While not inherently unsafe, MnDOT’s fracture-critical bridges — those having critical, nonredundant components — must be inspected on a regular basis. To help track the health of these bridges, MnDOT has developed a bridge health monitoring system that uses electronic instrumentation to provide advance warning of structural distress.
The system detects acoustic emissions — stress waves caused when cracks form and propagate in the steel components of a bridge. Researchers recently deployed and tested the system on the Cedar Avenue/Highway 77 Bridge in Burnsville, enabling them to develop procedures for automatically collecting and processing data.
“Ever since the collapse of the I-35W bridge in Minnesota, many states have been interested in developing a bridge health monitoring system that will help engineers address the many challenges of managing infrastructure and ensure the longevity and safety of our bridges,” said Moises Dimaculangan, MnDOT bridge rating engineer.
MnDOT will continue to use the system to monitor the Cedar Avenue Bridge, a steel tier-arched bridge over the Minnesota River. It was chosen because it is fracture-critical, but has no history of cracking. The test deployment also led to guidelines for monitoring other fracture-critical bridges.
University of Minnesota researchers recommend further investigation into acoustic emission data analysis methods, as well as using the system developed in this project to monitor another steel bridge, one with a history of cracking.
*Note: This article was adapted from the upcoming May–June 2014 issue of our Accelerator newsletter. Sign up today to receive your free print edition or to receive email notification when new issues become available online. Subscribe here.
A new technology that uses 3D-imaging sonar will enable MnDOT engineers to visualize the substructure of a bridge in a way they never have before.
Until now, MnDOT has relied on human divers and depth finders to identify problems beneath the water.
Divers are limited by what they can see and feel in murky waters, however, and depth finders can only look down, not around.
“With this new technology, we will be able to provide high resolution three-dimensional images of underwater areas, structures and objects to show what is occurring, regardless of water clarity,” said MnDOT Bridge Waterway Engineer Petra DeWall, who has received funding from MnDOT’s Transportation Research Innovation Group to purchase the equipment.
Video imagery from a sonar inspection of Minneapolis’ Third Avenue bridge is above.
Currently, MnDOT hires engineer divers to physically inspect about 500 bridges every five years. They look for cracked concrete, exposed reinforcement and other detrimental conditions.
Although divers can spot issues, they can’t always thoroughly assess the scope of a problem, such as the amount of sediment being washed out around a bridge pier, a problem called bridge scour.
It can also be difficult — or dangerous — for divers to venture down for an inspection.
This was the situation last winter with the Third Avenue Bridge in downtown Minneapolis, where the streambed has degraded around a bridge pier, causing erosion to the pier.
“The Third Avenue inspection was not totally detailed. We knew there was a void under the bridge, but it was very hard to visualize,” DeWall said.
Early ice build-up halted further inspection in November, so MnDOT asked 3D sonar scanner manufacturer Teledyne BlueView to scan the area as a demonstration of its equipment.
A video of the inspection is below:
Multiple holes were cut in the ice sheet to deploy the sonar, which provided an image of the bridge scour by emitting sound-waves that created a point cloud.
“It gives you a large data set of where the sound reaches and comes back to the equipment,” DeWall explained.
The 3D image provides a level of detail that will enable repair and construction contractors to make more accurate bids, saving MnDOT money on projects.
Although dive inspectors are also beginning to invest in this new technology, MnDOT wants its own equipment to perform quick assessments of troublesome spots without going through the lengthy contracting process.
The Federal Highway Administration is conducting a pooled fund study to see if the technology eliminates the need for dive inspectors all-together.
MnDOT also plans to use its 3D scanning sonar to inspect repair projects and assess bridge construction.
One of DeWall’s first goals is to take a scan of the Hastings bridge after construction is complete, which will provide a baseline scan that can be compared against future inspections. The old bridge has had problems with the loss of rocks at its piers. It is unclear if the rock just sinks or is washed away downstream. Monitoring will let MnDOT see what is happening over time.
“Inspection is just one part of it,” DeWall said of the sonar equipment. “The big interest in this project is coming from our construction folks.”
Imagine building a new house and not being able to complete the final walk-through.
This is the situation that transportation departments face when they build a new bridge, due to the limitations of underwater inspections.
“With 3D technology, you can go back afterward and check to make sure things were done the way they were supposed to,” DeWall said.
DeWall wishes the state had the scanner many years ago when a bridge was built that required expensive correction.
A bridge construction crew left construction material behind under the water, which wasn’t discovered until the redirected water flow caused significant erosion to the bridge pier.
Divers picked up that something was going on during a routine inspection, but engineers still had to bring in depth finders to get a better look. Due to the water current, they were limited in how close they could get to the bridge pier, and turbulence crashed their boat against the pier, damaging the transducer.
Not only would this 3D technology have provided a more thorough assessment than the depth finder, it also could have captured the imagery from a safe distance away.
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.”
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.”
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.
In a new study funded by the Minnesota Department of Transportation, engineers are trying to ensure that new culverts do not degrade the habitat of an endangered fish in southern Minnesota.
The state has already researched how to better accommodate fish passage at river and stream crossings. Now it is looking at design guidelines for culverts that specifically impact the Topeka shiner, a small endangered fish found in five Midwestern states.
In Minnesota, the Topeka shiner is known to live in at least 57 streams, totaling 605 miles, within the Big Sioux and Rock River watersheds.
“The Topeka shiner is reported to have been erased from about 50 percent of its historic range in Iowa and much of its range in Minnesota, which is why Minnesota is so intent on doing what it can to help this fish thrive here,” said Alan Rindels, MnDOT’s project coordinator for the research.
The Topeka shiner is endangered due to the degradation of stream habitat, stream channelization, non-native predatory fishes and construction within waterways.
Culverts might impede the passage of this small minnow for a number of reasons, including that they might be too long, lack sufficient depth or carry water too fast.
In addition, long culverts block sunlight, which possibly discourages fish from swimming through. Typically, older culverts are replaced with longer culverts to improve road safety and minimize maintenance costs. To eliminate or minimize impacts to the Topeka shiner, the state is trying to determine if light mitigation strategies are necessary.
Researchers from the University of Minnesota’s St. Anthony Falls Research Laboratory will monitor a newly installed culvert (110 feet in length) and a few other culverts in critical Topeka shiner habitat streams during spawning and fall movement.
Additionally, a laboratory-based light manipulation experiment will examine the behavior of the warm-water fish when presented with a dark culvert.
Guidelines for culvert design in Topeka shiner habitat will be developed based on these results, as well as examples from neighboring states. The state is also collaborating with the U.S. Fish and Wildlife Service and affected Minnesota counties.
A research implementation project could provide MnDOT with a new set of tools to help combat a major source of bridge failure.
The MnDOT Bridge Office is testing several new methods of monitoring bridge scour — erosion that occurs around bridge piers and abutments during high water-flow events like floods. Acting Waterway Engineer Nicole Danielson-Bartelt said the project’s goal is to be able to monitor scour-critical bridges remotely rather than sending maintenance personnel out on the water during difficult or hazardous conditions.
“There are a number of bridges that are pretty difficult to monitor, especially during high water events,” she said. “Typically, you need to get out on a boat and do either sonar readings or drop weights. It’s dangerous work to be out on the water during those types of events unless you have the right training.”
The project will evaluate several different monitoring technologies, including continuous monitoring equipment like tilt meters and active sonar. The sonar systems, which allows continuous stream bed and water surface elevation data to be transmitted to a website for graphical display, could provide benefits that go beyond monitoring individual bridges.
“The ability to collect continuous, long-term data could help engineers understand short term scour-fill and long term aggradation-degradation cycles,” said Solomon Woldeamlak, a Bridge Office hydraulic engineer. He added that the data can be used to calibrate existing methods of estimating scour at bridges.
Other devices being tested include “float-out” devices, which are buried in the sand around the abutment and send out a signal only if washed to the surface by a scour. Danielson-Bartelt said these non-continuous monitoring devices might be appropriate for bridges where installing permanent sonar is not advisable due to the presence of debris that could damage the equipment.
Monitoring equipment has been installed at two locations: the Highway 43 Winona bridge over the Mississippi River and the Highway 14 Mankato bridge over the Minnesota River. A final report on the project is expected in late 2014/early 2015. You can learn more about some of the products that are being tested on the website of ETI Instrument Systems, Inc., which provided the equipment.
Over the last several years, MnDOT has been participating in a national pooled-fund study on using infrared cameras to spot subsurface damage on bridge decks. These damaged areas just below the deck surface are called “delaminations,” and they’re what causes potholes and cracks on the surface. Detecting them is a key part of what MnDOT bridge inspectors do, and it’s huge challenge.
Currently, one of the primary methods of locating delaminations is “chain-dragging” — literally, dragging chains across the surface of a bridge deck. Using this method, inspectors can listen for evidence of hollowed-out areas beneath the surface, which produce a different sound than solid areas. While it works, this practice forces bridge crews to close down lanes and work near moving traffic. These issues have led Minnesota and other states to look for alternatives, and infrared or “thermographic” imaging is one of the top contenders.
In the video above, MnDOT bridge inspector Eric Evens demonstrates how to inspect a bridge deck using an infrared camera (specifically, a FLIR T620 — the model selected for the study). The delaminated areas appear as white or “hot” spots in the image. Evens does a nice job of explaining some of the benefits and potential uses of the camera, including minimizing traffic delays. He also demonstrates the camera’s ability to simultaneously capture photos and infrared images, which could be useful for cataloging the conditions of bridge decks and programming schedules for repairs.
However, as Evens pointed out during the filming, there are both pros and cons to using infrared thermography. One downside is it’s really only effective as the bridge deck warms up in the morning. Another is that it takes some practice to be able to identify which of the “hot spots” are actual delaminations and which are merely dirt or debris on the deck surface, or some other kind of false positive.
If you weren’t able to attend the CTS Research Conference, or, if you simply want to check out presentations from other sessions, the videos of the keynote and luncheon speeches, as well as PPTs from most of the concurrent sessions, are now available on the CTS website. You won’t want to miss Minnesota Department of Health Commissioner Ehlinger’s tuneful take on the links between health and transportation and Elizabeth Deakin’s view of new ways to get around.