Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (2013) / Chapter Skim
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From page 92... ...
92 7.1 Overview This chapter provides detailed illustrative examples to demonstrate the full range of applicability of the Level I probability-based scour estimates using the procedures presented in Chapter 5. Given the unique nature of any bridge-stream intersection, these examples illustrate application of the methodology for a wide variety of bridge-stream scenarios in a range of physiographic regions across the country.
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From page 93... ...
Illustrative Examples 93 contraction and abutment scour. The bridge has been rated as scour critical, has scour countermeasures, and is scheduled for replacement.
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94 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Step 2. Determine the appropriate bridge size, hydrologic uncertainty, and pier size corresponding to standard scour factor table values.
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Illustrative Examples 95 7.3 Example Bridge No. 2: Nevada Great Basin Subregion Location: Nevada Physiographic region: Intermontane basins and plateaus; Great Basin Subregion Bridge length: 210 ft No.
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96 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction footings. Because of long-term degradation at this site, the spread footings are now exposed above the stream bed.
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Illustrative Examples 97 discharge estimate is assumed to be lognormally distributed. Consequently, given the 95% upper and 95% lower confidence limits (see Section 3.5.2)
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98 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Consequently, this bridge is best classified as a medium bridge with high hydrologic uncertainty and large pier size for the Level I analysis. However, the 19 ft wide pile cap is significantly larger than the 4.5 ft large pier assumed for a medium bridge, suggesting that this bridge may be a candidate for a Level II analysis.
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Illustrative Examples 99 The total scour difference from expected is the square root of the sum of the squares of the component scour differences (pier and contraction scour)
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100 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction b. Design-equation scour computations using the HEC-18 and Florida DOT methods for pier scour, the HEC-18 method for contraction scour, and the NCHRP Project 24-20 method as presented in HEC-18 for abutment scour were computed for this example.
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Illustrative Examples 101 2. For a 95% confidence limit, Zc = 1.645 (see Appendix A)
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102 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Consequently, this bridge is best classified as a large bridge with low hydrologic uncertainty and medium pier size for the Level I analysis.
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Illustrative Examples 103 No. spans: 7 ADT: 94,470 (2006)
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From page 104... ...
104 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction b. Design-equation scour computations using the HEC-18 method for pier scour, the HEC-18 method for contraction scour, and the NCHRP Project 24-20 method as presented in HEC-18 for abutment scour were computed for the 100-year scour design flood in this example.
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Illustrative Examples 105 2. For a 95% confidence limit, Zc = 1.645 (see Appendix A)
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106 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Consequently, this bridge is best classified as a large bridge with low hydrologic uncertainty and large pier size for the Level I analysis.
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Illustrative Examples 107 River planform: Meandering, low sinuosity (< 1.06) 100-year discharge: 249,100 ft3/s total (181,900 ft3/s main channel, 36,000 ft3/s west relief, and 31,200 ft3/s east relief)
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From page 108... ...
108 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction b. Design-equation scour computations using the HEC-18 method for pier scour, the HEC-18 method for contraction scour, and the NCHRP Project 24-20 method as presented in HEC-18 for abutment scour were computed for this example.
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From page 109... ...
Illustrative Examples 109 discharge estimate is assumed to be lognormally distributed. Consequently, given the 95% upper and 95% lower confidence limits (see Section 3.5.2)
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110 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Consequently, the main channel bridge is best classified as a large bridge, low hydrologic uncertainty, medium pier size, and the two relief bridges are best classified as large bridge, low hydrologic uncertainty, small pier size for the Level I analysis.
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From page 111... ...
Illustrative Examples 111 HEC-18 Pier Scour Contraction Scour Total Scour Abutment Total Scour Left Right Design scour (ft) 9.9 3.4 13.3 4.6 8.8 Bias 0.68 0.93 0.76 0.76 Expected scour (ft)
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