Fatigue Reliability Assessment of a Long-Span Cable-Stayed Bridge Based on One-Year Monitoring Strain Data

Structural strain responses, generally induced by traffic, wind, temperature, and so forth, can be used to evaluate the safety and serviceability of in-service bridges. According to one-year continuous monitoring of strain data recorded by the structural health monitoring system of the Sutong cable-stayed bridge, the lifetime fatigue reliability of three welded details of the orthotropic steel deck was investigated: rib-to-deck at the diaphragm (RTDD), rib-to-diaphragm (RTD), and diaphragm-to-deck (DTD) details. In this case study, detailed analysis of the separated components of the raw strain data was first conducted and included the slow-varying trend and the dynamic component. The strain variation trend in the bottom flange synchronized with the mean temperature, while that in the top flange did not. In addition, the strain dynamic component was mainly induced by the local vehicle axle loads. In the second step, rainflow counting was used to obtain the stress range histograms, which were then used to calculate the equivalent stress range according to the lognormal-fitting method. Finally, a time-dependent fatigue reliability evaluation of the described three welded details was conducted using one-year monitoring strain data. Results showed that the fatigue performance of the two welded details, RTDD and DTD, remained satisfactory after 100 years of operation because the failure probabilities were both lower than 10−5. The designed cutout of the diaphragm was applied to the RTD weld at the welded connection between the U rib and diaphragm. This cutout was validated as a means to help achieve better fatigue resistance for the RTD weld. In the future, long-term monitoring strain data (more than 5 years) need to be investigated for better assessment of the fatigue performance of the orthotropic steel deck.