Reliability and redundancy of structural systems with application to highway bridges

For over three decades, researchers have been investigating applications of system reliability concepts and techniques in structural design and evaluation. Over the past twelve years, probability-based system redundancy concepts have been introduced and used to check the availability of warning before collapse occurs. System reliability and redundancy are priority topics in the effort to enhance the development of practical methods for safety assessment and safety assurance in structural system evaluation and design. At present, the Load and Resistance Factor Design (LRFD) provisions in most design specifications are based on the so-called level-one safety checking requirements using load factors and strength reduction factors. These requirements are checked at the member or component level only. For this reason, any formulation of this type, based on single component analysis, results in an inappropriate use of limited funds available at all levels for inspection, repair, and rehabilitation of existing structures, and/or design of new structures. Therefore, in spite of significant progress in the development and application of system reliability theory, the requirements for evaluation and design of structural systems have remained incomplete. The main objective of this study is to provide a general framework for consideration of reliability and redundancy of structural systems with application to highway bridges and to identify critical members in structural systems using a system reliability basis. This study presents a system reliability-based framework for assessing structural performance and structural importance of components in a structural system. Both idealized systems, such as series, parallel, and series-parallel systems, and existing highway bridges are used to exemplify the approach. The proposed methodology takes into account the system performance and quantifies the influences of individual components on the overall performance. The importance of a component is defined as the impact of that component on the overall system reliability. Various results concerning importance measures and ranking of individual components in a structural system are presented. The results from detailed reliability analyses of different topologies of a three-member model showed that system reliability could be vastly different from component reliability. Both the system reliability and the system redundancy (defined as the difference between the system reliability and the reliability of the weakest member) depend on various factors such as the failure model assumed [series, parallel, or mixed (series-parallel)], resistance sharing, residual strength, member reliability level, failure mode correlation, and load and strength variabilities, among others. This framework is applied to the safety assessment of highway bridge superstructures which are classified as series-parallel structural systems. The sensitivity of the system reliability to changes in bridge member reliability is quantified, and critical members are identified for existing bridges in Colorado. The results of this study can be applied to the development of importance factors in codified bridge design, and in the selection of maintenance strategies for bridges. In conclusion, it should be mentioned that the introduction of system reliability and redundancy measures in codified bridge evaluation and design is a practical possibility. However, for implementation of these probabilistic measures in bridge codes further research is needed.