Transportation Research Board 2002 Annual Meeting Presentations from Workshop on Plan of Action for Scour Critical Bridges
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Slide 1
RiskBased Countermeasure Design
Dave Pearson Stuart Stein, P.E. GKY and Associates, Inc.
J. Sterling Jones, P.E. Federal Highway Administration
Slide 2
Source

 FHWA Publication No. FHWARD92030
 "Strategies for Managing Unknown Bridge Foundations"
 Chapter 3

Slide 3
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.
Slide 4
Methodology
Slide 5
The Full Equation
K  Structural Factors P  Probability of Failure EC_{1}WL  Cost to Rebuild C_{2}DAd  Cost of Alternate Operation C_{3}O  Cost of Lost Time
Slide 6
Detour Duration
Slide 7
Cost Multiplier for Early Replacement
Slide 8  9
HYRISK Computer Program
 Operates under Windows 95 or later
 Imports "raw" NBI data directly
 Implements the methodology described
 Allows selecting a subset of bridges for analysis
 Computes quickly on large sets of bridges
 Accommodates userspecified economic criteria
 Makes logical, userspecified adjustments for missing or outofrange
NBI data


Slide 10
Establish Analysis Parameters
 Set default values for missing or outofrange NBI data
 Save settings in a recallable scenario file
Slide 11
Make Necessary Assumptions
Slide 12
Select Bridges
 Limits analysis to targeted characteristics
Slide 13
Set Step Function Values
 Detour duration and emergency rebuilding cost multiplier are based on ADT
Slide 14
Run the Analysis
Slide 15
Study Analysis Results
 Examine analysis results
 Rectify with underlying data
 Note any "exceptions"
 Navigate to any or all record(s)
Slide 16
Run Scour Countermeasure Calculator
 Examine analysis results
 Rectify with underlying data
 Note any "exceptions"
 Navigate to any or all record(s)
Slide 17
Scour Countermeasure Calculator
 Assists in evaluating the risk and economic feasibility of proposed or existing countermeasures
 Accounts for lossoflife costs
 Accounts for a finite service life
 Accounts for the time value of resources
 Allows adjustment of any calculation parameter
Slide 18
Step 1
 Applies to a specific bridge
 Uses NBI data as a default
 Data may be modified or new data added
 Assumptions may be modified
Slide 19
Step 2
 Set cost multiplier
 Uses step function values previously set
Slide 20
Step 3
 Establish economic assumptions
 Lossoflife cost assumptions
 Defaults based on historical statistics
 Discount rate
 Costs
Slide 21
Step 4
 Specify an annual failure probability
 Use HYRISKestimated P_{f} OR
 A known better number
Slide 22
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?
Slide 23
Bridge Life Expectancy
P_{L} = 1  (1 P_{A})^{L}
Where:P_{L} = Probability of failure during expected lifeP_{A}= Annual probability of failureL = Expected life, years
Slide 24
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 lossoflife
Slide 25
Present Value of Replacement Costs
\Where: C_{E} = Present value of future cost C_{R} = Current rebuilding cost i = Discount rate* L = Remaining life expectancy of bridge
*Discount rate is the time value of money over and above inflation
Slide 26
Step 6
 Review the economic risks of failure
 Look at present value of future costs
 Quantify benefits of protection
 Evaluate the impact of lossoflife
Slide 27
Step 7
 Specify Countermeasures
 Net benefit accounts for countermeasure costs
 Countermeasures need not be construction
 Doing nothing is always an option
Slide 28
Compare Countermeasures
Slide 29
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
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