Fatigue Reliability Assessment for Long-Span Bridges under Combined Dynamic Loads from Winds and Vehicles

During a bridge’s life cycle, the stresses from multiple dynamic loads might be superposed and progressive fatigue damage might accumulate and induce serious fatigue damage issues. This paper presents a comprehensive framework for fatigue reliability analysis of long-span bridges under combined dynamic loads from vehicles and wind. After modeling the complicated structure details with equivalent orthotropic material, dynamic stress ranges of a long-span bridge are obtained via solving the equations of motion for the vehicle-bridge-wind dynamic system with multiple random variables considered, for instance, vehicle speed, road roughness conditions, and wind velocity and direction. After counting the number of stress cycles at different stress range levels using the rain flow counting method, fatigue damage increments are obtained using the fatigue damage accumulation rule. The probability of failure for the fatigue damage at the end of each block of stress cycles and the cumulative probability of failure can be obtained. As a result, the fatigue life and reliability for the given structure details can be obtained. Based on the results from a case study, the dynamic effects from vehicles are found to be relatively small for long-span bridges and the effects from vehicle speeds and road roughness conditions can be neglected. Nevertheless, even though the stresses from either the vehicle loads or wind loads may not be able to induce serious fatigue problems alone, the superposed dynamic stress ranges cannot be ignored for fatigue reliability assessment of long-span bridges.

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