Key Findings

• Bridge design loading is a very important factor because spans designed for lesser design loadings generally have less fatigue capacity for current loadings.
• Span length and predominant car length are critical factors. The estimated service life can increase for spans that are longer than the inside axle spacing of the cars because they do not experience full unloading under each car and thus see smaller stress cycles.
• A service life analysis should be performed at cover plate terminations as the cyclic stress range at cover plate terminations can be greater than the cyclic stress range near mid-span. For spans longer than the inside axle spacing of typical cars, this different stress range can be particularly important.
• The estimated service life based on strain gage measurements is typically higher than the estimated service life based on theoretical calculations.
• Compared to the simple use of the basic fatigue design Category D, the most recent AREMA fatigue rating recommendations can often provide a much longer life estimate.