Risk-Based Countermeasure Design

1
Risk-Based Countermeasure Design

• Dave Pearson
• Stuart Stein, P.E.
• GKY and Associates, Inc.
• J. Sterling Jones, P.E.
• Federal Highway Administration

2
Source

• FHWA Publication No. FHWA-RD-92-030
• Strategies for Managing Unknown Bridge
  Foundations
• Chapter 3

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The Evolved Objective

• To provide a rational and practical tool for
  finding scour risky bridges using data already
  available in the NBI database.
• To provide a tool for examining the economic
  feasibility of alternate scour countermeasures.

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Methodology
Risk ()
Bridge Situation Scour Criticality
(113) OR Route Class (26) Substructure
Condition (60) Channel Protection (61) Waterway
Adequacy (71)
Failure Probability Baysian Posterior Failure
Probability
Expected Losses
Economic Factors Length (49)Width (52)
Classification (26)Detour Length (19)Average
Daily Traffic (29)
Revised Probability Accounts for age if outside
binomial expectations
Bridge Age (27)
( x ) NBI Item
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The Full Equation
Cost of lost time
Cost of alternate operation
Cost to rebuild
Probability of Failure
Structural factors
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Detour Duration
7
Cost Multiplier for Early Replacement
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HYRISK Computer Program

• Operates under Windows 95 or later
• Imports raw NBI data directly
• Implements the methodology described
• Allows selecting a sub-set of bridges for
  analysis
• Computes quickly on large sets of bridges
• Accommodates user-specified economic criteria
• Makes logical, user-specified adjustments for
  missing or out-of-range NBI data

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HYRISK Computer Program
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Establish Analysis Parameters

• Set default values for missing or out-of-range
  NBI data
• Save settings in a recallable scenario file

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Make Necessary Assumptions

• Current Year
• Computational parameters
• Economic factors
• Note Assumptions used for all bridges analyzed.

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Select Bridges

• Limits analysis to targeted characteristics

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Set Step Function Values

• Detour duration and emergency rebuilding cost
  multiplier are based on ADT

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Run the Analysis
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Study Analysis Results

• Examine analysis results
• Rectify with underlying data
• Note any exceptions
• Navigate to any or all record(s)

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Run Scour Countermeasure Calculator

• Examine analysis results
• Rectify with underlying data
• Note any exceptions
• Navigate to any or all record(s)

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Scour Countermeasure Calculator

• Assists in evaluating the risk and economic
  feasibility of proposed or existing
  countermeasures
• Accounts for loss-of-life costs
• Accounts for a finite service life
• Accounts for the time value of resources
• Allows adjustment of any calculation parameter

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Step 1

• Describe the bridge

• Applies to a specific bridge
• Uses NBI data as a default
• Data may be modified or new data added
• Assumptions may be modified

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Step 2

• Set cost multiplier

• Uses step function values previously set

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Step 3

• Establish economic assumptions

• Loss-of-life cost assumptions
• Defaults based on historical statistics
• Discount rate
• Costs

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Step 4

• Specify an annual failure probability

• Use HYRISK-estimated Pf
• OR
• A known better number

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Step 5

• Establish a life expectancy for the bridge

• Be reasonable Is it reasonable to protect
  against the 100 yr flood when the bridge is
  scheduled to be replaced in 5?

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Bridge Life Expectancy

• Where
• PL Probability of failure during expected life
• PA Annual probability of failure
• L Expected life, years

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Step 6

• Review the economic risks of failure

• Look at present value of future costs (without
  inflation)
• Quantify benefits of protection
• Evaluate the impact of loss-of-life

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Present Value of Replacement Costs

• Where
• CE Present value of future cost
• CR Current rebuilding cost
• i Discount rate
• L Remaining life expectancy of bridge

Discount rate is the time value of money over
and above inflation
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Step 6

• Review the economic risks of failure

• Look at present value of future costs
• Quantify benefits of protection
• Evaluate the impact of loss-of-life

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Step 7

• Specify Countermeasures

• Net benefit accounts for countermeasure costs
• Countermeasures need not be construction
• Doing nothing is always an option

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Compare Countermeasures
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Estimating the Risk of Scour Vulnerable Bridges

• Dave Pearson
• Stuart Stein, P.E.
• GKY and Associates, Inc.
• J. Sterling Jones, P.E.
• Federal Highway Administration