Engineering researchers at the University of Arkansas have developed a new and inexpensive system to test the structural condition of short- to medium-span bridges.
Three hundred and fifty feet above the East River, Kirk Grimmelsman inspects the Throgs Neck Bridge in New York City.
The system employs a network of tactile transducers – small-scale, inexpensive and off-the-shelf devices that create the sensation of shaking through feedback of low-frequency sound waves. These devices, known as “shakers,” are normally used in home entertainment systems and amusement park rides to enhance user experience.
“The compact size of the tactile transducers and their supporting electronics makes them ideal for executing controlled vibration testing of bridges without disrupting the traffic on the structures,” said Kirk Grimmelsman, assistant professor of civil engineering. “These devices can replicate practically any type of dynamic excitation, including random noise, impulses and harmonic signals.”
There are roughly 600,000 bridges in the United States, the overwhelming majority of which are 300 feet or shorter. The Federal Highway Administration maintains the National Bridge Inventory, a database containing safety and structural information on all U.S. bridges that carry vehicles. The data are used to analyze and judge the condition of bridges. As part of the requirements of the inventory, specially trained engineers must visually inspect each bridge every two years. Many bridge engineers consider the qualitative data provided by visual inspections to be less than optimal for cost-effective and reliable maintenance of the nation’s inventory of aging bridges. In recent years there has been a greater effort to use modern technology to provide more quantitative data for assessing the condition and safety of deteriorating bridges.
Grimmelsman’s work is part of this effort. He has performed a variety of full-scale testing programs on numerous bridges, including the Brooklyn Bridge and New York City’s heavily traveled Throgs Neck Bridge. His research focuses on investigating scientific, quantitative methods for testing the safety and structural integrity of bridges. The method he uses is called dynamic testing, an experimental approach that quantitatively characterizes and evaluates bridges. The two main approaches for dynamic testing are experimental modal analysis, also called forced-vibration testing, and operational modal analysis, frequently referred to as ambient vibration testing.
With forced vibration testing, the bridge is dynamically excited by a controlled and measurable source, such as shakers and impact hammers. This allows engineers 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. However, this approach has depended on a single vibration-inducing device, which is large, heavy and expensive, costing a minimum of $20,000. The device and its supporting equipment also interfere with traffic on bridges and are not practical for long-term measurements to track the condition of bridges as they age and deteriorate.
Ambient vibration testing, by far the most popular form of dynamic testing for bridges, relies on natural environmental sources such as wind, microtremors, waves and operating traffic on and near the structure, all of which make the bridge vibrate. Although it has the important advantages of being inexpensive and not disruptive to traffic, ambient vibration testing is more uncertain because researchers cannot control or measure the forces that are making the structure vibrate.
In recent years, Grimmelsman has sought to develop a more reliable, practical and less expensive way to perform forced vibration testing. He considered using small and inexpensive shakers to vibrate a bridge from many input locations spread out across the structure. He originally planned to modify subwoofer speakers to serve as shakers, until a graduate student in his laboratory mentioned “bass shakers,” devices that create the sensation of shaking with low frequency audio signals.
Grimmelsman and students Jessica Carreiro and Eric Fernstrom modified and experimented with a variety of available types of bass shakers, also known as tactile transducers. The devices they studied were all small and portable – weighing less than 10 pounds. Grimmelsman designed and built a bridge-testing system with these 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. 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 due to wind and other natural sources.
“The bridge test demonstrated that a system of these devices could dynamically excite a full-scale structure in a controlled manner to produce vibration responses with less uncertainty and more uniformity than those resulting from natural sources and traffic,” Grimmelsman said.
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.
Grimmelsman recently presented the research at the 2013 American Society of Civil Engineers Structures Congress. He and his team are conducting further bridge tests with their system and preparing their results for publication.CONTACTS:
Matt McGowan | Newswise
Smart homes will “LISTEN” to your voice
17.01.2017 | EML European Media Laboratory GmbH
Designing Architecture with Solar Building Envelopes
16.01.2017 | Fraunhofer-Institut für Solare Energiesysteme ISE
In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport
Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...
The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.
The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...
Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...
Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".
Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...
13.02.2017 | Event News
10.02.2017 | Event News
09.02.2017 | Event News
20.02.2017 | Materials Sciences
20.02.2017 | Health and Medicine
20.02.2017 | Health and Medicine