March/April 2001
Reliability
of Visual Bridge Inspection
by
Brent M. Phares, Dennis D. Rolander, Benjamin A. Graybeal, and Glenn
A. Washer
![photo of inspectors using a man-lift](images/img015.jpg) |
Inspectors, using a man-lift, conduct an in-depth inspection
at the Route 1 bridge. Note the inspectors on the ground in
the background, performing a routine inspection.
|
Visual inspection techniques are the primary methods
used to evaluate the condition of the majority of the nation's highway
bridges. These subjective assessments may have a significant impact
on the safety and maintenance of a bridge. However, until a study
was performed at the Federal Highway Administration's Nondestructive
Evaluation Validation Center (NDEVC), a complete study of the reliability
of these inspections had not been undertaken.
This article is the second of two on the visual inspection
study conducted at NDEVC and describes the results of this recently
completed study. The first article, "Studying the Reliability of Bridge
Inspection," appeared in the November/December 2000 issue of Public
Roads (Vol. 64, No. 3) and describes how the study was conducted.
The general approach taken in the field investigation
was to have a representative group of practicing bridge inspectors
complete a battery of predefined inspection tasks at NDEVC test bridges.
The subject population consisted of 49 bridge inspectors from 25 state
departments of transportation. These inspectors were asked to complete
seven routine inspections and three in-depth inspections at NDEVC
test bridges while being monitored by NDEVC staff. Information about
the inspector and the inspection environment was collected to assess
their influence on inspection reliability.
Study Results
Routine Bridge Inspection Results
One set of information generated during a routine inspection is a
series of "condition ratings" assigned to the bridge deck, superstructure,
and substructure. These condition ratings give an overall measure
of the condition of a bridge by considering the severity of deterioration
in the bridge and the extent to which it is distributed throughout
each component. The ratings assigned to each element are based on
a standard set of definitions associated with numerical ratings between
zero (failed) and nine (excellent condition). Inspection agencies
can use these ratings to track deterioration and to allocate maintenance
funds.
Table
1 - Routine Inspection Condition Rating Statistical Information |
Bridge
|
Element
|
Average
|
Standard
Deviation
|
Mini-
mum
|
Maxi-
mum
|
Mode
|
N
|
Ref-
erence
Rating
|
B521
|
Superstructure
Substructure
Deck
|
5.9
6.1
5.8
|
0.78
0.79
0.81
|
4
3
3
|
8
7
7
|
6
6
6
|
49
49
49
|
5
6
5
|
B101A
|
Superstructure
Substructure
Deck
|
4.2
4.3
4.9
|
0.77
0.76
0.94
|
2
3
2
|
6
6
7
|
4
4
5
|
49
49
48
|
4
4
4
|
B111A
|
Superstructure
Substructure
Deck
|
4.6
5.5
5.2
|
0.86
0.77
0.92
|
2
4
3
|
7
7
7
|
5
5,6
6
|
49
48
49
|
4
5
4
|
B543
|
Superstructure
Substructure
Deck
|
5.3
6.1
4.8
|
0.88
0.89
0.94
|
4
4
2
|
7
8
6
|
5
6
5
|
44
47
48
|
5
6
5
|
B544
|
Superstructure
Substructure
Deck
|
5.8
5.3
4.5
|
0.72
0.83
0.74
|
4
3
3
|
7
7
6
|
6
5
5
|
48
47
48
|
6
6
4
|
Route
1
|
Superstructure
Substructure
Deck
|
6.7
7.2
7.1
|
0.66
0.57
0.53
|
5
6
6
|
8
8
8
|
7
7
7
|
49
49
49
|
7
8
7
|
Van
Buren
|
Superstructure
Substructure
Deck
|
6.8
6.7
5.8
|
0.64
0.62
0.92
|
6
6
4
|
9
8
7
|
7
7
5
|
24
24
24
|
7
8
7
|
The condition rating results for this element did not pass C2
test for goodness-of-fit.
This task was performed by a team of two inspectors who
could collaborate to reach their findings.
|
The condition rating results for the seven routine
inspection tasks are given in table 1. This table summarizes the average,
standard deviation, maximum, and minimum condition rating results
from the participating inspectors. A reference rating is also given
in the table. This is the condition rating given to each element following
thorough assessments by NDEVC inspectors. Further analysis of these
data revealed that the condition ratings were normally distributed
in all but two instances.
|
Figure
1 - Bridge B111A experimental and theoretical condition rating
distributions. |
Note that table 1 includes the inspection results
generated during the inspection of the Van Buren Road bridge. For
this inspection, inspectors were allowed to work in two-person teams
and to collaborate in determining their inspection results. The following
discussion will exclude these results and will focus on condition
ratings assigned by individual inspectors, except where otherwise
noted.
Table
2 - Results of the t-Test at Five-Percent Significance Level for
the Average Condition Ratings |
|
Bridge
|
Element |
B521
|
B101A
|
B111A
|
B543
|
B544
|
Route
1
|
Deck |
Fail
|
Fail
|
Fail
|
Pass
|
Fail
|
Pass
|
Superstructure |
Fail
|
Fail
|
Fail
|
Fail
|
Fail
|
Fail
|
Substructure |
Pass
|
Fail
|
Fail
|
Pass
|
Fail
|
Fail
|
Pass
= average inspector Condition Rating and reference Condition Rating
can not be considered statistically different.
Fail = average inspector Condition Rating and reference Condition
Rating are statistically different. |
Because the reference ratings given in table 1 and
the inspector-assigned ratings were frequently different, a statistical
analysis was performed. This analysis examined whether or not the
two ratings were statistically different by applying what is known
as the t-test. In a t-test, "fail" indicates that the two ratings
are different, and "pass" indicates that the two ratings are the same
from a statistical standpoint. From table 2, it is apparent that in
most cases, the average inspector condition ratings are statistically
different from the reference ratings.
![photo of inspectors conducting a delamination survey](images/dcp_0618.jpg) |
These
inspectors are conducting a delamination survey of the Van Buren
Road bridge deck. |
The distribution of sample condition ratings was found
to be normal; an example of which is shown in figure 1, indicating
that the sample condition ratings can be used to predict how the general
population of bridge inspectors would perform. From these analyses,
it was found that 95 percent of condition ratings would be assigned
over a distribution of five discrete ratings or �from the mean.
Furthermore, only 68 percent would be distributed over three discrete
ratings or �from the mean.
During the inspection of the Van Buren Road bridge,
the inspectors were asked to use their respective state inspection
forms, and several teams also submitted element-level inspection data.
Element-level inspections rely on specific definitions of elements
to classify the bridge structure and to describe any observed deterioration
using the defined condition states. One of the most common element-level
inspection systems uses the Pontis bridge management system, but other
systems also exist. Fourteen inspection teams reported results consistent
with the commonly recognized (CoRe) elements. The CoRe element system
is a standardized set of descriptions of common bridge elements and
conditions.
![photo of inspector examining bridge superstructure](images/dcp_0636.jpg) |
An
Inspector examines the bridge superstructure. |
The major deck, superstructure, and substructure elements
were used very consistently by each of the teams reporting element-level
data. The "other superstructure/substructure" elements were recorded
much less consistently. As an example, there was significant confusion
in the use of CoRe elements for the joints with three different definitions
being used to describe the same joint. Another example is bridge railings,
which were defined by three different elements, with only three out
of 14 teams defining the element correctly.
As expected, the greatest variability in the element-level
inspection data occurred with the non-CoRe elements. For example,
five teams used five different elements to track wingwall information.
Another four teams used five different elements to track slope protection.
![table of inspection deficiency detection results](images/vistab3.gif)
Although not necessarily required, inspection notes
were often generated during an inspection to supplement condition
ratings and/or condition state assignments. As such, the use of inspection
notes was investigated. Although the inspectors participating in this
study may have taken a large number of inspection notes during each
of the tasks, this analysis focused only on a small set of notes deemed
to be of principal importance. These notes generally described poor
to very poor condition elements. Although not described here in great
detail, when analyzed, it was found that most inspectors made note
of the severe deficiencies, but typically, at least one-fifth of the
inspectors did not note a specific condition. It should be pointed
out that the level of deterioration precipitating each of these notes
is so severe that one could expect a nearly 100-percent note-taking
rate.
One other way to document inspection findings is through
the use of photographs. During one of the inspections, the inspectors
were provided with a camera and asked to document their findings.
The photographs could generally be grouped into 18 different types.
Of these 18 photographs, 13 were identified as needed to fully document
the bridge conditions. On average, each inspector took just over seven
photographs with a maximum of 19 and a minimum of one. The wide variability
in the number and specific types of photographs taken illustrates
the differences in the documentation policies of the agencies as to
what constitutes "full" documentation for a routine inspection.
One important aspect of the experimental study was
the quantitative measurement of human and environmental factors thought
to potentially have a relationship with inspection results. The quantitative
measurements were made through a series of written questionnaires,
oral interviews, environmental measurements, and first-hand observations.
The relationship of these factors to the inspection results was then
studied. A multivariate, nonlinear analysis was required to find correlation
between these factors and the inspection results. The analysis revealed
that several factors appear to have a relationship with the inspection
results. Specifically, the inspector's fear of traffic; near visual
acuity; color vision attributes; formal bridge inspection training;
and the inspector's perception of the bridge's maintenance, accessibility,
and complexity were found to have a consistent relationship with inspection
results.
In-Depth Inspection of Steel Superstructure Bridges
The in-depth inspection tasks of the superstructures of two steel
bridges performed during this study were intended to provide insight
into the accuracy and reliability of close-up, hands-on inspection
performed by bridge inspectors. As the goal of this type of inspection
is the specific identification of global and localized deficiencies,
the accuracy and reliability were studied in the context of correctly
noting the presence of known deficiencies.
During the in-depth inspection of bridge B544, inspectors
were asked to inspect approximately one-fifth of the superstructure
of this riveted plate girder bridge. To access the bridge, inspectors
were provided with a ladder and a man-lift. Of the 49 inspectors participating
in the study, 42 completed this inspection.
Two basic classes of deficiencies are present in bridge
B544. First, there are general, recurring deficiencies: paint system
failure, moderate to severe corrosion, and extensive corrosion and
section loss of rivet heads. Second, local deficiencies also exist:
an implanted crack indication at the root of a tack weld, a missing
rivet head, impact damage at two locations, and an abnormal rocker
bearing rotation.
Table 3 summarizes the defect-detection results for
the notable deficiencies. These data show that the inspectors reported
the general, recurring deficiencies with a relatively high frequency,
however, a much lower percentage of inspectors noted the local deficiencies.
For example, all inspectors reported the paint system failure, which
is obvious throughout the structure. However, only half of the inspectors
noted bearing misalignment, and only three inspectors noted a crack
indication.
![graphic of frequency of delamination cells during inspection](images/colormap.jpg) |
Figure
2 - Frequency of delamination cells during inspection of the Van
Buren Road bridge deck. |
The second in-depth inspection of a steel superstructure
bridge was performed at the Route 1 bridge. The inspectors were asked
to inspect a single bay of one span of the bridge. As with bridge
B544, the inspectors were allowed to use a man-lift to gain access
to the superstructure during the inspection. The Route 1 bridge is
a medium-span bridge with 1.83-meter-deep welded plate girders. The
superstructure framing consists of welded transverse and longitudinal
stiffeners, bolted angle diaphragms, bolted and welded flange transitions,
and a lateral bracing system of angle and T-members bolted to lateral
gusset plates welded to the girder web.
The Route 1 bridge has deficiencies in three general
categories: general deficiencies, welded connection deficiencies,
and bolted connection deficiencies. Obviously, the welded and bolted
connection deficiencies pertain to those specific connection types,
and the general deficiencies include all other deficiencies. Specifically,
the general deficiencies are paint system failure, corrosion, member
distortions, and fabrication errors. The welded connection deficiencies
consist of crack indications that occur in or close to a weld. In
the Route 1 bridge, there were four recurring locations that were
likely to produce welded connection deficiencies. These locations
had seven crack indications. Three bolted connection deficiencies
occurred at cross-frame-to-vertical stiffener connections in the form
of bolts with nuts at least four millimeters removed from the plate
that they were to bear against.
Table 3 also summarizes the deficiency-detection results
for the Route 1 bridge. In total, the overall accuracy rate for correctly
identifying crack indications was only 3.9 percent. In addition, there
was a false-positive rate of 0.6 percent for identifying non-cracked
welds as having crack indications. With respect to the bolted connection
deficiencies, the overall accuracy rate was 24 percent with a false-positive
rate of 0.5 percent. As with the results from the in-depth inspection
of bridge B544, these data indicate that a far greater percentage
of inspectors identify the general deficiencies than the local deficiencies.
In fact, deficiencies such as crack indications were correctly identified
by less than 20 percent of the inspectors. More than half of the inspectors
noted more general conditions such as corrosion and paint failure.
![photo of two inspectors using a man-lift](images/img127.jpg) |
Using
a man-lift, these two inspectors perform an in-depth inspection
of bridge B544. |
While each inspector was performing the inspection
of the Route 1 bridge, NDEVC staff noted how the task was performed
and what specific items were inspected. This information was used
to make a pseudo-quantitative measure of the thoroughness of each
inspector with respect to the inspection of welded connections. To
accomplish this, four parts of the test bridge were considered based
on the locations that were likely places for crack indications to
occur. Inspectors were assigned rating points contingent on the thoroughness
of the inspection of these areas. This rating system allows each inspector
to achieve a rating between zero and 10 based on the overall thoroughness
of the weld inspection.
The inspector thoroughness ratings were used to classify
the inspectors into profile groups. The groups were defined as those
inspectors who received a score of eight to 10, those who received
a score of five to seven, and those who received a score of zero to
four. Forty-five percent of the inspectors earned a rating of eight
or higher. These inspectors could be considered to have completed
a thorough in-depth inspection of the superstructure. Of the inspectors
who correctly identified a crack indication, 86 percent were from
this group. Eighteen percent of the inspectors earned a rating of
between five and seven, and thus, they are considered to have completed
an in-depth inspection on part of the structure. Fourteen percent
of the inspectors who correctly identified a crack indication were
from this group. Finally, 36 percent of the inspectors earned a rating
between zero and four; these inspectors can be considered to have
performed an incomplete in-depth inspection. None of the inspectors
in this group correctly identified a crack indication.
Table 4 shows the results by profile groups for a
number of factors. These factors are summarized because they are correlated
with the inspectors in the three inspector profile categories. Various
trends in the table are evident. Specifically, the inspectors who
earned the higher profile ratings tended to take longer to complete
the inspection, were generally more mentally focused, and were more
comfortable than average when performing the inspection. These inspectors
were also more likely to use a flashlight, to expect fatigue-related
deficiencies, and to be closer to the welds that they were inspecting.
The converse is true for each of these factors for the inspectors
who earned the lower inspection profile ratings.
Bridge Deck Delamination Assessments Using Mechanical
Sounding
A delamination assessment was conducted on the two southern spans
of the Van Buren Road bridge. This assessment was conducted by teams
of two inspectors using only visual inspection techniques, including
mechanical sounding. The Van Buren Road bridge has a 175-millimeter-thick
concrete deck that has significant delaminations with very few visible
indications that those deficiencies exist.
Of the 22 teams of inspectors completing the assessment,
20 provided maps of the delaminations that they found, and some of
the teams also provided a numerical estimate of the amount of delaminated
area. The two remaining teams provided only a numerical estimate.
In total, five teams were within five percentage points of the delamination
percentage determined by NDEVC - 19 percent of the deck area. Fourteen
teams were within 10 percentage points, and all of the teams were
within 20 percentage points. Three teams indicated that the deck was
less than 5 percent delaminated. Although these teams fall within
20 percentage points of the correct delamination percentage, it is
obvious that these teams failed to detect large areas of the deck
that were delaminated.
The relationship between areas of the bridge deck
that the inspectors indicated to be delaminated and the areas found
to be delaminated by NDEVC is also indicative of the accuracy of this
type of deck inspection. Figure 2 provides the delamination results
for the 20 teams that produced delamination maps. This figure presents
the locations where various numbers of teams indicated the presence
of a delamination.
As can be seen in this figure, most teams performed
relatively poorly at locating individual delaminations. In total,
69 percent of the deck was indicated to be delaminated by at least
one inspection team. In addition, only approximately 1 percent of
the deck was indicated as delaminated by at least 15 teams, and no
areas were indicated as delaminated by all inspection teams. By examining
the shading levels, the consensus of five or more teams shows the
best correlation with the delamination area found by NDEVC. The consensus
of five or more teams showed 21 percent of the deck delaminated compared
to 19 percent determined by NDEVC.
Concluding Remarks
Although significant advances have been made in the development of
nondestructive evaluation technologies, visual inspection is still
the predominant tool used to assess bridge conditions. However, this
study shows that there are aspects of bridge inspection that need
significant improvement. For routine inspections, condition ratings,
element-level inspection results, inspection notes, and photographs
are used with significant variability. Of greatest importance is the
amount of variability found in the assignment of condition ratings.
For in-depth inspections, it appears that when an in-depth inspection
is prescribed, the inspection may not yield any findings beyond those
that could be noted during a routine inspection. The results of the
deck-delamination survey conducted during this investigation indicate
that this type of inspection does not consistently provide accurate
results.
Brent M. Phares, Ph.D., is a research engineer
for Wiss, Janney, Elstner Associates Inc., a consultant to the Infrastructure
and Inspection Management Team in the Office of Infrastructure Research
and Development at the Federal Highway Administration's Turner-Fairbank
Highway Research Center in McLean, Va. He received his doctorate in
structural engineering from Iowa State University.
Dennis D. Rolander is a principal research
engineer for Wiss, Janney, Elstner Associates Inc. He received his
master's degree in structural engineering from North Carolina State
University.
Benjamin A. Graybeal is a research engineer
for Wiss, Janney, Elstner Associates Inc. He received his master's
degree in structural engineering from Lehigh University.
Glenn A. Washer, Ph.D., 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 and a doctorate from The Johns Hopkins University.
Washer is a licensed professional engineer in Virginia.
The authors thank the inspectors who participated
in the field portion of the visual inspection study; the input of
the inspectors was invaluable. In addition, the authors gratefully
acknowledge the contributions to the study made by Alabama, Alaska,
Arizona, California, Colorado, Delaware, District of Columbia, Florida,
Georgia, Hawaii, Illinois, Indiana, Kansas, Kentucky, Louisiana, Maine,
Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri,
New Hampshire, New Jersey, New Mexico, New York, North Carolina, North
Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South
Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia,
Washington, Wisconsin, and Wyoming.
Other
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DOT's Comprehensive Truck Size and Weight Study — A Summary
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FORETELL
— Finally, someone is doing something about the weather!
Steel
Fabrication Technologies Observed in Japan and Europe
Reliability
of Visual Bridge Inspection
For the Common Good: The 85th
Anniversary of a Historic Partnership
Telecommunications
— Getting More for Your Money
Celebrating
National Transportation Week, May 13-19