Reliability-based performance indicators for structural members

The implementation of reliability methods for designing new structures and assessing the safety and evaluating the performance of existing structures and infrastructure systems has gained widespread acceptance. Consequently, reliability-based design specifications in the form of load and resistance factor design (LRFD) methods have dominated the development of current codes and standards. This paper reviews the reliability-based performance criteria used to calibrate design and evaluation codes and standards for assessing the strength, serviceability, and fatigue resistance of structural components. The review shows that large differences exist in the target reliability levels adopted for evaluating the strength of various types of structural members and materials. These differences result from many factors, including (1) intended structure design and service life; (2) expected member modes of failure (e.g., ductile or brittle); (3) importance of the individual member to overall system integrity (secondary member, column, or connection); (4) experiences with previous designs; (5) material and construction costs; (6) structure type and occupancy; and (7) risk tolerance of the engineering community and the public within a code’s jurisdiction. For other than seismic hazards, current specifications remain primarily focused on the evaluation of individual structural members and components, although recently proposed performance-based design (PBD) procedures apply varying target member reliability levels that depend on structure categories, modes of failure, and required levels of structural performance. The implementation of reliability-based durability criteria in design standards is still a subject of research owing to difficulties encountered in modeling material degradation mechanisms and their interactions and in the collection and mapping of long-term site-specific data on degrading agents. Because of large epistemic uncertainties, the evaluation of the fatigue safety of structural components in engineering practice still relies on conservative basic models of damage accumulation using S-N curves or basic fracture mechanics crack growth models. Overall, reliability-calibrated structural standards are producing designs that offer a good balance between safety and cost. The future implementation of risk-based methods will further enhance the ability to meet structure-specific performance requirements set by owners and users.