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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

cover of publication Strategies for Managing Unknown Bridge Foundations
  • 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

Methodology used by HYRISK software to determine the risk (in dollars) based on the combination of parameters such as scour criticallity, route classification, substructure condition, channel protection, waterway adequacy, and bridge age.

Slide 5

The Full Equation

equation risk= kp[EC(sub1)WL = C(sub2)DAd = (C(sub3)O(1-(T/100))+C(sub4)(T/100))(DAd/S)]
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

graph of detour duration

Slide 7

Cost Multiplier for Early Replacement

graph of 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

Screen Print of HYRISK opening screen

screen print of HYRISK edit menu

Slide 10

Establish Analysis Parameters

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


 

Screen print of HYRISK Options/General Tab to establish analysis parameters

Slide 11

Make Necessary Assumptions

  • Current Year
  • Computational parameters
  • Economic factors

    Note: Assumptions used for all bridges analyzed.



screen print of HYRISK options/assumptions tab

Slide 12

Select Bridges

  • Limits analysis to targeted characteristics


Screen Print of HYRISK options/bridge selection screen

Slide 13

Set Step Function Values

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


Screen print of HYRISK option/step functions screen

Slide 14

Run the Analysis

HYRISK screen print showing tools-run analysis selection

Slide 15

Study Analysis Results

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


HYRISK screen print of results

Slide 16

Run Scour Countermeasure Calculator

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


HYRISK screen print of results with scour countermeasures economic analysis button circled

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



HYRISK screen print scour countermeasures economic analysis step 1 describe the bridge

Slide 19

Step 2

  • Set cost multiplier
    • Uses step function values previously set



HYRISK screen print scour countermeasures economic analysis step 2 cost multipliers

Slide 20

Step 3

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

    • Discount rate
    • Costs



HYRISK screen print scour countermeasures economic analysis step 3 establish economic assumptions

Slide 21

Step 4

  • Specify an annual failure probability
    • Use HYRISK-estimated Pf OR
    • A known better number



HYRISK screen print scour countermeasures economic analysis step 4 specify an annual failure probability

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?



HYRISK screen print scour countermeasures economic analysis step 5 Establish a life expectancy for the bridge

Slide 23

Bridge Life Expectancy

PL = 1 - (1- PA)L

Where:PL = Probability of failure during expected lifePA= 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


HYRISK screen print scour countermeasures economic analysis step 5 review the economic risks of failure

Slide 25

Present Value of Replacement Costs

equation C(subE) = C(subR) over (1 + i)raised to power L
\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

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


Benefit versus Return Period Graph

Slide 27

Step 7

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


HYRISK screen print scour countermeasures economic analysis step 7 specify countermeasures

Slide 28

Compare Countermeasures

screen print of output. Bar chart comparing various 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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Updated: 04/07/2011
 

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