Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007) / Chapter Skim
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From page 34... ...
NCHRP 24-25 Page 34 Phase II Final Report 3. QUANTIFYING RISK OF SCOUR FAILURE This section gives an overview of the HYRISK methodology and then moves on to discuss the annual probability of scour failure assumptions, the scour risk equation, and the lifetime risk of scour failure that are used in the Scour Risk Management Guidelines section.
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NCHRP 24-25 Page 35 Phase II Final Report 33/161,000 = 0.000205, or about 1 in 5,000 per year. Scaling this to all bridges over water (i.e.
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NCHRP 24-25 Page 36 Phase II Final Report Table 12 Annual Probability of Scour Failure Overtopping Frequency (from Table 13) Scour Vulnerability (from Table 14)
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NCHRP 24-25 Page 37 Phase II Final Report Table 14 Scour Vulnerability versus NBI Items 60 and 61 Substructure Condition (NBI Item 60 Code) Channel Protection (NBI Item 61 Code)
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NCHRP 24-25 Page 38 Phase II Final Report because they have enough information to evaluate the relationship between the HYRISK scour vulnerability and NBI item 113. Figure 2 plots the relationship between the HYRISK scour vulnerability and the NBI item 113 in such a way that the size of the dot is directly proportional to the number of bridges that correspond to these integer values.
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From page 39... ...
NCHRP 24-25 Page 39 Phase II Final Report 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 NBI Item 113 Code Sc ou r V ul ne ra bi lit y C od e Figure 2 HYRISK scour vulnerability versus NBI item 113
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NCHRP 24-25 Page 40 Phase II Final Report y = 0.4472x + 3.403 R2 = 0.8582 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 0 1 2 3 4 5 6 7 8 9 NBI Item 113 Code A ve ra ge S co ur V ul ne ra bi lit y C od e Figure 3 Average HYRISK scour vulnerability versus NBI item 113 3.3. The Scour Risk Equation The HYRISK equation developed by GKY & Associates, Inc.
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From page 41... ...
NCHRP 24-25 Page 41 Phase II Final Report RiskA = annual risk of scour failure ($/year) , K = risk adjustment factor based on foundation type and type of span based on NBI items and where available from more developed databases, foundation information, PA = annual probability of failure based on NBI items 26, 60, 61, 71, and 113 (see Table 12)
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From page 42... ...
NCHRP 24-25 Page 42 Phase II Final Report The values recommended for K2, given below, should be developed for both abutment and pier condition, selecting the largest value for the analysis. 1.0: unknown foundations or spread footings on erodible soil above scour depth with pier footing top visible or 1- to 2 ft below stream bed 0.8: pile foundations when length is unknown, are less than 19 ft, or are all-wood pile foundations 0.2: foundations on massive rock These factors are again subjective and should be revised or adjusted using local experience or further forensic studies.
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