Peggy A. Johnson

Quantifying Uncertainty and Reliability in Bridge Scour Estimations

AbstractThe majority of the bridges in the U.S. National Bridge Inventory (NBI) are built over waterways. Many will experience problems with scour, bank erosion, and channel instability during their design life. A number of studies in the recent past have attempted to quantify the uncertainty in predicting these erosional processes, particularly in pier scour, and developing probabilistic estimates of scour as a means of incorporating uncertainty. However, none of these studies have examined the overall uncertainty in local pier or abutment scour in combination with contraction scour. This study quantifies the model uncertainty in commonly used scour equations as well as parameter uncertainty. The overall reliability of the scour equations is then assessed for the individual components of scour and combined scour. The results lead to a set of scour design factors that are based on the reliability of the design estimate.

Applying Risk-Benefit Analysis to Select an Appropriate Streambank Stabilization Measure

AbstractStream stabilization designers are often faced with the challenge of selecting effective bank stabilization measures. The potential benefits of stream stabilization measures can be economic, environmental, or social. Depending on the level of the potential benefit, a designer may be willing to take higher risks in implementing a given measure. A risk-benefit analysis is presented here that involves a qualitative analysis of risk and benefit (using failure modes and effects analysis) and risk and benefit quantification in terms of cost. The initial result of the method is the establishment of risk priority numbers (RPNs) and benefit priority numbers (BPNs), which provide a relative qualitative measure of the potential risk and benefit and can be used to prioritize and rank measures. The results of the qualitative analysis are then used to estimate risk and benefit quantitatively in terms of cost. These quantitative values are then compared for several stabilization alternatives to provide justifica...

Performance of Two Real-Life California Bridges under Regional Natural Hazards

AbstractThe performance of two real-life California bridges is assessed under a possible regional multihazard condition involving floods and earthquakes. For flood events with varied frequency, expected scour depths at bridge piers are calculated and incorporated in finite-element analyses (FEAs) of the bridges under earthquakes that represent regional seismic hazards. Based on FEA results, fragility curves of bridges are developed at component and system levels. Fragility surfaces are generated to acquire comprehensive knowledge on bridge failure probability for the combined effect of earthquake and flood events of varying frequency. Quantified bridge vulnerability is applied to a risk evaluation framework that combines hazard probability with bridge failure consequences. Obtained results depict that the occurrence of flood events can increase the seismic vulnerability and risk of bridges, although the amount of increase depends on bridge attributes. Bigger (large-diameter) foundations tend to reduce the...

Uncertainty Analysis for Vapor Pressure Measurement

Experimental work in building science/physics involves various kinds of measurements. Uncertainties associated with these measurements are essential for evaluating the accuracy and quality of the experimental work and more importantly, for evaluating the validity of the experimental conclusions. Performing an uncertainty analysis for a complex measurement system that involves various uncertainty sources and requires multiple levels of uncertainty propagation is not an easy task. A general procedure to perform uncertainty analysis of vapor pressure measurements is described. A sensitivity study is included to demonstrate that it can be used to determine the contribution of each uncertainty source. The influence and validity of an important experimental conclusion is evaluated by considering the uncertainty in measurements obtained in an ASHRAE research project.

Risk-based framework for stream repair projects.

The phrase “stream restoration” has become commonplace to describe a wide variety of stream intervention practices. Rather than using the term “restoration” to cover the broad range of stream interventions, many of which are not restoration at all, the Center for Watershed Protection has moved to using the phrase “stream repair”. In this paper, a risk-based framework is presented for planning and designing stream repair projects, particularly in the vicinity of infrastructure. The method incorporates a wide variety of uncertainties, including non-quantifiable types, such as aesthetics, failure consequences, etc., providing a systematic approach to reducing risk prior to implementation. Individual components of a stream repair project, as well as the overall design, are considered. The results provide a basis for decisionmaking in determining the best option for stream repair and provides a powerful tool for communication between practitioners engaged in design, environmental funding and permitting agencies, infrastructure owners, and other stream stake-holders.

Restoration of Stream Meanders

Stream meander restoration design is often based on empirical equations or other empirical information. The appropriateness of these methods depends on the local physiographic and hydrologic setting and on the ability of the method to adequately describe the planform geometry. Freely formed meander planforms can be described by two equations for the meander arc length and sinuosity. Regional regression equations can be used to verify the use of free-form meander dimensions or as the basis for meander design or stabilization.

Uncertainty in risk of highway bridges assessed for integrated seismic and flood hazards

Abstract Probabilistic risk assessment for bridges under natural hazards is of great interest to engineers for the development of risk mitigation strategies and implementation plans. The present study evaluates risk of an existing highway bridge in California, USA, under the integrated impact of regional seismic and flood hazards. A sensitivity study combining tornado diagram and first-order second moment reliability analyses is conducted to screen significant uncertain parameters to which bridge response is mostly sensitive. A rigorous uncertainty analysis, employing random sampling and Monte Carlo simulation techniques, is performed to obtain variations in fragility and risk curves of the bridge. Observed variations in risk curves at various risk levels are quantified through 90% confidence intervals and coefficients of variation (COV) of risk. It is observed that uncertainty in the estimated risk increases due to the presence of flood hazard at the bridge site, although mean risk does not vary with flood hazard level. Research outcome signifies that the variation in risk due to parameter uncertainty and varied flood hazard level should not be ignored to ensure bridge safety under the stated multi-hazard condition.

Risk Assessment for Stream Modification Projects in Urban Settings

AbstractThe dynamic and stochastic nature of riverine ecosystems creates considerable uncertainty within the practice of stream modification. For streams in urban environments, this uncertainty results in risk to the environment and infrastructure. Bridges, culverts, sewers, fish populations, and drinking water sources could be negatively impacted affected by the response of a stream channel to modification. Communication about the associated risk is critical to minimize effects on water resources and infrastructure. This article describes a vulnerability-criticality risk-logic matrix that can be used to determine the relative risk of stream modification projects. Risk assessment can aid practitioners in understanding and communicating the risk associated with modifying stream channels, and provide guidance for allocating resources to project maintenance or rehabilitation. Two examples are provided to demonstrate the use of the risk assessment.

Probablistic Bridge Pier Scour Factors

Over the past 40 years, numerous studies have been conducted and equations developed to predict bridge-pier scour. Although it is desired to have a conservative approach to scour to increase our sense of safety and reliability, field experience often leads bridge and hydraulics engineers to suspect that the equations over-predict scour at a significant level. To date, few studies have been undertaken to quantify the reliability of the scour equations in terms of probability of exceedance. In this study, the authors used available laboratory data to determine the probability that the design scour will be exceeded using two pier scour equations, both recommended by the Federal Highway Administration: (1) HEC-18 equation and (2) FDOT equation.

APPLYING COST-BASED RISK ASSESSMENT TO STREAM RESTORATION DESIGN

A low-risk, stream restoration design includes methods that validate design assumptions, incorporate uncertainty in the decision-making process during the project design phase, and reduce uncertainty by checking the final design. A twostep method of incorporating uncertainty and risk in stream restoration design has been developed as a combination of Design Failure Modes and Effects Analysis (DFMEA) and risk quantification. As a first step, DFMEA is applied to identify risk in terms of ratings with respect to consequence of failure, the likelihood of occurrence of a failure, and the ability to detect a failure. Due to its evolutionary nature, the DFMEA can be revised to account for design modifications and relative ratings are re-evaluated to examine reductions in uncertainty, and thereby, risk. The second step of the method is quantifying risk using initial and expected failure costs. Expected failure cost is defined as the product of probability of failure and the cost associated with failure. Since failure probability and cost of failure are both difficult to determine directly in stream restoration, the consequence and likelihood of occurrence ratings from the DFMEA can be used to estimate the expected cost and probability of failure, respectively. Using this method, risk can be estimated for several restoration design alternatives and compared to provide justification and guidance on selecting the most cost effective design alternative. The two-step, riskbased method is illustrated through application to a stream relocation project in Pennsylvania and a stream restoration project in North Carolina. Overall, the twostep method presented here can prove valuable in decision-making and will improve the likelihood of success in stream restoration design.