Bridges built using prestressed concrete girders are among the most common in Minnesota and throughout the U.S. because of their good performance, lower initial material costs, and relatively low ongoing maintenance costs. However, the federal requirements for these bridges have changed considerably over the years. As a result, bridges built to older specifications may score poorly when subjected to new bridge rating standards even though they are actually in good condition.
“One area in which this discrepancy between ratings and reality can cause problems is determining safe legal load limits for bridges, which are used to decide whether larger trucks may cross the bridge with an overload permit,” says Catherine French, CSE Distinguished Professor in the Department of Civil, Environmental, and Geo- Engineering and the study’s principal investigator.
“Our goal was to evaluate whether the current guidelines regarding shear forces (which transfer the loads to the supports) may be overly conservative for these older concrete bridges that are in good condition.”
Sponsored by MnDOT, the study was conducted by a team of U of M researchers including Carol Shield (co-investigator) and Benjamin Dymond.
Researchers used a multipronged approach consisting of numerical modeling and tests in both the laboratory and the field. The numerical modeling was used to apply the results of the laboratory and field tests to a study examining the effects of key parameters on the distribution of shear in a bridge system. Parameters included span length, girder spacing and depth, deck thickness, and load position.
Results showed that the shear forces for some bridges are not as high as those predicted by distribution factors in the current specifications—at least partially explaining why some MnDOT bridges with low shear ratings show no signs of distress, French says. The researchers provided recommendations for more refined methods of evaluating prestressed concrete girder bridges that rate low for shear and developed a screening tool to identify which bridges that rate low for shear should be further analyzed.
“The results of this project will help us re-evaluate aging bridges in our inventory, to distinguish those that really do have shear problems from those that don’t, and make decisions about whether they need to be replaced or rehabilitated for extra capacity,” says Yihong Gao, bridge designer with MnDOT’s Office of Bridges and Structures.
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.
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.”
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.
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.