Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (2013)

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1 Overview NCHRP Report 761: Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction is based on the results of NCHRP Project 24-34, “Risk-Based Approach for Bridge Scour Prediction.” The goals of NCHRP Project 24-34 were (1) to develop a risk and reliability-based methodology that can be used to link scour depth esti- mates at a river crossing to a probability and (2) to extend this methodology to provide a preliminary approach for determining a target reliability for the service life of the bridge that is consistent with load and resistance factor design (LRFD) approaches used by struc- tural and geotechnical engineers. The uncertainties associated with bridge scour prediction—including hydrologic, hydrau- lic, and model/equation uncertainty—are described and evaluated, as is the development of a software tool that links the most widely used 1-dimensional (1-D) hydraulic model (HEC-RAS) with Monte Carlo simulation techniques. Tables of probability values (scour factors) are presented that allow associating an estimate of scour depth with a conditional (single event) probability of exceedance when a bridge meets certain criteria for hydrologic uncertainty, bridge size, and pier size. The tables address pier scour, contraction scour, abutment scour, and total scour. For complex foundation systems and channel conditions, a step-by-step procedure is pre- sented to provide scour factors for site-specific conditions. An integration technique that incorporates the uncertainties associated with the conditional probability of a limited num- ber of return-period flood events provides a reliability analysis framework for estimating the unconditional probability of exceeding a design scour depth over the service life of a bridge. Detailed illustrative examples demonstrate the full range of applicability of the methodologies. The research on which this reference guide is based developed probabilistic procedures that are consistent with LRFD approaches used by structural and geotechnical engineers. 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. These load and resistance factors are calibrated from actual bridge statistics to ensure a uniform level of safety over the life of the bridge. LRFD allows a bridge designer to focus on a design objective or limit state, which can lead to a similar probability of failure in each compo- nent of the bridge. Bridges designed with the LRFD specifications are intended to have relatively uniform safety levels, which helps ensure superior serviceability and long-term maintainability. A widespread belief within the bridge engineering community is that unaccounted-for biases, together with input parameter and hydraulic modeling uncertainty, lead to overly S U M M A R Y Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction

2 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction conservative estimates of scour depths. The perception is that this results in design and construction of costly and unnecessarily deep foundations. This reference guide is intended to close the gap between perception and reality and provide risk and reliability-based con- fidence bands for bridge scour estimates that align the hydraulic design approach with the design procedures currently used by structural and geotechnical engineers. Hydraulic engineers now have the option to perform scour calculations that incorporate probabilistic methods into the hydraulic design of bridges. Research Approach NCHRP Report 761 is oriented toward the practitioner. The Contractor’s Final Report that documents the investigation and results of the research project is available on www. trb.org by searching on “NCHRP Project 24-34.” The research supporting this reference guide involved the following steps: 1. Completion of a literature review and evaluation of current practice in the areas of hydro- logic and hydraulic analyses for bridge scour prediction, including the use of probabilistic methods in hydrologic and hydraulic engineering. The review included other disciplines where risk and reliability analyses have been incorporated into engineering design, with emphasis on LRFD approaches used by structural and geotechnical engineers. 2. Investigation of the application of reliability theory to the determination of bridge scour prediction and the quantity and quality of data available to support the objectives of this project. 3. Identification and evaluation of uncertainty associated with the variables and approaches used in bridge scour prediction, including hydrologic, hydraulic, and model/equation uncertainty. 4. Development of a conceptual approach for the implementation phase of the research and production of research-level software that links the most widely used 1-D hydraulic model (HEC-RAS) with Monte Carlo simulation techniques. 5. Development of a set of tables of probability values (scour factors) that can be used to asso- ciate an estimate of scour depth with a conditional (single event) probability of exceedance when a bridge meets certain criteria for hydrologic uncertainty, bridge size, and pier size. 6. For complex foundation systems and channel conditions, development of a step-by-step procedure that provides an approach for developing probability-based estimates and scour factors for site-specific conditions. 7. Development of an integration technique that incorporates the uncertainties associated with a conditional probability prediction into a reliability analysis framework to estimate the unconditional probability of exceedance for a selected service life of a bridge. 8. Providing a set of detailed illustrative examples to demonstrate the full range of applicability of the methodologies. 9. Production of this stand-alone reference guide. 10. Identification of additional research that would expand the findings of the project and sug- gestions for implementing the results of the research. Appraisal of Research Results The primary purpose of the research project supporting this reference guide was to analyze the probability of scour depth exceedance, not the probability of bridge failure. The latter requires advanced analyses of the weakened foundation under the effects of the expected applied loads, which was beyond the scope of this project.

Summary 3 The work plan that was developed and implemented for this research project yielded significant results of practical use to practitioners. The goals of the research study were achieved. A methodology is now available that can be used to link scour depth estimates to a probability and determine the risk associated with scour depth exceedance for a given design event. The probability linkage considers the propagation of uncertainties among the parameters that are used to quantify the confidence of scour estimates for a design event (e.g., a 100-year flood) based on uncertainty of input parameters and considering model uncertainty and bias. In addition, this methodology has been extended to provide an initial estimate of target reliability for the design life of a bridge consistent with LRFD approaches used by structural and geotechnical engineers. The Level 1 approach described in this reference guide consists of a set of tables of proba- bility values or scour factors that can be used to associate an estimated scour depth provided by the hydraulic engineer with a probability of exceedance for simple pier and abutment geometries. For more complex bridge or hydraulic situations, or for different return-period design events, the Level II approach can be used. The Level II approach consists of a step-by- step procedure that hydraulic engineers can follow to provide probability-based estimates of site-specific scour factors. A Level II approach will also be necessary if the unconditional probability of exceeding design scour depths to meet a target reliability over the life of a bridge is desired. During NCHRP Project 24-34, a research-level software engine called rasTool© also was developed. The rasTool© software was not developed for distribution, nor is it thoroughly documented or supported for general use. It is, however, considered robust and could be applied to a range of bridge and/or open-channel applications.