Bridge Study Analyzes Accuracy of Visual Inspections
For all the sophisticated technology employed in bridge design and construction
today, the maintenance and preservation of bridges still depends largely
on regular visual inspection of the structures. The visual inspection
method, in fact, is the predominant nondestructive evaluation technique
used for bridge inspections in the United States. Beginning in 1998,
the Federal Highway Administration's (FHWA) Nondestructive Evaluation
Validation Center undertook a comprehensive study to examine the reliability
of this method for highway bridges. The study was designed to measure
the accuracy and reliability of both routine and indepth inspections,
study the influence of key factors that might affect bridge inspector
performance, and examine differences in State inspection procedures
and reporting styles. The results have shown that the methods used and
data collected in routine inspections can vary considerably from State
to State.
The study
consisted of a review of existing literature on the topic, a survey
of bridge inspection agencies, and a series of performance trials involving
49 State department of transportation bridge inspectors. The performance
trials were conducted on bridges in Northern Virginia and south central
Pennsylvania. Researchers collected data on the results of the inspections,
the characteristics of the inspectors, and the inspection environment.
The primary data used to evaluate the routine inspections were the National
Bridge Inspection Standards Condition Ratings assigned by the inspectors
to the primary bridge components (deck, superstructure, and substructure).
The condition ratings, which range from 0 to 9, describe both the degree
of bridge deterioration and the extent to which it is distributed throughout
the structure's components. The primary data used to evaluate the indepth
inspections were the inspectors' field notes, which summarize specific
deficiencies identified in the bridge.
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As
part of FHWA's visual inspection performance trials, inspectors
work together to perform a deck delamination survey (left above),
while another inspector measures joint seperation during a routine
inspection (right above). |
The study
showed that routine inspections are completed with significant variability,
particularly with respect to the assignment of condition ratings. During
the performance trials, on average four or five different condition
rating values were assigned to the bridges' primary components. The
frequency with which field notes are taken also varies considerably
from State to State. Inadequacies with the indepth inspections were
noted as well-specifically, most of the inspectors did not make note
of the types of defects, such as weld cracks, that this type of inspection
is designed to identify. Factors that may affect indepth inspection
results include the structure's complexity and accessibility, the inspector's
comfort with the working height and access equipment used, and the inspector's
visual acuity.
Recommendations made by the study for improving the accuracy of visual
inspections include:
- Revising the condition rating system to increase accuracy and reliability;
- Increasing the training of inspectors with regard to frequently
reoccurring types of defects that they are likely to encounter and
methods that would allow them to more easily identify these faults;
- Examining further the types and sizes of specific defects that are
likely to be identified during an indepth inspection;
- Placing greater emphasis during the design phase of bridges on building
a structure that is accessible and can be more easily inspected; and
- Performing more research to determine if setting minimum vision
standards for inspectors would benefit the inspection process.
As these recommendations are reviewed by State highway agencies, contractors,
and others, "I think we will see some changes in bridge inspections
in the next 5 years," says Brent Phares of FHWA's Nondestructive
Evaluation Validation Center. "I think we will also see some major
revisions to the National Bridge Inspection Standards."
For more information or to obtain a copy of the study, contact Brent Phares
at 202-493-3121 (fax: 202-493-3126; email: brent.phares@fhwa.dot.gov).