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APPLICATION OF ACOUSTIC EMISSION AND STRAIN
GAGE MONITORING TO STEEL HIGHWAY BRIDGES
Dave
Prine, ITI Chief Research Engineer
ABSTRACT
Current bridge condition determination is based almost entirely
on the use of visual inspection. This approach to bridge
inspection provides data that is subjective and not traceable.
Nondestructive evaluation (NDE) is a tool that in actuality
is little used on bridges, but could eliminate much of the
subjectivity of the input data for the bridge condition determination.
A critical task for NDE on these structures is to detect
and locate flaws that are growing which may eventually lead
to serious impairment of the structures ability to perform
its designed function. This problem area is the focus for
a bridge NDE program currently being conducted by Northwestern's
Infrastructure Technology Institute, (ITI). Under this program,
all elements of the bridge inspection problem are being investigated
by an interdisciplinary group consisting of members of Northwestern's
faculty and BIRL staff. One of the major research areas of
the program is the application of acoustic emission (AE)
monitoring to steel bridges. AE monitoring is being combined
with strain gage monitoring to develop a practical bridge
inspection tool. This paper will presents the latest results
of field tests conducted recently on bridges in California,
and Wisconsin.
INTRODUCTION
Steel bridges may develop cracks in structural
members resulting from a variety of causes. The cracks may
have been produced
during fabrication, or grow from fabrication flaws, or they
may be the result of fatigue damage. Not all cracks grow
to failure. Most NDE methods currently in use can detect,
locate, and to some degree size a crack, but they cannot
determine if the crack is growing. Acoustic emission (AE)
monitoring has the capability of detecting crack growth in
real-time. In fact, AE only responds to active flaws. This
unique feature of AE makes it a prime candidate for crack
characterization in highway bridges.
The acoustic emission
(AE) monitoring discussed in this paper was done using
a monitor that has 6 input channels
and is computer based. This device is a hardened field
portable unit. The key feature of this AE monitoring system
is the
powerful pattern recognition system that is applied in
real time to the AE signals. This pattern recognition algorithm
was originally developed for in-process weld monitoring.
It is based on empirical results that key on signal characteristics
which allow crack related information to be separated from
a noisy background. The algorithm tests the rate of occurrence
of the AE bursts and when a group of bursts is received
that
exceeds the pre-programmed rate limit (typically 3 Hz),
the algorithm evaluates the locational spread of the group
of
signals. If the high rate group all came from a tightly
clustered location (typically less than 1 inch spread), the
algorithm
counts this group as one indication. The algorithm has
been successfully applied to in-process weld monitoring on
virtually
every type of weld process and material that is commonly
encountered in heavy fabrication. Since 1982, the same
approach has been successfully applied to the in-service
monitoring
of steel highway bridges.
This approach is the only known
way that AE can be successfully applied to details that
are adjacent to or part of bolted
splices. The fundamental problem with AE monitoring of
these details is the noise produced by the bolts. The bolt
fretting
imitates AE very well and if the area to be monitored is
not locationally isolatable from the bolts, the noise rejection
algorithm must be used to eliminate the irrelevant bolt
noise.
The addition of strain gage monitoring in conjunction
with the AE information provides additional useful information
on a cracks nature. The strain gage data can indicate the
magnitude of the live stresses in the vicinity of the flaw
being characterized.
In the following sections we will discuss
the application of this AE monitoring system to four different
steel bridge
NDE problems.
WIDOT STRUCTURE B-5-158,
GREEN BAY, WISCONSIN
Click here for
list of I-43 Bridge pictures.
Wisconsin Department of Transportation Bridge
B-5-158 is located in the city of Green Bay in Brown County,
Wisconsin.
The structure carries east and westbound I-43 traffic over
the Fox River at the southern end of Green Bay. Total length
of the structure is 7982 feet including a 450 foot long tied
arch. The bridge was constructed in 1980.
In-depth inspection
of the bridge by WIDOT personnel detected visual cracks
inside the tie girders in the tied arch. The
cracks were located in welds at the ends of 1 by 6 inch
bars that join the bars to the hanger diaphragms at two sites.
The bars, which serve as horizontal stiffeners, are welded
to the inside of the tie girder at the point of attachment
of the floor beams. The welds that join the 1 by 6 inch
bars
to the tie girder web and the hanger diaphragm were fabricated
using shielded metal arc welding (SMAW) and have rough
unfinished reinforcements which makes ultrasonic inspection
very difficult
to perform. The welds were supposed to have been full penetration
and both visual as well as ultrasonic inspection indicate
that this is not true. WIDOT expressed an interest in gaining
a better understanding of the nature of the visible cracks
as well as additional information on the condition of the
stiffener to web welds. Following discussions between BIRL
and WIDOT this project was initiated. The test program
utilized a combination of acoustic emission and strain gage
monitoring
to provide the needed information on live load and crack
activity. The tests were performed by BIRL with assistance
from The Kentucky Transportation Center (KTC).
On May 3,
1993 BIRL commenced testing. Test sites included hangers
4 and 6 on both the north and south tie girders.
A total of six sites were monitored (the west side of hanger
4 on the north girder, the east side of hanger 6 on the
south girder, and the east and west sides of hanger 6 on
the north
and hanger 4 on the south girder). These sites included
all of the known cracks, sites that were adjacent to known
cracks
but with no known cracks, and sites that had no known cracks
present either in or adjacent to the test site. The acoustic
emission setup monitored both the stiffener to diaphragm
and stiffener to web welds at each test site. Two strain
gages were monitored at each of the test sites. Testing
continued through May 13, 1993. Traffic loading during the
tests included
many large and obviously heavy loads. A wide range of environmental
conditions were encountered, including high gusty winds
and temperatures ranging from 37 degrees F. to 80 degrees
F.
Test results (which are summarized in TABLE
1) showed no detectable crack related activity from
any of the six test sites and very small live loads (30
to 50
microinches/inch strain) from the strain gage tests. These
test results imply that some mechanism other than fatigue
is responsible for the visible cracks.
CALTRANS STRUCTURE
B-28-153, BENICIA MARTINEZ, CALIFORNIA
Click here for
list of Benicia-Martinez Bridge pictures.
California Department of Transportation structure
B-28-153 carries Interstate Highway 680 traffic across the
Sacramento
River at the east end of Carquinez Strait thirty miles northeast
of San Francisco, California. This 1.2 mile long, high-level
structure consists of ten steel deck-truss spans ranging
in length from 330 to 528 feet. The bridge was designed by
CALTRANS in the late 1950's and has been in continuous service
since it was opened in 1963. The design uses built-up steel
H-sections for the truss members and bolted connections.
The steel H-sections were fabricated using T-1 and ASTM A242
plates.
Stay plates were welded to the flanges of the
H-sections at the joints of the truss members. Cracks have
been detected
in these welds. Subsequent retrofit was performed on these
details that included removal of the ends of thecracked
plates by coring and adding bolted doubler plates. We applied
AE
and strain gage monitoring to two of these crack sites.
The first site was located in span 7 of the west truss at
location
L11, bottom plate. This crack was approximately 1/2 inch
long. The second test site was located in span 5 of the
west truss at location U14, top. This crack was approximately
3/4 inch long. The test results are summarized in TABLE
2.
The U14 Flaw indications were at the end of
the weld and near its midpoint. The AE indications coupled
with
relatively
high live stresses indicates that this site (U14) has an
active fatigue crack and should be closely watched.
CALTRANS STRUCTURE
B-22-26 R/L, BRYTE BEND, SACRAMENTO, CALIFORNIA
Click here for
list of Bryte Bend Bridge pictures.
Click here for list
of updated Bryte Bend Bridge pictures (1996).
Click here for list
of updated Bryte Bend Bridge pictures (2004).
The Bryte Bend Bridge carries I-80 traffic
over the Sacramento River near Sacramento CA. The bridge
consists of two 4050
foot trapezoidal steel boxes, thirty six feet wide. Its approaches
are 146.5 foot simple spans 8.5 feet deep with main spans
of 370 feet and 281.5 feet in length at a depth of 15.5 feet.
Flanges on the sloped side and vertical center web support
the composite concrete deck. In-depth inspection by Caltrans
personnel led to the discovery of cracks in the web of the
trapezoidal box at the lower attachment point for the stiffener
cross frames.
BIRL engineers applied acoustic emission and
strain gage monitoring to three crack sites to determine
the nature of
the cracks. Since this bridge is an all welded structure,
we were able to apply the acoustic emission using a simple
guard channel approach (no extraneous noise sources were
in the immediate vicinity of the crack). This approach
also allowed us to circumvent source location problems caused
by dispersive acoustic propagation that result from the
thin
plate (3/8 inch) used on this bridge. The guard channel
setup consisted of 4 sensors. A sensor was located at the
visible
crack tip and three others were placed in a triangular
array surrounding the crack tip. The AE data was recorded
and analyzed
post test. Any signal originating at the crack will reach
the crack mounted sensor first. AE signals arriving from
outside the array will be received at a guard sensor first.
An additional sensor was mounted at the tip of the vertical
stiffener to catch any AE generated by fretting from the
end of the stiffener on the bottom flange. Post test analysis
showed this precaution to be unnecessary for all but the
third test. A large portion of the 12,233 events came from
the vertical stiffener fretting. A summary of the AE results
is shown in TABLE
3.
Strain gages were mounted on the web of the
girder near the crack site. Two gages were mounted at each
test
site
and data was recorded in the rainflow mode. In the relatively
short period of time taken for these tests significant
live strains were recorded with ranges of 200 microstrain
and
higher. The conclusion reached from the combined AE and
strain gage tests were that the cracks were defiantly growing
under
fatigue loading. Discussions with Caltrans engineers subsequently
led us to apply strain gages over longer time periods to
obtain more statistically significant live strain histograms.
This work was performed in June of 1994 during the period
that the users group meeting was held. A preliminary analysis
of the strain gage data further confirmed the fatigue findings.
A summary of these tests is shown in TABLE
4. Channel 1 was mounted on the horizontal stiffener
transverse to the bridge axis. Channels 3 and 4 were mounted
on the vertical web with 3 horizontal and 4 vertical. The
counts to date are based on the life of the bridge assuming
uniform traffic volume.
WIDOT STRUCTURE B-70-97-93,
MENASHA, WISCONSIN
Click here for
list of Tayco Street Lift Bridge illustrations.
At the request of WIDOT, engineers from Northwestern University's
Industrial Research Laboratory, BIRL performed AE and strain
gage tests on the east bascule girder and segmental casting
assembly of structure B-70-97-93 (the Tayco St. lift bridge)
over the Fox River in Menasha, WI. The purpose of these tests
was to determine the origin of the loud impact noises that
accompany the lifting and lowering of the bridge.
Acoustic emission testing
We initially applied AE monitoring using a
4 sensor array. Three of the sensors were located on the
individual casting
segments and one was mounted near the pinion bearing. Analysis
of the time of arrival of the AE signals clearly showed that
the acoustic sources were located in the vicinity of the
segmental casting and the bascule girder flange. They were
not coming from the bearing area.
A second AE test was done
using a two channel array placed along the bottom edge of
the bascule flange at each end of
the center segmental casting (casting #2). The sensors were
separated by 50 inches. The purpose of this test was to utilize
linear source location to determine if the acoustic sources
tracked the contact of the center segmental casting with
the track casting. As the bridge was raised, the AE sources
were first detected at the right end of segment 2 (near Sensor
#2) as this segment engaged the track casting. The detection
of the sources coincided with the loud audible impacts and
appeared to coincide with the approximate locations of the
bolts. Upon lowering of the bridge the process reversed and
the source locations "walked" back toward the right end of
the casting segment.
Strain gage tests
Strain gages were applied near the center of
the center casting segment (casting #2). One gage was mounted
radially
(perpendicular to the casting-flange mating surface) while
the other gage was mounted at approximately 15 degrees to
this surface with the mating surface bisecting the gage diagonally
in the long gage dimension. This approach was used to determine
whether the casting was moving with respect to the flange.
The radial gage showed relatively small strains as the bridge
was exercised. The diagonal gage response was remarkably
different. Recorded data for this gage clearly shows displacement
of the casting with respect to the flange. The strain swings
negative during the rising cycle and then positive as the
bridge lowers. The final reading does not return to zero
which indicates that the casting is probably not returning
to its original position. Additionally, we mounted a second
inclined gage on the left most casting segment (casting #3).
This gage showed similar data as the bridge was raised and
lowered with the primary difference that the strain was unidirectional.
These tests were repeated several times with the gages being
re-zeroed prior to each run. The shape of the curve remains
the same. During live observation of the strain gage signals
we saw clear correlation between the occurrence of the impact
noises and large jumps in the strain values.
The combination
of AE and strain gage testing clearly confirms the WI-DOT
concerns that friction bolts are not properly
attaching the segmental casting to the bascule flange.
SUMMARY AND CONCLUSIONS
The four tests discussed in this paper are
examples of the useful information that application of AE
and strain gage
testing can provide to the bridge owner. The I-43 Green Bay
bridge had both visible as well as suspected flaws. AE and
strain gage tests clearly showed that the visible cracks
were not of fatigue origin. The lack of crack related AE
coupled with low live stresses indicate that the cracks were
most likely an example of early failure of a weld flaw. AE
further confirmed that there were no active cracks in the
stiffener to web welds. The tests performed on the Benicia
Martinez bridge, on the other hand clearly confirmed that
one of the visible cracks is being driven by fatigue. Similarly,
the tests conducted on Bryte bend confirmed that all of the
cracks monitored are active fatigue cracks that are being
driven by high live stresses.
The Menasha lift bridge is an
example of an application of the AE and strain gage monitoring
technology that is quite
different from the usual crack characterization. In this
example, these NDE tools were used to diagnose a problem
in a new structure that was simply a case of poor mechanical
design. The AE was able to clearly pinpoint the source
of the loud impact noises while strain gage monitoring confirmed
that the castings were moving tangentially with respect
to
the bascule flange. This diagnosis was made early enough
to allow corrective action to be taken before bolt failure
and potential jamming of a casting which would render the
bridge in-operable.
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