The National Academies Press

Currently Skimming:

Pages 41-58

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.

From page 41... ... 41 4.1 Introduction This chapter includes a brief summary of the data sets used in developing model bias and COV for each of the three individual scour components. For pier scour, both the HEC-18 and Florida DOT equations are assessed using comprehensive data sets from both laboratory and field studies. Read the entire page →
From page 42... ... 42 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Nearly all of the laboratory tests involved cylindrical piers; only 36 tests (about 5% of the data points) used square, rectangular, or multiple-column piers. Read the entire page →
From page 43... ... Bridge Scour Equations and Data Screening 43 test; no predictive techniques were used to discriminate among data points. Of the original 699 data points, 119 were identified as outliers (approximately 17% of the total data points) Read the entire page →
From page 44... ... 44 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction DOT equation combines pier geometry, shape, and angle of attack to compute an effective pier width, a* Read the entire page →
From page 45... ... Bridge Scour Equations and Data Screening 45 The effective pier width, a* , is the projected width of the pier times the shape factor, Ksf. Read the entire page →
From page 46... ... 46 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction • Scour Region II is for clear-water conditions with flow velocity large enough to produce pier scour (Vc > V1 > 0.4Vc) as defined by Step 9. Read the entire page →
From page 47... ... Bridge Scour Equations and Data Screening 47 graphically. Table 4.2 provides the final results of the pier scour prediction for the laboratory data using the Florida DOT methodology. Read the entire page →
From page 48... ... 48 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Contraction scour data obtained under controlled laboratory conditions were assembled from eight sources, yielding 182 independent measurements of contraction scour in cohesionless soils. Only long contractions were considered, because short contractions include an abutment scour effect in addition to the contraction scour. Read the entire page →
From page 49... ... Bridge Scour Equations and Data Screening 49 section, but instead assumed that the depth of scour was equal to the difference in flow depths, y2 – y1. Dey and Raikar reported those results as "observed scour"; however, their assumption is not valid because the drawdown effect on the water surface in the contracted section is not accounted for. Read the entire page →
From page 50... ... 50 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction entrance to the contraction. The threshold of choking occurs when the actual contraction ratio is less than the critical ratio s, defined as follows by Wu and Molinas (2005) Read the entire page →
From page 51... ... Bridge Scour Equations and Data Screening 51 scour is underpredicted by about the same magnitude and frequency as it is overpredicted. Table 4.3 provides a summary of the prediction statistics for the HEC-18 clear-water contraction scour equation. Read the entire page →
From page 52... ... 52 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction subsequently endorsed by NCHRP Project 24-27(02) was investigated. Read the entire page →
From page 53... ... Bridge Scour Equations and Data Screening 53 Figure 4.7. Abutment scour conditions A, B, and C from NCHRP Project 24-20 (Ettema et al. Read the entire page →
From page 54... ... 54 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction width or by the product of velocity and depth. The contraction scour equation for Scour Condition A is: y y q q (4.26) Read the entire page →
From page 55... ... Bridge Scour Equations and Data Screening 55 of velocity and depth. Two clear-water contraction scour equations may be applied. Read the entire page →
From page 56... ... 56 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction The value of q2f should be estimated including local concentration of flow at the bridge abutment. The value of qf is the floodplain flow upstream of the bridge. Read the entire page →
From page 57... ... Bridge Scour Equations and Data Screening 57 abutment scour process. For large values of q2f/qf, contraction scour dominates the abutment scour process and the amplification factor is small. Read the entire page →
From page 58... ... 58 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction The reliability index, b, for the NCHRP Project 24-20 abutment scour equation was determined to be 1.53 and 1.44 for normal and lognormal distributions, respectively. These relatively high values of b reflect the fact that the curves for the amplification factors aA and aB for both spill-through and wingwall abutments (Figures 4-8 through 4-11) Read the entire page →

From page 41...

... 41 4.1 Introduction This chapter includes a brief summary of the data sets used in developing model bias and COV for each of the three individual scour components. For pier scour, both the HEC-18 and Florida DOT equations are assessed using comprehensive data sets from both laboratory and field studies.