Ultra-low cycle fatigue evaluation method for unstiffened steel piers using fiber model

Abstract

The safety of steel bridge piers is threatened by ultra-low cycle fatigue (ULCF) damage, which dominates the failure mode in those with relatively compact section under strong seismic action. Existing methods for calculating such ULCF damage face the dilemma between accuracy and efficiency. To this end, this paper proposes a simplified evaluation method based on fiber model to expediently evaluate the ULCF damage of unstiffened steel piers, evenly with sound precision. Specifically, the average stress triaxiality is assumed as 0.63 for steel piers, and it is validated by cyclic loading experiments with the modified Coffin-Masson formula and solid element sub-model. The disparity between local plastic strain range in fiber and solid models is compensated by introducing a magnified coefficient. The analyzed results indicate the slenderness ratio, axial compressive ratio, especially the flange thickness and the width-to-thickness ratio, strongly influence the strain concentration effect occurring in the pier bottom. In addition, the proposed close equation for the magnified coefficient fits well with the discrete calculated points. The 3.05% mean absolute error is realized on the predicting half cycles when applying this method in random cyclic loading. Its favorable applicability in practical engineering is also verified with considering different ground motion types.