November/December 2000
Ultrasonic Inspection of Bridge Hanger Pins
by: Benjamin A. Graybeal, R.A. Walther, Glenn A. Washer, and Amy M. Waters
Adapted
from TRB Paper No. 00-0918 with the same title by Graybeal, Walther,
and Washer. The original paper was prepared for the 79th Annual Meeting
of the Transportation Research Board on Jan. 9-13, 2000, and was published
in Transportation Research Record (TRR) No. 169,, Maintenance and Management
of Bridges and Pavements, July 31, 2000, pages 19-23.
In
June 1983, a failed hanger pin initiated the tragic collapse of one
span of the Mianus River Bridge on the Connecticut Turnpike near Greenwich,
Conn. This incident resulted in the deaths of three motorists. Following
the collapse, there was an immediate increase in interest in the inspection
and condition evaluation of bridge hanger pins.
Ultrasonic inspection is one of the most reliable methods used to inspect
hanger pins, and it has become the primary method of performing a detailed
inspection of an in-service hanger pin.
This article describes a study conducted by the staff of the Federal
Highway Administration's (FHWA) Nondestructive Evaluation Validation
Center (NDEVC) to determine the reliability of contact ultrasonic techniques
in the field to accurately locate defects in hanger pins. The study
examined and compared two ultrasonic techniques. The first technique
involves testing in-service pins using a contact ultrasonic method.
This type of inspection would generally be used during an in-service
inspection.1 The second technique involves testing decommissioned pins
through a noncontact ultrasonic method using an immersion tank. While
this type of inspection is not practical for field use, it does provide
highly repeatable and reliable results that are intended to help verify
the contact ultrasonic inspection methods.
This article provides some background information regarding hanger pins
in general, as well as the hanger pins studied in this paper, and it
discusses the field ultrasonic technique, including methods, results,
and limitations; the immersion tank ultrasonic testing, also including
methods, results, and limitations; comparisons between the two testing
methods; and conclusions.
Background
Hanger pins are the structural elements connecting the suspended span
of the bridge to the fixed cantilever arm of that same bridge. A diagram
of a pin-and-hanger connection is shown in figure 1. Photographs of
one of the hanger pins examined for this study are shown in figures
2 and 3. Note that one of the exterior faces of the pin can be seen
in figure 2 and that the exterior face and the shear planes are identified
in figure 3. The exterior face is defined as either of the ends of the
pin that are visible while the pin is still in the pin-and-hanger connection.
The exterior faces are the only surfaces through which ultrasonic pulses
can be transmitted while the pin is in situ.
|
Figure
1 - Schematic diagram of an in-service pin-and hanger connection
prior to pin removal. |
The primary function of a pin-and-hanger connection is to allow for
longitudinal thermal expansion and contraction in the bridge superstructure.
These connections are designed to support the transfer of shear forces
from the suspended span into the anchor span. As long as the connection
is operating properly, neither shear forces from the anchor span nor
moments from either span can be transmitted across the connection.
|
Figure
3 - Photographs of hanger pin prior to and following removal from
bridge. |
In
general, loads from the suspended span are transmitted into the anchor
span as follows. The loads travel from the suspended girder web reinforcement
plate to the lower hanger pin and into the hanger plates. From the hanger
plates, the load is then transferred into the upper hanger pin and finally
into the anchor girder web reinforcement plate. This load path creates
two shear planes within each pin - one at each of the intersections
of the web reinforcement plate and the hanger plate. If a pin were to
fail along both shear planes, the portion of the bridge section suspended
by that pin would be unsupported.
Pin-and-hanger connections are typically located directly beneath bridge
deck expansion joints. Consequently, they are frequently exposed to
water and debris that falls through the joint. Water and debris can
accumulate behind hanger plates and around pins. The presence of moisture
in the confined region between the hanger plates and girder web can
lead to corrosion of both of these elements and of the pin at the critical
shear planes.
|
Figure
4 - Cracked section results from field ultrasonic inspection. |
This corrosion can have two detrimental effects on the pin. First, the
cross-section of the pin can decrease due to corrosive section loss.
This corrosion can produce pitting that may act as crack-initiation
sites. Second, corrosion can effectively lock the pin within the connection
so that no rotation about the pin is permitted. This can lead to large
torsional stresses within a reduced section of the pin. The torsional
stresses, combined with the shear stresses, provide a likely location
for the development and propagation of cracks and the eventual failure
of the pin.
Locating cracks that start on the pin barrel at the shear plane perimeter
is a difficult task. The shear plane is not visible unless the pin is
removed from the connection - a labor- and equipment-intensive task.
Ultrasonic field inspection provides the requisite combination of simplicity
of inspection procedure and accuracy of results to make it a preferred
method for many bridge owners. Furthermore, ultrasonic techniques access
internal pin regions that otherwise are inaccessible except by more
costly nondestructive techniques.
Bridge
Description
The bridge studied for this research is a 12-span, 324.5-meter- (1064-foot-)
long structure that provides access to a heavy-freight transfer facility
in the midwestern United States. The bridge deck accommodates two lanes
of traffic and is supported by a superstructure consisting of four welded
steel plate girders. The bridge contains 12 pin-and-hanger connections,
all of which occur at expansion joints approximately 1.5 meters (5 feet)
from an adjacent pier. The hanger pins in the bridge all have a barrel
diameter of 76 millimeters (3 inches) and an overall length of 216 mm
(8.5 in). The distance from the exterior face of the pin to the pin
shoulder is 32 mm (1.25 in), and the distance from each exterior face
to the closer shear plane is 76 mm (3 in). The bridge is owned and maintained
by a state department of transportation (DOT).
The bridge serves as the primary roadway access point to a freight transfer
facility and, as such, is subjected to extremely heavy and frequent
truck traffic. Permit loads routinely use this structure. A structural
analysis of the bridge conducted in 1994 revealed that the pin-and-hanger
detail was being subjected to higher stresses than originally intended.
The bridge was last inspected in 1996, at which time a National Bridge
Inspection Standards (NBIS) routine inspection was conducted. In conjunction
with this routine inspection, the DOT contracted a consulting engineer
to perform a special inspection of the pin-and-hanger detail; the inspection
included detailed measurement of member sizes and expansion-joint gaps.
The purpose of these measurements was to determine if the pin-and-hanger
detail was "frozen" or locked. In addition, in 1994, a field
ultrasonic inspection of the pin members was carried out. The combined
results of the ultrasonic testing and visual examination revealed shallow
defects in some of the pins and no defects in the hanger links. In general,
these shallow defects were reported as wear grooves and/or corrosion
section loss having a depth ranging from minimal to 3 mm (one-eighth
inch).
In spring 1998, a DOT employee noted that one of the hangers was no
longer aligned correctly with its lower pin. An immediate close-up inspection
was performed and confirmed that the pin had fractured. As a result,
both pins were removed from that connection and were replaced with new
pins.
In summer 1998, NDEVC staff members had the opportunity to view the
salvaged pin, which had been removed by flame-cutting. The flame-cutting
had obscured most of the failed surface; however, enough was visible
to conclude that fatigue had played a part in the failure mechanism.
The state DOT agreed to permit the NDEVC staff to access to the remaining
pins to investigate whether or not additional pins within the structure
were cracked.
Field
Ultrasonic Inspection
With the state's permission, the FHWA NDEVC conducted ultrasonic testing
of the remaining 22 pins on the bridge. The inspection team was led
by an American Society of Nondestructive Testing- (ASNT-) certified
Level III ultrasonic inspector. The ultrasonic testing was conducted
with the intent of identifying cracked or failed pins. The inspections
were conducted from a boom lift positioned below the structure. Prior
to ultrasonic testing, the pin exterior faces were prepared by grinding
to remove paint and smooth the surface to facilitate sound transmission
into the pin. The procedures for ultrasonic testing outlined in the
Bridge Welding Code were followed throughout this inspection.2
Ultrasonic
examinations were performed using a Krautkramer/Branson USN 52 model
ultrasonic flaw detector. This instrument features state-of-the-art
digital electronics, an electro-luminescent display, and 70 data set
storage registers. Preliminary scans were performed using a straight-beam
(0 degrees) transducer, with subsequent scans using an incident angle
of 15 degrees. An incident angle of 15 degrees was selected to avoid
the pin shoulder and to provide a better angle of incidence to the near-side
shear plane. A transducer frequency of 5 MHz and a 12.7-mm (0.5-in)
element size were used throughout the testing. Prior to testing, the
transducer and flaw detector were calibrated to establish sensitivity
and time base. Standard American Welding Society (AWS) calibration blocks
and specially prepared pin calibration standards, developed by the ultrasonic
inspector, were used.
|
Figure
7 - Ultrasonic pulse showing beam spread around the pin shoulder
and defects under the pin exterior face at the shear plane level.
|
A straight-beam scan was initially used to confirm pin geometry. Measurements
of component length were made using a tape measure to corroborate the
ultrasonic findings. The straight-beam scan was also used to identify
large cracks or complete failure. The angled-beam scans were then conducted
to more accurately resolve reflectors located near the pin barrel surface
and to examine areas shielded from the straight-beam scan, such as the
pin shoulder region. Angled-beam scans were concentrated on pin shear
planes where the potential for wear grooves, corrosion, and cracking
indicators is greatest. All scanning proceeded in a systematic manner
to ensure complete coverage of the pin interior. Scanning included movement
of the transducer over the entire pin end with the angle oriented toward
the pin barrel. All pins were examined from both exterior faces. In
several instances, the nut was not fully seated on the pin end, thereby
limiting transducer positioning near the perimeter of the pin end. After
testing, all pin ends were painted using a rust-inhibiting primer.
|
Figure
8 - Cscan images of defect indications in hanger pins S106 and S108 |
The field inspection indicated that two of the 22 original pins were
cracked. This inspection also found five additional pins that had wear-groove
indications at the shear plane. All seven of these pins were removed
from the bridge structure and were shipped to NDEVC. These pins, labeled
S104 through S110, are now part of NDEVC's Component Specimen Database.
This study primarily focuses on the two cracked pins. The results from
field ultrasonic testing of the two cracked pins, labeled S106 and S108,
are shown in figure 4. The approximate defect profile at the shear plane
is indicated in the figure for each pin. The original sketches were
produced in the field by hand-sketching.
|
Figure
9 - Cracked section results from immersion tank ultrasonic inspection.
|
Immersion
Tank Ultrasonic Testing
Immersion tank ultrasonic testing of the seven hanger pins was performed
at NDEVC. The purpose of this testing was to accurately detect and quantify
crack-like defects that may be present in any or all of the pins.
|
Figure
10 - The image produced by a computed tomography scan clearly shows
that a significant crack exists at the level of the shear plane
in this specimen. |
The
immersion tank ultrasonic examinations were performed using a system
developed by Scanning Systems International Inc. (formerly Infometrics
Inc.). TestProÔ, release 6.0, was the software used. The basic
setup of the scans can be seen in figure 5. The pin was placed on end
while immersed in a water-filled tank. The ultrasonic transducer was
positioned above the pin with its face parallel to the end face of the
pin. A 5-MHz, 12.7-mm (0.5-in) unfocused transducer was used, with a
60-mm (2.35-in) separation between the transducer face and the pin face.
The movement of the transducer for the scanning process was computer-controlled,
allowing the electric motors to systematically move the transducer within
the plane parallel to the exterior face of the pin. This equipment setup
was used for all scans performed in this study.
The system was calibrated with a standard distance-area amplitude block.
This block consists of a 51-mm- (2-in-) diameter stainless steel cylinder
that is 89 mm (3.5 in) tall. A 3.2-mm- (0.125-in-) diameter flat-bottomed
hole (FBH) that is 12.7 mm (0.5 in) deep was present in the center of
one end of the reference block. This reference block was chosen since
it provides a pulse path from transducer to defect that is similar to
that used in hanger pin specimens. This reference block was used to
determine the beam spread and the required threshold levels for various
transducer gain and attenuation configurations. As a result, a defect
location and the extent on a particular plane within the pin (as related
to the transducer location) could be determined.
Initial scans were performed on all seven pins. The pins were scanned
from both exterior faces to provide full coverage within the pin barrel.
This setup allowed for the detection of virtually all reflectors, including
most defects shrouded from direct detection by the shoulder of the pin.
The identification of defects in the shrouded area through the use of
beam spread is critical because this portion of the pin is not directly
visible to the straight-beam ultrasonic transducer.
These
initial scans indicated that only pins S106 and S108 contained cracks
and that the cracks in these pins occurred at the level of the shear
plane. Detailed scans were then performed on these two specimens to
precisely locate the defects. This additional testing was conducted
in two phases. First, the testing apparatus was set up to generally
locate small defects near the perimeter of the pin body, below the shoulder.
Second, the testing apparatus was set up to precisely locate any defect
that occurred within the body of the pin directly under the exterior
face.
The specimen preparation and test setup for the detailed phases of testing
were as follows. First, the end of each pin that was closer to the defect
was milled flat and perpendicular to the body of the pin. This was done
to optimize transmission of the ultrasonic pulses into the pin. In addition,
because the approximate location of the defects had been determined
by the previous scans to be at the level of the shear plane, these further
scans were optimized for a 19-mm- (0.75-in-) deep zone at the level
of the shear plane. Finally, these scans were set to provide a much
finer spatial resolution (0.25 mm by 0.25 mm).
As mentioned previously, the first phase of the detailed testing focused
on the identification of the location of defects near the perimeter
of the body of the pin. These scans used the same gain, attenuation,
and threshold settings as were used for the initial preliminary scans
of all seven pins. Through the use of a calibration block, it was determined
that these settings allowed for the scan to receive a reflection from
a defect at the level of the shear plane, which was 6.4 mm (0.25 in)
outside of the direct path of the transducer. Figure 6 shows the beam
spread of the ultrasonic pulse that allows for these defects to be located.
Accordingly, the procedure could detect a defect that was near the perimeter
of the pin body at the level of the shear plane. However, it must be
noted that crack-like indications at the limits of this range were difficult
to differentiate from wear grooves or corrosion.
The results of these scans are presented in the form of a Cscan image
in figure 7. (Within ultrasonics, there are different ways of looking
at the ultrasonic data, and these are called Ascans, Bscans, and Cscans.)
For reference, the Cscan image of the defects has been superimposed
over a darkened Cscan image of the exterior face of the pin. This allows
for clear identification of the travel path of the ultrasonic pulse.
The outline of the pin barrel is also presented.
The second phase of the detailed investigation of the cracked shear
plane regions was then conducted. The goal of this set of scans was
to accurately locate the defects that lay directly under the exterior
face of the pin. The calibration block was used to determine what settings
were required so that only defects that were at least partially under
the transducer were reported. The Cscan results for these tests are
shown in figure 8. Once again, for reference, the exterior face of the
pin and the pin barrel are presented.
Using the results shown in figures 7 and 8, a schematic representation
of the defects present at the shear plane level of S106 and S108 can
be determined. These results are shown in figure 9. The cracked portion
of the shear plane is shaded in the figure.
Comparison
of Field and Immersion Tank Ultrasonic Results
Comparison between the field ultrasonic results (figure 4) and the immersion
tank ultrasonic results (figure 9) indicates that the two methods provide
similar conclusions as to the extent of cracking at the shear plane
level in these two specimens. In pin S106, the immersion tank findings
indicate that 79 percent of the cross-section is cracked, and the field
ultrasonics indicate that 83 percent of the section is cracked. For
S108, the immersion tank and field ultrasonic findings indicate, respectively,
that 30 and 22 percent of the section is cracked. Due to the automated
nature and the higher spatial resolution of the immersion tank scan,
the results from this portion of the testing are believed to provide
a more refined and more accurate representation of the extent of the
cracking. However, the results from the field ultrasonic testing were
found to be quite accurate, given the logistical challenges present
in the acquisition of field ultrasonic data.
Computed
Tomography
Computed tomography (CT) was also used to inspect pin S106. The CT data
were acquired with a 9-MV LINAC x-ray source and an area-array digital
detector. Figure 10 shows a small portion of the CT results. The images
produced by the CT scans clearly show that a significant crack exists
at the level of the shear plane in this specimen. At this time, a full
reconstruction of the data has not been performed, and therefore, a
quantitative assessment of the size of the crack is not possible. However,
the location, general shape, and general size of the crack correspond
well with the results obtained from the ultrasonic testing.
Conclusions
Field and immersion tank ultrasonic inspection techniques were used
to investigate possible defects within hanger pins removed from an in-service
bridge. The field inspections identified crack-like defects within two
pins. These pins were sent to the FHWA's NDEVC for further testing.
The results from the immersion tank testing correlated well with the
field ultrasonic testing. Both the defect location and defect size findings
show a high level of consistency between the two ultrasonic techniques.
References
1. R.A. Walther and R.D. Gessel. "Ultrasonic Inspection of Bridge
Pin and Hanger Assemblies," Proceedings, Structural Materials Technology,
An NDT Conference, 1996, San Diego, Calif., p. 23-28.
2. Bridge Welding Code, ANSI/AASHTO/AWS D1.5-98, American Association
of State Highway and Transportation Officials, Washington, D.C., 1998.
Benjamin A. Graybeal is a research engineer employed by Wiss,
Janney, Elstner Associates Inc and working at the Federal Highway Administration's
Nondestructive Evaluation Validation Center. He received his master's
degree in structural engineering from Lehigh University and is currently
enrolled in the doctoral program in structural engineering at the University
of Maryland.
R.A.
Walther is a consultant for Wiss, Janney, Elstner Associates Inc.
Glenn
A. Washer is the program manager of the Federal Highway Administration's
Nondestructive Evaluation Validation Center at the Turner-Fairbank Highway
Research Center in McLean, Va. He has a master's degree in civil engineering
from the University of Maryland. He is currently a doctoral candidate
at The Johns Hopkins University Center for Nondestructive Evaluation.
Washer is a licensed professional engineer in Virginia.
Amy
M. Waters of the Lawrence Livermore National Laboratory was instrumental
in the performing of the computed tomography work.
Other Articles in this Issue:
Using Monte Carlo Simulation for Pavement Cost Analysis
ITS Peer-to-Peer Program
Design Evaluation and Model of Attention Demand (DEMAnD): A Tool for In-Vehicle Information System Designers
Studying the Reliability of Bridge Inspection
Ultrasonic Inspection of Bridge Hanger Pins
The Northwest Transportation Technology Exposition
Faster, Easier, Cheaper - Pyrotechnical Anchoring
Practical Research Answers Real-Life Questions
A Nondestructive Impulse Radar Tomography Imaging System for Timber Structures
Strategic Work-Zone Analysis Tools