Risk-Based Approach for Bridge Scour Prediction: Applications for Design

NCHRP Project 24-34 was completed in September 2013 with the publication of NCHRP Report 761, "Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction." The project accomplished its objectives of developing risk/reliability-based methodologies that can be used in calculating bridge pier, abutment, contraction, and total scour at waterway crossings so that scour estimates can be linked to a probability. The developed probabilistic procedures are consistent with LRFD approaches for bridge design used by structural and geotechnical engineers.\nAs a necessary first step in developing the statistical parameters for risk and reliability analyses, the key bridge scour equations (HEC-18 pier scour, Florida DOT pier scour, HEC-18 contraction scour, and NCHRP 24-20 abutment scour) were tested against available laboratory and field data sets. The results which are summarized in this presentation include: · The pier scour equations (HEC-18 and FDOT) are design equations which do not under predict observed scour very often. Consequently, the probabilistic reliability indexes for pier scour compare favorably with those used by structural and geotechnical engineers in LRFD applications for bridges. · In contrast with the pier scour equations, the HEC-18 contraction scour equations are essentially predictive, given that they are derived from sediment transport principles and theory. Therefore, under predictions of observed scour are much more common, and the resulting reliability is very low compared to typical target values used in LRFD applications. · The NCHRP 24-20 equations for live-bed and clear-water abutment scour both use a calculation for contraction scour and then apply an amplification factor to account for the additional scour caused by local effects at the tip of the abutment. The scour predicted by this method is the total scour at the abutment. The reliability of the abutment scour equations was found to be intermediate between those of the pier scour and contraction scour equations. \nThis presentation will be followed by Part II which will provide an example application of the basic risk-based methodology in a typical design situation.