MIMO (Multiple Input/Multiple Output) Dynamic Data Saves Bridges and Lives
Pictured above: Kirk Grimmelsman, left, and students install tactile transducers on the underside of a rural highway bridge. The system produces significant vibrations over a broad range of frequencies. The bridge vibrations induced by the shakers allow researchers to evaluate the structural integrity of a bridge. | Photos submitted
There are no sad songs about Silver Bridge. There should be. During rush hour on Dec. 15, 1967, 46 people died on the bridge when it collapsed into the Ohio River. That’s 17 more than the entire crew of the SS Edmund Fitzgerald, the Great Lakes freighter eponymously immortalized in 1976 by Gordon Lightfoot.
But structural and civil engineers do not need an elegiac ballad to be reminded. For them, the Silver Bridge lives in infamy. In fact, the purpose of Kirk Grimmelsman’s research can be traced back to the day it collapsed into the Ohio River.
“Prior to that event, there really weren’t any national bridge inspection standards or requirements,” says Grimmelsman, assistant professor in the College of Engineering. “The Silver Bridge collapse changed everything.”
In reaction, Congress passed legislation creating the National Bridge Inventory, a database maintained by the Federal Highway Administration and containing safety and structural information on all U.S. bridges and tunnels that carry vehicles. Today there are approximately 600,000 bridges in the United States, most of which are 300 feet or shorter.
The database includes information on bridge types and specifications, operational conditions and other data, such as materials, loading, hydraulics and preservation. Engineers use the data to analyze and judge the condition of bridges. But they also provide the data as part of the biannual inspections required by law.
Grimmelsman is one of these engineers. In his office, on a wall next to his desk, there is a picture of him at the top of one of towers of the Throgs Neck Bridge in New York City. The Bronx is behind him. He is wearing all the appropriate safety gear, including a fluorescent vest, a hard hat and a headlamp, not to mention a harness and cable that tether him to the bridge. He isn’t unhappy, but he looks focused and serious, the way you might expect someone working 300 feet above the East River to look.
For 17 years, Grimmelsman has participated in scores of bridge evaluation studies, but he is one of many engineers nationwide who consider visual inspection data to be primarily qualitative and therefore less than optimal for cost-effective and reliable maintenance of the nation’s inventory of aging and deteriorating bridges. In recent years there has been a greater effort to use modern technology to provide quantitative data for assessing the condition and safety of deteriorating bridges.
Grimmelsman is at the core of this effort. He has performed a variety of full-scale testing programs on several long-span bridges in New York City and elsewhere. One method he uses to evaluate bridges is called dynamic testing, an experimental approach that quantitatively characterizes bridges. The two main approaches for dynamic testing are operational modal analysis, frequently referred to as ambient vibration testing, and experimental modal analysis, also called forced-vibration testing.
Ambient vibration testing is by far the most popular form of dynamic testing for bridges. It relies on the natural environment, sources such as wind, microtremors, waves and especially operating traffic on and near the structure, all of which make the bridge vibrate. Although it has the important advantages of being inexpensive and non-disruptive to traffic, ambient vibration testing is less exact because researchers cannot control or measure the forces that make the structure vibrate.
With forced vibration testing, engineers dynamically “excite” the bridge with controlled and measurable sources, such as shakers and impact hammers, which allow researchers to control the inputs used for testing. The relationship between the dynamic inputs and structural response provides a meaningful description of how the bridge is currently behaving – a behavior that can’t necessarily be viewed.
But there’s a downside. While it is a superior approach, the effectiveness of forced vibration testing depends on a single vibration-inducing device. These devices are large, heavy and expensive. Most weigh 100 pounds or more and cost a minimum of $15,000. More importantly, deploying these bulky devices
and their supporting equipment interferes with traffic on bridges, and they are not practical to use for long-term measurements
that will continuously track the condition of bridges as they age and deteriorate.
The crux of Grimmelsman’s work attempts to solve this problem.
FROM WOOFER TO BASS SHAKER
He spent several years chasing solutions to this problem, thinking about how he might develop a more reliable, practical and less expensive system to perform forced vibration testing of full-scale bridges. He was searching for something light and flexible, a system he could design as a distributed network, something that would vibrate a bridge from many input locations without interrupting traffic.
And then one day, he had an idea. What about speakers? Nearly everyone has experienced the shaking sensation caused by car stereo’s low-frequency loudspeaker. In the ’70s, they were called woofers or subwoofers. Today, most people call them bass speakers. Except engineers.
“I had been thinking about modifying voice-coil actuators for use in dynamic testing of bridges,” Grimmelsman said. “You know, a bass speaker.”
He knew he would have to modify speakers to make them suitable for use in testing bridges, but Grimmelsman thought he could build a distributed network of shakers with these off-the-shelf devices. As he prepared to start the project, another thing happened, the kind of synergetic experience that occurs when people share ideas, exactly the kind of thing that would not happen if scientists toiled in a vacuum.
“So that was the original idea,” Grimmelsman says, “to go and get some speakers and put some mass on them and put them on our grid structure in the lab. I was telling my grad student at the time, Jason Herrman, what I was planning, and he said, ‘Well, there’s already something called a bass shaker that does the same thing.’”
Through low-frequency vibrations, tactile transducers, popularly called bass shakers, provide haptic feedback of audio signals at or below the audible range. In other words, they make sounds you can feel rather than hear.
Which isn’t absolutely true. “You can hear them,” Grimmelsman says.
“Depending on the tuning… If you allow the device to use the full lower band on the audio spectrum, you can hear the sounds produced by these devices. But the pitch is odd. You have to listen carefully and know what to listen for.”Normally integrated with audio speakers, tactile transducers enhance user experience by adding shaking, vibrating and jarring movements. They are used for flight simulation, computer games, home-entertainment systems and amusement park rides. These are the tools that make you “feel” like you are flying through turbulence when you’re actually sitting in a theater chair.
It is true that transducers do the same thing – produce low-frequency vibrations – as his graduate student said, but for the purposes of Grimmelsman’s research, they do it better.
“Once I figured out what these things were, I did some market research,” he says. “I found some devices that might have worked for dynamic testing, maybe a little more expensive than the speakers I was originally going to use, but I quickly realized that we wouldn’t have to modify them the way we would have had to if we used a regular audio speaker.”
Grimmelsman and students Jessica Carreiro and Eric Fernstrom experimented with and evaluated the capabilities of a variety of available types of tactile transducers for dynamic testing purposes. All of the devices they studied were small and portable – weighing less than 10 pounds. After this vetting process, Grimmelsman designed and built a prototype bridge-testing system with a series of devices that cost less than $500 per shaker.
The researchers later installed 12 tactile transducers on the underside of a rural highway bridge to evaluate how the shakers would operate in the field. As a network, the system produced vibrations with reasonable force over a broad range of frequencies. The bridge vibrations induced by the shakers were also much larger than those caused by wind and other natural sources. The testing was the first attempt by any researchers to dynamically excite a full-scale bridge in the field using a large number of controlled inputs at the same time.
“The bridge test demonstrated that this system could dynamically excite a full-scale structure in a controlled manner to produce vibration responses with less uncertainty and more uniformity than from natural sources and traffic,” Grimmelsman said.
Five students at three academic levels are currently working on various facets of Grimmelsman’s bridge-shaking research. These projects use the shaker system to investigate multiple dynamic testing methods and different characterization approaches.
Two undergraduate students are gathering field data from ambient-vibration testing on two bridges: a truss bridge with virtually no traffic and a girder bridge with a high volume of traffic and other environmental forces. A third undergraduate student is performing laboratory tests to determine the ideal number and location of shaker devices for optimal performance.
In another project, a master’s student is using a modified transducer to deliver and measure how a bridge response to impact dynamic loading. These tests are similar to those in which engineers drop heavy weights onto bridges. Essentially, with this project, Grimmelsman says, the shakers act as large hammers striking the bridge at multiple locations.
Fernstrom’s doctoral research project focuses on using the shaker system for forced vibration testing of typical highway girder bridges. His research represents the first attempt to perform what Grimmelsman calls “MIMO” – multiple input and multiple output – forced vibration testing of a bridge using more than two excitation devices simultaneously. With this type of vibration testing, engineers could “excite” a structure at many locations while simultaneously measuring the structure’s vibration responses at many different locations.
Taken together, all of these projects constitute the first efforts to characterize the structural integrity of a bridge using controlled and known dynamic excitation at many locations on the structure at once. The testing system could lead advance ambient and forced-vibration testing of bridges.
This inclusive approach is part of what Grimmelsman would like to see incorporated into new testing standards for the National Bridge Inspection Program. He knows that quantitative testing would not replace visual inspections, but the two approaches could be combined to improve the reliability and utility of the existing testing and evaluation programs.
“The beauty of quantitative assessments is that it can be done continuously, so you don’t have to wait for the end of the two-year inspection cycle,” Grimmelsman says. “And it gives you data about the performance of certain bridge details relative to others over time. This means you might not have to wait for a failure. With minor damage and deterioration over time, the quantitative assessment will change and may reveal things might not be visible in a regular inspection.”
The other major benefit of quantitative assessments is that they enable repairs and retrofits of existing bridges to be implemented cost-effectively. The dynamic testing results allow researchers to calibrate analytical models developed by architects and engineers to better reflect the actual behavior of the structure. Incorporating measurements of actual structural behavior into the modeling process provides a more realistic and reliable representation of the structure, which helps engineers analyze important metrics such as loading, force, stress and hydraulics.
In short, Grimmelsman says, “It forces the model to reflect reality.”
Kirk Grimmelsman is an assistant professor in the department of civil engineering. He earned his bachelor’s and master’s degrees at the University of Cincinnati and his doctorate at Drexel University. At the University of Arkansas’ College of Engineering, he teaches courses and conducts research on structural analysis, structural mechanics and field instrumentation.