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.
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.
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