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ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.
NCHRP 24-25 Page i Phase II Final Report CONTENTS LIST OF FIGURES .....................................................................................................................ii LIST OF TABLES .......................................................................................................................ii ACKNOWLEDGEMENTS........................................................................................................ iii ABSTRACT................................................................................................................................ iii EXECUTIVE SUMMARY...........................................................................................................1 1. Introduction ..........................................................................................................................5 1.1. Bridges with Unknown Foundations ..........................................................................5 1.2. Performance-Based versus Traditional Design Practice .........................................10 1.3. Report Overview ........................................................................................................12 2. General Approach to Risk Management ...........................................................................13 2.1. Probability of Failure.................................................................................................13 2.1.1. Hazardous Potential ..........................................................................................13 2.1.2. Vulnerability to Failure.....................................................................................14 2.1.3. Correlations with Observed Failures................................................................16 2.1.4. Multiple Failure Modes .....................................................................................16 2.2. Cost of Failure............................................................................................................18 2.2.1. Expenses per Mile for the Motor Carrier Industry ..........................................18 2.2.2. Bridge Costs .......................................................................................................21 2.2.3. Price Elasticity of Demand................................................................................22 2.2.4. Loss of Life .........................................................................................................23 2.2.5. HYRISK Cost of Failure Equation....................................................................24 2.3. Risk of Failure............................................................................................................25 2.4. Mitigating Activities..................................................................................................26 2.5. General Guidelines for Risk Management ...............................................................27 3. Quantifying Risk of Scour Failure ....................................................................................34 3.1. HYRISK Background.................................................................................................34 3.2. Annual Probability of Scour Failure.........................................................................34 3.3. The Scour Risk Equation...........................................................................................40 3.4. Lifetime Risk of Scour Failure ..................................................................................42 4. Mitigating Actions for Scour..............................................................................................43 4.1. Pertinent Findings from Experience ........................................................................43 4.2. Foundation Reconnaissance......................................................................................44 4.3. Scour Monitoring .......................................................................................................49 4.4. Scour Countermeasures ............................................................................................53 5. Scour Risk Management Guidelines .................................................................................56 5.1. Can the Foundation Be Inferred? .............................................................................58 5.2. Is the Bridge a High Priority Structure?..................................................................59 5.3. Screening Bridges According to Risk........................................................................60 5.3.1. Does the Bridge Meet Minimum Performance Level? .....................................61 5.3.2. Is Automated Scour Monitoring Warranted? ...................................................62 5.3.3. Are Scour Countermeasure Warranted? ..........................................................63 5.3.4. Is Foundation Reconnaissance and Scour Analysis Warranted? ....................63 5.4. Install Countermeasures without Field Reconnaissance and Scour Analysis .......65 5.5. Develop a Bridge Closure Plan .................................................................................66 5.6. Is Significant Scour Occurring? ................................................................................68 6. Scour Management Case Studies......................................................................................69
NCHRP 24-25 Page ii Phase II Final Report 6.1. Information Search and Preliminary Screens..........................................................69 6.2. The Minimum Performance Level Criterion ............................................................70 6.3. Scour Risk Assessment..............................................................................................72 6.4. Management Alternatives.........................................................................................74 7. Conclusions and Recommendations ..................................................................................81 8. References...........................................................................................................................83 LIST OF FIGURES Figure 1 General risk management guidelines flow chart......................................................32 Figure 2 HYRISK scour vulnerability versus NBI item 113...................................................39 Figure 3 Average HYRISK scour vulnerability versus NBI item 113....................................40 Figure 4 Scour risk management guidelines flow chart..........................................................57 LIST OF TABLES Table 1 Numbers of Bridges with Unknown Foundations by State .........................................7 Table 2 Numbers of Bridges with Unknown Foundations by Age............................................9 Table 3 Detour Duration versus ADT ......................................................................................18 Table 4 Occupancy per Vehicle Mile by Daily Trip Purpose ...................................................19 Table 5 Comparison of Total and Variable Costs per Mile .....................................................19 Table 6 Values of Time Used in the Derivation of Road User Costs ......................................19 Table 7 Estimates of the Values of Travel Time......................................................................20 Table 8 Values of Time..............................................................................................................21 Table 9 Cost of Bridge Construction.........................................................................................22 Table 10 Cost Multiplier for Early Replacement.....................................................................22 Table 11 Assumed Number of Lives Lost in Bridge Failure...................................................24 Table 12 Annual Probability of Scour Failure .........................................................................36 Table 13 Bridge Overtopping Frequency versus NBI Items 26 and 71..................................36 Table 14 Scour Vulnerability versus NBI Items 60 and 61 ....................................................37 Table 15 Effectiveness of NDT Methods ..................................................................................47 Table 16 Fixed Scour Monitoring Methods ..............................................................................49 Table 17 Comparison of Instrument Types..............................................................................51 Table 18 Comparison of Fixed Instrumentation......................................................................51 Table 19 Comparison of Portable Instrumentation.................................................................52 Table 20 Estimated Instrument Cost.......................................................................................52 Table 21 Stone Riprap weights.................................................................................................54 Table 22 Bedding stone sizes ....................................................................................................54 Table 23 Material costs (Florida)..............................................................................................54 Table 24 Material costs (New York State) ...............................................................................54 Table 25 Material costs (Colorado) ...........................................................................................54 Table 26 Average Total Armor Costs per Pier (Florida)..........................................................55 Table 27 Minimum Performance Levels for Bridges ...............................................................61 Table 28 Summary of Required Data.......................................................................................70 Table 29 Annual Probability of Failure, Example 1................................................................71 Table 30 Annual Probability of Failure, Example 2................................................................72 Table 31 Case Study Management Decisions by Functional Classification ..........................77 Table 32 Summary of Bridge Case Studies..............................................................................79
NCHRP 24-25 Page iii Phase II Final Report ACKNOWLEDGEMENTS The research reported herein was performed under NCHRP Project 24â25 by GKY & Associates, Inc (GKY&A). Mr. Stuart M. Stein, President of GKY&A, was the Principal Investigator. The other contributors to this report include Mr. Karsten A. Sedmera, Water Resources Engineer at GKY&A, and Mr. David R. Pearson, Vice President of GKY&A. Larry Olsen, President of Olson Engineering, Inc. helped evaluate non-destructive testing methods; Martin W. McCann, Jr. of Jack R. Benjamin and Associates, Inc. researched existing risk methodologies and helped formulate the risk-based guidelines; Dr. Max Shepherd, President of OEA, Inc., helped evaluate methods for scour monitoring and scour countermeasures; Libby Ogard, a sub-contractor for Tioga Group, helped evaluate the economic assumptions; Jorge Pagan, Principal Bridge Engineer (Hydraulics) of the Federal Highway Administration, provided invaluable input at the start of the project, including contacts for data collection and the Federal perspective on unknown foundations; and J. Sterling Jones, formerly of the Federal Highway Administration, provided National Bridge Inventory data and analysis as well as contact information for State Tranportation Agency personnel who currently make management decisions for bridges with unknown foundations. All work was performed under the general supervision of Mr. Stein at GKY&A. ABSTRACT The US currently has over 60,000 bridges over water with unknown foundations. This report presents a risk-based approach to managing these bridges in the absence of foundation information. The general framework in this report, which is primarily applied to scour failure, can easily be applied to other hazards such as earthquakes and tsunamis. The guidelines illustrate how to collect appropriate data, estimate risk of failure from an estimated failure probability and associated economic losses, and use risk in a structured approach to select an appropriate management plan. Risk analysis is specifically used to select appropriate performance standards for various bridge classifications and justify the costs of nondestructive testing of foundations, monitoring activities, and countermeasures. The scour guidelines were then applied to sixty case studies in the US to validate the management plan that it selected for bridges with known foundations, and to illustrate its specific application in a variety of settings.