Study on ultra‐low cycle fatigue fracture of thick‐walled steel bridge piers

Steel structures with thick‐walled section tend to have local fracture before the occurrence of instability under strong earthquakes, which is termed as ultra‐low cycle fatigue (ULCF). In this study, a series of specimens made with structural steel Q345qC for bridge in China were firstly tested to investigate the ULCF fracture behaviour in material level. Based on the test data, three models for ULCF fracture were calibrated. Subsequently, the fracture behaviour of thick‐walled square section steel bridge piers were also investigated in component level. Moreover, the finite element global and submodels of steel bridge piers were established, and ULCF fracture initiation life was predicted by calibrated models and validated against test results. The research results showed that the crack firstly occurred at the bottom of piers under cyclic oblique loading, which was at the junction between base plate and bottom weld, and then, cracks propagated along the bottom weld metal which did not cause a decrease in strength capacity. Finally, the strength capacity of steel bridge piers began to rapidly declined as cracks propagated into base metal. The presented finite element global model can accurately simulate the cyclic behaviour of steel bridge piers, and the submodel has the ability of capturing the local stress and plastic strain. The fracture models accurately predicted ULCF fracture initiation life of steel bridge piers with the average discrepancy of 7.14%. The developed finite element analysis method considering ULCF fracture provides a solution for the further parametric analysis on ULCF fracture performance of steel bridge piers.