A Parametric Investigation on Ultra-low Cycle Fatigue Damage of Steel Bridge Piers Under Horizontal Bi-directional Seismic Excitations

Ultra-low cycle fatigue (ULCF) damage is one of the main failure modes of steel bridge piers when subjected to severe earthquake. However, existing experimental and numerical studies aiming at ULCF damage of steel piers almost adopt the uniaxial loading strategy, which is different from the real seismic motion. To make up for this problem, the ULCF behavior of steel piers under horizontal bidirectional cyclic loads was investigated in this paper. A two-level zooming analytical system was presented first to save computational cost of the ULCF assessment of steel piers. Its applicability and cost-efficiency were numerically and experimentally verified through detailed elaboration. And based on this, a series of numerical work of piers under horizontal bi-directional and unidirectional cyclic loads were carried out. The relationship between the ductile durability and pier parameters was studied with the help of a micro-damage mechanism-based ULCF assessment method and an energy assumption-based evaluation index. Results showed that compared with the unidirectional loads, bi-directional loads could bring about significant deterioration in the ULCF resistance of piers. The relative ductility capacity of steel piers under bi-directional cyclic loads is approximately consistent as width-to-thickness ratio and slenderness ratio change, and enhances as axial compression ratio increases.