FATIGUE-RELIABILITY EVALUATION OF STEEL BRIDGES

The fatigue reliability of steel-bridge components is evaluated. The most commonly used S-N curve-based American Association of State Highway and Transportation Officials (AASHTO) method is considered first. The limit-state equation and the uncertainty associated with all the basic random variables are quantified. Using the advanced first-order second-moment method, the corresponding probability of failure is calculated in terms of the reliability index. It is observed that the reliability index is very similar to other design problems involving steel structures. Since the AASHTO approach cannot incorporate the crack-size information even if it is known at the time of evaluation, an alternative linear-elastic fracture mechanics (LEFM) method is proposed. The corresponding limit-state equation and the basic variables are identified, and the uncertainties associated with them are quantified. The reliability index is calculated similarly. For comparison purposes, the reliability indexes according to the AASHTO and LEFM approaches are evaluated for full-penetration butt welds in the tension flange of a steel-box girder used in a public-transportation system. The reliability indexes are almost identical, at least around the design life of 50 years. The LEFM approach still retains the simplicity of the AASHTO method, yet it is extremely powerful and efficient and can incorporate information on crack size as inspection results become available. It is proposed that the LEFM can be used as an alternative to the AASHTO method in the fatigue-damage evaluation of steel bridges. The LEFM method is extended in the companion paper to incorporate information from inspections and the corresponding updating of the underlying reliability.