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Healthy Bridges

 


Healthy Bridges
A sensor-based structural monitoring system could be a key to improving highway safety

About the Photo: Penobscot Narrows is only the second major cable-stayed bridge of its kind in the country. Its continuous cable stays, each containing 5070 epoxy-coated steel strands, stretches from one span of the bridge deck, through a cradle on the 400-foot concrete pylon and down to the other bridge deck. Six steel strands in three of the cable stays were replaced by high-strength, noncorrosive carbon fiber composite strands. UMaine researchers, led by Roberto Lopez-Anido, right, then implemented a comprehensive structural health monitoring system.

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Student Engineers Making Connections
Development and implementation of a sensor-based systems monitoring network for the Penobscot Narrows Bridge involved University of Maine mechanical engineering Ph.D. student Keith Berube and undergraduate mechanical engineering major Kate Wheeler.
 

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The catastrophic collapse of the I-35W steel deck truss bridge over the Mississippi River in Minneapolis Aug. 1 focused national attention on the safety and security of aging bridges across the country. The tragedy that claimed more than a dozen lives emphasized the need for safer, more reliable designs for new bridges, and more comprehensive and reliable monitoring and maintenance programs for existing spans.

The new Penobscot Narrows Bridge in Maine, which opened to the public last Dec. 30, is a shining example of what new designs and technologies have to offer.

Working in cooperation with a Bridge Team the Maine Department of Transportation (MaineDOT), Federal Highway Administration, Figg Engineering Group (the Florida-based company that designed the bridge), CTL Group, Cianbro-Reed & Reed LLC (the bridge contractor), Dywidag Systems International (DSI), Lawrence Technological University, and Tokyo Rope of Japan University of Maine researchers have been instrumental in making the bridge something more than just a new way to get from Prospect to Verona Island. They've given it a voice.

In late June, six epoxy-coated steel strands in three of the bridge cable stays were replaced by high-strength, noncorrosive carbon fiber composite strands, developed and installed by the Bridge Team. UMaine researchers then implemented a comprehensive structural health monitoring system.

UMaine civil and environmental engineering professor Roberto Lopez-Anido, mechanical engineering professor Vince Caccese, Ph.D. student Keith Berube and a small team of undergraduate engineering students have taken advantage of the 2,120-foot-long structure's unique design to help install a sensor-based structural health monitoring system. The system, in effect, allows the bridge to communicate with its maintenance team, providing such information as tension levels in the structure's carbon composite and epoxy-coated steel strands, and temperature fluctuations in the surrounding environment. The sensors help inspectors determine whether the bridge is safe, and provide an unprecedented opportunity to measure the reliability of new materials.

"The design of this cable-stayed bridge allowed us, working in partnership with the Maine Department of Transportation, the Federal Highway Administration, Figg Engineering Group and other collaborators, to monitor the recently installed carbon fiber-reinforced composite strands, which has never been done in this type of bridge," says Lopez-Anido.


Penobscot Narrows is only the second major cable-stayed bridge of its kind in the country. Its continuous cable stays, each containing 5070 epoxy-coated steel strands, stretches from one span of the bridge deck, through a cradle on the 400-foot concrete tower (pylon) and down to the other bridge deck. In the cradle system assembly, developed by Figg Engineering Group, each strand passes through its own stainless steel sleeve. The cradle system separating each strand facilitates individual inspection, adjustment and replacement.

A pair of strands can be detensioned and replaced in each stay without compromising the structural safety of the bridge. At different heights through the pylon supporting the observation tower, crews replaced two strands at three stays with experimental strands of carbon fiber composite, a material that could greatly improve the strength and durability of bridges around the world. Each carbon fiber strand was tensioned between 20,00026,000 pounds, as indicated by the load cells monitored by UMaine researchers.

Working with Caccese and a team of student research assistants, Lopez-Anido and Berube designed a novel sensor monitoring system for the carbon fiber composite strands, manufactured by Tokyo Rope. By building and testing a support structure (anchorage chair) for the carbon fiber strands equipped with displacement sensors and load cells, they developed a unique system for measuring changes in the strands' tension performance.

In addition, the team developed an effective new method for measuring strain within the cables. By embedding existing fiber optic strain sensors in E-glass/vinyl ester composites, Lopez-Anido created a tube-like sheath for the composite strands to provide additional data on changes in the tension force.

The Penobscot Narrows Bridge gave Lopez-Anido and collaborators the ability to test the performance of carbon composite strands in a real-world environment, generating invaluable data that will allow researchers to compare the carbon composite test strands to the more traditional epoxy-coated steel strands that currently support the bridge. Using a battery of sensors, including their own devices as well as sensors installed by the Bridge Team and construction crews, the UMaine researchers will be able to gather data regarding temperature, tension forces and strand strains from throughout the cable-stayed structure.


To simplify the monitoring process and assist the MaineDOT in making the bridge a kind of "living laboratory," where trends can be measured and new technologies tested, Lopez-Anido's team is helping to coordinate the sensors in a way that would allow remote access to the sensor data. Once completed, the complex system of multiplexers, data loggers and fiber optic cables will allow researchers on campus or MaineDOT officials in Augusta to look at performance indicators in real time over the Internet, complementing on-site safety inspections and other research.

UMaine's Advanced Engineered Wood Composites (AEWC) Center is widely recognized as a leader in the development of sensor technologies and novel materials for use in bridges. Lopez-Anido has been involved in a number of AEWC projects. Materials and techniques developed at the center, incorporating the latest in cutting-edge sensor technologies, are helping to extend the life and improve the safety of a broad range of civil engineering projects.

Currently, Lopez-Anido is fine-tuning a long-term monitoring program for the bridge. Recording data at one minute intervals 24 hours a day, 365 days a year, the comprehensive program will provide the kind of information that engineers and bridge maintenance crews need to prevent tragedies like the Minneapolis collapse.

"We are developing a proposal for a long-term monitoring plan for the MaineDOT that would not only coordinate the use of the existing sensors in the system, but also explore new technologies, including wireless systems, that could ultimately be used in a variety of projects," says Lopez-Anido. "This is a unique structure with very innovative technologies. (Coupled with) the cooperative approach that has been taken, (it) has allowed us to go from the lab to the field with important new technologies.

"When you look at an old steel truss bridge, you can easily see the challenges involved in making an accurate inspection. There are so many members and joints, it's like looking through a jungle. Today when we build a bridge, we have to think about monitoring and maintenance, and sensor technologies help us test new materials and monitor safety issues so that repairs and renovations can be made," he says. "We're creating bridges that are a lot different from those that were built 40 or 50 years ago."

by David Munson
November-December, 2007

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