3 of Drone Bridge Inspection Research Focuses on Confined Spaces
MnDOT recently entered into a contract with Collins Engineers Inc. to complete a third phase of research testing drones for bridge inspections, with a new focus on confined spaces.
This Phase 3 project is titled “Improving Quality of Bridge Inspections Using Unmanned Aircraft Systems.” Jennifer Wells, MnDOT maintenance bridge engineer, will serve as the project’s technical liaison. Barritt Lovelace, regional manager for Collins Engineering, will serve as principal investigator.
“Phase 3 will allow us to utilize a new drone specific to confined space inspections,” Wells said. “This new drone is meant to reach places the prior drones could not, which will supplement our efforts nicely. Also, Phase 3 will include more bridge inspections in order to get a more comprehensive feel for cost and time savings.”
The increasing costs of bridge inspections are a concern for MnDOT. The use of unmanned aircraft systems (UAS) has been shown to reduce costs, improve the quality of bridge inspections, and increase safety. The UAS can deploy a wide range of imaging technologies including high definition still, video, and infrared sensors, and data can be analyzed using 3D imaging software.
MnDOT completed a small research project in 2015 to study the effectiveness of UAS technology applied to bridge safety inspections. The project team inspected four bridges at various locations throughout Minnesota and evaluated UAS’ effectiveness in improving inspection quality and inspector safety based on field results.
A second research effort demonstrated UAS imaging on the Blatnik Bridge and investigated UAS use for infrared deck surveys. Additionally, a best practices document was created to identify bridges that are best suited for UAS inspection.
It is the goal, based on this next phase of research, to implement a statewide UAS bridge inspection plan, which will identify overall cost effectiveness, improvements in quality and safety, and future funding sources for both state and local bridges.
Collins Engineering will also investigate a collision tolerant drone — the Flyability Elios — for confined space inspections.
As part of the Phase 3 project, Collins Engineering will:
- Review current Federal Aviation (FAA) rules, technical literature, owners and industry experiences, and ongoing UAS research.
- Develop bridge inspection list based on Phase II research regarding best practices. Approximately 20-25 bridges will be inspected under this contract depending on location and size.
- Develop a field work plan for the bridge inspection list. If approvals for these bridges cannot be obtained, suitable alternatives will be chosen. This field work plan will address safety concerns, FAA, and other agency requirements.
- Establish a work schedule and deliverable submission schedule.
- Establish methods of access and schedule equipment.
- Receive training on the Flyability collision tolerant drone for use in the study.
- Perform field work at the selected bridges to collect imagery and evaluate the technology to accomplish the project goals.
- Inspect known deficiencies identified during previous inspections with the use of the UAS to evaluate the ability to identify deficiencies using photos and video.
- Enter bridge inspection data in Minnesota’s Structure Information Management System (SIMS) providing element condition ratings, photos, videos, etc. based on UAS imagery and information.
- Prepare a draft report to document project activities, findings and recommendations.
The Phase 3 project is scheduled to be complete by July 2018.
MnDOT Improves on Award-Winning Use of Drones for Bridge Inspection
MnDOT’s efforts to study whether drones can help bridge inspectors are progressing, and the second phase project has been completed. (Meanwhile, a third project has just begun.)
Phase 1 of this research project demonstrated that drones can reduce safety risks and inconvenience to bridge inspectors and the traveling public. Phase 2 shows that new drones, designed with vertical and horizontal camera and sensor capabilities for structure inspections, give bridge inspectors safe access to under-deck areas that were previously difficult or impossible to reach. The new drones cost even less than the unit tested in Phase 1.
“Using a drone rather than snoopers for bridge inspection can save significant time and cost. The FHWA approves of this use as well. It’s another tool for inspectors to employ,” said Jennifer Wells, Principal Engineer on Mobility, MnDOT Office of Bridges and Structures.
“We were one of the first transportation agencies and contractors to test and use this new technology for bridge inspections. Drones let bridge inspectors collect more data and collect it more safely and efficiently,” said Barritt Lovelace, Regional Manager, Collins Engineers, Inc.
What Was the Need?
MnDOT and local bridge owners have 600 bridge inspectors who monitor more than 20,000 bridges in Minnesota. Each bridge must be inspected once every 24 months. Bridges in poor condition and those considered fracture-critical (where failure of a single component could cause collapse) must be inspected every 12 months. Large bridges can take weeks to fully inspect and often require inspectors to dangle from ropes or stand in buckets on the end of “snoopers,” cranes that reach from the bridge deck to below-deck level to put inspectors within sight of under-deck elements.
Snoopers are expensive and require traffic lane closures, presenting safety risks to the traveling public and inspectors. MnDOT established in a Phase 1 study that unmanned aircraft systems (UAS) significantly augment inspection findings with infrared and imaging data while reducing safety risks to inspectors and the public. The project earned a 2016 Minnesota State Government Innovation Award as well as awards and recognition from such groups as the American Public Works Association.
UAS designed specifically for structure inspections were unavailable during Phase 1. The UAS used in that phase had key operational limitations, including the inability to proceed when concrete and steel bridge components blocked Global Positioning System (GPS) signals. When that happened, the drone simply returned to base automatically.
What Was Our Goal?
In Phase 2, MnDOT wanted to test the use of an upgraded UAS to examine larger and more challenging bridges. The new UAS, which was specially designed for structure inspections, featured more robust imaging and infrared data-gathering capabilities, and was more flexible to control. Its operational capabilities also were not diminished by the loss of GPS signals. Results from UAS inspections and traditional bridge inspection methods would be compared for quality and cost-effectiveness.
What Did We Do?
Investigators selected a prototype senseFly albris UAS to inspect four bridges:
- The Blatnik Bridge over the St. Louis River between Duluth, Minnesota, and Superior, Wisconsin, a 7,980-foot-long steel through-arch bridge with steel deck trusses.
- A 362-foot-long two-span steel high truss bridge over the Red River in Nielsville, Minnesota.
- A 263-foot-long corrugated steel culvert in St. Paul.
- The Stillwater Lift Bridge, a 10-span structure over the St. Croix River with six steel through-truss spans and one movable span.
For each bridge or structure, researchers prepared detailed safety and inspection plans to identify and mitigate potential hazards, inspection needs and Federal Aviation Administration (FAA) requirements. Researchers conducted and evaluated UAS and standard inspection methods for each inspection site, analyzing results in terms of access technique, data collection and usefulness for interim and special inspections.
What Did We Learn?
The senseFly albris UAS offered a clear operational upgrade over the Phase 1 unit. It can operate without GPS; the camera lens can turn up and down at 90-degree angles; and protective shrouds and ultrasonic sensors prevent the propellers from striking bridge elements.
For some inspection functions, lane closures can be curtailed or eliminated altogether. The drone worked well in the high, confined spaces of the Blatnik Bridge and should provide under-deck inspection details otherwise unavailable or too costly for any tall bridge in the MnDOT system. This UAS identifies and measures clearances, rope access anchor points and other pre-inspection conditions for planning large-scale or emergency inspections. Photogrammetry software can be used with the UAS to develop three-dimensional models of bridges and bridge sites. Using infrared thermal sensors, the UAS can detect delamination of concrete while flying adjacent to lanes of traffic. For smaller, confined spaces on bridges and culverts, the senseFly albris may not be ideal. Despite its protective shrouds, it is not as collision-tolerant as needed for very tight spaces.
Currently no UAS replicates hands-on inspection functions like cleaning, sounding, measuring and tactile testing. But the UAS is an additional tool that provides conventional and improved data safely. The FAA and the MnDOT Office of Aeronautics no longer require private pilot certification for drone operators. A new, streamlined certification and licensing procedure makes drone use more practical.
Costs were significantly lower with UAS inspections than with conventional approaches. Conventional inspection of the Blatnik Bridge would have required four snoopers, an 80-foot lift and eight days of inspection, at a cost of about $59,000 (without the cost of mobilizing equipment and traveling). The UAS Blatnik Bridge inspection would contract as a five-day, $20,000 project.
What’s Next?
Phase 3, which began in the summer of 2017, uses the senseFly albris and the Flyability Elios, a collision-tolerant drone more suited to confined spaces such as box girders or culverts. During this phase, researchers will identify which situations are best suited for drone use, what parameters should govern drone use in bridge inspections, and how UAS can be integrated into standard inspection operations at a county and district level.
This Technical Summary pertains to Report 2017-18, “Unmanned Aircraft System Bridge Inspection Demonstration Project Phase II,” published June 2017.
I-35W ‘Smart Bridge’ Test Site Uses Vibration Data to Detect Bridge Defects
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.
What’s Next?
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.
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This post pertains to Report 2017-01, Feasibility of Vibration-Based Long-Term Bridge Monitoring Using the I-35W St. Anthony Falls Bridge, published January 2017.
Using drones to inspect bridges
In recent years, drones made headlines for fighting wars overseas, detecting crop conditions, keeping an eye on power lines and even delivering retail goods.
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.”
Project goal
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.”
FAA approval
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.
The team, which included personnel from Collins Engineers Inc. and Unmanned Experts, also worked closely with the MnDOT Office of Aeronautics to plan the project and gain the necessary approvals. The Aeronautics Office recently published an official MnDOT drone policy.
In the air
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.
Related links
- Upcoming workshop: Unmanned Aircraft Systems in Traffic and Infrastructure Surveillance: Opportunities and Challenges (Oct. 15)
- Press release: Recent study evaluates drone use in bridge inspections
- Research report: Unmanned Aerial Vehicle Bridge Inspection Demonstration Project
- Technical summary: Unmanned Aerial Vehicles Enable Safe and Cost-Effective Bridge Inspection
- Policy: MnDOT Policy OP006 (Unmanned Aircraft System)
- YouTube video: MnDOT tests drones for bridge inspections
High-Tech Inspections to Keep Minnesota’s Timber Bridges Safe
Across Minnesota, hundreds of wooden bridges are reaching the end of their lifespan, but counties don’t know which ones to repair and which ones to replace.
In 2010, a timber bridge partially collapsed in Nobles County, heightening concerns about the state of inspections statewide.
“A lot of it right now is just visual and sounding the wood – striking it with a hammer and interpreting dull or hollow sounds,” said MnDOT State Aid Bridge Engineer David Conkel.
Timber bridges are at a critical point in Minnesota, not only because of the sheer number built in the 1950s and 1960s, but because it’s difficult to judge their structural soundness without advanced equipment.
While current inspection methods adequately identify areas of advanced decay, they do a poor job of detecting early decay or internal deterioration, especially in the timber substructure.
MnDOT and the Local Road Research Board have partnered to develop better inspection and repair methods on behalf of Minnesota counties. Training will be held in May and June for county inspectors. [Register here]
Identifying internal deterioration early is essential because once significant rot is noted, a timber bridge can slip into a severe condition within just two to three years.
Early bridge makers treated timber bridge elements with creosote to prevent decay from fungi and insect damage. However, because it was typically applied to the shell, a good external condition may hide severe internal deterioration.
“The timber bridge elements typically decay from the inside out due to the lack of preservative in the center of the timber,” explained Matt Hemmila, St. Louis County Bridge Engineer. “The outside will look okay, but the inside may be highly deteriorated.”
Better Inspection
Resistance microdrills and stress wave timers are two proven inspection tools that counties can use to see past the surface of a timber bridge and identify the actual amount and area of internal rot. But Minnesota counties have lacked this equipment and the training.
“These tools will enable us to identify the bad bridges before the decay shows up visually– but it will also tell us which bridges are still good so we can allocate the funds we have to replace the worst bridges,” Hemmila said.
A stress wave timer (video above) locates bad areas on a bridge by using probes to measure the time it takes for sound to travel through the material. A decayed piling will have a time that is more than double that of a sound piling.
A resistance microdrill (video below) can then be used to determine how much good wood is left in a piling or timber element by drilling a bit into the wood and measuring the resistance.
MnDOT and the LRRB are developing a customized inspection manual and standardized inspection protocols, which can be integrated into the state’s bridge data management software.
“Good inspections can catch potential problems early and possibly avoid emergency closures or load postings,” Conkel said. “It enhances safety while also helping stretch available funding for bridge repair and replacement.”
Baby Boomers
Minnesota has one of the highest concentrations of timber bridges in the country — 1,600 (down from 1,970 in 2001), more than half built before 1971.
These bridges typically start experiencing issues in their substructure when they reach 40 to 60 years old, with decay usually occurring where the piling meets the ground or water line – a perfect environment of air and moisture for rot to thrive and propagate.
Unfortunately, some bridges were unwisely built on the pilings of former bridges.
“Well-maintained, well-designed and well-treated bridges can last a long time, equivalent to other materials,” said Brian Brashaw, director of Wood Materials and Manufacturing Program at the University of Minnesota-Duluth.
Because bridge engineers have been unable to fully assess the internal cross-sections of timber bridges, they have been very conservative when assessing timber bridges, Brashaw said, resulting in load limit reductions and bridge replacements.
“The use of advanced techniques will take the guess work out of the equation, allowing for better decision-making on which bridges need repair or replacement now,” Brashaw said.
With no formal national or state guidance, MnDOT and the Local Road Research Board undertook a research project to identify state-of-the-art inspection practices and marry those techniques with the needs of Minnesota county engineers.
“We don’t have enough money to just replace all the timber bridges, so we want to provide county engineers with more advanced inspection tools so they can determine how much decay there is in the piling, and other susceptible areas,” Conkel said.
A second LRRB project, led by Iowa State University, is advancing the development of cost-effective repair techniques that counties can use to lengthen a bridge’s service life.
“We can’t build them fast enough, so we have to find a way to make them last longer so we can catch up,” Hemmila said.
Upcoming courses (see flyer) *
May 14: SE Minnesota – Register Here for Welch, MN
May 15: SW Minnesota – Register Here for Windom, MN
June 23: NE Minnesota – Register Here for Aitkin, MN
June 24: NW Minnesota – Register Here for Bemidji, MN
*All classes 9:00 a.m. to 3:30 p.m.
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