Transportation Research Board 2002 Annual Meeting Presentations from Workshop on Plan of Action for Scour Critical Bridges

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Slide 1

Risk-Based 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. FHWA-RD-92-030 "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
EC₁WL - Cost to Rebuild
C₂DAd - Cost of Alternate Operation
C₃O - 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 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

Slide 10

Establish Analysis Parameters

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

Slide 11

Make Necessary Assumptions

• Current Year
• Computational parameters
• Economic factors

  Note: Assumptions used for all bridges analyzed.

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 loss-of-life costs
• Accounts for a finite service life
• Accounts for the time value of resources
• Allows adjustment of any calculation parameter

Slide 18

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
  
  

Slide 19

Step 2

• Set cost multiplier
  • Uses step function values previously set
  
  

Slide 20

Step 3

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

Slide 21

Step 4

• Specify an annual failure probability
  • Use HYRISK-estimated 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 loss-of-life
  
  

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 loss-of-life

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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