Featuring developments in Federal highway policies, programs, and research and technology.
|This magazine is an archived publication and may contain dated technical, contact, and link information.|
|Federal Highway Administration > Publications > Public Roads > Vol. 64 · No. 3 > Studying the Reliability of Bridge Inspection|
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.
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:
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.
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.
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.
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.
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.
Page Owner: Office of Corporate Research, Technology, and Innovation Management
Scheduled Update: Archive - No Update
Technical Issues: TFHRC.WebMaster@dot.gov