Fatigue Reliability of Steel Highway Bridge Details

About the report:

The expected life of a steel highway bridge subjected to random, variable-amplitude traffic cycles is highly dependent on damage accumulation caused by various fatigue mechanisms. This study addressed some of the issues associated with developing probabilistic reliability models for steel bridge structures under vehicular traffic loadings. Specifically, methods for incorporating inspection data (e.g., the presence and size of cracks) into classical and fracture mechanics-based fatigue models to predict fracture-critical element damage accumulation were analyzed. The block loading method introduced here assigns a damage state to the cracked detail after each vehicle passage. This method is limited to loading blocks where the plastic zone at the crack tip developed by the pseudo-static response of the bridge must be larger than the crack growth caused by the subsequent dynamic cycles for each vehicle passage. It must also be assumed that the crack length is fixed during a loading block. This method accounts for closure effects by predicting the occurrence of damage only when the crack is opened. When a bridge inspection reveals fatigue cracks, field data in the form of strain measurements can be collected in the vicinity of the fracture detail to identify the characteristic stress block and the fluctuations in the curve attributes attributable to randomness of the traffic loading. Then, the data can be analyzed to identify the statistical properties of the attribute parameters, which include the magnitude of the pseudo-static response, the number of dynamic cycles in each block, and the duration of the vehicle passage. With this information, the distribution of stress blocks can be estimated, and the block loading method can be employed to calculate the fatigue lifetime of each critical detail.

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