Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (2013) / Chapter Skim
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11 2.1 Introduction This chapter provides a discussion of the various types and sources of uncertainty that must be considered in the assessment of bridge scour. Citations from the literature provide relevant background information on the current state of practice.
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12 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction to the bridge location using area-weighting and other techniques. Where gaging station records on the particular stream or river are unavailable, data are used from stations in nearby watersheds of similar size and nature to the watershed of interest.
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Uncertainty in Hydraulic Design 13 The data from other gaging stations allowed the period of record to be extended from 1935 through 2008 (74 years) for purposes of quantifying the cumulative hydraulic loading from the time the bridge was built to the present.
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14 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction The USGS software package PKFQWin can be used to determine hydrologic uncertainty when dealing with data from gaged sites. The software is a public-domain, Windows-based program that allows the user to access annual peak flow records in standard USGS format.
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Uncertainty in Hydraulic Design 15 combination of the standard error of estimate, the standard error of prediction, the equivalent years of record, or 90% prediction intervals, depending on what was provided by the authors of the equations. NSS is a public-domain software program that can be used to: • Obtain estimates of flood frequencies for sites in rural (non-regulated)
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16 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 0 1 2 3 4 Ve lo ci ty a t B rid ge (m /s) Left Embankment Right Embankment Bridge Cross Section Bridge Velocity Distribution Approach Cross Section Approach Velocity Distribution 0 1 2 3 0 200 400 600 800 1000 1200 1400 Distance, m A pp ro ac h Ve lo ci ty (m /s)
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Uncertainty in Hydraulic Design 17 models (e.g., 2-D models) for more complex situations or by calibrating the model to measured conditions.
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18 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction or by comparison with published values. Since its original publication, this table has been cited numerous times in risk, reliability, and other studies, and it has been the basis for parameter input for bridge scour, levee and dam overtopping, and other hydrodynamic studies.
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Uncertainty in Hydraulic Design 19 Jones (1984) compared numerous pier scour equations using laboratory data and limited field data.
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20 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction and distributions described in Section 2.3.2 and Table 2.2. They also accounted for model uncertainty using a model correction factor and its COV and distribution.
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Uncertainty in Hydraulic Design 21 The Scour Design Check Flood Frequencies are larger than the Scour Design Flood Frequencies using the same logic and for the same reasons as outlined above (Arneson et al.
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22 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 2.5 LRFD -- A Hydraulic Engineering Perspective 2.5.1 Introduction The LRFD methodologies for bridge design were initially developed using concepts derived from structural engineering procedures. LRFD incorporates state-of-the-art analysis and design methodologies with load and resistance factors based on the known variability of applied loads and material properties.
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Uncertainty in Hydraulic Design 23 where: br = Resistance bias Rn = The nominal value as specified by the design code For example, A36 steel has a nominal design yield stress of 36 ksi (248,220 kPa) , but coupon tests show an actual average value close to 40 ksi (275,800 kPa)
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24 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Thus, the reliability index, , which is often used as a measure of structural safety, gives in this instance the number of standard deviations that the mean margin of safety falls on the "safe" side. The reliability index, b, defined by Equation (2.8)
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Uncertainty in Hydraulic Design 25 code-writing groups for recommending appropriate load and resistance safety factors for new structural design or evaluation specifications. One commonly used calibration approach is based on the principle that each type of structure should have uniform or consistent reliability levels over the full range of applications.
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26 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction During the calibration of a new design code, the average reliability index from typical "safe" designs is used as the target reliability value for the new code. That is, a set of load and resistance factors as well as the nominal loads (or return periods for the design loads)
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