Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (2013) / Chapter Skim
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From page 80... ...
80 6.1 Approach 6.1.1 Background The calculations performed for the probability-based scour estimates described in Chapter 5 are for a single discharge rate that corresponds to a design return period (e.g., the discharge rate having a return period of 100 years)
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Calibration of Scour Factors for a Target Reliability 81 event. Although there are an infinite number of hydraulic events, these can be combined into discrete segments where each segment has a probability of occurrence Pi.
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82 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction periods. The probability of exceedance within a 1-year period is calculated to be Pex(Tn = 1 - yr)
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From page 83... ...
Calibration of Scour Factors for a Target Reliability 83 only a limited number of discrete return periods and the results are integrated to obtain estimates of the reliability of the bridge over the entire service period. The objective of the reliability analysis is to find the reliability index, b, which as defined in Equation (6.4)
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84 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction will have a lower COV. Figure 6.1 provides a flow chart for evaluating the reliability index, b, using the simplified procedure.
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Calibration of Scour Factors for a Target Reliability 85 Start 1. Assign scour design parameters 2.
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86 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction bridge would have a probability of 60.07% of exceeding the design scour within a 75-year service period (b = -0.25)
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Calibration of Scour Factors for a Target Reliability 87 6.3.2 Pier Scour Designed Using Florida DOT Method The approach was executed to find the pier scour that would be obtained if the bridge foundation is designed for the scour depth determined using the Florida DOT pier scour equation. The Florida DOT method leads to a design scour depth equal to 11.2 ft (see Section 5.4.2)
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From page 88... ...
88 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 6.3.4 Total Pier and Contraction Scour Using HEC-18 Methods The simulations were performed to find the combined (total) pier and contraction scour that would be obtained if the bridge foundation is designed for the scour depth determined using the HEC-18 methods for pier and contraction scour.
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From page 89... ...
Calibration of Scour Factors for a Target Reliability 89 of 16.5 ft is 21.75% with a reliability index of b = 0.78. This value is very low compared to typical reliability indexes that have been deemed acceptable for bridges under extreme events.
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From page 90... ...
90 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction its COV on the reliability index, which varies from an acceptable value of 2.67 (obtained when the HEC-18 pier scour equation is used to design the foundation) to the very low value of 0.07 (obtained when the HEC-18 equations are used for designing the foundation for contraction scour)
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From page 91... ...
Calibration of Scour Factors for a Target Reliability 91 For the case analyzed, the scour factors shown in Table 6.4 indicate that no additional safety factors would be required for the HEC-18 pier scour equation if the target reliability index is set at 2.50 or lower. A scour factor equal to 1.04 would be needed to reach a target reliability index of b = 3.0.
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