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Nondestructive Bridge Evaluation
The Infrastructure Technology Institute of
Northwestern University was established in 1992 with a mission
of research, technology transfer, education, and management
and policy studies related to American physical infrastructure.
Located on the University's Evanston campus, the Institute
is an interdisciplinary center within Northwestern's Robert
R. McCormick School of Engineering and Applied Science. It
is supported by Northwestern and a grant from the U. S. Department
of Transportation.
Research
Since its inception, the Institute has been
supporting research at Northwestern in cooperation with public
and private sector technology deployment partners throughout
the United States to address significant challenges identified
by the infrastructure community. A major emphasis of the
Institute's research program is nondestructive evaluation
(NDE) of bridges. We are currently active in six general
areas:
- Remote Monitoring
- Fatigue Crack Evaluation in Steel Structures
- Structural Stability Evaluation
- Lift Bridge Diagnostics
- Deep Foundation Evaluation
- Retrofit Evaluation
Remote Monitoring of the Infrastructure
Bridge and highway owners are faced with the
reality of shrinking resources and increasing pressure to
maintain and extend the life of existing infrastructure.
A major problem is providing adequate inspection to maintain
cost effective and safe operation. Under these conditions,
continuous remote monitoring becomes an attractive alternative
to periodic inspection conducted by human on-site inspectors.
Institute researchers have pioneered efforts
to develop and deploy remote monitoring systems for bridges
and highways. These systems utilize various sensors coupled
with computers and digital communications technology to provide
vital structural condition data to inspection and maintenance
personnel without the need for the inspectors to actually
visit the structure.
The ability to continuously monitor critical
components of a structure also opens up new opportunities
to change the paradigm of how bridges are managed. The continuous
acquisition of bridge condition data allows the owner to
record and clearly observe performance and detect deterioration.
This approach is well suited to preventive maintenance. Furthermore,
these techniques are the first step toward the "smart" structure
that informs the owner that a problem exists.
To date, the Institute team has installed nine
remote monitoring systems in Wisconsin, Indiana, Ohio, and
California. The applications include continuous "health" monitoring
of a 70-year-old rolling bascule lift bridge, temperature-strain
dependence monitoring of a large tied arch bridge, monitoring
of rock and soil stability beneath an interstate highway
that passes over abandoned coal mines, and structural stability
of several scour prone bridges. Sensor types include strain
gauges, clinometers, thermocouples, and TDR cables.
Commercially available data loggers and signal
conditioners, originally developed for other applications,
have been adapted to the severe environment encountered in
continuous infrastructure monitoring. Both hard-wired and
radio-linked local area networks have been used successfully.
Fatigue Crack Evaluation in Steel Structures
Steel structures such as highway bridges frequently
have cracks in weldments and connections. These cracks may
be due to a variety of causes; not all of the cracks may
be growing under operating live load conditions. A crack
may result from improper welding conditions. If it is not
oriented orthogonal to significant live stresses, it will
not be driven by fatigue. Owners and operators of these structures
need NDE tools that can provide them with a better understanding
of the nature of cracks to allow them to develop effective
retrofit procedures from the standpoint of both preventing
further deterioration of the structure and applying limited
maintenance budgets in the most cost effective manner.
Institute researchers have been successfully
applying acoustic emission and strain gauge measurements
to this problem. Acoustic emission (AE) is a passive acoustic
technique that uses a fixed array of sensors to detect acoustic
activity from actively growing cracks. Unique proprietary
AE monitoring techniques have been developed that allow the
crack-related activity to be effectively separated from the
background noise present in a structure under live loading
conditions. The addition of strain gauges mounted in the
immediate crack vicinity quantifies the live strains that
the crack is experiencing. The information can be combined
to provide a reliable evaluation of the nature of the crack
in question.
One very promising use of this technology is
the evaluation of a given retrofit procedure. AE and strain
gauge monitoring can be applied before and after the application
of a prototype retrofit to assess the effectiveness of the
retrofit before large expenditures are made to apply this
procedure to a multiplicity of details on the structure.
This technology was recently successfully applied to a major
interstate bridge in California.
Lift Bridge Diagnostics
In the last 10 years in the U.S. a major design
change has been made in the method of attachment of the segmental
castings in rolling bascule lift bridges. The design change
is a cost saving measure that replaces machined and fitted
bolts with high strength friction bolts in oversized holes.
This new method of attachment is highly dependent
on such things as surface finish and treatment of the casting/bascule
girder mating surfaces and the relative fit of the inter-casting
wedges that function as load distributing devices. Some rolling
bascule bridge owners have experienced problems with bolt
failures and slippage of the castings, both of which can
be serious safety concerns and operational problems.
Institute engineers have developed a simple
diagnostic technique that uses acoustic emission and a non-contact
laser displacement gauge to evaluate the segmental casting
performance. The AE technique detects slippage and fretting
of the high strength bolts as well as localizing mechanical
noise sources. The laser gauge quantitatively measures the
elastic response of the casting segments to the high tangential
loads that accompany bridge operation. These measurements
can easily detect permanent displacements of the castings
with respect to the girder flange as well as assess the integrity
of the inter-casting wedges. The test is easy to perform,
provides real time results, and can be applied to a double
leaf bridge in one eight hour work period.
Wanted: Technology Deployment Partners
The Institute points with pride to a variety
of successful partnerships it has undertaken with public
and private sector agencies around the United States. The
Institute's professional engineering research staff gives
it the capacity to respond in a timely manner to the needs
of infrastructure practitioners, while at the same time accessing
the world-class infrastructure technology resources on Northwestern's
faculty.
The Institute is careful not to engage in unfair
competition with private engineering consultants and testing
services. But where the opportunity exists to advance the
state of the art through a successful field application of
an innovative technology, the Institute is prepared to work
with infrastructure owners to design, install, and evaluate
experimental systems. If your agency has a challenging infrastructure
problem in one of the areas outlined above, please contact
us to determine whether we can be helpful.
NORTHWESTERN UNIVERSITY FACULTY
The Institute works closely with Northwestern
faculty members on advanced NDE development work. Regularly
participating faculty include:
- Professor J. D. Achenbach: Director of The Center for
Quality Engineering and Failure Prevention (QEFP), (Quantitative
NDE and Advanced Ultrasonics)
- Professor S. Shah: Director of The Center for Advanced
Cement Based Materials, (NDE of Concrete)
- Professor C. Dowding: Civil Engineering, (Time Domain
Reflectometry)
- Professor R. Finno: Civil Engineering, (NDE of Deep Foundations)
Contact
David W. Prine
Chief Research Engineer
Infrastructure Technology Institute
Northwestern University
2133 Sheridan Road
Evanston, IL 60201-3140
Telephone: (847) 491-2873
FAX: (847) 467-2056
E-mail: dprine@northwestern.edu
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