The research project will help the State of Minnesota’s Bridge Office develop a guidance document and a tool for bridge construction time estimation to be used by MnDOT District project managers and construction staff. The tool will provide a range of production rates based on specific design criteria, being more concise based on the level of information available and will aid in evaluating the potential benefit for accelerated bridge construction (ABC) techniques.
“This research will enable District project managers, who may not have bridge knowledge or background, to complete project planning and scoping more effectively,” said Paul Johns of MnDOT’s Office of Construction and Innovative Contracting.
Mike Rief of WSB & Associates will serve as the project’s principal investigator. Johns will serve as technical liaison.
According to the initial work plan, the project is scheduled to be completed by early March 2018, and WSB & Associates will complete the following tasks:
Conduct an existing practices literature review of current departments of transportation processes around the United States for bridge time and cost estimation.
Review and compile actual case study bridge construction production rates and cost data for major bridge components from state-provided diaries, schedules and bridge plans.
Evaluate and select the best software format and style for a bridge construction time estimation tool. Load state case study production rate data into estimation tool and run validation using bridges currently under construction.
Produce a research report summarizing the literature review on best practices. Produce a user guide for bridge time estimation tool and training presentation.
An optional task, if the budget allows, will include the development of a cost estimating tool. Cost estimation data would be gathered from the literature review and case study analysis during the development of bridge construction time estimation tool for efficiency.
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.
The research will determine if a bridge repair to the TH 169 Nine Mile Creek Bridge near Edina and Minnetonka was sufficient to restore the original strength of a girder end in shear. Load testing to failure will be conducted on two repaired girder ends and two unrepaired girder ends that will be removed from the bridge. Objectives include a comparison of the failure load between the repaired ends and the unrepaired ends. The test results also will provide some answers to questions on whether shotcrete is a structural repair or if it is just a covering over of deterioration.
Carol Shield, professor at the University of Minnesota’s Department of Civil Engineering, Civil, Environmental, and Geo- Engineering, will serve as the research project’s principal investigator. Paul Pilarski, MnDOT bridge engineer, will serve as the study’s technical liaison.
According to the initial work plan in the contract, the project is scheduled to be completed by the end of March 2018.
Over time, the south bound exterior girder ends on each side of Pier 4 and Pier 26 of the TH 169 Nine Mile Creek Bridge have suffered significant corrosion damage that exposed shear reinforcement, exterior flange prestressing strands, and the sole plate anchorages. Girder ends were repaired in September 201 3 by encasing a 4-foot length of the end using a system of dowels, additional shear reinforcement, and shotcrete. The bridge is scheduled for replacement in 2017.
There is interest in determining if the repair was sufficient to restore the original strength of the girder end in shear. Load testing to failure will be conducted on two repaired girder ends and two unrepaired girder ends that will be removed from the bridge. Objectives include a comparison of the failure load between the repaired ends and the unrepaired ends.
The ability to effectively repair corrosion damaged girder ends extends the useful life of prestressed concrete bridges. These repairs are significantly less expensive than replacing the bridge. Repairing bridges is also beneficial to the traveling public as travel is not interrupted, or interrupted for a significantly shorter time than for bridge replacement. Experimentally demonstrating that the repair restores the girders up to the design strength enhances the safety of the bridge and provides MnDOT with a documented substantiated repair method that can be applied to other bridge girders in a similar state.
When the southbound lanes of the TH 169 Nine Mile Creek Bridge are taken out of service, the contractor will remove four prestressed girders from the structure and deliver the south ends of them to the University’s Department of Civil Engineering Theodore V. Galambos Structures Lab. Two of the girders will have ends that have been repaired. The other two girders will be of the same shear design but will not have been repaired, nor show significant signs of corrosion. Once at the Galambos Laboratory decks will be cast on the girders. One end of each girder will be tested to failure using a setup designed to precipitate a shear failure. Failure loads between the repaired and original undamaged girder ends will be compared. The development of crack patterns under load will also be documented to further understand the behavior of the repair.
MnDOT will make arrangements for transportation of the girder ends to the University’s Civil Engineering Building. MnDOT will request that the contractor provide weights of the cut girders prior to delivery. MnDOT will make arrangements with the contractor to take concrete cores from the short end of the cut girders and provide the existing bridge bearing pads. MnDOT will provide calculations for determining the required deck width and concrete strength to avoid a flexural failure.
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.
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.
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.
New video, below, shows how explosions are used to test the bedrock for the new Highway 43 bridge in Winona.
Bridge engineers use “pile load testing” to find out how much weight and resistance the ground will bear. It not only saves time and money, but helps design a bridge that will sit securely on the bedrock, below the river.
The statnamic test used in the video is one part of this process.
Winona Bridge Statnamic Test
How load testing works:
It begins with digging and pounding.
Two different kinds of piles are put into the ground:
A hollowed shaft, which is filled with rebar and concrete. It goes 30 to 50 feet below the bedrock to create a solid pillar that can assess how much weight and sway the ground will bear.
A steel pipe that is hammered into the ground. Since the bedrock is about 100 to 150 feet below the river, these pipes are welded together end-to-end to reach that length.
Once the piles are in, they’re tested two different ways.
Pile Dynamic Analysis, with gauges affixed to the top of the pile to read the pressure put on it when hit with a pile driver.
A Statnamic test (shown in videos), which involves accelerating a heavy weight by setting off a controlled combustion reaction. This shows how much resistance the pile can take.
Once the data is collected for the bridge design, the piles are cut off two feet below the river bed.
Using Dawn dish soap to grease the rails, MnDOT crews inched the new Larpenteur Avenue Bridge into place two weeks ago using an innovative construction method.
As the bridge reopens to traffic tonight over I-35E, MnDOT celebrates the success of its first slide-in place bridge construction.
“The slide-in worked very well,” said David Herzog, MnDOT’s project manager for the I-35E Corridor – MnPass Project. “I think the process has given us the confidence to possibly use it again in the future.”
The slide-in method has been used in the past for railroad bridges and large bridges with high traffic and limited construction options. Now, state agencies and the Federal Highway Administration are applying the method to smaller, more routine bridges to minimize impacts to the traveling public.
Whereas the typical phased construction of a bridge builds one-half of the structure at a time, slide-in bridge technology allows the entire superstructure to be built at once, requiring just a brief, temporary closure of the highway.
Crews constructed the 3.5-million-pound Larpenteur Bridge right next to the existing bridge and then slowly slid it into place during the course of two nights. This effectively sped up construction from 110 days to 47 and reduced traffic impacts to drivers. (Watch video of the slide.)
The quality of the bridge also improves with this method, since it eliminates the deck construction joints and girder camber problems associated with phased construction, according to the FHWA. The pressure to use faster concrete cure times is also reduced.
With a quarter of the nation’s bridges in need of repair or replacement, the FHWA is pushing the slide-in method as a cost-effective technique that can cut construction time in half. It has previously been used in Oregon, Utah, Missouri, Michigan, Colorado and Massachusetts.
The concept has been around for more than a century, but slide-in technology is relatively new for small or medium-sized bridges, and it’s the first time MnDOT has attempted it on a state bridge.
Although MnDOT staff had flown out to Utah to view a slide-in, it was Burnsville-based Ames Construction that proposed reconstructing the Larpenteur Avenue bridge that way when it made its successful bid for the corridor project.
The slide-in method is about 15 percent more expensive, Herzog said, but it allowed the bridge to re-open in 47 days, versus 110 days.
Earlier this summer, Ames replaced the Wheelock Parkway and Arlington Avenue bridges in conventional fashion, although they were only closed for 65 days because they were constructed on a very accelerated timetable.
“Larpenteur is more of a major thoroughfare and we thought shortening the duration of its closure would be more valuable to MnDOT,” said Steve McPherson of Ames Construction, who was brought in from Utah to oversee the corridor project.
The fast reconstructions will allow the company to complete the bridge replacements and highway reconstruction in just 120 days. Next year it’ll finish the other half of the corridor.
All three bridges are being replaced to make room for the new MnPASS lane on I-35E.
One of the drawbacks to slide-in technology is that it requires ample room to build the bridge on-site. An alternative is to construct off-site.
The new Maryland Avenue/I-35E bridge was built off-site, as was the Hastings Hwy. 61 bridge. It was then loaded onto a barge, floated down the Mississippi River and lifted into place.