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 50–70 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 20,000–26,000 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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