November/December 2000
Studying the Reliability of Bridge Inspection
by:
Brent M. Phares, Dennis D. Rolander, Benjamin A. Graybeal, and Glenn A.
Washer
In
1967, the Silver Bridge over the Ohio River collapsed during rush hour,
killing 46 people. This catastrophic failure was found to have been
initiated by fracture of an eyebar at a pin connection. This bridge
failure drew attention to the safety of the nation's infrastructure,
and the following year, the National Bridge Inspection Program was initiated,
requiring regular and periodic inspections of all highway bridges. As
a result of the National Bridge Inspection Program, the National Bridge
Inspection Standards (NBIS), which were implemented in 1971, established
the minimum bridge inspection criteria. These standards prescribe how,
with what frequency, and by whom bridge inspection must be completed.
Bridge inspection, in basic terms, is simply an educated assessment
and extrapolation of the condition of a bridge. Bridge inspectors have
a wide variety of tools to help them complete these inspections more
accurately, efficiently, and reliably. However, even though many highly
sophisticated evaluation tools may be available, the most common means
of evaluating a bridge is to simply assess the condition visually.
There are a number of different types of bridge inspections that are
typically completed, each reflecting a different intensity and scope
of inspection. The first inspection completed on a bridge is known as
an "initial inspection." The goal of this inspection is to
establish baseline structural conditions and to identify potential or
known problem areas. The most common type of inspection is what NBIS
calls "routine inspection." The goal of a routine inspection
is to assess the physical and functional condition of a bridge and to
ensure that the bridge continues to satisfy all applicable serviceability
requirements. Alternatively, another NBIS-defined inspection type is
a close-up, hands-on "in-depth inspection" to identify deficiencies
not normally detected during a routine inspection. A "damage inspection"
is similar to an in-depth inspection with the exception that it is an
unscheduled inspection manifested from reported damage to a bridge resulting
from human actions or from the environment. Typically, "special
inspections" are scheduled inspections completed solely to monitor
a known or suspected deficiency.
Statewide bridge inspection programs require the dedication of many
resources. The findings of these inspection programs are heavily dependent
on subjective visual assessment, and the results can have an impact
on the safety and maintenance of a bridge. For these and other reasons,
a comprehensive study was initiated at the Federal Highway Administration's
Nondestructive Evaluation Validation Center (NDEVC) to study the reliability
of visual inspection (VI) of highway bridges. The general objective
of this study was to provide an overall measure of the reliability and
accuracy of routine and in-depth inspections and to study the influence
of human and environmental factors on inspection reliability. Two important
components of this study were a state-of-the-practice survey and a field
investigation.
State-of-the-Practice
Survey
To
ensure that the VI study would benefit bridge owners, a survey was developed
to determine current policies and practices related to bridge inspection.
Three groups were targeted for the survey: State departments of transportation
(DOTs), county DOTs, and inspection contractors. The 50 state DOTs,
the District of Columbia, and Puerto Rico were the primary focus of
the survey. However, in an attempt to understand bridge inspection on
the secondary road system, the 99 counties of Iowa were solicited for
participation. In addition, 15 bridge inspection contractors were contacted
to gain a complete understanding of bridge inspection organizations.
The survey had three discrete sections - each with a different focus.
The first section was developed to determine the composition of a "typical"
inspection team. In particular, questions assessed how many inspectors
are typically used, how frequently licensed professional engineers are
part of the onsite inspection team, and the average experience level
of inspectors. The second section was developed primarily to assess
the influence of administrative requirements on inspection, primarily
VI. These questions dealt with issues related to the size of the bridge
inspection unit, total number of bridges inspected, inspector training
requirements, and vision testing requirements. The final section focused
on the general use of nondestructive evaluation (NDE) technologies.
The goal of this series of questions was to ascertain current NDE usage
as well as current and anticipated needs.
Approximately 72 percent of the surveys were returned, and the general
conclusions are:
- Professional engineers are typically not part of the onsite inspection
team.
- There are no direct vision testing requirements for bridge inspectors.
- Review of inspection reports and data entry are the most common
forms of quality control.
- VI is, by far, the most common form of NDE used in bridge inspection.
Field
Investigation
The
general approach taken in the field investigation was to have a representative
group of practicing bridge inspectors complete a battery of pre-defined
inspection tasks at the NDEVC test bridges. The subject population consisted
entirely of bridge inspectors from state DOTs. In all, 49 inspectors
representing 25 states participated in the study. Inspectors were asked
to complete 10 inspections of the 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. To
ensure that specific conclusions could be drawn regarding VI, the condition
of each of the test bridges was fully characterized by the NDEVC staff
prior to use in the study.
Human
and Environmental Factors Measurement
One
of the most important aspects of this study was the consistent and unbiased
measurement of the independent variables - the human and environmental
factors. Measurement of these factors was essential to establishing
accurate and meaningful cause-effect relationships with the inspection
results.
Measurement of the environmental conditions was completed with relative
ease. Measurements of temperature, humidity, wind speed, noise level,
and light intensity were made at consistent locations during each inspection.
These measurements were made using standard equipment. In addition to
these measurements, the general weather conditions were also recorded
(e.g., rain, cloud cover, etc.) to gain a full understanding of the
environmental conditions at the bridge site during the inspection.
Measuring the human factors proved much more challenging; thus, four
independent tools were developed for this purpose. These tools included
written questionnaires, physical measurements, oral interviews, and
first-hand observations.
All participating inspectors were asked to complete two voluntary questionnaires
intended to assess many "non-measurable" human factors. The
intent of these questionnaires was to give pseudo-quantitative evaluations
of the inspectors' physical and psychological attributes. Typically,
this included general physical health, general mental health, perception
of dangerous situations, training, etc. In addition, information was
collected regarding the inspectors' perception of the current state
of bridge inspection. One questionnaire was administered before the
inspectors completed any bridge inspections while the second was given
after completing all inspection tasks. This "before and after"
system allowed gross changes in attitude and condition to be recorded.
Three direct physical measurements were made of attributes thought to
possibly influence VI reliability. These measurements related specifically
to the inspectors' vision. During routine inspections, inspectors must
often evaluate the condition of a bridge from a distance. In light of
this, inspectors were asked to take a distance visual acuity test. This
test is similar to vision tests commonly given in a doctor's office
and is a reliable tool for quickly measuring distance visual acuity.
Conversely, during in-depth inspections, inspectors generally make their
assessments from a closer range and must generally rely on their near
visual acuity. Thus, a near visual acuity test was administered. To
fully describe vision attributes, inspectors were also given a color
vision test to assess any color vision deficiencies. The goal of this
test is to orient 16 colored caps so that adjacent caps are closest
in color to one another. Through this type of testing, the presence
of color vision deficiencies can rapidly be identified.
Human factors were also measured immediately before and after each inspection
was completed. These assessments were made through an interview between
the inspector and the NDEVC staff. Many of the questions asked during
these interviews related to the inspector's physical and psychological
condition immediately before and after each inspection. Other questions
were developed to assess an inspector's perception of various bridge
characteristics - maintenance, complexity, etc. By using this type of
measurement tool, a greater understanding of what influenced each inspection
could be gained.
The final method of collecting human factor data was through first-hand
observation. A member of the NDEVC staff closely monitored and documented
each inspector's activities and behavior during each inspection task.
Information was collected about how the inspection was performed, where
the inspector's attention was focused, and what tools were used.
|
An
inspector takes the near visual acuity test. |
Inspection
Tasks
Inspectors were asked to complete a total of 10 inspection tasks on
a wide variety of bridge types. The inspection tasks included both routine
and in-depth inspections of concrete and steel bridges of a variety
of construction types.
A
total of seven routine inspection tasks were completed on both in-service
and decommissioned bridges. During these tasks, inspectors were asked
to provide condition ratings for the deck, superstructure, and substructure
of each bridge. Condition ratings are single numbers for the deck, superstructure,
and substructure describing the overall bridge condition, considering
both the severity of deterioration and the extent to which it is distributed
throughout the bridge. In addition to the condition ratings, inspectors
were asked to prepare written documentation (i.e., inspection notes)
in accordance with their normal practice to supplement the condition
ratings.
|
To
test color vision, inspectors were required to align 16 colored
disks so that the closest colors were adjacent to one another. |
Inspectors
were also asked to complete three in-depth inspections of portions of
two bridge superstructures and one reinforced concrete deck. Where needed,
special equipment, such as a boom lift or ladder, was provided to allow
inspectors full access to the bridge components.
Concluding
Remarks
This NDE reliability study is an effective means of evaluating the overall
reliability of VI as it is currently practiced in bridge inspection.
By collecting data related to inspector qualities and the inspection
environment, specific conclusions can be drawn regarding their influence.
It is envisioned that the results of this study will help define the
accuracy of normal bridge inspection practices and will help bridge
owners allocate and use bridge inspection resources more effectively.
These
inspectors are conducting an in-depth inspection of a bridge deck. |
This
inspector uses a boom lift to make an up-close, in-depth inspection
of a bridge superstructure. |
This
is the first in a planned series of articles on the VI study. Subsequent
articles will focus on the data collected during the study and the resulting
conclusions.
Dr. Brent M. Phares is a research engineer for Wiss, Janney, Elstner
Associates Inc., a consultant to the Infrastructure and Inspection Management
Team of the Office of Research, Development, and Technology at the Federal
Highway Administration. 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 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's Center for Nondestructive Evaluation.
Washer is a licensed professional engineer in Virginia.
The
authors gratefully thank the participating inspectors for their valuable
input during this study. Forty-two states and the District of Columbia
participated in the survey and field investigation: 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 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