Two Alternative System Reliability Approaches to the Serviceability Condition Assessment of Spillway Gate Systems on Dams
The U.S. Army Corps of Engineers is one of the Nation’s largest managers of infrastructure. It is responsible for maintaining and operating the Nation’s navigable waterways and is the primary agency for maintaining federal flood control dams. This includes a vast amount of infrastructure that includes roughly 270 navigation dams, 350 reservoir dams, and 238 lock chambers (Bullock and Foltz 1995). The navigable inland waterways carry roughly 17% of the Nation’s intercity cargo – an important economic role (USACE 2004). Over half of the locks and dams are over 50 years old. The entire inventory is deteriorating over time and requires billions of dollars to upgrade, maintain, and repair. The Corps of Engineers requires that a strength-based reliability analysis be completed to justify major rehabilitation projects. Reliability methods are preferred for cost-benefits analyses and for quantifying risk. More often, however, it is the general serviceability of a structure that dictates the requirements for maintenance and repair. The Corps of Engineers has developed a Condition Index (CI) rating system for a variety of structures that assesses the general serviceability condition of a structure based on periodic visual inspections. A Condition Index (CI) is a rating between 0 and 100 that describes the condition of a structure at a point in time. The CI is based on a series of observations by an inspector. At the component level, the inspector classifies what he or she sees into the predefined descriptive category that best matches the observation. Some CIs also include measurements. More often, the condition scores are based on descriptive word pictures that are often vague and difficult to quantify. Chouinard et.al. (2003) developed a deterministic CI rating system for spillway gate systems on dams. The gate system contained 122 separate inspectable components, each with its own condition rating table. The structure was decomposed into a seven-level structural hierarchy of systems, sub-systems, and components. The highest levels are shown in Figure 1. Estes et.al. (2005) developed a reliability-based approach that assigns probabilistic CI ratings for groups of components, systems, and projects. The approach accounts for the considerable uncertainty associated with the CI process which includes: • Uncertainty in the ability of different inspectors to reliably choose the correct condition state and to a greater degree, the appropriate score within a condition state 17 ANALYSIS AND COMPUTATION SPECIALTY CONFERENCE th
Estimating Risk from Spillway Gate Systems on Dams Using Condition Assessment Data
Abstract : Inland waterways maintained by the U.S. Army Corps of Engineers carry about 17% of the nation's intercity cargo, so service interruptions related to waterway infrastructure failure can cause substantial economic loss. Maintenance and repair (M&R) requirements for navigation structures compete for funding with every other national priority, however, and Federal budgeting decisions are determined largely on the basis of net benefit to the nation per dollar invested. The Corps uses analytical tools and methods to help objectively determine project benefits versus costs. In Corps cost/benefit analyses for navigation structures, reducing the risk of failure through repair or rehabilitation is quantified as a benefit. Conventional reliability-based risk analysis is costly and complex, however. This study investigated the adaptation of an existing condition indexing (CI) methodology to assess overall structural risk and failure probability. It was concluded that a risk based analysis, using the well established concepts of a reliability index, failure probability, hazard function, and cost/benefit analysis, is possible and feasible. Because CI data have not been systematically collected and are not available, the methodology proposed here remains untested. A number of reasonable assumptions made in this study have not yet been verified using actual CI data collected over time from existing projects.
Updating Bridge Reliability Based on Bridge Management Systems Visual Inspection Results
Bridge management systems have become increasingly sophisticated over the past decade and provide valuable information about the structural condition of all bridges in the national database. At the same time, reliability methods have gained increasing prominence and are used to forecast life-cycle performance over many decades of structural life. Such reliability analyses need to be updated based on the results of inspections. Specifically targeted nondestructive evaluations are the preferred solution, but are not always available for every bridge. This paper examines how the visual inspection data provided from bridge management systems already in place can be used to update the reliability of a bridge. The limitations and necessary modifications to current practice are discussed. The superstructure of a Colorado highway bridge deteriorating due to corrosion is used as an example.