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Preservation Role in Risk-Based
Inspections
Glenn A. Washer, PhD
University of Missouri
Columbia, MO
November 13, 2013
Midwest Bridge Preservation Partnership
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NCHRP 12-82
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This investigation was sponsored by TRB under the
NCHRP Program. Data reported are work in
progress. The contents of this presentation has not
been reviewed by the project panel or NCHRP, nor
do they constitute a standard, specification, or
regulation.
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NCHRP 12-82 Goals
• Goal: Improve the safety and reliability of bridges – focusing inspection efforts where most needed
• Optimize the use of resources – Better match inspection requirements to
inspection needs
– Develop a rational process for assessing inspection needs using reliability theories
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Agenda
• What is Risk-Based Inspection
• How does Preservation fit in?
• Conclusion
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Risk-Based Bridge Inspection
• Inspections that consider
– The reliability of bridge elements • Likelihood of deterioration and damage
• Condition, design, materials and loading
– The consequences of that damage • Minor serviceability issues, safety issue?
• Inspection interval and scope
– Match inspection requirements with inspection needs for a bridge
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Motivation
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Time to corrosion initiation for RC Typical lifetime performance
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Example Guidelines
• ASME (2007). Inspection Planning Using Risk-Based Methods, The American Society of Mechanical Engineers: 92.
• API (2002). "Risk-Based Inspection, API Recommended Practice 580." 45. (updated)
• API (2008). "Risk-Based Inspection Technology, API Recommended Practice 581.“
• (2003). Surveys Using Risk-Based Inspection for the Offshore Industry Houston, TX, American Bureau of Shipping: 62.
• INTERIM ADVICE NOTE 148/12, Risk Based Principal Inspection Intervals
• Transport Wales Framework Lot 5,Task Order 5/2, BD 63/07 Risk Based Inspections Guidance Note February 2010
• NRC docs
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Definitions
• Reliability: Ability of an item, component or
system to operate safely under designated
operating conditions for a designated period
of time or number of cycles.
• Risk: Combination of the probability of an
event and its consequence.
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Risk – Based Approaches
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R = POF x C
R = Likelihood x C
R = Frequency x C
R = Occurrence x C
Consequences:
• Economic
• Environmental
• Safety
Requires time interval
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Reliability-Based Inspection
(RBI) • What can go wrong?
– Identify damage modes for elements
– Deterioration mechanisms
• How likely is it? • Categorization based on reliability characteristics of
bridge elements
• Based on expert judgment and expert elicitations • Past experience
• Analysis of existing or potential damage modes
• Deterioration data if available (and relevant)
• Preservation activities
• What are the consequences? – How important is it?
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Factors in RBI
• Damage Modes and deterioration mechanisms
– Attributes affecting likelihood
• Occurrence factor (POF)
• Consequences
– Experience, redundancy, situation (ADT, traffic speed), analysis, etc.
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Damage Modes
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Damage Mode: Section Loss
Det. Mechanism: Corrosion Damage Mode: Impact?
Det. Mechanism: Impact
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Identifying Damage Modes
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http://thewritepractice.com/emergency-your-creativity-is-dying/
Cause of death Likelihood (%)
Heart attack ●●●●●○○○○○
Hit by car ●●●○○○○○○○
Murdered ●●○○○○○○○○
Brain Aneurism ○○○○○○○○○○
Lightning ○○○○○○○○○○
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Concept - Likelihood
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Concept - Likelihood
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Bad attributes Unique
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Where does a bridge fall on
the distribution?
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25.221.618.014.410.87.23.60.0
80
70
60
50
40
30
20
10
0
CR=4
Fre
qu
en
cy
Loc 1.544
Scale 0.7701
N 357
Histogram of CR=4Lognormal Yr. in Condition Rating
Years
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Concept - Consequences
• Water = low consequence
• Nuclear waste = Severe
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Consequences…
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• Ex. Multi-girder 3 span PS vs. pin and hanger in two-girder (fracture critical) bridge
• Low, moderate, high and severe
• Design characteristics, scenario, documented experience, calculation
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Risk Matrix
• Plot values of likelihood and consequence
• Components in the top right corner are “high risk”
• High likelihood may not mean high risk, if consequence is small
• High consequence may not be high risk, if the likelihood is low
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Preservation Effect
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Conditio
n
Time (yrs)
Preservation
Effect
Failure
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Preservation Effect
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Conditio
n
Time (yrs)
Preservation
Effect
Failure
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Example
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Potential Benefits of RBI • Better, more effective and purposeful inspections
– Inspection plan (scope and interval) supported by engineering assessment by RAP
• Vs. Calendar-based inspection strategy
– Rational inspection strategies • Flexible intervals based on need and engineering analysis
• Allocate resources more effectively
– Focus inspections resources where most needed
• Value-added to preservation activities
• Improved bridge reliability and safety
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Preservation Role in RBI
• Preservation can reduce the POF and
thereby reduce the risk
– Reduce unnecessary inspections
– Focus inspection where most needed
– Allocate resources more effectively
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Questions
• Questions?
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